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Transformer Power Rating

Electronics & PhysicsPower and Energy
Supply
Required transformer rating (kVA) —
Load apparent power (kVA)
—
Next standard size
—
Full-load current of that size (A)
—
Load as share of that size
—

A transformer's power rating is its apparent power in kVA. For single phase, kVA = volts × amps ÷ 1000; for three phase, kVA = 1.732 × line voltage × amps ÷ 1000. A 480 V three-phase load drawing 100 A needs 83.14 kVA, so the next standard size is 100 kVA. Add a margin for growth before picking the size.

About this tool

Electricians, panel designers and facility engineers size distribution transformers from the load they must carry. This calculator takes the supply type, the secondary voltage and the load current, works out the apparent power in kVA, adds the spare capacity you choose with the slider, and rounds up to the next standard rating from the common ANSI/IEEE list (15, 30, 45, 75, 112.5, 150 kVA and so on). It also reports the full-load current of that size and how much of it your load uses, so you can see whether the unit will run near its limit. It sizes on steady current only: motor starting inrush, harmonic-rich loads that call for K-rated units, ambient derating and altitude are not modelled and need the manufacturer's data.

How to use it

  1. Pick the supply

    Choose single phase or three phase; three phase applies the √3 factor.

  2. Enter the voltage

    Use the secondary (load-side) voltage, line to line for three-phase systems such as 208 V or 480 V.

  3. Enter the load current

    Use the total current the connected load draws per phase, in amps.

  4. Set the margin

    Drag the slider to add spare capacity for growth; 20–25% is a common design allowance.

Examples

Three-phase 480 V panel drawing 100 A

Supply
Three phase
Secondary voltage (line to line for three phase)
480
Load current
100
Spare capacity margin
0

Result Required transformer rating (kVA): 83.14
Load apparent power (kVA): 83.14
Next standard size: 100 kVA
Full-load current of that size (A): 120.3
Load as share of that size: 83.1%

  1. Load kVA = √3 × 480 V × 100 A ÷ 1000 = 83.14 kVA
  2. Required kVA = 83.14 × (1 + 0/100) = 83.14 kVA
  3. Next standard size ≥ 83.14 kVA is 100 kVA
  4. Full-load current of 100 kVA = 100 × 1000 ÷ (√3 × 480) = 120.3 A

With no margin the load is 83.14 kVA, so the smallest standard unit is 100 kVA, loaded to about 83%.

Single-phase 240 V, 50 A with 25% spare

Supply
Single phase
Secondary voltage (line to line for three phase)
240
Load current
50
Spare capacity margin
25

Result Required transformer rating (kVA): 15
Load apparent power (kVA): 12
Next standard size: 15 kVA
Full-load current of that size (A): 62.5
Load as share of that size: 80%

  1. Load kVA = 240 V × 50 A ÷ 1000 = 12 kVA
  2. Required kVA = 12 × (1 + 25/100) = 15 kVA
  3. Next standard size ≥ 15 kVA is 15 kVA
  4. Full-load current of 15 kVA = 15 × 1000 ÷ (240) = 62.5 A

12 kVA of load plus a quarter for growth lands exactly on the 15 kVA standard size, whose full-load current is 62.5 A.

Small 208 V three-phase load

Supply
Three phase
Secondary voltage (line to line for three phase)
208
Load current
30
Spare capacity margin
20

Result Required transformer rating (kVA): 12.97
Load apparent power (kVA): 10.81
Next standard size: 15 kVA
Full-load current of that size (A): 41.6
Load as share of that size: 72.1%

  1. Load kVA = √3 × 208 V × 30 A ÷ 1000 = 10.81 kVA
  2. Required kVA = 10.81 × (1 + 20/100) = 12.97 kVA
  3. Next standard size ≥ 12.97 kVA is 15 kVA
  4. Full-load current of 15 kVA = 15 × 1000 ÷ (√3 × 208) = 41.6 A

30 A at 208 V is 10.81 kVA; with 20% spare the requirement is 12.97 kVA, so a 15 kVA unit fits.

How it is calculated

S = V × I ÷ 1000 (single phase); S = √3 × V_LL × I ÷ 1000 (three phase); S_req = S × (1 + m ÷ 100)

S
load apparent power in kVA
V, V_LL
secondary voltage; line-to-line for three phase
I
load current per phase in amps
m
spare capacity margin in percent

Transformers are rated in apparent power because their heating depends on current and voltage, not on the power factor of the load. The required kVA is rounded up to the first standard size at or above it, and that size's full-load current is S × 1000 divided by V (or √3 × V). Loading above about 80% of nameplate leaves little room for heat or added circuits, so the tool flags it.

When not to use it

  • Do not size for large motors from running current alone; starting inrush can be six times higher.
  • Loads with heavy harmonics, such as many drives or rectifiers, need a K-rated or derated unit.
  • It does not cover ambient temperature, altitude or enclosure derating.

Common mistakes

  • Using line-to-neutral voltage (277 V) instead of line-to-line (480 V) for a three-phase load.
  • Adding the three phase currents together instead of entering the current per phase.
  • Treating kW as kVA when the load power factor is well below 1.

Frequently asked questions

Why are transformers rated in kVA rather than kW?

Winding losses and core heating depend on the current flowing and the voltage applied, whatever the power factor of the load. A transformer delivering 100 kVA at a power factor of 0.8 carries the same current as one delivering 100 kVA at unity, even though the first supplies only 80 kW of real power.

Where does the 1.732 factor come from?

In a balanced three-phase system the total apparent power equals three times the per-phase power. Since line-to-line voltage is √3 times line-to-neutral voltage, three × VLN × I simplifies to √3 × VLL × I, and √3 is about 1.732.

What margin is typical?

Many designers add 20 to 25% above the calculated load to allow for future circuits and to keep the unit running cooler. Critical or fast-growing sites sometimes use more; a fixed, well-known load may need less.

Which standard sizes does the tool round to?

It uses common ANSI/IEEE C57 ratings: 1 to 10 kVA for small units, then 15, 25, 30, 37.5, 45, 50, 75, 100, 112.5, 150, 167, 200, 225, 250, 300, 333, 500, 750 kVA and on up to 10,000 kVA. Single-phase and three-phase product ranges differ slightly, so check the catalogue.

Can I convert a kW load to kVA first?

Yes. Divide kW by the power factor to get kVA: a 40 kW load at a power factor of 0.85 is about 47.1 kVA. You can then enter the equivalent current, or compare that kVA directly with the result here.

What does the loading percentage tell me?

It is the load kVA divided by the chosen standard size. Running continuously near 100% raises winding temperature and shortens insulation life, which is why the tool warns above 80%.

Cite this tool

Using this tool in an assignment, article or report? Copy a citation, or a link for your website.

APA

Tools Directory Editorial Team. (2026, October 12). Transformer Power Rating. Tools Directory. Retrieved October 27, 2026, from https://tools.milunir.com/electronics-physics/power-and-energy/transformer-power-rating/

MLA

Tools Directory Editorial Team. “Transformer Power Rating.” Tools Directory, 12 Oct. 2026, https://tools.milunir.com/electronics-physics/power-and-energy/transformer-power-rating/. Accessed 27 Oct. 2026.

Chicago

Tools Directory Editorial Team. “Transformer Power Rating.” Tools Directory. Last modified October 12, 2026. Accessed October 27, 2026. https://tools.milunir.com/electronics-physics/power-and-energy/transformer-power-rating/.

Link<a href="https://tools.milunir.com/electronics-physics/power-and-energy/transformer-power-rating/">Transformer Power Rating – Tools Directory</a>