Transformer Calculations for Electrician Exams

The two formulas to know

Single-phase transformer current

I = (kVA × 1000) / V

Use 1000 to convert kVA to VA, then divide by voltage. Same formula for either side; just swap the voltage.

Example: 25 kVA single-phase transformer, 480V primary, 240V secondary.

• Primary current: 25,000 / 480 = 52.1 A
• Secondary current: 25,000 / 240 = 104.2 A

Three-phase transformer current

I = (kVA × 1000) / (V × 1.732)

Example: 75 kVA three-phase transformer, 480V primary, 208V secondary.

• Primary current: 75,000 / (480 × 1.732) = 75,000 / 831 = 90.3 A
• Secondary current: 75,000 / (208 × 1.732) = 75,000 / 360 = 208.3 A

Transformer OCPD sizing (450.3)

Table 450.3(B) sets the maximum percentage of rated primary current for the transformer’s overcurrent protection. The percentages depend on whether secondary protection exists and the type of OCPD.

• Primary only protection, primary current ≥ 9 A: 125% of primary current. The next standard size up is permitted if 125% doesn’t correspond to a standard rating.
• Primary only, primary current under 9 A: 167%.
• Primary and secondary protection: Primary up to 250%, secondary at 125% of secondary current.

The 125% rule is the same as the continuous-load rule across the NEC. It comes up everywhere: branch circuits, feeders, transformers. The exam tests whether you can recognize the same rule across different articles.

Conductor sizing on each side

Conductors connecting to the transformer follow the standard ampacity rules. Primary side conductors carry primary current. Secondary side conductors carry secondary current. Use the right current for the right side and you avoid 90% of transformer traps.

The unit-conversion trap

Some questions list the transformer in VA (e.g., 25,000 VA), some in kVA (e.g., 25 kVA). Same value, different unit. Use kVA × 1000 in the formula or VA directly. The trap is the candidate who divides 25 by 480 and gets a tiny number that doesn’t match any answer choice.

Turns ratio

The turns ratio relates primary voltage to secondary voltage (and inversely, primary current to secondary current).

Vp / Vs = Np / Ns = Is / Ip

For a 480V to 240V transformer, the turns ratio is 2:1. Voltage cuts in half; current doubles. (Power stays approximately the same, losses excluded for exam math.)