Maximum size circuit breaker permitted by NEC Table 450.3(A) to protect the primary of a 4160 to 480-V, 3Ø, 75 kVA transformer with 1.7% impedance in a nonsupervised location is:

Maximum size circuit breaker permitted by NEC Table 450.3(A) to protect the primary of a 4160 to 480-V, 3Ø, 75 kVA transformer with 1.7% impedance in a nonsupervised location is:

• A. 35 A
• B. 20 A
• C. 60 A
• D. 70 A

Answer: (D)

The protection of a transformer's primary is dictated by NEC 450.3(A), which ties the transformer’s kVA, primary voltage, percent impedance, and installation location to the allowable size of the overcurrent device on the primary. First, determine the transformer's primary full-load current: 75 kVA divided by (√3 × 4160 V) ≈ 10.4 A. Then use Table 450.3(A) for a 75 kVA, 4160 V primary with 1.7% impedance in a nonsupervised location. That table specifies the maximum primary overcurrent protective device size as 70 A. So, the largest device you may use on the primary is 70 A; smaller values like 60 A, 35 A, or 20 A would still protect, but they are not the maximum permitted by the table for this case.

The protection of a transformer's primary is dictated by NEC 450.3(A), which ties the transformer’s kVA, primary voltage, percent impedance, and installation location to the allowable size of the overcurrent device on the primary. First, determine the transformer's primary full-load current: 75 kVA divided by (√3 × 4160 V) ≈ 10.4 A. Then use Table 450.3(A) for a 75 kVA, 4160 V primary with 1.7% impedance in a nonsupervised location. That table specifies the maximum primary overcurrent protective device size as 70 A. So, the largest device you may use on the primary is 70 A; smaller values like 60 A, 35 A, or 20 A would still protect, but they are not the maximum permitted by the table for this case.