In a system where a 4160 to 2400 V three-phase transformer is fed by a feeder through a circuit breaker in a main distribution panelboard, and the transformer is 45 kVA with a 3% impedance, what is the maximum feeder circuit breaker size allowed by NEC 450.3?

In a system where a 4160 to 2400 V three-phase transformer is fed by a feeder through a circuit breaker in a main distribution panelboard, and the transformer is 45 kVA with a 3% impedance, what is the maximum feeder circuit breaker size allowed by NEC 450.3?

• A. 30
• B. 40
• C. 15
• D. 60

Answer: (B)

NEC 450.3 governs how large the overcurrent protection (the feeder breaker) on the transformer’s primary side can be. The idea is to protect the transformer from nuisance trips while still allowing it to deliver its rated load. First find the transformer's primary full-load current. With a 45 kVA transformer, primary voltage of 4160 V, the three-phase current is I_primary = S / (√3 × V_primary) = 45,000 VA / (1.732 × 4,160 V) ≈ 6.25 A. NEC 450.3 requires that the overcurrent protection on the primary not exceed 125% of this full-load current. 1.25 × 6.25 A ≈ 7.8 A. Since protective devices come in standard sizes, the practical maximum primary feeder breaker would be the next standard size up, which is 10 A. The transformer’s impedance (3%) can be used to estimate the fault current the transformer could contribute if the secondary were faulted: I_sc_primary ≈ I_primary / Z% = 6.25 A / 0.03 ≈ 208 A. This shows the fault current capability is much higher than the protection setting, so the breaker would trip during a fault, as intended. So, the maximum feeder circuit breaker size on the primary side per NEC 450.3 is about 10 A (rounded to the nearest standard size).

NEC 450.3 governs how large the overcurrent protection (the feeder breaker) on the transformer’s primary side can be. The idea is to protect the transformer from nuisance trips while still allowing it to deliver its rated load.

First find the transformer's primary full-load current. With a 45 kVA transformer, primary voltage of 4160 V, the three-phase current is I_primary = S / (√3 × V_primary) = 45,000 VA / (1.732 × 4,160 V) ≈ 6.25 A.

NEC 450.3 requires that the overcurrent protection on the primary not exceed 125% of this full-load current. 1.25 × 6.25 A ≈ 7.8 A. Since protective devices come in standard sizes, the practical maximum primary feeder breaker would be the next standard size up, which is 10 A.

The transformer’s impedance (3%) can be used to estimate the fault current the transformer could contribute if the secondary were faulted: I_sc_primary ≈ I_primary / Z% = 6.25 A / 0.03 ≈ 208 A. This shows the fault current capability is much higher than the protection setting, so the breaker would trip during a fault, as intended.

So, the maximum feeder circuit breaker size on the primary side per NEC 450.3 is about 10 A (rounded to the nearest standard size).