How this calculation works
Worth memorizing: a 75 kVA transformer stepping 480 down to 208 three-phase draws about 90 amps on the primary and delivers about 208 amps on the secondary. The 480 to 208 ratio is roughly 2.3 to 1, and the currents flip by the same factor.
Protection is where it gets complicated
Table 450.3(B) is a matrix, not a single rule. Which percentage applies depends on whether the transformer has secondary protection, and whether the primary current is above or below 9 amperes.
- Primary only, 9 amps or more: 125% of rated primary current, with a round-up to the next standard size permitted.
- Primary only, 2 to 9 amps: 167%.
- Primary only, under 2 amps: 300%.
- Primary and secondary protection: the primary can go to 250% when the secondary is protected at 125% or less.
The calculator shows the 125% figures because that is the common case. Check the table row that actually matches your installation before you order gear.
Secondary conductors and the tap rules
240.21(C) governs conductors on the secondary side, and it is unforgiving. Transformer secondary conductors are not protected by the primary overcurrent device in the way people assume — the turns ratio means a fault on the secondary may not draw enough primary current to trip. The 10-foot and 25-foot tap rules exist to bound that risk. Read them before running an unprotected secondary any distance.
Separately derived systems
A transformer with no direct electrical connection between primary and secondary creates a separately derived system, and 250.30 applies: a system bonding jumper, a grounding electrode conductor to a qualifying electrode, and the neutral bonded at one point only. Bonding it in two places creates parallel neutral paths through the building steel, and those are miserable to find later.
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Frequently asked questions
What size conductors do I run on the secondary?
Size them for the secondary current at 125%, then check 240.21(C) for how they are protected. The primary breaker does not adequately protect secondary conductors, because the turns ratio means a secondary fault may not draw enough primary current to trip it. The tap rules exist for exactly this.
Can I protect a transformer with the primary breaker alone?
Often yes. Table 450.3(B) permits primary-only protection at 125% of rated primary current when that current is 9 amps or more, with a round-up to the next standard size. Below 9 amps the allowable percentages go up. Secondary conductors still need their own consideration under 240.21(C).
Where does the neutral get bonded on a transformer?
At one point only, on the secondary side, per 250.30. The system bonding jumper connects the grounded conductor to the equipment grounding conductor and the grounding electrode conductor at either the transformer or the first disconnect — not both. Two bonds create parallel neutral current paths through the building steel.
Why is a transformer rated in kVA instead of kW?
Because the windings heat from total current regardless of power factor. The transformer has no idea how much of the current it carries is doing useful work, so its rating has to be stated in apparent power.
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nfpa.org for complete and authoritative code text. Always verify requirements with the code edition adopted by your jurisdiction and your Authority Having Jurisdiction (AHJ).