Electrical
How to read a three-phase motor nameplate
Every rating you need to wire, protect, size, and replace a motor lives on the plate. Read it right and it sets the conductor, the overload, the starter, and the spare.
Direct answer
A three-phase motor nameplate is the manufacturer's record of every rating you need to wire, protect, size, and replace the motor: horsepower, voltage, full-load amps, service factor, NEMA code and design letters, RPM, frame, insulation class, and enclosure. Read it right and it sets the conductor, the overload, the starter, and the spare you order.
Key takeaways
- NEC 430.6 splits the current: size conductors and branch protection from table FLC (Table 430.250), set the overload from nameplate FLA.
- Per NEC 430.32, a service factor of 1.15 or greater allows overload up to 125 percent of nameplate FLA; a 1.0 SF allows 115 percent.
- Synchronous speed is 120 times frequency divided by poles; at 60 Hz, 1750 RPM reads four-pole, 1160 six-pole, 3450 two-pole.
- Match a replacement on frame, full-load RPM, enclosure, voltage, duty, and design letter, not horsepower alone, or it will not fit, perform, or last.
- On a VFD, use an inverter-duty motor built to NEMA MG-1 Part 31; standard insulation may not survive the drive's voltage spikes.
The nameplate is the whole motor on one plate
A motor nameplate is the manufacturer's record of how the motor is built and how it has to be applied. Every decision you make about that motor traces back to it. The conductor and the breaker come off the data, the overload gets set from it, the starter is chosen against the code and design letters stamped on it, and the day it fails, the plate is what tells you which motor to order.
Read it wrong and the errors stack quietly. Wire it for the wrong voltage and it runs hot or burns. Set the overload off the wrong current and it either trips on nothing or fails to protect the windings. Order a replacement off horsepower alone and the new motor does not bolt to the base. None of these announce themselves at startup. They show up weeks later as heat, nuisance trips, and a motor that will not mount.
Two jobs lean on the plate hardest, and both have their own guide. Sizing the conductor, the overload, and the branch protection is its own set of rules under NEC Article 430, covered in the motor circuit conductor sizing guide. Picking how the motor gets up to speed, across the line or reduced voltage or on a drive, is covered in the motor starting methods guide. This guide is about reading the plate so those two decisions start from the right numbers.
Photograph the plate before you do anything else. They corrode, paint over, and rub illegible on the motors you most need to identify, and a clear photo in the job record has saved more replacement orders than any catalog.
Horsepower and kW: the rated output
Horsepower on the plate is the mechanical output the motor is rated to deliver at the shaft, not the electrical input it draws. A 25 HP motor turns 25 HP of work into the load and pulls more than that from the line, because no motor is 100 percent efficient. The difference between input and output is the efficiency, stamped separately.
HP and kW are the same rating in two unit systems. One horsepower is 0.746 kW, so a 25 HP motor is about 18.6 kW of output and a 50 HP motor is about 37 kW. NEMA plates lead with horsepower and often list kW alongside. IEC plates lead with kW. Same motor, same output, different label, and a 37 kW IEC motor and a 50 HP NEMA motor are close cousins.
The number to hold onto is that horsepower describes what the motor can drive, not what the load actually demands. A 25 HP motor on a load that only needs 12 HP runs lightly loaded, draws well under its full-load amps, and runs at a worse power factor for it. Oversizing is common and it costs efficiency and power factor every hour the motor runs. Size the motor to the load, then read the rest of the plate against that.
Rated voltage and the dual-voltage plate
Rated voltage is the supply the motor is designed to run on, and it has to match the system you are connecting to. A motor marked 460 V belongs on a 480 V system, because the motor rating sits below the nominal distribution voltage on purpose, to allow for the drop between the transformer and the motor terminals. The common pairings are a 230 V motor on a 240 V system and a 460 V motor on a 480 V system.
Many motors are dual voltage, marked 230/460. That motor runs on either system, but only if the leads are connected for the voltage you have. The windings are reconnected internally through the lead connections: the lower voltage parallels or wyes the coils, the higher voltage puts them in series. Connect a dual-voltage motor for 230 and put it on 480 and you have doubled the voltage on the windings. It will not survive that.
The dual-voltage plate lists a full-load amp value for each voltage, and the higher voltage draws roughly half the current of the lower. That is the same power at twice the voltage. It is also why the trade runs larger motors at 460 rather than 230 where the system allows: half the current means smaller conductors and less drop over the run. Match the connection to the supply, and confirm it against the connection diagram on the plate or inside the peckerhead before you energize.
Full-load amps: the current at rated load
Full-load amps, the FLA on the plate, is the current the motor draws at rated horsepower, rated voltage, and rated frequency. The manufacturer measured it on that motor design. It already accounts for that motor's real efficiency and power factor, which is what makes it specific to the motor in front of you rather than a generic value.
