Dry-type transformer sizing and installation field guide

The dry-type transformer and the four-part job

A dry-type transformer steps voltage from one level to another with no oil, using air and a solid insulation system to cool the windings. The workhorse in a commercial building is the 480 V delta primary to 208Y/120 V secondary step-down, the one that takes the building's 480 V distribution and makes the 208 V three-phase and 120 V single-phase that receptacles, lighting, and small equipment actually run on. Sized right and installed right, it disappears into the electrical room and runs for thirty years.

The job has four parts that all have to be right together. Size the transformer in kVA to the load it feeds, with headroom. Protect it on the primary, and on the secondary where the rules require, under NEC Article 450. Ground the secondary as a separately derived system, because that is exactly what it is. And give it room to shed heat. Miss any one of those and the other three do not save you.

What goes wrong is rarely the transformer itself. It is a unit sized to the connected load with no margin, a primary breaker picked off the conductor instead of the transformer rule, a secondary that nobody bonded as a separately derived system, or a 75 kVA can shoved into a closet with 4 in of clearance and the ventilation louvers facing a wall. The transformer runs hot, trips on energization, or fails inspection, and the cause traces back to one of those four parts done wrong.

How do you size a dry-type transformer?

You size a dry-type transformer in kVA, starting from the calculated load it has to feed, then pick the next standard kVA above that number with headroom. The transformer is sized to the secondary load, the connected and demand load on the panel it feeds, which comes out of the load calculation. The companion load-calculation guide covers how that demand number is built. The transformer takes that number as its input.

For a three-phase load, the kVA is the secondary voltage times the secondary current times the square root of three, divided by 1000. Run the panel's design load in amps through that and you get the kVA the transformer has to deliver. Then round up to a standard size. Standard dry-type ratings run 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, and 1000 kVA, and you take the first one at or above your calculated load.

Headroom is not padding. Pick the standard size right at the load and the first added circuit pushes it into overload, and a transformer run continuously over its rating ages the insulation fast. Most designs leave the transformer loaded around 80 percent of its rating at design load, which keeps it cooler and leaves room for growth. Size to the load you will have in a few years, not just the load on the day you energize it.

V

Secondary line-to-line voltage in volts, the level the transformer delivers

I

Secondary design load current in amps, from the load calculation

kVA

Rated apparent power in thousands of volt-amperes, the size of the transformer

What are the primary and secondary full-load amps?

The full-load amps on each side come straight from the kVA and the voltage on that side, and they size the conductors and the overcurrent protection. For a three-phase transformer, the full-load amps equal the kVA times 1000 divided by the voltage times the square root of three. Run it once for the primary voltage and once for the secondary voltage, and you have the two currents the whole install is built around.

The primary current is smaller because the primary voltage is higher. The same power moves through both windings, so the high-voltage side carries less current and the low-voltage side carries more. On a 480 V to 208 V transformer the secondary current is more than double the primary current, which is why the secondary conductors and the secondary panel are the big copper and the primary feeder is comparatively small.

This is the relationship that catches people who size both sides off one number. The primary breaker and primary conductors come off the primary FLA. The secondary conductors and the secondary panel come off the secondary FLA. They are different currents on the same transformer, and mixing them up is how a secondary gets fed with conductors sized for the primary, undersized for the current they actually carry.

Primary FLA

Rated current on the primary side, off the primary voltage, sizes the primary feeder and device

Secondary FLA

Rated current on the secondary side, off the secondary voltage, sizes the secondary conductors and panel

Field example: a 75 kVA 480 to 208/120 V transformer

Take a 75 kVA transformer, 480 V three-phase delta primary, 208Y/120 V secondary, feeding a panel. Run the full-load amps on each side. Primary: 75 times 1000 divided by 480 times 1.732, which is about 90 A. Secondary: 75 times 1000 divided by 208 times 1.732, which is about 208 A. So this is roughly a 90 A primary, 208 A secondary machine, and every sizing decision flows from those two numbers.

The primary feeder and primary breaker are built around 90 A. The secondary conductors and the secondary panel are built around 208 A. Watch the secondary main: 125 percent of 208 A lands at 260 A, so the secondary panel main is commonly a 225 A or 250 A device sized at or below that limit, and the secondary conductors are sized to carry 208 A continuously.

Change the kVA and both currents move in step. A 112.5 kVA unit on the same voltages runs about 135 A primary and 312 A secondary. A 45 kVA unit runs about 54 A primary and 125 A secondary. The voltages set the ratio. The kVA sets the size.

What overcurrent protection does a transformer need?

