Motor circuit conductor sizing per NEC Article 430

A motor circuit is three sizing problems, not one

A motor branch circuit is sized in three separate parts that each follow their own rule and pull from their own current, and treating it as one number is the mistake that runs through almost every bad motor install. The conductor is sized one way. The overload that protects the motor from running too hot is sized a second way. The short-circuit and ground-fault device, the breaker or fuse at the top of the circuit, is sized a third way, and it lands far higher than the other two.

Two of those parts size off the full-load current you look up in the NEC tables by horsepower and voltage. One of them, the overload, sizes off the full-load amps stamped on the motor nameplate. They are usually close but not equal, and the code is specific about which current goes where. This is NEC Article 430, the long article that covers motors, motor circuits, and controllers, and it is built around the idea that the wire, the overload, and the short-circuit device are protecting against three different things.

The trouble on the job is the reflex to grab one current, multiply it by one percentage, and size everything off it. That gives you a conductor that might be fine, an overload that is wrong, and a breaker that either nuisance-trips on every start or sits too small to ever let the motor run. Get the three rules straight and the rest is arithmetic and table lookups.

Do you size a motor circuit off the nameplate?

No, not the conductor and not the branch-circuit protection. You size those off the full-load current value in the NEC tables, looked up by horsepower and voltage, and commonly cited at 430.6(A). For a three-phase motor that table is 430.250, and for single-phase it is 430.248. The only part of the circuit that sizes off the nameplate full-load amps is the overload, under 430.32. This split is the single most misread rule in the article.

The reason the code does it this way is consistency. Two identical 25 HP motors from two manufacturers can stamp slightly different nameplate amps, but the code wants the same conductor and the same breaker on both, so it anchors those to a published table value instead of the nameplate. The overload, by contrast, is protecting that specific motor's windings, so it has to track that specific motor's actual nameplate current.

Carry the distinction as a sentence: table FLC sizes the wire and the short-circuit device, nameplate FLA sizes the overload. The currents are close, so swapping them often produces a number that looks plausible and passes a glance. It is exactly the kind of error that survives until an inspector reconstructs the calculation, or until the motor that should have been protected at its nameplate current is sitting under an overload set off a table value that did not match.

How do you size motor branch-circuit conductors?

Size the conductor at 125 percent of the table full-load current for a single continuous-duty motor, then pick a conductor from the ampacity table that meets or beats that number. This is NEC 430.22. You take the FLC from 430.250 or 430.248, multiply by 1.25, and that product is the minimum ampacity the conductor has to carry. Then you go to the ampacity table, commonly 310.16, and read the column for the conductor's temperature rating and the installation conditions.

The 125 percent is the same idea as any continuous load. A motor that runs for hours puts a steady current on the wire and the heat builds, so the code wants the conductor sized with headroom above the running current rather than right at it. A 25 HP, 460 V three-phase motor has a table FLC of 34 A, so the conductor has to carry at least 34 times 1.25, which is 42.5 A. That points you to #8 copper at the 75 degree C column, rated 50 A, with room to spare.

Two things move the answer after the 125 percent. The ampacity table itself derates for ambient temperature and for more than three current-carrying conductors in a raceway, so a hot space or a packed conduit pushes the conductor up. And on a long run, voltage drop can drive the conductor larger than either the ampacity or the 125 percent rule would, which is its own check covered below. The 125 percent is the floor, not always the answer.

Overload protection sized off the nameplate

The overload device protects the motor itself from running too hot, and it is the one part of the circuit sized off the nameplate full-load amps, under NEC 430.32. For a continuous-duty motor of more than 1 HP, the standard limits are tied to the motor's marked service factor and temperature rise. A motor with a service factor of 1.15 or higher, or a marked temperature rise of 40 degrees C or less, gets an overload at no more than 125 percent of nameplate FLA. Every other motor gets 115 percent.

