Transformers

Full-load current, the biggest breaker or fuse allowed, and how the common connections work. For transformers 1000 V and less.

Current and protection

Phase

Three-phaseSingle-phase

Size

kVA

Primary voltage

Secondary voltage

Protection plan	Side	Max %	Max rating

Max rating is the largest standard fuse or breaker allowed. At 9 A and up, 125% may round up to the next standard size; the other limits can't be exceeded. Secondary conductors also need protection under 240.21(C) (tap rules) — size them from the secondary current.

NEC: 450.3(B), Table 450.3(B) Note 1, 240.6(A), 240.21(C)

Full-load current formulas

Single-phase: I = kVA × 1000 ÷ V

Three-phase: I = kVA × 1000 ÷ (V × 1.732)

Common 480 to 208Y/120 V three-phase sizes

kVA	Primary A (480 V)	Secondary A (208 V)
15	18.0	41.6
30	36.1	83.3
45	54.1	124.9
75	90.2	208.2
112.5	135.3	312.3
150	180.4	416.4
225	270.6	624.5

Connections

Delta-wye (480 V delta to 208Y/120 V)

Primary H1, H2, H3 to the 480 V phases. No neutral on the primary.
Secondary X1, X2, X3 are the 208 V phases; X0 is the neutral (120 V to each phase).
It's a separately derived system: bond X0 to ground at the transformer or the first disconnect, and run a GEC to an electrode (250.30).
The secondary is shifted 30° from the primary. Two transformers feeding the same bus must match.

Single-phase 120/240 V

Two 120 V secondary windings in series: X1 to X4 is 240 V, the middle tap (X2-X3 tied) is the neutral.
Wired in parallel instead, the windings give 120 V only at double the current.

Buck-boost

A small transformer wired as an autotransformer raises (boosts) or lowers (bucks) the voltage 5 to 20%, like 208 V to 230 V.
Size it by the load kVA and the voltage change, using the maker's chart. It isn't a separately derived system.
Protect it under 450.4 (autotransformers).

Checks before you energize

Set the taps to match the measured supply voltage (each tap is usually 2.5%).
Torque the lugs, confirm the X0 bond is in one place only, and check ventilation clearances on the nameplate (450.9).
After energizing, measure every secondary phase-to-phase and phase-to-neutral voltage before landing loads.

NEC: 250.30, 450.4, 450.9, 450.14 (disconnecting means)