A transformer has its own overcurrent rule, separate from the conductors feeding it, because the device is protecting the transformer windings, not just the wire. The rule for transformers rated 1000 V or less lives in NEC 450.3(B) and Table 450.3(B), and it gives you two ways to do it.

Primary-only protection is a single overcurrent device on the primary, set at not more than 125 percent of the primary full-load current. Where 125 percent does not land on a standard device size, you are permitted to round up to the next standard size. On the 75 kVA example, 125 percent of the 90 A primary is about 113 A, which rounds up to a 125 A primary device, and that one device protects the transformer.

Primary-and-secondary protection is the scheme that buys you a bigger primary device to ride inrush. With an overcurrent device on the secondary set at not more than 125 percent of the secondary full-load current, the primary device is allowed up to 250 percent of the primary full-load current. On the 75 kVA unit, that lets the primary go up to about 225 A while a 250 A or smaller secondary main does the close-in protecting. The secondary device is usually the main breaker in the panel the transformer feeds, which is why the tap rule and the panel main tie directly into this.

A few cautions. The 125 and 250 percent figures are for transformers over 9 A of rated current, which covers nearly every distribution transformer. Small control transformers between 2 A and 9 A, and under 2 A, carry higher primary-only percentages. The percentages are maximums, not targets, so a smaller standard device that still carries the load and rides inrush is legal and often better. And these are the 450.3 transformer numbers only. The secondary conductors still have to be protected on their own under the conductor and tap rules. Confirm the table against the adopted code edition before you cite a number on a submittal.

The conductors: primary feeder and the secondary tap

The primary feeder is sized like any feeder: to the primary full-load amps, with the continuous-load 125 percent applied, and protected by the primary overcurrent device. Nothing exotic there. The secondary is where it gets specific, because transformer secondary conductors are not an ordinary feeder tap and they get their own treatment in NEC 240.21(C).

The catch on the secondary is that there is no overcurrent device at the source rated for the secondary conductors. The primary breaker protects the primary, not the secondary, because the voltage and current ratios across the transformer mean the primary device cannot see and protect the secondary conductors. So the code lets the secondary run to an overcurrent device downstream, within limits.

The common one is the 25-ft secondary conductor rule, 240.21(C)(6). Secondary conductors can run up to 25 ft without protection at the transformer if they have an ampacity, adjusted for the voltage ratio, of at least one-third the primary overcurrent device rating, they terminate in a single circuit breaker or set of fuses rated no more than the conductor ampacity, and they are protected from physical damage, typically by being in a raceway. There is also a 10-ft rule, 240.21(C)(2), with tighter conductor sizing, for short secondary runs. Either way, size the secondary conductors to 125 percent of the secondary FLA first, then confirm they satisfy the tap rule, not the other way around. Confirm the section numbers against the adopted edition before a submittal.

Is the transformer secondary a separately derived system?

Yes. A dry-type transformer secondary is a separately derived system every time, because the secondary winding has no direct electrical connection to the primary supply conductors. That makes the secondary a new source, and it has to be grounded and bonded as its own system under NEC 250.30, the same single-point logic as the service. The companion grounding guide covers the separately derived system rules in depth. This is how they land on a transformer.

Three connections make it right. One system bonding jumper, the neutral-to-ground bond for the new system, made at the transformer or at the first downstream disconnect, at exactly one point. One grounding electrode conductor from that same point to the nearest effective grounding electrode, commonly the building steel or the metal water pipe near the transformer, sized from the secondary conductors by Table 250.66. And the new neutral, run as a system bonded only at that one point and kept separate from ground everywhere downstream.

The error that undoes a clean service is the bond count at the transformer. No system bonding jumper at all, so the secondary has no neutral-ground reference and a secondary ground fault has no low-impedance path home. Or a system bonding jumper at the transformer and a second neutral-ground bond in the downstream panel, which puts normal neutral current on the equipment grounding conductors and the raceway. Bond the secondary once, at the source, exactly like the service.

The secondary panel and the main

The panel the transformer feeds is sized to the secondary full-load amps, and its main overcurrent device usually does double duty: the secondary protection required by 450.3 and the termination required by the 25-ft tap rule. That is the clean way to build it. Secondary conductors from the transformer run to a panel whose main breaker is rated at not more than 125 percent of the secondary FLA, and that one device satisfies the transformer secondary protection and the tap rule at the same time.

The neutral on the secondary is the new system neutral. It bonds to ground at the transformer through the system bonding jumper, and from there it runs to the panel isolated from ground, on its own neutral bar, the same as any subpanel downstream of the service. The bonding screw stays out of the secondary panel. The single neutral-ground bond is back at the transformer.