On a dual-voltage plate you get two FLA numbers, one per voltage, and you use the one that matches how the motor is connected. A 25 HP motor might read 64 amps at 230 V and 32 amps at 460 V. Read the wrong line and every downstream number is off.
The plate FLA has one primary job in the NEC: it sets the overload. It is not the number you size the conductor or the branch protection from. That distinction is the single most misread point on the whole plate, and it gets its own section next, because it trips up apprentices and experienced hands alike. The short version: the plate FLA protects the motor, the NEC table value sizes the wire.
What is the difference between FLA and FLC?
FLA is the nameplate full-load amps, the current your specific motor draws. FLC is the full-load current from the NEC tables, a standardized value the code assigns by horsepower and voltage regardless of the motor brand. They are usually close but rarely equal, and the NEC uses each for a different job. Using one where the code calls for the other is the most common Article 430 mistake on industrial work.
The rule, found at NEC 430.6, is a two-value rule. You size the branch-circuit conductors and the short-circuit and ground-fault protection, the breaker or fuse, from the table FLC, commonly the three-phase values in NEC Table 430.250. You set the overload, and only the overload, from the nameplate FLA. The conductor and breaker use the table. The overload uses the plate. Memorize it that way.
The reason is deliberate. The table values are conservative and built around typical motors, so the wire and the breaker get sized for a motor a little hungrier than yours might be, which builds margin into the parts that protect the building. The overload is the part that protects the motor itself, so it has to track the actual motor, and the actual motor is the nameplate. The full sizing walk-through, with the conductor at 125 percent and the high branch-protection percentages, is in the motor circuit conductor sizing guide. The point here is just which current feeds which calculation.
| Value | Source | What it sizes |
|---|---|---|
| Table FLC | NEC Table 430.250 (three-phase) | Conductors, branch short-circuit and ground-fault protection |
| Nameplate FLA | Stamped on the motor plate | Overload protection only |
What is the service factor on a motor?
Service factor is the overload margin the manufacturer designed into the motor, written as a multiplier such as SF 1.15. A 1.15 service factor means the motor can carry 15 percent over its rated horsepower continuously at rated voltage and frequency without immediate damage. A motor with no margin reads SF 1.0. Many general-purpose open motors carry 1.15, while a number of totally enclosed motors run at 1.0.
The margin is real but it is not free horsepower. Running in the service factor draws more current, runs the windings hotter, and shortens insulation life. It is headroom for the occasional surge or a hot day, not a place to live. Size the motor so the normal load sits at or below the rated horsepower and keep the service factor as the reserve it was meant to be.
The service factor changes the overload setting, and that is where it earns its place on the plate. The NEC, at 430.32, ties the allowable overload to the service factor and the temperature rise. For a motor with a marked service factor of 1.15 or greater, the overload is generally permitted at up to 125 percent of the nameplate FLA. For a 1.0 service factor motor, the allowance drops to 115 percent. Those are the common figures, but confirm the exact percentages and conditions against the adopted code edition, because the section has specific qualifiers.
What is the NEMA code letter on a motor?
The NEMA code letter is a single letter that tells you how much current the motor pulls at the instant of starting. It encodes the locked-rotor kVA per horsepower, the inrush the motor demands when the rotor is still and full voltage is applied. The NEC carries the letter-to-kVA ranges in Table 430.7(B). A letter early in the alphabet means low inrush. A letter late in the alphabet means a hard, current-hungry start.
This is not the same as the design letter, and people mix them up constantly. The code letter is about how much starting current. The design letter, covered next, is about the torque-and-slip shape of the motor. A plate carries both, and they answer different questions.
The code letter drives two field decisions. It feeds the branch short-circuit and ground-fault protection sizing, because a motor with high inrush needs a device that will not trip on the starting surge. And it tells you whether across-the-line starting will sag the supply or whether you need a reduced-voltage method. A large motor with a high code letter on a soft utility transformer is exactly the case where you reach for a soft starter or a drive. That tradeoff, the inrush against the starting torque and the supply stiffness, is the whole subject of the motor starting methods guide.
| Code letter | Locked-rotor kVA per HP | Reading |
|---|---|---|
| A | 0 to 3.14 | Lowest inrush |
| F | 5.0 to 5.59 | Moderate |
| G | 5.6 to 6.29 | Common on standard Design B |
| H | 6.3 to 7.09 | Higher inrush |
| J and later | 7.1 and up | Hard start, check the supply |
The NEMA design letter and the torque curve
The design letter, A through D, tells you the shape of the motor's torque-speed curve and how much it slips under load. It answers a question the horsepower cannot: how the motor behaves while it is coming up to speed and pulling against the load. Pick the wrong design for the load and the motor either cannot start it or runs in a way the load does not want.