The logic is in the motor's own ratings. A higher service factor or a lower temperature rise means the motor has thermal margin to spare, so the code allows the overload to sit a little higher before it trips. A motor without that margin gets the tighter 115 percent, because it has less room before the windings cook. Read the nameplate for the service factor and the rise before you pick the percentage, because guessing the wrong one undersizes or oversizes the protection.

The overload is a separate device from the breaker or fuse at the top of the circuit. It lives in the starter or the controller, classically as thermal heaters matched to the motor current, or as an electronic overload relay you set to the nameplate value. There is also a next-size-up allowance in 430.32 when the standard percentage does not protect the motor and it will not start or carry the load, letting you bump to a higher limit, but you start at 115 or 125 and only step up by the rule when you have to.

Why is the breaker bigger than the wire?

Because the breaker on a motor circuit is not protecting the wire from overload. It is protecting the circuit from a short circuit or a ground fault, and it has to be large enough to let the motor's starting current through without tripping. That is the part that confuses anyone used to general lighting and receptacle circuits, where the breaker and the conductor are sized to roughly the same number and the breaker is the conductor's overload protection.

On a motor circuit those two jobs are split across two devices. The overload, sized off the nameplate at 115 to 125 percent, is what protects the motor from running too hot. The branch-circuit short-circuit and ground-fault device, sized off the table FLC at percentages that can reach several hundred percent, is what clears a fault. So you legitimately end up with a #8 conductor rated 50 A protected by a 90 A breaker, and that is correct, intended, and code-compliant, because a different device is handling the overload.

This is the conceptual heart of Article 430. The conductor carries the running current at 125 percent. The overload watches the motor's actual current against its nameplate. The short-circuit device is sized high on purpose so the motor can start. Three numbers, three jobs, and the reason they look inconsistent is that they are doing three different things. Once that clicks, the high breaker stops looking like an error and starts looking like the design.

Branch-circuit short-circuit and ground-fault protection

The maximum size of the branch-circuit short-circuit and ground-fault device is set by NEC 430.52 as a percentage of the table full-load current, and the percentage depends entirely on the device type. An inverse-time circuit breaker, the common molded-case breaker, is allowed up to 250 percent of FLC. A non-time-delay fuse is allowed up to 300 percent. A dual-element time-delay fuse, the usual motor fuse, is allowed up to 175 percent. An instantaneous-trip breaker, used inside a listed combination motor controller, is allowed up to 800 percent.

The numbers track how well each device rides through inrush. A time-delay fuse tolerates the starting surge well, so it needs the least headroom at 175 percent. A non-time-delay fuse reacts faster and needs more room at 300 percent so it does not blow on every start. The inverse-time breaker sits at 250 percent. These are maximums, and 430.52 includes a next-standard-size-up allowance: when the calculated percentage does not land on a standard device rating from 240.6, you may go up to the next standard size. There is a further exception that lets you step higher still, up to a hard ceiling, when the motor will not start at the standard percentage.

Pick the device type first, because it sets the percentage. A 34 A table FLC on an inverse-time breaker is 34 times 2.5, which is 85 A, and since 85 is not a standard breaker size, the next-size-up allowance takes you to 90 A. The same motor on a time-delay fuse is 34 times 1.75, about 60 A. The device you choose changes the protection rating by a wide margin, and the right one for the job is usually the one that lets the motor start cleanly while still clearing a fault fast.

Starting current and why the device runs high

A motor at the instant of starting is effectively a short to the line, because the rotor is not turning yet and there is no back-voltage to limit the current. That locked-rotor or inrush current commonly runs six to eight times the motor's running full-load current for the first fraction of a second, then collapses as the motor comes up to speed. The branch-circuit device has to let that surge through without tripping, which is the entire reason it is sized at 175 to 800 percent instead of around 100 percent like a lighting circuit.