A main-lug-only panel with no main breaker is legal on the secondary only if the secondary protection lives somewhere else and the tap rule is satisfied another way. Most installs use a main breaker panel, because it gives you the secondary overcurrent device, the conductor termination, and a disconnect in one box.

Heat and the temperature-rise class

A transformer turns part of the power passing through it into heat, the core losses and the winding losses, and that heat has to leave the enclosure or the windings cook. The core loss is roughly fixed, and the winding loss rises with the square of the load, so a transformer run near its rating makes much more winding heat than one run at half load. This is the physical reason headroom matters: a lightly loaded transformer runs cooler and lasts longer.

Dry-type transformers carry a temperature-rise class, the average winding rise above a 40 C ambient at full load. The common classes are 80 C, 115 C, and 150 C rise. A lower rise class runs cooler at full load and has more overload headroom, but costs more. Most general-purpose dry-types are 150 C rise on a 220 C insulation system, which leaves margin between the operating temperature and the insulation limit. An 80 C rise unit on the same 220 C insulation runs much cooler and is the pick where overload capacity or long life matters, like a transformer that will see harmonic heating.

The ambient is part of the rating. The rise class assumes a 40 C ambient. Put the transformer in a hot, unventilated room and the actual winding temperature is the rise plus whatever the room runs above 40 C, which can push it past where the insulation is happy even though the load is within the rating. Heat is an install problem as much as a sizing problem.

How much clearance does a dry-type transformer need?

A dry-type transformer needs two different clearances, and people confuse them. The first is the ventilation and combustible clearance from NEC 450.21, which keeps the heat off nearby surfaces and lets air move through the unit. The second is the electrical working space from 110.26, which keeps the area in front of the equipment clear for a person to work on it safely. They are separate requirements and you have to meet both.

For ventilation, dry-type transformers rated 112.5 kVA or less must be kept at least 12 in from combustible material, unless they are separated by a fire-resistant heat-insulating barrier or the unit is listed and marked for a smaller separation. Larger units, over 112.5 kVA, generally require a fire-resistant transformer room, with similar listed-marking exceptions. The clearances marked on the transformer by the manufacturer govern, and you keep the ventilation louvers clear so the air can actually move. Box a transformer into a tight enclosure with the louvers against a wall and you starve it of cooling no matter what the catalog clearance said.

The working space is the 110.26 requirement, and it does not go away because the equipment is a transformer. You need the working depth in front of the unit, commonly 3 ft for these voltages, the working width, and the headroom, kept clear and not used for storage. The inspector checks the working clearance before the wiring, every time. A transformer crammed into a corner where you cannot stand in front of the secondary terminals is a violation before anyone looks at how it is wired. Confirm both the 450.21 ventilation distance and the 110.26 working space against the adopted code edition and the nameplate markings.

K-factor and harmonic loads

Nonlinear loads, the switching power supplies in computers, servers, drives, and LED drivers, draw current in pulses instead of a smooth sine wave, and those pulses are full of harmonics. Harmonic current heats a standard transformer more than its sine-wave rating accounts for, mostly through eddy losses that climb fast with frequency, and the triplen harmonics add up on the neutral instead of canceling. Feed a building full of IT load through a standard transformer sized only on kVA and it runs hot at a load the nameplate says is fine.

A K-rated transformer is built for that. It carries the extra thermal capacity to handle a defined level of harmonic current without overheating, and it comes with a neutral rated at 200 percent of the phase current to carry the additive triplen harmonics, a feature the listing standard UL 1561 ties to the K rating. K-ratings run K-4, K-9, K-13, K-20 and up, the number reflecting how much harmonic content the unit is built for. K-13 suits a typical office mix of computers and lighting. K-20 is the common pick for data processing, critical care, and UPS-fed loads with heavy sustained harmonics.

This is where the transformer ties into the power distribution unit and the data hall load. A PDU in a data center is a K-rated transformer plus distribution in one cabinet, sized for the harmonic profile of the racks. On any install feeding serious nonlinear load, specify the K rating from the load, and size the neutral conductor for 200 percent, not 100 percent, of the phase current. A standard transformer in that service is a callback waiting for the first hot summer.

Taps: adjusting for a high or low primary voltage

Dry-type transformers come with no-load taps on the primary, a set of connection points that adjust the turns ratio slightly to correct for a primary voltage that runs high or low. The common arrangement is full-capacity taps at 2.5 percent steps, typically two above and two below nominal, giving a range of plus or minus 5 percent. They are full-capacity taps, so the transformer carries its full kVA on any tap.

