Transformer calculations

Updated November 8, 2026 · Written for the 2023 NEC

Why transformers matter on the exam

Transformer questions combine theory and code, so they appear on almost every journeyman exam. Expect to find full-load current, use a turns ratio, size overcurrent devices, and name the voltages a wye or delta secondary produces. The math is short, but common slips like using 1.732 on a single-phase unit usually match one of the wrong choices.

How a transformer changes voltage and current

A transformer moves power between windings through a shared magnetic core. Ignoring losses, VA in equals VA out, so if voltage goes down, current goes up by the same proportion.

• The primary is the winding connected to the supply. The secondary is the winding connected to the load.
• A step-down transformer has a lower secondary voltage and a higher secondary current.
• Transformers are rated in kVA, not kW, because winding and core heating depend on voltage and current, not on the load's power factor.

The formulas

The turns ratio links the two sides. Voltage follows the turns ratio and current goes the opposite way.

Always use line-to-line voltage in the three-phase formula: 208 V for a 208Y/120 secondary, not 120 V. See three-phase power for where 1.732 comes from.

Delta and wye secondaries

The secondary connection decides which voltages are available to the building.

In a wye, line voltage is 1.732 times winding voltage and the currents are equal. In a delta, the voltages are equal and line current is 1.732 times winding current.

On a 4-wire delta, the high leg measures about 208 V to the neutral and is not usable for 120-volt loads. NEC 110.15 requires it to be identified with orange or another effective means, and Article 408 places it in the B position in panelboards.

A transformer secondary that has no direct connection to the supply conductors is a separately derived system. It needs its own system bonding jumper and grounding electrode conductor under 250.30. See grounding and bonding.

NEC 450.3(B): overcurrent protection basics

For transformers rated 1,000 volts or less, 450.3(B) lets you protect the transformer one of two ways. The percentages are based on the transformer's rated current, not the load.

1. Primary-only protection. If the rated primary current is 9 A or more, the primary device may be up to 125% of primary current. If 125% does not land on a standard size from 240.6(A), you may go up to the next standard size. Smaller transformers get higher limits (167% below 9 A, 300% below 2 A), and at those levels you round down, not up.
2. Primary and secondary protection. The primary device may be up to 250% of primary current, and the secondary device up to 125% of secondary current (next standard size up permitted when the secondary current is 9 A or more). Here the secondary device handles overload and the primary device mainly handles short circuits. The 250% value is a hard maximum, so round down to a standard size.

Protecting the transformer does not protect the conductors. Secondary conductors have their own rules in 240.21(C), and primary conductors still need protection per 240.4. See overcurrent protection and conductor ampacity.

Worked examples

Example 1: single-phase full-load current

A 37.5 kVA single-phase transformer has a 480-volt primary and a 120/240-volt secondary. Find both currents.

• Primary: 37,500 ÷ 480 = 78.1 A
• Secondary: 37,500 ÷ 240 = 156.3 A

Use the full 240 V winding voltage, and no 1.732, because it is single phase.

Example 2: three-phase full-load current and turns ratio

A 150 kVA three-phase transformer is 480 V delta primary to 208Y/120 V secondary.

• Primary: 150,000 ÷ (480 × 1.732) = 180.4 A
• Secondary: 150,000 ÷ (208 × 1.732) = 416.4 A

The line-to-line voltage ratio is 480 ÷ 208 = 2.31:1. The actual winding turns ratio is different: each delta primary winding sees 480 V, and each wye secondary winding produces 120 V, so the winding ratio is 480 ÷ 120 = 4:1.

Example 3: turns ratio

A transformer has a 7,200-volt primary and a 240-volt secondary that delivers 100 A.

• Turns ratio: 7,200 ÷ 240 = 30:1
• Primary current: 100 ÷ 30 = 3.33 A

Example 4: sizing protection under 450.3(B)

Use the 150 kVA transformer from Example 2.

Primary only: 180.4 × 1.25 = 225.5 A. That is just above the 225 A standard size, so the next standard size, 250 A, is the maximum.

Primary and secondary:

• Secondary: 416.4 × 1.25 = 520.5 A. Next standard size up is 600 A.
• Primary: 180.4 × 2.50 = 451.1 A. You may not round up at 250%, so the maximum standard size is 450 A.

Common exam mistakes

• Using 1.732 on a single-phase transformer. A 25 kVA, 480-volt single-phase unit draws 52.1 A, not 30.1 A.
• Dividing by 208 alone on a three-phase unit. 112.5 kVA at 208 V is 312 A, not 541 A.
• Using 120 V instead of 208 V for a 208Y/120 secondary.
• Rounding up at 167%, 300% or 250%. The next-size-up allowance only applies to the 125% values.
• Sizing protection from the load instead of the transformer's rated current.
• Forgetting the secondary conductors. They still need to meet 240.21(C).

Drill these on the practice test or in the exam calculations guide.

Practice questions

Frequently asked questions