Design B is the standard, and unless the plate says otherwise, assume B. It gives normal starting torque, around 150 percent of rated, with low starting current and low slip under 5 percent. It suits the loads most motors drive: fans, blowers, centrifugal pumps, and compressors that start unloaded or lightly loaded.
The others are for specific loads. Design C gives high starting torque, around 200 percent, for loads that start under load, like positive-displacement pumps, loaded conveyors, and reciprocating compressors. Design D gives very high starting torque and high slip, in the 5 to 13 percent range, for high-inertia and shock loads such as cranes, hoists, and punch presses. Design A looks like B but allows higher starting current, which can make it harder on the supply. When you replace a motor, match the design letter, because a Design B dropped onto a load that needed a Design C will struggle to break it away.
| Design | Starting torque | Slip | Typical load |
|---|---|---|---|
| A | Normal | Under 5 percent | Fans, pumps (higher inrush than B) |
| B | Normal, about 150 percent | Under 5 percent | Fans, pumps, compressors (the standard) |
| C | High, about 200 percent | Under 5 percent | Loaded conveyors, positive-displacement pumps |
| D | Very high | 5 to 13 percent | Cranes, hoists, presses, high-inertia loads |
Efficiency and the energy the motor costs to run
The nominal efficiency on the plate is the percentage of electrical input the motor turns into shaft output at full load. A 93.6 percent efficient 50 HP motor wastes the other 6.4 percent as heat, and over a year of running, that waste is the largest line item the motor has. The purchase price of a motor that runs continuously is small next to the power it burns.
Efficiency is regulated. In the United States, most general-purpose motors must meet NEMA Premium efficiency levels, defined in NEMA MG-1 and enforced through federal energy rules, so a new motor in that class will carry a NEMA Premium marking. The IEC equivalent is the IE class, where IE3 is premium and IE4 is super premium. A plate may carry both a percentage and a class.
Read the efficiency as money, not trivia. When an old standard-efficiency motor fails, the replacement is almost always a premium-efficiency motor, and the power savings often pay back the price difference well within the motor's life. Confirm the current efficiency requirement against the applicable energy rule and the project specification, because the floors have moved upward across recent cycles.
Power factor at full load
Power factor on the plate is the ratio of real power doing work to the apparent power the motor draws from the line, at full load. Induction motors are inductive loads, so the power factor runs below unity, commonly in the 0.80 to 0.90 range at full load for a motor near its rated output. The plate value is the full-load figure.
Power factor matters because the apparent power, the kVA, is what the conductors and the utility actually carry. At a power factor of 0.85, a motor doing 37 kW of real work pulls about 43.5 kVA from the supply, and the conductors and transformer have to handle the full kVA, not just the kW. A lower power factor means more current for the same work, and on a plant with many motors it can drive a utility power-factor penalty.
Here is the part people miss: power factor collapses on a lightly loaded motor. Run a 25 HP motor at a quarter of its load and the power factor can fall well below the plate value, dragging the current up out of proportion to the work done. It is one more reason oversizing a motor costs you, and one more reason the plate value assumes the motor is actually loaded to its rating.
RPM, synchronous speed, and the poles
The RPM on the plate is the full-load speed, the speed the shaft turns when the motor is loaded to its rating. It is not a round number on purpose. An induction motor always turns slightly slower than its magnetic field, and that gap is the slip that lets it make torque. A plate reading 1765 RPM is a four-pole motor whose field turns at 1800.
Synchronous speed, the speed of the rotating field, comes from the supply frequency and the number of poles. The relationship is 120 times the frequency divided by the number of poles. On 60 Hz, a two-pole motor synchronizes at 3600, four poles at 1800, six poles at 1200, and eight poles at 900. You read the poles straight off the full-load RPM: a plate near 1750 is four-pole, near 1160 is six-pole, near 3450 is two-pole.
The full-load RPM matters for replacement and for the load. A pump or fan is matched to a speed, and dropping in a motor one pole-count off, an 1800 where the system wanted a 1200, changes everything the load does. On a centrifugal load the flow follows the speed and the power follows the cube of the speed, so a speed mismatch is not a small error. Match the full-load RPM, not just the horsepower.
| Poles | Synchronous RPM (60 Hz) | Typical full-load RPM |
|---|---|---|
| 2 | 3600 | About 3450 |
| 4 | 1800 | About 1750 |
| 6 | 1200 | About 1160 |
| 8 | 900 | About 870 |
The frame size and why it controls the bolt-up
The frame number is the physical dimension code. It fixes the shaft height off the base, the shaft diameter, the keyway, and the bolt-hole pattern in the feet. Two motors with the same horsepower but different frames will not interchange, because one of them will not bolt to the base or line up with the coupling. The frame is the field that decides whether the replacement physically fits.