Use them when the measured primary voltage is consistently off nominal. If the 480 V primary actually sits at 504 V because the transformer is close to the service, the secondary runs proportionally high, and 208 V becomes about 218 V, which is hard on the connected equipment. Move to a tap above nominal and the higher turns ratio brings the secondary back down to where it belongs. Low primary voltage, common at the far end of a long primary feeder, gets corrected the other way, on a tap below nominal.

The word no-load is the safety part. These are off-load taps, changed with the transformer de-energized, not under load. Open the primary disconnect, verify it dead, move the tap links to the same position on all three phases, torque them, and document the position. Changing a no-load tap on a live transformer is how you arc a tap link and damage the winding. Set the taps at commissioning after you measure the actual secondary voltage, not by guessing at install.

Inrush and why the primary breaker rides through it

When you energize a transformer, the magnetizing inrush current spikes far above the full-load current for the first few cycles, commonly 8 to 12 times the primary full-load amps, decaying over roughly the first second as the core settles into normal magnetizing. That surge is why the primary overcurrent device cannot simply be set at full-load current. A breaker tight to the running current trips every time the transformer is switched on.

This is half the reason the 450.3 percentages exist. The primary-only 125 percent rule, with the round-up to the next standard size, gives a device that carries the load but is still loose enough to ride the inrush on most distribution transformers. The primary-and-secondary scheme, with its 250 percent primary device, gives even more room to ride inrush, because the close-in secondary device handles the actual fault protection. Either way the primary device is intentionally sized above the running current to let the transformer energize.

The device type matters as much as the rating. An inverse-time breaker or a time-delay fuse tolerates the brief inrush and still clears a sustained fault. An instantaneous-only or fast device sized for the load nuisance-trips on energization. If a transformer trips its primary every time it is switched on, the problem is almost never a fault. It is a primary device too small or too fast for the inrush.

Sound, vibration, and location

A transformer hums. The core laminations magnetostrict at twice the line frequency, a steady 120 Hz buzz on a 60 Hz system, and it carries through the structure if the unit is hard-mounted to a wall or a light floor. The sound is not a defect. It is the core doing its job, and it is louder on a fully loaded unit and on a cheaper core.

Location is the fix, not silencing the transformer. Keep it out of and away from quiet occupied spaces: not on the wall shared with a conference room or a bedroom, not directly above a quiet office on a lightweight floor. An electrical room with a masonry wall between it and the occupied space is the right home. Where the transformer has to sit near a noise-sensitive space, mount it on the manufacturer's vibration isolators, keep the conduit connections flexible so they do not transmit the hum into the structure, and avoid bolting it rigidly to a resonant wall.

The bigger the kVA, the louder the hum and the more the location matters. A 15 kVA unit in a closet is rarely a complaint. A 300 kVA unit on a shared wall is a callback the first night someone tries to sleep or meet next to it.

Energizing and commissioning

Before a transformer is energized, it gets the same receiving discipline as any piece of gear, scaled to its size. The test regimen that governs medium-voltage transformer acceptance is overkill for a 75 kVA dry-type, but the logic carries down: confirm it is undamaged, confirm the insulation is sound, confirm the connections are tight, then energize it under control and verify the output.

The core checks come before energizing. Insulation resistance with a megohmmeter, winding to winding and winding to ground, to confirm the insulation system is dry and intact, especially if the unit sat in a damp space during construction. A visual on the connections and the torque on every lug, primary and secondary, because a loose secondary lug at 208 A is a hot connection from day one. The tap setting, confirmed and matched on all three phases. And the grounding and bonding, the system bonding jumper and the grounding electrode conductor, confirmed in place before there is voltage on the secondary.

Then energize the primary first, with the secondary main open, and let the transformer magnetize with no load. Check the secondary voltage phase to phase and phase to neutral, all three phases, before you close the secondary main onto the panel. If the secondary voltage is off, that is the moment to recheck the taps, de-energized, not after the building is loaded and someone reports equipment running high or low. Record the readings as the energization record.

The grounding and bonding inspection

The inspector walks a transformer install for a short, predictable list, and most of it is the separately derived system grounding. They look for one system bonding jumper at the transformer, and they look hard for a second neutral-ground bond in the downstream panel that would put current on the metal. They look for the grounding electrode conductor, sized from the secondary conductors and landed on an effective electrode near the transformer, not just clipped to a nearby conduit. The companion grounding guide details what makes an electrode connection acceptable.

