Wire size and voltage drop

Size a wire — enter the load as amps, VA or watts (with the voltage, phase and power factor), tick continuous if it runs three hours or more, then the conductor: copper or aluminium, the insulation or cable (THHN/THWN-2, XHHW-2, THWN, NM-B, UF-B, SE/SER), the termination temperature (by default the 110.14© rule: 60 °C for circuits of 100 A or less, 75 °C above — choose 75 °C if your breaker and equipment are marked 60/75 °C), the ambient temperature in °F, whether the raceway sits on a sunny rooftop, how many current-carrying conductors share the raceway or cable, and parallel sets. The breaker is chosen for you (210.20(A)) or you pick one. Give the one-way length for the voltage-drop check and choose the target (3 % for a branch circuit or feeder, 5 % in all) and the method: the K-factor (DC resistance) or Chapter 9 Table 9 (AC impedance in PVC, aluminium or steel raceway, at a power factor).

Tick Motor branch circuit (Article 430) for a motor: enter the table full-load current (not the nameplate), and the conductor is sized at 125 % of it after corrections (430.22). The breaker or fuse is sized for starting by 430.52, so the 240.4 rules — including the 15/20/30 A limits on 14/12/10 AWG — don’t apply (240.4(G)); enter that device only for the ground wire. Check voltage drop in the motor circuits tool fills all of this in.

The answer bar stays at the top as you scroll: the size, the breaker, the rule that governs, the equipment grounding conductor (Table 250.122, upsized by 250.122(B) when voltage drop made the hots bigger) and the drop. Below it, every rule with the smallest size it allows and why.

What will this wire carry? — choose a size and the same conditions: the largest breaker, the largest load and continuous load, and for a given current and length, the drop, the longest run and the most current within the target.

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Related: box fill, conduit fill, grounding and bonding sizes, the shop power planner for whole workshops, the dwelling load calculation and motor circuits (motors follow Article 430, not these rules), and the wire gauge table.

Ampacities are from Table 310.16 (copper and aluminium, 60, 75 and 90 °C columns), corrections from Table 310.15(B)(1)(1) and Table 310.15©(1), standard ratings from 240.6(A), resistance from Chapter 9 Table 8 (K = 12.9 for copper, 21.2 for aluminium) and impedance from Chapter 9 Table 9 (Ze = R·pf + X·sin θ). Single-phase drop uses 2·I·L, three-phase √3·I·L. NM-B corrections start from the 90 °C column but stop at the 60 °C one (334.80); UF is 60 °C (340.80). The 2020 edition has the same numbers; the ambient table was 310.15(B)(1) there.

Not covered: motor, air-conditioning and welder circuits (Articles 430, 440, 630), the 83 % rule for dwelling services (310.12), underground and medium-voltage tables, and conductors in free air.

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