🧲What size transformer do you need?

✓ Tested & verified Updated: Aug 10, 2026 How we calculate this

Transformer sizing

Transformer rating

kVA

Supply

Primary voltage

Secondary voltage

60.1 amps on the secondary

and 30.1 A on the primary · ratio 2.00 to 1

Primary device at 125%: 40 A
Secondary device at 125%: 80 A
Copper secondary conductor: 4 AWG

Use line-to-line voltages: a 480Y/277 primary is 480 here, not 277.

A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.

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How do you size a transformer?

The AmpSizer transformer result for a 25 kVA three-phase transformer stepping 480 volts down to 240 is 30.1 amps on the primary and 60.1 amps on the secondary, with a 40 amp primary device and an 80 amp secondary device.

The current ratio is the inverse of the voltage ratio. Halving the voltage doubles the current, because a transformer moves power rather than making it, and 25 kVA in is 25 kVA out minus a small loss.

Both sides get protected and for different reasons. The primary device protects the transformer and the supply conductors; the secondary device protects the secondary conductors, which the primary device cannot see well enough to protect through the winding.

The transformer sizes that exist, at the voltage pair that covers most US commercial work.

Rating	480 V primary, 3-ph	208 V secondary, 3-ph	Primary device	Secondary device
15 kVA	18.0 A	41.6 A	25 A	60 A
25 kVA	30.1 A	69.4 A	40 A	90 A
30 kVA	36.1 A	83.3 A	45 A	110 A
45 kVA	54.1 A	124.9 A	70 A	175 A
75 kVA	90.2 A	208.2 A	125 A	300 A
112.5 kVA	135.3 A	312.3 A	175 A	400 A
150 kVA	180.4 A	416.4 A	225 A	600 A

Devices are 125% of the calculated current rounded up to a standard rating. Local conditions, inrush behaviour and coordination with upstream devices can all move those figures, which is why the number on a drawing is sometimes different from the number here.

Step-down, step-up and the buck-boost case

Step-down is the normal job: a higher distribution voltage into a lower utilisation voltage, and the secondary current is the larger of the two.

Step-up happens where a piece of imported machinery wants 400 or 415 volts on a 480 volt supply, or 230 where there is 208. The arithmetic is identical and only the direction of the ratio changes.

Buck-boost is a small autotransformer correcting a voltage by 10 or 20 volts rather than transforming it. Its kVA rating covers only the corrected portion, which is why a tiny buck-boost unit can handle a large load — and why sizing one from the load’s full kVA buys something enormous and unnecessary.

Frequently asked questions

How many amps is a 25 kVA transformer?

30.1 A on a 480 V three-phase primary, 60.1 A on a 240 V secondary, or 69.4 A if the secondary is 208 V three-phase. On single phase the same 25 kVA is 52.1 A at 480 V and 104.2 A at 240 V. The secondary is always the higher current on a step-down.

Do I need protection on both sides of a transformer?

In most cases yes. 450.3(B) requires primary protection at up to 125% of the primary current, and where the secondary conductors are not protected by the primary device — which for most step-down transformers they are not — 240.21(C) requires secondary protection as well. The exception for a two-wire to two-wire transformer is the reason people think it is optional.

Why is the transformer rated in kVA and not kW?

Because a transformer is limited by current and voltage, not by what the load does with them. Its windings heat according to the current they carry, whatever the power factor of the load. Rating it in kW would mean the rating changed depending on what you plugged in.

How much bigger should a transformer be than the load?

Enough that it is not running at 100% continuously — typically 20 to 25% spare, so a 20 kVA load takes a 25 kVA transformer. Oversizing much further wastes money and increases no-load losses, which run 24 hours a day whether anything is connected or not.

What is inrush on a transformer?

The magnetising current at the instant it is energised, which can be eight to twelve times full-load current for a few cycles. It is why a transformer primary breaker sometimes trips on switch-on and why 450.3(B) allows the primary device to be larger than the primary current — the same logic as a motor breaker.

How we calculate this

Current on either side is kVA × 1,000 ÷ volts for single phase and kVA × 1,000 ÷ (1.732 × volts) for three phase, using line-to-line voltages. Losses are not deducted: a transformer is 97 to 99% efficient at full load, which is inside the tolerance of the rating itself. Protection shown is 125% of the calculated current rounded to the next standard rating in NEC 240.6(A), which follows 450.3(B) for the primary of a transformer over 9 amps; where the calculated 125% figure does not correspond to a standard rating, 450.3(B) permits the next higher standard size. Secondary conductors are then sized from the secondary current with Table 310.16 ampacity and the ceiling of 240.4(D), and secondary overcurrent protection follows 240.21(C). Inrush, impedance and available fault current are design inputs beyond a sizing calculation.

Worked example

With Transformer rating 25, Supply Three phase, Primary voltage 480 V, Secondary voltage 240 V, this page works out 60.1 amps on the secondary. and 30.1 A on the primary · ratio 2.00 to 1

Primary device at 125%40 A
Secondary device at 125%80 A
Copper secondary conductor4 AWG

Sources:

The three things a nameplate tells you

kVA. The continuous throughput. Not kW, and not affected by your power factor.

The voltage pair and the connection. 480 delta primary to 208Y/120 secondary is the workhorse of US commercial work, because it produces a neutral for 120 V loads out of a supply that has none.

Impedance, as a percentage. Usually ignored on small units and important on large ones: it sets the available fault current on the secondary, which is what the downstream gear has to be rated to interrupt.