This is the trade-off that makes a motor circuit different. Size the short-circuit device tight enough to act like an overload and it trips on inrush every time the motor starts. Size it to ride the inrush and it is far too large to protect the conductor from a sustained overload, which is why the separate overload device exists. The two devices divide the work along the time axis: the overload handles the slow heat of a running motor, the short-circuit device handles the fast fault, and the inrush passes under both.

Inrush is also why a marginal supply shows up at start. The motor pulls its heaviest current exactly when the voltage at its terminals is already sagging under that current, so a circuit that runs fine can still fail to start, with the contactor dropping out or the motor stalling and growling. When a motor starts hard or trips its breaker only on starting, the answer is usually inrush meeting an undersized device or a soft supply, not the motor itself.

The disconnecting means and where it sits

Every motor needs a disconnecting means, and its ampere rating is set by NEC 430.110 at not less than 115 percent of the table full-load current, with a horsepower rating at least equal to the motor. So a 34 A table FLC needs a disconnect rated for at least 34 times 1.15, about 39 A, sized to the next standard switch and rated for motor duty at the motor's horsepower. The horsepower rating matters as much as the ampere rating, because it certifies the switch can break the motor's inrush, not just carry its running current.

Location is the other half of the rule. The disconnecting means generally has to be in sight of the motor and the driven machinery, meaning visible and within about 50 ft, so a worker servicing the equipment can lock it open and know it is off without trusting a panel three rooms away. There is an allowance to use a single lockable disconnect elsewhere under specific conditions, but the default expectation, and what an inspector looks for first, is a disconnect you can see from the motor.

This is a safety device, not a sizing afterthought. The whole point is that someone working on the motor can kill it locally and verify it is dead. A disconnect out of sight, or one without the horsepower rating to break the load, is a finding and a hazard at the same time. Size it at 115 percent of table FLC, rate it for the horsepower, and put it where the person on the motor can reach it.

Voltage drop on the motor feeder

Motor runs are often long, and a motor is more sensitive to low voltage than most loads, so voltage drop is a real constraint on the conductor and not just a formality. The 125 percent conductor from 430.22 might satisfy ampacity comfortably and still arrive at the motor terminals low, because over a few hundred feet the resistance of the wire eats voltage the ampacity table never accounts for. On a long feeder, voltage drop can drive the conductor larger than the motor sizing rules alone would.

A motor starved for voltage draws more current to make the same torque, runs hotter, and edges toward the overload trip point or shortens its winding life. The starting dip is worse, because the inrush current pulls the terminal voltage down hardest exactly when the motor needs voltage to develop starting torque. A long run that holds an acceptable running voltage can still sag too far on start, and the motor stalls or the contactor chatters. That is a voltage-drop problem wearing a mechanical disguise.

Run the voltage-drop calculation with the routed length and the running current on any motor feeder of real length, and check the starting condition separately on a hard-starting load. The voltage-drop field guide covers the formula, the resistance values, and the targets. The point here is that the motor sizing rules give you a minimum conductor, and on a long run the voltage-drop check is what tells you whether that minimum is actually big enough.

How do you size a feeder for several motors?

A feeder supplying a group of motors is sized under NEC 430.24 at 125 percent of the largest motor's full-load current plus the sum of the full-load currents of all the other motors in the group. You apply the 125 percent once, to the largest motor only, because only one motor's starting and continuous headroom needs to be carried in the feeder at a time. The rest are added at 100 percent of their table FLC. Non-motor loads on the same feeder are added at their own rules, continuous at 125 percent and noncontinuous at 100 percent.

The example shows why it matters. Three motors at 40 A, 34 A, and 28 A table FLC give a feeder ampacity of 40 times 1.25, which is 50, plus 34 plus 28, for 112 A. Size everything at 125 percent and you would carry 127.5 A and over-buy the feeder. Forget the 125 percent on the largest and you would carry 102 A and under-size it. The rule is specific about applying the adder once, to the biggest motor.

