When can I use the 250% rule?

Table 450.3(B) lets you go to 250% of primary current on the primary OCPD only when no secondary protection is provided AND the primary current is 9 A or more. For currents under 9 A, the limit drops to 167%.

Why round up the OCPD?

NEC 450.3(B) explicitly permits the next higher standard rating from 240.6(A) when the calculated value does not match a standard, regardless of the size of the transformer. This is more permissive than the general 240.4(B) rule for branch circuits.

Does this calculator size the conductors too?

It reports the primary and secondary FLA, which is the starting point for conductor sizing per NEC 450.3, but you still need to apply 215.2 feeder sizing rules and any derating for the actual conductor run conditions.

Transformer Sizing Calculator

NEC 450.3(B)

Size transformers and calculate overcurrent protection per NEC 450.3(B). Includes primary and secondary breaker sizing.

Load

Input Mode

WattskVA

Load

Power Factor

kVA = Watts / (Power Factor x 1000). Typical power factor ranges from 0.8 to 1.0.

Transformer

Primary Voltage

Secondary Voltage

Phase

Single-PhaseThree-Phase

Results

Calculation Breakdown (NEC 450.3(B))

kVA = W / (PF x 1000) | I = kVA x 1000 / V

Load:10,000W

Required kVA:11.11 kVA

Selected Transformer:15 kVA

Primary Current (480V):31.25A

Secondary Current (240V):62.50A

Primary OCP (125% rule):40A breaker

Secondary OCP (125% rule):80A breaker

15 kVA → 40A primary / 80A secondary

Required kVA

11.11kVA

Selected Transformer

15kVA

Primary Current

31.3A

Secondary Current

62.5A

Primary OCP

40A

Secondary OCP

80A

About this calculator

Transformer sizing has two halves: pick a kVA rating big enough for the connected load, then size the primary and secondary overcurrent protection per NEC 450.3(B). The OCPD rules are the trickier half because they depend on whether secondary protection is provided, and they include rounding-up allowances that the breaker sizing rules elsewhere in the code do not.

Formula

Transformer kVA equals load amps times voltage (single-phase) or √3 times line-to-line voltage times line current (three-phase), divided by 1000. Primary OCPD allows up to 125% of primary current when both primary and secondary protection are provided, or up to 250% when only primary protection is used (with conditions).

Single-phase kVA = (V × I) ÷ 1000
Three-phase kVA = (√3 × V × I) ÷ 1000
Primary FLA = kVA × 1000 ÷ V (single-phase) or kVA × 1000 ÷ (√3 × V) (three-phase)

Reference: NEC 450.3(B), Table 450.3(B)

How to use

Pick single-phase or three-phase.
Enter primary voltage, secondary voltage, and the connected kVA load.
The calculator computes primary and secondary full-load amps.
Pick whether you have secondary protection only, primary only, or both.
Read the maximum allowed primary and secondary OCPD per Table 450.3(B), rounded to the next standard size from NEC 240.6(A).

Worked example

Setup

A 45 kVA, 480V to 208Y/120V three-phase transformer with both primary and secondary protection.

Calculation

Primary FLA = 45,000 ÷ (1.732 × 480) = 54.1 A. Secondary FLA = 45,000 ÷ (1.732 × 208) = 124.9 A. Primary OCPD allowed at 125% × 54.1 = 67.6 A → next standard up = 70 A. Secondary OCPD at 125% × 124.9 = 156 A → next standard up = 175 A.

Answer

Use a 70 A primary breaker and a 175 A secondary breaker. Sized the secondary feeders for at least 124.9 A (continuous adjustment if the load qualifies) and verify the panel rating downstream.

Related study guides

Frequently asked questions

When can I use the 250% rule?: Table 450.3(B) lets you go to 250% of primary current on the primary OCPD only when no secondary protection is provided AND the primary current is 9 A or more. For currents under 9 A, the limit drops to 167%.
Why round up the OCPD?: NEC 450.3(B) explicitly permits the next higher standard rating from 240.6(A) when the calculated value does not match a standard, regardless of the size of the transformer. This is more permissive than the general 240.4(B) rule for branch circuits.
Does this calculator size the conductors too?: It reports the primary and secondary FLA, which is the starting point for conductor sizing per NEC 450.3, but you still need to apply 215.2 feeder sizing rules and any derating for the actual conductor run conditions.