Reading a transformer nameplate in amps
Transformers, generators, and UPS units are rated in kVA — apparent power — but electricians need amps to size conductors, breakers, and disconnects. The conversion depends only on the kVA and the voltage, never on power factor, because kVA already bundles the real and reactive components together. This converter handles single-phase and three-phase systems at all the common North American voltages and shows the exact formula it used so you can check the work.
How it works
For a single-phase (or split-phase line-to-line) system the line current is:
I = kVA × 1000 / VFor a balanced three-phase system the apparent power is √3 × V × I, so solving for current gives:
I = kVA × 1000 / (√3 × V)with V always the line-to-line voltage and √3 ≈ 1.732. Because three-phase spreads the same apparent power across three conductors, the same kVA at the same voltage produces a noticeably lower per-line current than single-phase would.
Common voltages and what they imply
| System | Typical voltage | Notes |
|---|---|---|
| Single-phase residential | 120 V or 240 V | 240 V = line-to-line on a split-phase 120/240 panel |
| Light commercial three-phase | 208 V | Delta or wye from a 120/208 V distribution transformer |
| Commercial three-phase | 480 V | Most common industrial three-phase voltage in North America |
| High-voltage distribution | 600 V | Canada and some industrial sites |
For three-phase always enter the line-to-line voltage — not the line-to-neutral. The √3 factor in the formula already accounts for the 30-degree phase relationship between conductors, so using line-to-neutral would double-count it and give an inflated current that is wrong by a factor of √3.
Worked example
A 75 kVA three-phase transformer at 480 V has a full-load current of 75 × 1000 ÷ (1.732 × 480) ≈ 90.2 A. The same 75 kVA single-phase at 240 V would draw 312.5 A — over three times as much — which is exactly why distribution uses three-phase at higher voltage.
From full-load amps to breaker size
The tool gives you the full-load current (FLA), which is the starting point for protection sizing, not the final answer. The standard workflow from there:
- For continuous loads (loads energised for 3 hours or more), multiply FLA by 1.25 per NEC Article 215.
- Round up to the next standard overcurrent device (common sizes: 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 125, 150, 175, 200 A and larger).
- Verify conductor ampacity — check that the chosen wire gauge can carry the derated current at the installation temperature and accounting for any bundling derating.
- Compare against the transformer’s nameplate ampacity if listed; some nameplates already express FLA directly as a check.
The converter shows you step one. Steps two through four require the NEC tables and your specific installation conditions.