sparkytools

Second year Transformers Free · works offline once loaded

Single-phase transformer calculator

Enter the kVA and the two voltages off the nameplate. You get the ratio and the full-load current on each side. Optionally, give an actual secondary load to see the primary current, or a primary turns count to see the secondary turns.

Nameplate first; the last two boxes are optional.

Ratio—
Full-load primary—
Full-load secondary—
Primary at load—
Secondary turns—

Ideal transformer, losses ignored. Conductor and overcurrent sizing on each side come from your code book, not from this.

Keep these results

We send this one email through our mail provider and don't keep your address.

How it works

  • a = Vp ÷ Vs= Np ÷ Ns
  • I = kVA × 1,000 ÷ Veach side, single-phase
  • Ip = Is ÷ acurrent goes the other way

Each turn of wire on the core picks up the same slice of voltage, so the voltage ratio is the turns ratio. Power in is (nearly) power out, so the current ratio is the same number upside down. The kVA rating is volts times amps the windings can carry without overheating; divide it by either side’s voltage and you have that side’s full-load current.

Worked examples

A 3 kVA control transformer, 480 V to 120 V

  1. Ratio: 480 ÷ 120 = 4 : 1, step-down.
  2. Full-load primary: 3,000 ÷ 480 = 6.25 A. Secondary: 3,000 ÷ 120 = 25 A.
  3. With only 10 A of load on the secondary, the primary carries 10 ÷ 4 = 2.5 A.
  4. If the primary has 400 turns, the secondary has 400 ÷ 4 = 100 turns.

A 25 kVA pole-top unit, 2,400 V to 240 V

  1. Ratio 10 : 1. Primary full load: 25,000 ÷ 2,400 = 10.4 A.
  2. Secondary full load across the 240 V: 25,000 ÷ 240 = 104.2 A.
  3. A small current on the high side, a large one on the low side — the same power.

Questions people ask

Is this the current I size the conductors and protection for?

It is the full-load current — the starting number. How big the conductors and the overcurrent protection on each side must be, and where the protection goes, are code rules with their own multipliers and conditions. Take this current to your code book (CSA C22.1 in Canada, the NEC in the US) and your journeyman.

Why does the low-voltage side carry more current?

Power in is about the same as power out. If the voltage goes down by four, the current has to go up by four to carry the same volt-amps. A step-down transformer is a current step-up.

Does it matter which side I call the primary?

The primary is the side you feed. Swap the two voltages and the calculator shows a step-up with the ratio written 1 : n. The full-load currents do not change — they belong to the windings, not to which one is fed.

Related

Spot something wrong on this page? Report it

We’ll include this page and the numbers you entered. We read every report but can’t reply.

Thanks — your report was sent.

That didn’t send. Please try again later.