Most current motors carry a T frame, the standard set in 1964, like 184T or 254T. The number is not arbitrary. The first digits encode the shaft height: divide the first two digits of the frame by four and you get the shaft centerline height in inches, so a 254T sits 6.25 inches off the base. Letter suffixes carry meaning too. A plain foot-mount frame differs from a C-face or flange motor that bolts directly to a pump or gearbox, and special suffixes flag nonstandard mounting or shaft dimensions.
This is the field that catches people ordering a replacement off horsepower alone. The new 25 HP motor shows up, the horsepower matches, and the frame is one size off, so the shaft sits at the wrong height and the coupling will not align. Record the full frame designation, suffix letters included, and match it. If the frame on the old motor is illegible, the dimension that narrows it fastest is the shaft height measured to the centerline.
The insulation class and the thermal limit
The insulation class sets the maximum temperature the winding insulation can take before it degrades. The common classes and their limits are A at 105 degrees C, B at 130, F at 155, and H at 180. The class is a ceiling on total winding temperature, the ambient plus the rise plus a hotspot allowance, not a rating you read in isolation.
Most modern general-purpose motors are built with Class F insulation, and many are then applied at a Class B temperature rise. That gap between the insulation's capability and the actual rise is deliberate margin. It buys insulation life, because every roughly 10 degrees C of sustained overtemperature cuts insulation life by about half, and it gives the motor headroom for a hot location or an occasional run into the service factor.
On the plate, read the insulation class together with the temperature rise and the ambient. The insulation class is what the windings can survive. The rise and ambient, covered next, are what the motor is rated to actually reach. A Class F motor run at a Class B rise has a long life ahead of it. The same motor run hard against its Class F limit, hot day after hot day, does not.
| Insulation class | Maximum winding temperature |
|---|---|
| A | 105 degrees C |
| B | 130 degrees C |
| F | 155 degrees C |
| H | 180 degrees C |
Temperature rise and the rated ambient
Two thermal numbers on the plate work together: the ambient the motor is rated for and the temperature rise it is allowed to add to that ambient. The standard rated ambient is 40 degrees C, about 104 degrees F. The temperature rise is how much the windings climb above the ambient when the motor runs at full load, measured by the resistance method after the motor reaches thermal equilibrium.
The rise stacks on the ambient and has to stay under the insulation class limit. At the standard 40 degree C ambient, a Class B rise is about 80 degrees C, a Class F rise about 105, and a Class H rise about 125, each leaving a hotspot margin below the class ceiling. Add the rise to the ambient and you land near the insulation limit, which is how the system is meant to balance.
The number that bites in the field is the ambient. A motor rated for a 40 degree C ambient that lives in a boiler room or a closed equipment space at 50 degrees C has lost margin before it makes any heat of its own, and it has to be derated. Run a 40 degree C motor in a hotter space without derating and you spend its insulation life early. When the location runs hot, check the ambient against the plate and derate, or spec a motor rated for the higher ambient.
Duty: continuous or intermittent
The duty marking tells you whether the motor is rated to run forever or only for a stretch. Most general-purpose motors are marked CONT or S1 for continuous duty, meaning they can carry rated load indefinitely and reach a stable temperature. That is the default you want for a pump or fan that runs all day.
A time-rated or intermittent motor is a different animal. A plate marked with a time, like 30 minutes, or an IEC duty code from S2 through S10, is rated to carry its load only for that period before it has to cool. These show up on loads that work in bursts, like a hoist or a valve actuator. Put an intermittent-duty motor on a continuous load and it overheats, because it was never designed to shed the heat of an all-day run.
When you replace a motor, match the duty. It is an easy field to skip, since most motors are continuous and you stop expecting anything else. The intermittent ones are exactly where that assumption costs a burned motor.
The enclosure and the environment
The enclosure tells you what the motor is sealed against, and it has to match where the motor lives. The two you meet most are ODP and TEFC. An open drip-proof motor, ODP, draws ambient air through the windings to cool itself and is built so dripping water cannot enter at an angle. It runs cool and costs less, but it pulls the room's air through the motor, so it belongs in a clean, dry, indoor space.
A totally enclosed fan-cooled motor, TEFC, is sealed so no outside air passes through the windings, and an external fan blows air over the housing to cool it. It handles dust, moisture, washdown, and outdoor and dirty industrial locations. TENV, totally enclosed non-ventilated, is sealed with no fan, cooling by its surface alone, used on small or low-load motors. An explosion-proof motor is built to contain an internal ignition so it cannot light the atmosphere around it, and it carries a Class and Division or Zone rating for the hazardous location it is listed for.
