SizingKitTransformers & gensets
kVA to Amps Calculator
Divide volt-amperes by the winding voltage — and by 1.732 as well on three-phase — and no power factor appears anywhere in it; this free calculator does that for both windings of the same unit at once, with no signup. A 75 kVA transformer stepping 480 V down to 208 V is 90.2 A on the primary and 208 A on the secondary, and knowing which of those two the question meant is most of the work.
- 100% free
- No signup
- Both windings at once
- Preferred kVA series
- No power factor needed
Kilovolt-amperes unless you type another unit. VA and MVA are both accepted.
A 480 V to 208Y/120 V unit takes 208 here — the wye’s 120 V is line to neutral, and full-load current is a line quantity.
Default 1.00 — Unity, which is the only assumption a supply rating can honestly start from: kW is kVA multiplied by the power factor of whatever gets connected, so at 1.00 the whole rating is available as real power and every lower figure is the share of it your particular load will convert into work. A generator set is sold the other way round. ISO 8528-1 rates engine-driven alternating-current sets at 0.8 lagging, so a 100 kVA set is marketed as 80 kW and its kilowatt figure is already the derated one — do not multiply it down a second time. A consumer UPS marks both numbers for the same reason, and the ratio between them on the box is that unit's assumed power factor. This field changes the kilowatt row only; it cannot change either current, because apparent power is volts times amps and nothing else.
Primary 480 V
90.2 A
Secondary 208 V
208 A
| Rating entered | 75 kVA |
|---|---|
| Nearest rating that is actually manufactured | 75 kVA |
| Real power at the load power factor | 75 kW |
| Voltage ratio, and the current ratio inverted from it | 2.308 : 1 · currents 1 : 2.308 |
The two currents are in the inverse of the voltage ratio and nothing else decides them: the winding turns down the volts by 2.308 and turns the amps up by the same figure, because both windings pass the same volt-amperes. That is the check worth doing before you trust any transformer number — if the currents are not in that ratio, one of the two voltages is wrong.
The device ahead of a transformer is not this arithmetic. NEC Table 450.3(B) sets the maximum primary and secondary protection as a percentage of the full-load current computed here — the percentage depends on the winding, on whether secondary protection is provided, and on which current band the winding falls into — and this site does not reproduce that table, because its 2 to 9 ampere and under-2 ampere bands and its supervised-location column could not be confirmed to the standard the rest of these pages are held to. Take the percentages from the code book and apply them to the currents above.
Three-phase full-load amperes, by rating and winding voltage
Every rating in the preferred series against the winding voltages these units are usually built for. Read the row for the plate and the column for the winding you are standing at.
| kVA | 208 V | 240 V | 480 V | 600 V |
|---|---|---|---|---|
| 3 | 8.3 | 7.2 | 3.6 | 2.9 |
| 6 | 16.7 | 14.4 | 7.2 | 5.8 |
| 9 | 25 | 21.7 | 10.8 | 8.7 |
| 15 | 41.6 | 36.1 | 18 | 14.4 |
| 30 | 83.3 | 72.2 | 36.1 | 28.9 |
| 45 | 124.9 | 108.3 | 54.1 | 43.3 |
| 75 | 208.2 | 180.4 | 90.2 | 72.2 |
| 112.5 | 312.3 | 270.6 | 135.3 | 108.3 |
| 150 | 416.4 | 360.8 | 180.4 | 144.3 |
| 225 | 624.5 | 541.3 | 270.6 | 216.5 |
| 300 | 832.7 | 721.7 | 360.8 | 288.7 |
| 500 | 1387.9 | 1202.8 | 601.4 | 481.1 |
| 750 | 2081.8 | 1804.2 | 902.1 | 721.7 |
| 1000 | 2775.7 | 2405.6 | 1202.8 | 962.3 |
| 1500 | 4163.6 | 3608.4 | 1804.2 | 1443.4 |
| 2000 | 5551.4 | 4811.3 | 2405.6 | 1924.5 |
| 2500 | 6939.3 | 6014.1 | 3007 | 2405.6 |
Preferred three-phase ratings of ANSI/IEEE C57.12.00 and the NEMA ST 20 dry-type series. These are manufacturing conventions, not a code list — the NEC does not specify transformer kVA ratings.
How to read full-load current off a transformer or genset plate
Two windings, one rating, and a check you can do in your head afterwards.
Enter the rating, or the load you want the unit to carry
Working from a plate, type its kVA. Working from a load instead, switch the first field and give the amperes the secondary has to serve — the page turns that back into volt-amperes and then rounds up to a rating somebody actually manufactures, since an arithmetic answer of 43 kVA is not orderable and 45 kVA is.
Give both winding voltages, line to line
Full-load current is a line quantity, so a 208Y/120 V secondary is entered as 208 and not as 120. Getting this wrong is the single most common error on this calculation and it lands the secondary current out by 1.732, which looks entirely plausible on a clamp meter until the conductor warms up.
Check the two currents against the voltage ratio, then go to the code book for the device
The amperes should come out in exactly the inverse of the volts: step down by 2.308 and the current steps up by 2.308, because both windings pass the same volt-amperes. What the page will not tell you is the breaker or fuse ahead of it — that is Table 450.3(B) in the NEC, applied as a percentage of the currents shown here.
