Wire Ampacity and Wire Size Calculator (NEC Table 310.16)

How it works

Design current = load × 1.25 (continuous) · Allowable ampacity = min(table value at the insulation rating × ambient factor × bundling factor, table value at the termination rating) · Pick the smallest size with allowable ampacity ≥ design current and breaker ≤ the 240.4(D) limit · Voltage drop = 2 (single-phase) or √3 (three-phase) × I × R × L ÷ 1000, R in Ω per 1000 ft (NEC Chapter 9 Table 8)

A conductor’s ampacity is the current it can carry continuously without its insulation going over its temperature rating. NEC Table 310.16 lists it by size, metal (copper or aluminum) and insulation rating (60, 75 or 90 °C) for an ambient of 30 °C with no more than three current-carrying conductors together. The calculator then applies the corrections an inspector checks. Ambient temperature uses Table 310.15(B)(1)(1), and a raceway less than 7/8 in above a sunny roof adds 33 °C. More than three current-carrying conductors in a raceway or cable use the Table 310.15(C)(1) adjustment (80% for 4–6 down to 35% for 41 or more). Continuous loads, running three hours or more, are sized at 125%. The termination rule in 110.14(C) caps the result at the table value for the breaker or lug rating, usually 75 °C, so 90 °C THHN only helps when you have derating to absorb. NM cable is held to the 60 °C column. The small-conductor rule in 240.4(D) limits 14, 12 and 10 AWG copper to 15, 20 and 30 A breakers, and 12 and 10 AWG aluminum to 15 and 25 A. Above 1000 kcmil, or when you ask for it, the calculator splits the load into parallel sets of 1/0 AWG or larger. Last, it checks voltage drop with the Chapter 9 Table 8 DC resistance at 75 °C and, if needed, moves up sizes until the drop is within your limit (3% is the usual branch-circuit target). The breaker shown is the smallest standard rating at or above the design current. Values follow the 2020 and 2023 NEC; local amendments, equipment instructions and Articles 430/440/625 for motors, HVAC and EV chargers can change the answer.

Worked example

Default: a 40 A continuous load (an EV charger, for example), copper, 75 °C insulation and terminations, 30 °C ambient, three conductors, 50 ft at 240 V single-phase. Design current = 40 × 1.25 = 50 A. 8 AWG copper is 50 A in the 75 °C column × 1.00 × 1.00 = 50 A, which meets 50 A, so 8 AWG copper (≈ 8.37 mm²) on a 50 A breaker. 10 AWG (35 A) is too small. Voltage drop = 2 × 40 A × 0.778 Ω/1000 ft × 50 ft = 3.11 V, 1.30% of 240 V, within 3%. In a 40 °C attic the factor is 0.88: 8 AWG falls to 44 A, so 6 AWG (65 × 0.88 = 57.2 A) is needed.

Sources: Elliott Electric: Table 310.15(B)(1) ambient temperature correction factors (2020 NEC) · Schneider Electric: conductor ampacity based on the NEC (Table 310.16 values) · Encore Wire: wire size and voltage drop calculator (method notes) · Elek: how to size wires using the NEC (2023 NEC worked examples)

Frequently asked questions