They check the overcurrent protection against 450.3: the primary device rating, and the secondary device where the scheme requires one, against the transformer full-load currents, not the conductors. They check the secondary conductors against the tap rule, the length and the termination in a single device. And they check the two clearances, the 450.21 ventilation and combustible separation and the 110.26 working space, because those are the violations you cannot wire your way out of after the fact.

Walk it yourself before they do. The bond count, the grounding electrode conductor, the overcurrent ratings, the tap setting, and the clearances are all cheap to fix with the cover off and the room still open, and expensive once the space is finished and energized.

Field checklist

What to document

A transformer install hides most of its decisions inside the can and the wall, so the record is what answers the questions later. The point of the documentation is that the inspector, the next electrician, and the commissioning agent can each confirm the install was sized and grounded right without pulling it apart.

Capture the kVA and both voltages, the full-load amps on each side, the primary and secondary overcurrent ratings against those currents, the secondary conductor size and which tap rule was used, the system bonding jumper location and the grounding electrode conductor, the no-load tap position, and the clearances held. Where you made a judgment call, like the K rating chosen or the secondary main size, write down why. The table below is the minimum a defensible transformer record carries.

Common mistakes

• Sizing the kVA right at the connected load with no headroom, so the first added circuit overloads the transformer.
• Picking the primary breaker off the feeder conductor instead of the 450.3 transformer rule, then nuisance-tripping on inrush or leaving the transformer unprotected.
• Sizing both sides off one current, feeding the 208 A secondary with conductors picked for the 90 A primary.
• Leaving the secondary with no system bonding jumper, so a secondary ground fault has no low-impedance path back to the source.
• Bonding the neutral to ground both at the transformer and in the downstream panel, putting normal neutral current on the equipment grounding conductors and raceway.
• Boxing the transformer into a tight space with the ventilation louvers against a wall, or coming up short on the 110.26 working clearance.
• Running a standard transformer on heavy nonlinear load instead of a K-rated unit, so it overheats and the neutral runs hot.
• Leaving the no-load taps on the wrong position, so the secondary voltage runs high or low and the connected equipment pays for it.
• Changing a no-load tap on a live transformer instead of de-energizing and verifying it dead first.

Standards and references

The framework is NEC, NFPA 70, Article 450, Transformers. Overcurrent protection for transformers 1000 V and less is in 450.3(B) and Table 450.3(B), with the 125 percent primary-only and the 250 percent primary with 125 percent secondary schemes for currents of 9 A or more. Dry-type transformer installation, including the 12 in ventilation and combustible clearance for units 112.5 kVA and under and the transformer-room requirement for larger units, is in 450.21, with the general ventilation requirement at 450.9.

The secondary conductor and tap rules are in 240.21(C), including the 25-ft rule at 240.21(C)(6) and the 10-ft rule at 240.21(C)(2). The secondary is a separately derived system grounded under 250.30, with the system bonding jumper and a grounding electrode conductor sized by Table 250.66. The equipment working space is the 110.26 requirement. Continuous-load sizing at 125 percent and the conductor ampacity tables apply to the feeders the same as any other feeder. K-rated transformers and general-purpose dry-types are listed to UL 1561.

Section numbers and percentages shift between code cycles, so confirm each against the edition the jurisdiction has adopted and any local amendments before citing it on a submittal. The manufacturer's instructions for clearance, ventilation, and tap setting, and the project specification, control where they are stricter than the general code. The AHJ governs.

Units, terms, and abbreviations

Transformer ratings carry a stack of terms that show up across a nameplate, a one-line, and a spec, and a few of them describe the same quantity in different forms.

The set below is what shows up on a dry-type install. The pair worth keeping straight is kVA versus FLA: kVA is the size of the transformer, and the FLA is the current that size produces on a given side at that side's voltage. Run the kVA and the voltage and you get the FLA, and the FLA is what sizes the copper and the breakers.

kVA

Rated apparent power in thousands of volt-amperes, the size of the transformer

FLA

Full-load amps, the rated current on one side at rated kVA and that side's voltage

450.3

The NEC transformer overcurrent rule, sized off the transformer current, not the conductor

SDS

Separately derived system, the secondary grounded as its own source under 250.30

Temperature rise

Average winding rise above a 40 C ambient at full load, commonly 80, 115, or 150 C

K-rated

A transformer built with extra thermal capacity and a 200 percent neutral for harmonic load

No-load taps

Off-load primary connection points, commonly plus or minus 5 percent in 2.5 percent steps

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