Voltage drop

Voltage drop, the smallest conductor for a run, or the longest run for a conductor — copper or aluminium, single-phase, three-phase or DC, in PVC, aluminium or steel conduit, with paralleled sets. Worked from NEC Chapter 9 Tables 8 and 9 at your power factor, every step shown.

Calculate

Pick the NEC edition your state enforces. Tables 8 and 9 print the same values in 2020 and 2023; the voltage-drop note's clause number moved.

Single-phase is two conductors carrying the load current; three-phase is the line-to-line drop of a balanced load; 120/208 V 3-wire is two phases and the neutral of a 208Y/120 V system, where the neutral carries the 120 V load current. The size is not needed for the smallest-conductor answer.

Power factor, temperature, parallel sets, limit, terminations

Left blank, the tables' 75 °C resistance is used — the larger drop. A run sized at its 60 °C ampacity may be taken at 60 °C (Table 8 Note 2).

Left blank, the answer is the worst drop for any power factor from 0.85 (Table 9's basis) to 1.0 (a resistive load) — small conductors are worst at 1.0, large ones near 0.85. Paralleling is allowed at 1/0 AWG and larger (310.10(G)(1)).

How it works

For AC, Chapter 9 Table 9 gives each conductor's resistance R and reactance X per 1000 ft; its Note 2 turns them into an effective impedance at your power factor, Ze = R × PF + X × sin(arccos PF). The drop is 2 × I × Ze × L for single-phase and √3 × I × Ze × L line to line for three-phase. DC uses Table 8's resistance.

The smallest-conductor answer is the larger of the size that meets the drop and the size whose Table 310.16 ampacity carries the current at the 110.14(C) termination column — so it never suggests a wire too small to carry the load.

What it will not tell you

120/208 V 3-wire is computed as circuit analysis on Table 9's R and X: the neutral carries the line-to-neutral current, and the drop is taken on the worse phase. Unbalanced 120 V loads on the two phases are not modelled.

Resistance is Table 8 / Table 9's, at 75 °C, unless a conductor temperature is entered: then it is corrected by Table 8 Note 2. A conductor running cooler drops less.

Table 9 is three single conductors in one conduit at 60 Hz. Cable assemblies, cable tray and direct burial are close but not the table.

The minimum-size mode checks ampacity at 30 °C with three or fewer current-carrying conductors. For derating, burial and the rest of a feeder, use the NEC feeder sizing calculator.

The 3 % is an informational note, not a requirement. A larger drop is reported, never called a violation.

No report, PDF or saved project — the answer is the whole of it.

Wiring a whole house?

The paid module sizes every feeder in the house on the same tables — and upsizes a run for voltage drop when you ask it to — alongside the load calculation, panel schedules, the riser and the DXF.

See the plans