Match the enclosure to the environment, not to the price. Dropping an ODP motor into a wet or dusty space because it was cheaper and on the shelf is how you buy a second motor in a year. The explosion-proof rating is not negotiable: in a classified area, the enclosure and its listing are a safety requirement, and a non-rated motor in that space is a hazard, not a substitution.
| Enclosure | Cooling | Where it belongs |
|---|---|---|
| ODP (open drip-proof) | Ambient air through the windings | Clean, dry, indoor |
| TEFC (totally enclosed fan-cooled) | External fan over a sealed housing | Dust, moisture, washdown, outdoor |
| TENV (totally enclosed non-ventilated) | Surface cooling, no fan | Small or low-load, dirty or wet |
| Explosion-proof | Sealed to contain internal ignition | Classified hazardous locations only |
Phase and frequency: the supply it expects
Phase and frequency on the plate state the supply the motor was built for. A three-phase motor marked 3 PH and 60 Hz expects three-phase power at sixty cycles. These look like the most obvious fields on the plate, which is exactly why they get glossed over.
Frequency is not cosmetic. A 50 Hz motor on a 60 Hz supply runs faster than its rating and its other ratings shift with it, and a 60 Hz motor on 50 Hz runs slower and can overheat at the same load. Motors built for export markets often carry dual frequency ratings with different voltage and speed for each. Read the line that matches your supply.
Frequency also matters the moment a drive enters the picture, because a VFD makes its own frequency to vary the speed. The plate frequency is then the base point the drive works around, which is part of why the VFD section later treats the plate as the starting reference for the drive's parameters.
Bearings and lubrication for maintenance
Better plates and the motor's data sheet list the bearing numbers for the drive end and the opposite end, often something like a 6309 at the drive end, along with the grease type and a relubrication interval. This is maintenance data, not sizing data, but it is the difference between servicing the motor and guessing at it.
Bearings are the most common mechanical failure on an induction motor, and they fail from the wrong grease, too much grease, mixing incompatible greases, or a missed interval as often as from age. Knowing the bearing number lets you stock or order the exact replacement instead of pulling the bearing to read it. Knowing the grease and interval lets you service it on schedule rather than running it to failure.
On a regreasable motor, follow the manufacturer's grease and interval, and resist the urge to pack it full. Overgreasing forces grease into the windings and churns heat into the bearing, and it kills more bearings than undergreasing does. Sealed bearings have no fitting and are not serviced; they run to the end of the grease and get replaced with the bearing.
The connection diagram and the lead numbers
The connection diagram shows how to wire the motor leads for the voltage and the starting method you want, and the lead numbers on the wires correspond to it. The diagram lives on the plate, on a separate plate, or on a sticker inside the conduit box. Wire the leads to the diagram, not from memory, because the same physical leads connect differently for different voltages and different starters.
Lead count tells you the motor's flexibility. A three-lead motor, T1 through T3, is single-voltage and connects one way. A nine-lead motor, T1 through T9, is the common dual-voltage motor, reconnected for low or high voltage by the diagram, typically in a wye or delta arrangement. A twelve-lead motor opens up more options, including wye-delta starting and part-winding starting, because the extra leads let a starter reconfigure the windings during the start.
Those extra leads are where the starting method lives. Wye-delta starting and part-winding starting both work by switching the winding connections through the motor leads as the motor comes up to speed, which is why they need the lead count to support it. If the job calls for one of those reduced-voltage starts, the motor has to have the leads for it, and the connection diagram is what tells you. How each method trades inrush against starting torque is the subject of the motor starting methods guide; reading the plate just tells you whether the motor can do it.
Field example: reading a sample plate
Take a typical industrial plate and read it line by line. The motor is 25 HP, 230/460 V, 64/32 A, SF 1.15, code G, Design B, 1765 RPM, frame 284T, Class F insulation, 40 degree C ambient, continuous duty, TEFC, three-phase, 60 Hz, power factor 86 percent, efficiency 93.6 percent.
Here is what each line tells the person applying it. The 25 HP is about 18.6 kW of shaft output. The 230/460 V is dual voltage, so on a 480 V system you wire it for 460 and the FLA you care about is the 32 A, not the 64. That 32 A is what sets the overload, and at SF 1.15 the overload can generally go up to 125 percent of it. The conductor and breaker, though, come from the NEC table value for a 25 HP three-phase motor at 460 V, not from the 32 A.
