Transformers and three-phase

A planning aid. Based on the NEC 2023 — check the edition your jurisdiction has adopted and its amendments; the inspector decides. Each tab shows its formula and the sections it uses.

How to use it

kVA ↔ amps — full-load current on each side of a transformer (kVA × 1000 ÷ (√3 × V), or ÷ V for single-phase), the reverse for a load current, and a table of standard sizes at your voltages.

Protection 450.3(B) — for transformers of 1000 V and less: with primary protection only, 125 % of the primary current (the next standard size up is allowed when the current is 9 A or more), 167 % under 9 A, 300 % under 2 A. With secondary protection too, the primary may go to 250 % and the secondary is 125 % (or 167 % under 9 A). Standard sizes are those of 240.6(A).

Secondary 240.21© — enter the primary OCPD, the voltages, the secondary conductors' ampacity and length, and the OCPD they land on; each rule says whether it fits and what the conductors need: protected by the primary (single-voltage secondaries only), 10 ft, industrial 25 ft, outside, and 25 ft with one-third of the primary OCPD.

Buck-boost — supply voltage, the voltage you want and the load. The tool tries the common 120 × 240 / 12-24 V, 120 × 240 / 16-32 V and 240 × 480 / 24-48 V units in every winding connection, ranks them by how close they get, and sizes each unit by line current × its secondary voltage. A diagram shows the boost or buck connection with polarity marks and the jumpers. Three-phase uses two units in open delta or three in wye.

Delta / wye — line and phase voltage and current for either connection, and the four-wire delta with its high leg: 208 V to neutral on a 240 V delta, marked orange (110.15), in the B position in panelboards (408.3(E)), with the caution label of 408.3(F).

Power triangle — any two of kW, kVA, kVAR and power factor give the others; with a target power factor, the capacitor kVAR (kW × (tan θ₁ − tan θ₂)), its capacitance and current, and the 135 % conductor size of 460.8.

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Related tools: available fault current (including a second transformer in the chain), wire size and voltage drop, motor circuits, dwelling load calculation, panel schedule, shop power planner and Ohm’s law.

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