How do you size a transformer secondary feeder?
Start from full-load amps: three-phase FLA = kVA × 1000 ÷ (V × 1.732). Continuous loads (3 hours or more) require conductors at 125% of the continuous current, then select a conductor whose ampacity (NEC Table 310.16, at the termination temperature — typically 75°C) meets the design current; small conductors take the 240.4(D) caps (14 to 15A, 12 to 20A, 10 to 30A). Secondary conductor location and length fall under 240.21(C) (10-ft and 25-ft rules); the secondary overcurrent device is sized per 450.3(B) with conductor protection per 240.4. This tool computes the reference size; ambient temperature corrections and terminal ratings are separate checks.
When do I need parallel feeder conductors?
When a single conductor would need to be larger than 600 kcmil — roughly 420A copper or 350A aluminum at 75°C — the tool recommends parallel sets of 250–600 kcmil conductors (typically 2–4 sets, up to 6 for the largest gear). NEC 310.10(G) permits paralleling sizes 1/0 AWG and larger; the sets must be identical in size, length, and routing.
Why the 75°C column?
Most modern equipment terminations are rated 75°C, and NEC 110.14(C) limits conductor ampacity to the lowest temperature rating of any termination in the circuit. The 90°C column is used for derating calculations but rarely for final ampacity. Check your gear — older terminals may be 60°C.
Does this account for voltage drop?
No — this is the ampacity check only. Long runs need the voltage-drop check too (3% branch / 5% total reference), which often upsizes the conductor. Run the result through our voltage drop calculator before finalizing.
