Transformer Calculator

The Transformer Equations

In an ideal transformer, voltage scales with turns and power is conserved, so current scales inversely:

Np / Ns = Vp / Vs   and   Vp × Ip = Vs × Is

The apparent-power rating (kVA) is what you actually buy. Single-phase: kVA = V × I / 1,000. Three-phase: kVA = √3 × V_LL × I / 1,000 — the √3 appears because the three windings sit 120° apart, so their vectors don't simply add. Get the phase wrong and you mis-size by 73%.

Worked Example: A 240 V → 24 V Control Transformer

A 24 V thermostat relay draws 10 A. The turns ratio is 240 ÷ 24 = 10:1, so the secondary delivers 240 VA (24 × 10). On the primary that's only 240 ÷ 240 = 1 A. The unit is a 0.24 kVA transformer — you'd specify the next standard size, a 1 kVA dry-type. At 97% efficiency the input is about 247 W, so roughly 7 W is lost to heat (core + copper). Note the wire sizes: the 24 V side carries 10 A and needs thick conductors; the 240 V side carries 1 A and can be thin.

Sizing With Headroom

Never size a transformer to exactly the running load. Motor and inductive loads pull 5–10× their running current for a fraction of a second at startup, and that inrush must fit without saturating the core. A common rule is to rate at 1.25–1.5× the continuous VA, and always round up to a standard kVA. Undersizing causes overheating and premature failure; oversizing only costs a little upfront and runs cooler (longer life).

Real-World Case Study: Panel Upgrade for a Home Workshop

A homeowner added a 240 V, 30 A subpanel for a workshop. The transformer feeding it needed to handle 240 × 30 = 7,200 VA continuous. With a 1.25× safety factor: 7,200 × 1.25 = 9,000 VA. The nearest standard size was a 10 kVA dry-type transformer. At 97% efficiency, it dissipates about 300 W of heat — manageable with passive ventilation. Wire sizing: 240 V side at 30 A needed 10 AWG; the secondary at 12 V for control relays drew 100 A and needed 1/0 AWG copper. Total transformer cost: $680 installed. Without the calculator, the homeowner almost bought a 5 kVA unit that would have overheated within months.

Key takeaway: Always size transformers at 125% of continuous load and round up to the nearest standard kVA. Undersizing causes insulation breakdown and fire risk.

Transformer Sizing Quick Reference

The table below shows common transformer applications and their typical kVA requirements based on load.

Key insight: The "Recommended kVA" includes 125–150% headroom for startup inrush on inductive loads (motors, compressors). Running at exactly rated capacity shortens transformer life.

Frequently Asked Questions