The code G says moderate starting inrush, around 5.6 to 6.29 kVA per horsepower, so across-the-line starting is likely fine on a normal supply. The Design B says normal starting torque for a fan or pump. The 1765 RPM makes it a four-pole motor whose field turns at 1800, slipping under load. The 284T frame fixes the mounting and shaft, so a replacement must match it. Class F insulation at a 40 degree C ambient gives long life with margin. TEFC means it can live in a dusty or damp space. Read top to bottom, the plate has told you how to wire it, how to protect it, how to start it, and what to order when it dies.
| Nameplate field | Sample value | What it tells you |
|---|---|---|
| HP | 25 (about 18.6 kW) | Shaft output the motor can drive |
| Voltage | 230/460 V | Dual voltage, wire for the system you have |
| FLA | 64/32 A | Current at rated load; sets the overload |
| SF | 1.15 | 15 percent margin; raises the overload allowance |
| Code | G | Starting inrush, 5.6 to 6.29 kVA per HP |
| Design | B | Normal starting torque, low slip |
| RPM | 1765 | Four-pole motor, 1800 synchronous |
| Frame | 284T | Mounting and shaft for the replacement |
| Insulation / ambient | Class F / 40 C | Thermal limit and rated ambient |
| Enclosure | TEFC | Sealed, fan-cooled, dirty or damp ok |
Setting the overload from the plate
The overload relay protects the motor from running overcurrent, the slow kind of overload that cooks windings over minutes and hours, and it is set from the nameplate FLA. Not the table value, not the breaker rating, the plate FLA for the voltage the motor is connected at. This is the one protection device that tracks the actual motor.
The setting follows the service factor. Under NEC 430.32, a motor with a marked service factor of 1.15 or greater generally allows the overload at up to 125 percent of the nameplate FLA, and a 1.0 service factor motor at up to 115 percent. So a motor reading 32 A with SF 1.15 sets near 40 A, while the same FLA at SF 1.0 sets near 37 A. Modern electronic overloads and many starters let you dial the exact FLA in, which is cleaner than picking a heater element, but the input is still the plate FLA.
Confirm the percentages and the conditions against the adopted code edition, because the section carries qualifiers and an allowance to step up if the motor will not start or run at the lower setting. The fuller treatment of overload sizing alongside the conductor and the breaker is in the motor circuit conductor sizing guide. The point that belongs on the plate-reading side is simple: the overload comes off the nameplate, every time.
Using the plate to troubleshoot a running motor
The plate is the baseline you measure a sick motor against. Clamp the running current and compare it to the nameplate FLA. A motor pulling well over its FLA at a steady load is overloaded, and the question is whether the load grew, the voltage sagged, or the motor is failing mechanically. A motor pulling far under FLA is lightly loaded or unloaded, which points at a broken coupling, a closed valve, or a load that walked away.
Voltage is the next reading, and the plate is the reference. Measure the voltage at the motor terminals under load and compare it to the rated voltage. A motor starved for voltage draws more current to make the same torque and runs hot, so low terminal voltage and high current together point at a voltage-drop problem on the feeder rather than a bad motor. That is where the voltage drop field guide picks up.
Check the current balance across the three phases while you are there. The three leg currents should read close to each other. A meaningful imbalance points at a supply problem, a loose connection, or a developing winding fault, and it drives extra heating that the overload may or may not catch in time. Read the plate, read the meter, and the gap between them tells you which system to chase.
The nameplate when a drive is in front of the motor
A variable frequency drive needs the motor's plate data to do its job. You program the drive with the nameplate voltage, FLA, horsepower, base frequency, and full-load RPM, and the drive uses those to model the motor and to set its own protection. Get the plate values into the drive wrong and the motor's protection is wrong from the first run.
The drive also changes what the plate has to be. A PWM drive puts fast voltage spikes on the windings that a standard motor's insulation was never asked to take, and on a long cable run those spikes can double at the motor terminals. An inverter-duty motor built to NEMA MG-1 Part 31 has an insulation system rated for that voltage stress, commonly to a 1600 volt peak on a 460 V motor, along with bearing protection against the shaft currents a drive induces. A plate or data sheet that states MG-1 Part 31 compliance is what you want on a drive.
Watch the marketing language. Inverter ready, drive duty, and VFD rated are not the same as MG-1 Part 31 compliant, and only the standard reference on the plate or the documentation tells you the insulation is actually rated for it. Watch the cooling too: a TEFC motor's shaft-driven fan slows with the motor, so a motor run slowly on a drive may need a separately powered blower to stay cool. How the drive starts the motor and varies its speed, against the other starting methods, is covered in the motor starting methods guide.
Speccing a replacement from the plate
When a motor dies, the plate is the spec sheet for its replacement, and the failure of new hands is to match the horsepower and call it done. Horsepower is necessary and far from sufficient. A replacement has to match the horsepower, the voltage, the full-load RPM, the frame, the enclosure, the mounting, and the duty, and it should match the design letter and service factor too.
Work the list in the order that bites. The frame decides whether it bolts up and the shaft aligns, so a frame mismatch stops you at the base. The full-load RPM, read as the pole count, decides whether the load behaves, because a centrifugal pump or fan is matched to a speed. The enclosure decides whether it survives the environment. The voltage decides whether you can even wire it to the supply. Miss any one of these and the new motor either will not fit, will not perform, or will not last.