The feeder overcurrent protection is a separate rule, 430.62. The feeder device is limited to the largest branch-circuit short-circuit device in the group, the one you sized at 430.52, plus the sum of the full-load currents of the other motors. So the feeder protection is built up from the branch protections, not calculated fresh off the feeder ampacity. Size the conductor by 430.24 and the protection by 430.62, and keep them straight, because they use different inputs.

The starter, the contactor, and the VFD

The control gear sits between the branch circuit and the motor, and on a conventional across-the-line start it does not change the sizing logic. The starter holds the contactor that switches the motor and the overload relay that protects it, so the overload that 430.32 calls for usually lives right there in the starter, set to the nameplate FLA. The branch-circuit device and conductor are still sized off the table FLC the same way whether the start is a plain contactor, a soft starter, or a reduced-voltage start.

A variable frequency drive changes the picture because it has an input side and an output side that carry different currents. The conductor and protection feeding the drive are sized to the drive's rated input current, which is not the same as the motor's full-load current and which the drive's listing and instructions specify. The conductors from the drive to the motor are sized to the motor, but the drive's own listed requirements and the manufacturer's instructions govern that output, and they often call for specific conductor types and lengths to manage the switching waveform.

The drive also brings its own protection. Most VFDs provide electronic motor overload protection internally, and many are listed as the branch-circuit and ground-fault protection or specify exactly what upstream device to use, so you follow the drive's listing rather than applying the bare 430.52 percentages to the motor. The rule with a drive is the same as with any listed equipment: the listing and the manufacturer's instructions control, and you size to the input current the drive is rated for, not to the motor on the output.

The equipment grounding conductor for the motor

The motor circuit needs an equipment grounding conductor sized by NEC 250.122, off the rating of the branch-circuit short-circuit and ground-fault device, not off the phase conductors directly. So the same breaker or fuse rating you set at 430.52 is the value you take into the 250.122 table to find the minimum ground. A 90 A breaker on the worked example points to a 250.122 ground sized for that device rating, even though the phase conductors are only #8.

The complication on a motor circuit is upsizing. When you increase the phase conductors above the minimum, commonly for voltage drop on a long run, 250.122(B) requires the equipment grounding conductor to grow in proportion by the same ratio of circular-mil area. The ground exists to carry fault current long enough to trip that high-rated breaker, so on a circuit where the breaker is already large relative to the conductors, an undersized or unupsized ground is exactly the wrong place to cut a corner.

Treat the ground as part of the same decision, not a leftover. Size it off the branch device rating, upsize it with the phase conductors when you grow them, and record the size you actually installed. The voltage-drop field guide covers the upsizing-the-ground step in detail, because that is where it usually gets dropped on a real pull.

Worked example: 25 HP, 460 V three-phase motor

Take a 25 HP, 460 V three-phase continuous-duty motor with a nameplate full-load amps of 32 A and a service factor of 1.15. From NEC Table 430.250, the table full-load current for a 25 HP, 460 V three-phase motor is 34 A. That 34 A table value, not the 32 A nameplate, is what sizes the conductor and the branch device. The 32 A nameplate sizes the overload. Hold those two currents apart and the rest falls out.

The conductor sizes at 125 percent of the 34 A table FLC, which is 42.5 A minimum ampacity, pointing to #8 copper at the 75 degree C column, rated 50 A. The overload sizes off the 32 A nameplate at 125 percent, because the service factor is 1.15, giving a 40 A maximum overload setting. The branch short-circuit device, an inverse-time breaker, sizes at 250 percent of the 34 A table FLC, which is 85 A, and the next-standard-size-up allowance takes that to a 90 A breaker. The disconnect sizes at 115 percent of 34 A, about 39 A, rated for 25 HP.

