Two windings, two currents

When to use this: specifying a dry-type transformer, checking an existing one has capacity, or sizing the conductors and devices either side of it.

A transformer has a different full-load current on each winding, and they are not interchangeable. A 45 kVA 480→208 V unit draws 54.1 A on the primary and delivers 124.9 A on the secondary — a ratio of 2.31, which is exactly the inverse of the voltage ratio.

Size the primary conductors from the primary current and the secondary conductors from the secondary current. Using one for the other gives conductors less than half the size they need to be, and it is a mistake that survives inspection because nothing looks wrong until the transformer is loaded.

Full-load current, three phase

I = kVA × 1000 / (√3 × V). The √3 is geometry, not power factor — kVA is apparent power, so power factor does not enter this at all.

Where the kVA comes from

Transformers are rated in kVA rather than kW because the winding heats on current, and current follows apparent power. Your load is probably quoted in kW, so divide by the power factor first: 40 kW at 0.8 is 50 kVA of transformer, and the same 40 kW at unity is 40 kVA.

Then add spare capacity — 20 to 25% is usual — and round up. Note how widely spaced the standard sizes are above 100 kVA: 45, 75, 112.5, 150, 225, 300. A 50 kVA requirement takes a 75 kVA transformer, so the spare you actually get is often far more than you asked for.

Protection: the 125% / 250% distinction

This is the part that decides whether the installation stays energised, and it turns on a single question: is the secondary separately protected?

The reason for 250% is magnetising inrush. Energising a transformer draws 8 to 12 times full-load current for a few cycles while the core flux settles. It does the transformer no harm and it is entirely capable of tripping a device sitting at 125% of full load.

With a secondary device in place the primary no longer has to protect against overload — the secondary does that — so it is free to be large enough to ride through inrush. With primary protection only, you are held to 125% and nuisance tripping on switch-on becomes your problem. It is the most common reason a correctly sized installation will not stay on.

Secondary protected at 125%, so the primary device only has to survive a fault and may go to 250% — which is what lets magnetising inrush through without nuisance tripping.

Rounding goes different ways

450.3(B) note 1 permits the next standard size up where 125% does not land on one — 54.1 A × 1.25 = 67.7 A, so 70 A. The higher percentages are hard ceilings that may not be exceeded, so they round down: 54.1 A × 2.5 = 135.3 A takes a 125 A device, not the next size above.

Secondary conductors

Transformer secondary conductors are one of the few places the code lets a conductor run without overcurrent protection at its origin — 240.21(C), and the allowance depends on length.

• 10 ft tap. Conductor ampacity at least 10% of the device protecting them, terminating in a single overcurrent device, enclosed in a raceway.
• 25 ft tap. Ampacity at least one third of the primary device rating multiplied by the turns ratio, terminating in a single device, protected from physical damage.

Neither is a general permission to leave conductors unprotected, and the conditions are where installations fail inspection — particularly the requirement to land in a single device rather than a small distribution board.

Details worth knowing

• Impedance sets fault current. A transformer marked 5% Z will deliver roughly 20 times its full-load current into a bolted secondary fault. That figure decides the interrupting rating of everything downstream, and it is on the nameplate.
• A separately derived system needs its own grounding. 250.30 requires a system bonding jumper and a grounding electrode conductor at the transformer. The neutral and ground are bonded there and nowhere downstream.
• Delta-wye shifts the phase by 30°. It matters when paralleling transformers or connecting to an existing system, and it is invisible until it goes wrong.
• Nonlinear loads may need a K-rated unit. Harmonic currents heat the windings well beyond what the kVA rating suggests. A standard transformer feeding heavy electronics is a common cause of unexplained overheating.

Once you have the two currents, our wire size calculator handles the conductors either side, and panel capacity checks whether the secondary distribution has room.

How this calculator is checked

Technical reviewer

Salamot Hok

Electrician · 10+ years of installation work in Bangladesh and the wider South Asian region

He reads the result the way an installer would: are the defaults values people actually meet, does the warning fire where you would stop and think, and is the answer something you could buy and fit? The code figures themselves come from the published standards cited below, not from him — that boundary is set out on his profile.

• The maths lives in a pure function with its own test suite, asserted against worked examples from published references and standards. A calculator does not ship until those tests pass.
• 2 sources cited by name and linked, so any figure on the page can be traced back to the document it came from.
• Last reviewed 17 August 2026. Review dates are advanced only when the page is actually re-read, never to look fresh.
• Unusable input returns no answer. Where the inputs do not describe a real design, the calculator says so and withholds the number rather than printing a plausible-looking wrong one.

The full process is written up in the methodology and editorial policy. Results are engineering guidance, not a code sign-off — see the disclaimer. If a result looks wrong, tell us; corrections are answered before anything else.