Two fields are worth upgrading rather than matching blind. Efficiency has been pushed up by energy rules across recent cycles, so the replacement for an old standard-efficiency motor is normally a premium-efficiency motor, and that is an improvement to take. And if the old motor is going on a drive, the replacement should be an inverter-duty motor even if the original was not. Match the physical and electrical fit, photograph and record the old plate, and confirm anything illegible by measuring before you order.
- Match horsepower and the connected voltage.
- Match the full-load RPM, read as the pole count, to the driven load.
- Match the frame, suffix letters included, so it bolts up and aligns.
- Match the enclosure to the environment, ODP, TEFC, or an explosion-proof rating.
- Match the mounting, foot, C-face, or flange, and the shaft and keyway.
- Match the duty and the design letter, and match or improve the service factor.
- Upgrade to premium efficiency, and to inverter-duty if it runs on a drive.
Reading the plate by the load: pumps, fans, chillers, and data centers
The same plate reads differently depending on what the motor drives, because the load decides which fields you weigh hardest. On a centrifugal pump or fan, the load starts light, so a Design B motor across the line is the usual answer, and the full-load RPM is the field you protect, since the flow and pressure follow the speed. An ODP motor suits a clean mechanical room; a TEFC suits a wet or dusty one.
Chiller and large compressor motors push you to the high end of the plate. They are large enough that the code letter and the starting method matter, because the inrush of a big motor across the line can sag the building supply, and many run on soft starters or drives for that reason. Their service factor, insulation class, and ambient all get a hard look, because they run long and hot, and a chiller plant is exactly where a premium-efficiency motor pays back fastest.
Data center cooling motors, the fans and pumps on CRAC and CRAH units and the chilled-water plant, get read for reliability and for the drive. Most run on VFDs to track the cooling load, so the inverter-duty rating and the cooling at reduced speed are the fields that decide whether the motor lasts. In a space where a cooling failure is an outage, the plate is read to confirm the motor was built for continuous, drive-fed duty, not just to confirm the horsepower.
What to document
A motor record built from the plate is what makes the next decision fast, whether that decision is a replacement order, an overload setting, or a troubleshooting call. Capture the plate once, completely, and you stop pulling motors apart to read corroded stamps later.
Record the make and model, the horsepower, the voltage and the FLA for each voltage, the service factor, the code and design letters, the full-load RPM, the frame with suffix, the insulation class, the ambient, the duty, the enclosure, the phase and frequency, the efficiency and power factor, and the bearing numbers if the plate carries them. Keep the photo of the plate with the record, because a number you transcribed wrong is worse than no number at all.
| Nameplate field | What it means | How it is used |
|---|---|---|
| Horsepower / kW | Rated shaft output | Match the load and the replacement |
| Voltage | Rated supply, single or dual | Wire to the system; select the FLA line |
| FLA | Current at rated load | Set the overload |
| Service factor | Continuous overload margin | Raises the overload allowance |
| Code letter | Locked-rotor kVA per HP | Starting inrush and branch protection |
| Design letter | Torque and slip shape | Match to the load type |
| RPM | Full-load speed | Pole count and load match |
| Frame | Physical dimensions | Bolt-up and shaft for replacement |
| Insulation / ambient / rise | Thermal ratings | Life and derating in hot spaces |
| Enclosure | Environmental sealing | Match to the location |
| Efficiency / PF | Energy performance | Operating cost and PF correction |
Common mistakes
- Sizing the conductor or the breaker from the nameplate FLA instead of the NEC table FLC.
- Setting the overload from the table value or the breaker instead of the nameplate FLA.
- Ignoring the service factor when setting the overload allowance.
- Wiring a dual-voltage motor for the wrong voltage, or reading the wrong FLA line.
- Ignoring the code letter and starting a high-inrush motor across the line on a soft supply.
- Ordering a replacement on horsepower alone and missing the frame, RPM, or enclosure.
- Putting an ODP motor in a wet or dusty space, or a non-rated motor in a classified area.
- Running a standard motor on a VFD instead of an inverter-duty motor built to MG-1 Part 31.
Field checklist
Want this checklist to run itself on every job — with photo proof and a signed record crews can hand the customer? That's FieldOS.
Standards and references
The NEC, NFPA 70, governs how the plate data is applied. Article 430 covers motors end to end. The nameplate markings and the rule that you size the overload from nameplate FLA while sizing conductors and branch protection from the table values are found around 430.6, the full-load current tables sit at 430.250 for three-phase motors and 430.248 for single-phase, the code-letter kVA ranges are in Table 430.7(B), and the overload sizing tied to service factor is at 430.32. The exact section numbers and percentages shift between code cycles, so confirm them against the adopted edition and any local amendments before you cite them on a submittal.
NEMA MG-1 is the motor standard behind the plate itself. It defines the design letters and their torque and slip characteristics, the frame dimensions, the temperature rise allowances, the efficiency levels including NEMA Premium, and, in Part 31, the requirements for inverter-duty motors run on drives. IEC standards carry the parallel definitions, including the IE efficiency classes, on motors built to the metric system.
