The maximum size fuse for the primary of a 480 to 240 V, 1Ø, 15 kVA dry-type transformer is:

The maximum size fuse for the primary of a 480 to 240 V, 1Ø, 15 kVA dry-type transformer is:

• A. 70 A
• B. 35 A
• C. 60 A
• D. 30 A

Answer: (D)

Sizing the primary protection comes down to the transformer’s full-load current on the primary side. The full-load current is found from S = 15 kVA and Vp = 480 V: IFL = 15000 / 480 ≈ 31.25 A. The protection on the primary is typically set at 125% of the full-load current to prevent nuisance tripping while still guarding the winding. That gives a maximum fuse rating of 1.25 × 31.25 ≈ 39.1 A. Fuses come in standard sizes, so the largest standard rating not exceeding 39.1 A is 35 A (since 40 A would be too high). Therefore, the practical maximum primary fuse size in this case is 35 A.

Sizing the primary protection comes down to the transformer’s full-load current on the primary side. The full-load current is found from S = 15 kVA and Vp = 480 V: IFL = 15000 / 480 ≈ 31.25 A.

The protection on the primary is typically set at 125% of the full-load current to prevent nuisance tripping while still guarding the winding. That gives a maximum fuse rating of 1.25 × 31.25 ≈ 39.1 A.

Fuses come in standard sizes, so the largest standard rating not exceeding 39.1 A is 35 A (since 40 A would be too high). Therefore, the practical maximum primary fuse size in this case is 35 A.