Set the three numbers side by side and the lesson is obvious. The wire is rated 50 A. The overload trips around 40 A. The breaker is 90 A. A 50 A wire under a 90 A breaker looks wrong to anyone trained on lighting circuits, and it is exactly right here, because the overload at 40 A is what actually protects that conductor and motor from a sustained overload, while the 90 A breaker is there only to clear a fault and let the motor start.

Duty cycle and special motors

The 125 percent conductor rule and the standard overload percentages assume a continuous-duty motor, the most common case. A motor on a genuine intermittent, periodic, or short-time duty cycle is sized differently, because it does not put a steady current on the conductor for hours. NEC 430.22 and its tables apply a duty-cycle factor based on the service classification, so a motor that runs five minutes out of every hour can take a smaller conductor than a continuous motor of the same horsepower. Use the duty-cycle provision only when the duty is genuinely rated and documented, not because the motor happens to cycle.

Multispeed motors carry more than one full-load current, one per speed, and the conductor and protection generally have to suit the speed that draws the most. The nameplate lists the separate currents, and the article has specific rules for sizing each winding's conductors and protection. Part-winding and wye-delta starting arrangements split the motor current across multiple conductor sets during start, which changes how those conductors are sized and protected, and they have their own sections in Article 430.

Wound-rotor motors, synchronous motors, and DC motors each have their own full-load current tables and their own protection percentages in 430.52, which differ from the standard AC squirrel-cage values. A wound-rotor motor also has a secondary circuit, between the rotor and the resistor or controller, that has to be sized for the secondary current. When the motor is anything other than a plain continuous-duty AC induction motor, stop and read the section that applies to that type before you reuse the standard percentages.

Why does my motor breaker trip on starting?

A breaker that holds while the motor runs but trips the instant it starts is almost always too small to pass the inrush, not a sign of a fault. The starting current is several times the running current, and if the branch device was sized like a normal load breaker at roughly 125 percent instead of the 250 percent that 430.52 allows for an inverse-time breaker, the inrush blows right past it on every start. The fix is to size the device by the motor rule, not the load rule.

The next suspect is device type. A non-time-delay fuse or a standard breaker reacts faster than a time-delay fuse, so a circuit that nuisance-trips on a hard-starting motor sometimes just needs a time-delay fuse or a breaker sized at the correct percentage. The next-size-up and the further step-up allowances in 430.52 exist precisely for the motor that will not start at the first calculated rating, but you apply them by the rule and up to the ceiling, not by reaching for a bigger breaker until the tripping stops.

Before you blame the device, rule out a soft supply and a mechanical bind. A long or undersized feeder drops the terminal voltage on start, which raises the current the motor pulls and makes the trip worse. A motor that is hard to turn by hand, a jammed driven machine, or a failing bearing all raise the starting current too. Check the device sizing against 430.52 first, then the supply voltage on start, then the mechanical load, in that order.

Common errors and what the inspector checks

An inspector reconstructing a motor circuit looks at the inputs most likely to be wrong, and the first is which current sized which part. They check that the conductor and the branch device came off the table FLC, not the nameplate, and that the overload came off the nameplate, not the table. Sizing the whole circuit off one current is the error they see most, because it is the natural mistake and it leaves a trail in the numbers.

The second thing checked is whether the breaker is too small to start the motor or, less often, oversized past what 430.52 allows. A device sized like a load breaker nuisance-trips and gets called in; a device sized past the percentage with no documented step-up reason is over the limit. The third is the overload: present, sized to the nameplate, and set to the right percentage for the service factor or temperature rise. A missing or wrong overload is a safety finding, because nothing else on the circuit protects the motor from a slow overload.

After those, the inspector looks at the disconnect in sight and rated for the horsepower, the equipment grounding conductor sized to the branch device and upsized if the phase conductors were, and on a long run, whether voltage drop was considered. The clean way through is the same as any inspection: have the calculation written down, with the table FLC, the nameplate FLA, and each of the three sizing results next to the section that drove it. When the numbers are on paper with their sources, the inspection is a check, not an argument.

