How to Use the Transformer Primary Fuse Calculator (September 2026 basis)
Sizing a transformer primary fuse takes three separate checks: the ampere rating allowed by code, a standard rating that actually exists, and an interrupting rating above the available fault current. Getting the first two right and missing the third can rupture the fuse instead of clearing the fault. This calculator works through all three for transformers rated 1,000 V nominal or less, the scope of NEC 450.3(B).
Primary full load amps is kVA times 1,000 divided by the voltage, with the square root of three included for three-phase. The allowable percentage then comes straight from Table 450.3(B). Where the secondary has overcurrent protection set at 125 percent or less of secondary current, the primary fuse may reach 250 percent. With primary protection only, the limit is 125 percent at 9 A or more, 167 percent from 2 A up to 9 A, and 300 percent below 2 A.
Only the 125 percent case may be rounded up, because Note 1 to 450.3(B) permits the next higher standard rating from NEC 240.6(A). The 167, 250 and 300 percent values are ceilings, so the selection rounds down to the largest standard size that fits. A 75 kVA transformer on a 480 V three-phase primary draws about 90 A, giving 125 A with primary protection only and 225 A where the secondary is protected. Interrupting ratings follow UL 248 and ANSI C37.46 classes, and the published time-current curve still has to be coordinated with transformer inrush and downstream devices by a qualified engineer.
Frequently Asked Questions
With overcurrent protection on the secondary set at 125 percent or less of secondary current, the primary fuse may go up to 250 percent. With primary protection only, the limit is 125 percent when primary current is 9 A or more, 167 percent from 2 A up to 9 A, and 300 percent below 2 A.
Only in the 125 percent case. Note 1 to NEC 450.3(B) permits the next higher standard rating from 240.6(A) when 125 percent does not land on a standard size. The 167, 250 and 300 percent figures are maximums, so the selection must round down.
Ampere rating controls when the fuse opens, while interrupting rating is how much fault current it can safely clear. If available fault current exceeds it, the fuse can rupture instead of interrupting, so check the utility fault current and pick a UL 248 class rated above it.