Identities, series and the tables behind them
| Single-phase identity | I = VA ÷ V, with no power-factor term in it at all. 25 kVA at 240 V returns 104.2 A on that winding. |
|---|---|
| Three-phase identity | I = VA ÷ (√3 × V). A 75 kVA unit returns 90.2 A at 480 V and 208 A at 208 V — the same machine, both windings. |
| Single-phase preferred ratings | Seventeen sizes from 1 to 500 kVA, including the 167, 333 and 500 kVA units that exist so three of them make a bank. |
| Three-phase preferred ratings | Seventeen sizes from 3 to 2,500 kVA. The 15/30/45/75/112.5/150/225/300 run is where light-commercial answers land. |
| What that series is and is not | ANSI/IEEE C57.12.00 and the NEMA ST 20 dry-type series — manufacturing conventions. The NEC does not specify transformer kVA ratings at all. |
| Overcurrent protection | Not computed. NEC Table 450.3(B) sets the primary and secondary percentages, and this site does not reproduce that table; its low-current bands could not be confirmed. |
| Kilowatt row | kW = kVA × power factor, with the field starting at 1.00 so the whole rating shows, and the current rows deliberately unaffected by it. |
| Printed chart | 68 computed cells three-phase (17 ratings against 208, 240, 480 and 600 V) and 85 single-phase (against 120, 208, 240, 277 and 480 V). |
| Where it runs | In the browser. Ratings and voltages you type stay on the device. |
Frequently asked questions
The nameplate says kVA but my load is stated in kW. Which one sizes the wire?
The kVA figure, every time — a transformer has no idea what you are going to connect to it. What heats a winding and what limits a conductor is current, and current is settled by volt-amperes regardless of how much of it does useful work. A 75 kVA unit feeding a purely resistive load delivers 75 kW; the same unit feeding a rack of motors at 0.8 delivers 60 kW and runs exactly as hot doing it, which is why the honest rating for a supply is the one that ignores power factor.
Does the load's power factor change the secondary amperes?
No, and that is the whole point of the rating. Apparent power is volts times amps, so a full-load secondary current is fixed the moment you know the kVA and the voltage. Power factor only decides how many of those volt-amperes turn into kilowatts on the load side; it never reduces the current the winding and the conductor have to carry.
Do I size the wire on the primary current or the secondary current?
On whichever side that conductor is — they are different circuits with different currents and usually different conductor sizes. The high side of a 480 V to 208 V unit carries under half of what the low side carries, which is why the secondary conductors and the panel they feed are so much larger than the feeder arriving at the transformer. Both circuits also have their own overcurrent requirements under Article 450.
My calculation says 43 kVA. Can I order that?
No — you order 45 kVA, the next size in the preferred series. Transformer ratings are a manufacturing convention rather than a continuous scale, and the standard three-phase dry-type run goes 30, 45, 75, 112.5, 150, 225, 300. Below about 3 kVA and above 300 kVA the series thins out and diverges between manufacturers, so check a catalog rather than trusting any list at those ends.
My generator plate says 100 kVA and 80 kW. Which figure do I work from?
Use the kVA for current and the kW for the engine, and do not derate twice. ISO 8528-1 rates engine-driven sets at 0.8 lagging power factor, so 80 kW is already 100 kVA times 0.8 — multiplying that 80 kW down again by a power factor is a mistake that undersizes the set by a fifth. The alternator's current limit comes from the 100 kVA; whether the diesel can hold the load comes from the 80 kW.
Does a delta-wye connection change the full-load current?
Not the line current, which is all this calculation reports. Both windings pass the same volt-amperes whatever their internal connection, so line current is still the rating divided by the line-to-line voltage and 1.732. What the connection does change is the winding current inside the transformer — in a delta winding it is the line current divided by 1.732 — along with the phase shift and whether a neutral exists at all.
What does the kVA on the plate actually promise?
It is a continuous output rating at a stated average ambient temperature and a stated winding temperature rise, per ANSI/IEEE C57.12.00 — not a peak and not a limit that anything enforces. Harmonic-rich loads such as switch-mode supplies and drive rectifiers heat a winding more than their volt-amperes suggest, which is what a K-factor rating exists to address. Whether a particular installation can sit at nameplate output is a question for the engineer of record and the local inspector, not for arithmetic.
About apparent power, two windings and the ratings you can actually buy
Apparent power is the quantity that stops being an abstraction the moment you touch a winding. It is volts multiplied by amps with nothing removed, and it is what decides how hot a copper conductor gets, which is why the equipment that has to survive the current — transformers, alternators, uninterruptible supplies — is rated in it, while the equipment that consumes energy is rated in watts. Everything on this site that ends in a conductor size traces back to apparent power for that reason, and the three phase power calculator is where the line and phase quantities behind it are laid out.
One transformer having two full-load currents catches people out constantly, and the arithmetic makes it obvious once it is written down: the same volt-amperes cross both windings, so if the voltage drops by a factor the current has to rise by the same factor. Step 480 V down to 208 V — a ratio of 2.308 — and a 90 A primary becomes a 208 A secondary. This is also the fastest sanity check available on any transformer number somebody hands you: if the two currents are not in the inverse of the two voltages, one of the four figures is wrong, and it is usually a wye secondary entered as its line-to-neutral 120 V instead of its line-to-line 208 V.
The last step is the one arithmetic cannot take. A computed 43 kVA is not a thing that exists; ratings come from a preferred series that manufacturers build to, so the answer is 45 kVA and the spare capacity is a fact about the catalog rather than a design margin you chose. Neither can this page pick the overcurrent device — NEC Table 450.3(B) governs that, as a percentage of the very currents shown here, and its low-current bands are among the figures this site declines to reproduce from memory. If what you are really assembling is a list of loads rather than a single rating, build it on the amperage calculator first and bring the total back here.
Where the plate readings you enter are handled
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The preferred-rating series and both full-load identities are compiled into the page, so switching between single- and three-phase, or between windings, involves no network request at all.