Where the manufacturer's data sheet or the equipment listing states a tolerance or a requirement tighter than the general rule, that listing governs for that motor. Read the plate, apply the code that controls the point, and let the manufacturer's instructions and the project specification override a rule of thumb when they are stricter.
Units and terms
Motor data spans two unit systems and a stack of abbreviations, and the same motor reads differently across a NEMA plate, an IEC plate, and a spec.
Horsepower is the NEMA output unit; kW is the IEC unit, at 0.746 kW per horsepower. Speed is in RPM on both. Frame is a NEMA dimension code; IEC motors use a metric frame in millimeters. Efficiency shows as a NEMA percentage and class or an IEC IE class. The terms below are the ones a plate makes you know cold.
- FLA
- Full-load amps, the nameplate current at rated load; sets the overload
- FLC
- Full-load current from the NEC tables; sizes conductors and branch protection
- Service factor (SF)
- The continuous overload margin, such as 1.15 for 15 percent over rated
- Code letter
- Locked-rotor kVA per horsepower, the starting-inrush indicator
- Design letter
- NEMA A through D, the torque-and-slip class of the motor
- Slip
- The gap between synchronous and full-load speed that lets the motor make torque
- Frame
- The NEMA dimension code fixing shaft height, shaft, and bolt pattern
- TEFC / ODP
- Totally enclosed fan-cooled and open drip-proof, the two common enclosures
FAQ
What is on a motor nameplate?
A three-phase motor nameplate carries horsepower or kW, rated voltage, full-load amps, service factor, the NEMA code and design letters, full-load RPM, frame size, insulation class, ambient and temperature rise, duty, enclosure, phase, frequency, efficiency, and power factor. Together they tell you how to wire, protect, start, and replace the motor.
What is the difference between FLA and FLC?
FLA is the nameplate full-load amps, the current your specific motor draws at rated load. FLC is the full-load current from the NEC tables, a standardized value by horsepower and voltage. You size conductors and branch protection from the table FLC, and you set the overload from the nameplate FLA.
What is the service factor on a motor?
Service factor is the continuous overload margin built into the motor, written as a multiplier like SF 1.15, meaning it can carry 15 percent over rated horsepower continuously. It runs hotter doing so, shortening insulation life. A 1.15 or greater service factor also raises the allowable overload setting to 125 percent of nameplate FLA.
What is the NEMA code letter on a motor?
The NEMA code letter encodes the locked-rotor kVA per horsepower, the inrush current the motor pulls at startup. Early letters mean low inrush; late letters mean a hard, current-hungry start. It is found in NEC Table 430.7(B) and tells you whether across-the-line starting suits the motor and the supply.
How do I find the motor speed and poles from the nameplate?
The full-load RPM on the plate is the loaded speed, slightly below synchronous. Synchronous speed is 120 times the frequency divided by the poles, so at 60 Hz a plate near 1750 RPM is four-pole (1800 synchronous), near 1160 is six-pole, and near 3450 is two-pole. The gap is the slip.
How do I set the overload from the nameplate?
Set the overload from the nameplate FLA for the connected voltage, not the table value or the breaker. Under NEC 430.32, a service factor of 1.15 or greater generally allows up to 125 percent of FLA, and a 1.0 service factor up to 115 percent. Confirm the percentages against the adopted code edition.
What does TEFC mean on a motor nameplate?
TEFC means totally enclosed fan-cooled. The motor is sealed so no outside air passes through the windings, and an external fan blows air over the housing to cool it. It handles dust, moisture, washdown, and outdoor locations. ODP, open drip-proof, runs cooler and cheaper but needs a clean, dry indoor space.
What is the NEMA design letter on a motor?
The design letter, A through D, sets the motor's torque-speed curve and slip. Design B is the standard, with normal starting torque around 150 percent and low slip, for fans and pumps. Design C gives high torque for loaded starts, Design D gives high torque and slip for high-inertia loads like hoists.
How do I spec a replacement motor from the nameplate?
Match more than horsepower. The frame must match so it bolts up and the shaft aligns, the full-load RPM must match the driven load, the enclosure must suit the environment, and the voltage must match the supply. Also match duty, design letter, and service factor, and consider upgrading to premium efficiency or inverter-duty.
Does a VFD use the motor nameplate data?
Yes. You program the drive with the nameplate voltage, FLA, horsepower, base frequency, and RPM so it can model and protect the motor. On a drive, use an inverter-duty motor built to NEMA MG-1 Part 31, because standard insulation may not survive the drive's voltage spikes, and verify cooling at reduced speed.
People also ask
Codes cited in this guide
This guide is written and reviewed against the published standards below. Always confirm the current adopted edition with the authority having jurisdiction.