What to document

A motor circuit calculation nobody wrote down is one you cannot defend at inspection or reproduce when the motor gets swapped for a bigger one. The record answers the question that comes up later: which current sized each part, and against what rule. Because a motor circuit has three independent results off two currents, the record has to show both currents and all three results, or the next person cannot tell whether the breaker that looks oversized is correct.

Capture the motor horsepower and voltage, the table full-load current and its table, the nameplate full-load amps and service factor, the conductor size and the 125 percent basis, the overload device and its setting, the branch short-circuit device type and rating, the disconnect rating and horsepower, and the equipment grounding conductor. Note any next-size-up or step-up allowance you used and why. One row per motor keeps a whole motor schedule auditable.

Common mistakes

• Sizing the conductor or the breaker off the nameplate FLA instead of the table FLC.
• Sizing the overload off the table FLC instead of the nameplate FLA.
• Using one current and one percentage to size all three parts of the circuit.
• Sizing the branch breaker like a load breaker, too small to pass the starting inrush.
• Leaving the overload out, or setting it to the wrong percentage for the service factor.
• Ignoring the device type when applying 430.52, so the wrong percentage gets used.
• Skipping the voltage-drop check on a long motor feeder that passes ampacity.
• Failing to upsize the equipment grounding conductor when the phase conductors are upsized.

Field checklist

Standards and references

Motor circuits are governed by the NEC, NFPA 70, Article 430, which is one of the longer articles because it splits the circuit into parts that each get their own rule. The full-load current that sizes the conductor and the branch device comes from the tables, 430.250 for three-phase and 430.248 for single-phase, applied through 430.6. The conductor sizing at 125 percent for a single continuous-duty motor is 430.22. The branch-circuit short-circuit and ground-fault percentages by device type are 430.52 and its table.

Overload protection off the nameplate at 115 or 125 percent is 430.32. The disconnecting means and its 115 percent rating are 430.102 and 430.110. Feeder conductor sizing for several motors is 430.24, and the feeder overcurrent protection built up from the branch devices is 430.62. The equipment grounding conductor is 250.122, sized off the branch device rating and upsized in proportion when the phase conductors grow. The conductor ampacity itself comes from the ampacity tables, commonly 310.16, with the correction and adjustment factors in 310.15.

These Article 430 section numbers have been stable across several code cycles, but the exact table values, the standard device sizes in 240.6, and the local amendments all shift, so confirm the numbers against the edition the jurisdiction has adopted before you put them on a submittal. Where the motor is fed by a listed drive or controller, the equipment listing and the manufacturer's instructions govern the input sizing and the protection, and they override the bare percentages.

Units, terms, and conversions

A motor circuit trades in a handful of terms that travel between the nameplate, the NEC tables, and the controller, and keeping them straight is most of the work. The two currents are the ones to never confuse: the table full-load current that sizes the conductor and the branch device, and the nameplate full-load amps that sizes the overload.

Motor size is given in horsepower in the NEC tables and often in kilowatts on imported nameplates, where roughly 0.75 kW equals 1 HP. Current is in amps throughout. Conductor size is AWG for smaller conductors and kcmil for larger ones. The terms below are the ones that move between the schedule, the tables, and the inspection.

FLC (full-load current)

The table current from NEC 430.250 or 430.248 by horsepower and voltage; sizes the conductor and the branch device

FLA (full-load amps)

The current stamped on the motor nameplate; sizes the overload only, under 430.32

Overload

The device that protects the motor from running too hot, set to 115 or 125 percent of nameplate FLA

SCGF protection

Branch-circuit short-circuit and ground-fault device, sized off table FLC at the 430.52 percentage by device type

Locked-rotor / inrush current

The high current at the instant of starting, commonly six to eight times running current, which the branch device must pass

Service factor

Nameplate multiplier of thermal margin; 1.15 or higher allows the overload at 125 percent instead of 115 percent

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