Feeder Sizing Calculator

This calculator sizes a feeder two different ways and keeps the more conservative result. First it looks up the smallest conductor whose ampacity meets Load current (amps) directly against Table 310.16 of NFPA 70, the National Electrical Code (NEC). Second, it computes Min cmil for VD limit from the voltage-drop formula Vd = phaseFactor x K x I x L / cmil (K = 12.9 ohm-cmil/ft for copper, 21.2 for aluminum; phaseFactor is 2 for single-phase or the square root of 3 for three-phase) and picks the smallest conductor whose circular-mil area meets that requirement. Whichever of the two picks is the larger conductor becomes the final selection, and Sizing Basis reports which check actually drove it. Min cmil for VD limit itself is a clean product of three inputs: Load current (amps) and One-way distance (ft) both raise it in direct proportion (doubling either one doubles the required cmil area), while Max voltage drop (%) lowers it, since a looser allowed drop needs less copper or aluminum to stay within it. Which of the two sizing checks (ampacity or voltage drop) actually governs the final conductor size depends on the combination of load, distance, and the allowed drop percentage -- at this calculator's default inputs (200 A, 150 ft, 480 V three-phase, 3% max drop), the ampacity check governs for both conductor materials, and One-way distance, System voltage, and Max voltage drop have no effect on the selected conductor size while ampacity governs (they still move the displayed Min cmil for VD limit and Actual voltage drop figures, just not the selection itself).

Conductor material is different: it still moves the final selection directly at the defaults, because copper and aluminum have their own separate ampacity table rows -- a 200 A copper load lands on 3/0 AWG (200 A ampacity) while the same 200 A aluminum load lands two full table rows higher on 250 kcmil (205 A ampacity), even though the ampacity check governs for both. At longer distances or a tighter drop limit the voltage-drop check takes over and distance, voltage, and drop percentage start to matter directly as well. If the required ampacity or cmil area exceeds the largest listed conductor (750 kcmil), Exceeds conductor table flags it and Sizing Basis reports "(exceeds table)" so the displayed single-conductor selection is never mistaken for an adequate one. Parallel conductor runs is derived from ampacity alone (load current divided by the largest conductor's ampacity, rounded up), so it only tells you how many sets to run in parallel when Sizing Basis reads "Ampacity (exceeds table)" -- the calculator does not account for conduit fill limits, ambient temperature derating, or bundling adjustments that a full NEC 310.15 analysis would require. When Sizing Basis instead reads "Voltage drop (exceeds table)," Parallel conductor runs can still read 1 even though a single run isn't adequate, because voltage drop and ampacity are independent checks; the fix there is a shorter run, a higher system voltage, a higher allowed Max voltage drop, or splitting the load, not more parallel conductors. Actual voltage drop (V) and (%) account for Parallel conductor runs by dividing the single-conductor drop by the number of parallel runs, since each run only carries a fraction of the total current.