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. This tool computes the reference size; ambient temperature corrections and terminal ratings are separate checks.
When do I need parallel feeder conductors?
Above roughly 500–600 A, a single conductor becomes impractical and NEC 310.10(G) permits paralleling sizes 1/0 AWG and larger. The tool flags when your design current exceeds the largest single-conductor ampacity and suggests parallel sets — 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.
