Cable Size & AWG Calculator
Pick the minimum copper or aluminum conductor from load current, ambient temperature, bundling, terminal rating and conduit fill.
Step-by-step
- Enter valid values to begin.
Based on NEC Table 310.16 (30°C ambient, not more than three current-carrying conductors in raceway, 60/75/90°C columns). Table values are for reference only: always confirm with the adopted code edition, local amendments and the equipment listing.
Cable Size and AWG: Ampacity, Derating and What the Inspector Looks For
Core Engineering Principles
Ampacity is just a temperature limit written in amps. Current heats the wire at I²R, the heat has to escape into the air or the conduit, and when the conductor gets hotter than the insulation can stand it ages quickly and eventually fails. Bigger copper has less resistance and more surface, so each step up the AWG ladder carries more. Everything else in the sizing process is a correction for how well that heat gets away: a hot attic, a conduit with nine wires in it, or a wire buried in insulation all trap heat, so the wire has to be bigger.
The sequence I teach apprentices never changes. Start with the load, add 125% if it runs three hours or more, then pick a conductor from Table 310.16. Apply the ambient temperature and bundling factors to the insulation column you started in (usually 90°C for THHN), and finally make sure the result doesn’t exceed what the terminal can handle. That last step is the one that fails inspections.
Terminal check: table ampacity in the 60°C or 75°C column ≥ Idesign (110.14(C))
NEC & Standard References
Table 310.16 gives ampacities for 60, 75 and 90°C columns at a 30°C ambient with no more than three current-carrying conductors. 310.15(B)(1) gives the temperature correction (0.87 at 41–45°C for 90°C wire, 0.82 for 75°C). 310.15(C)(1) reduces ampacity to 80% for four to six conductors and 70% for seven to nine. 210.19(A)(1) and 215.2(A)(1) require 125% of continuous load. And 110.14(C) says that unless the equipment is marked otherwise, circuits of 100 A or less (or 14–1 AWG) use the 60°C column, and larger circuits use 75°C. You may start derating from the 90°C column but you can never end above the terminal column.1. Design current: 40 × 1.25 = 50 A.
2. Try 8 AWG: 55 A (90°C) × 0.87 = 47.9 A, below 50 A — fails.
3. Try 6 AWG: 75 A × 0.87 = 65.3 A, passes. Terminal check: 6 AWG at 75°C = 65 A, above 50 A — passes.
4. Fill: 3 × 0.0507 in² = 0.152 in²; 1/2 in EMT allows 0.122 in² at 40%, 3/4 in allows 0.213 in². Use 6 AWG copper in 3/4 in EMT.
- Don’t finish in the 90°C column. It’s a starting point for derating only. The lug on the breaker is almost always rated 75°C or less.
- Skipping the 125%. EV chargers, heaters and lighting run for hours and need it. Undersized wire shows up as a warm panel.
- Sun-baked conduit. Conduit on a roof can sit at 50°C or more. Apply the ambient correction.
- Neutrals and grounds. A neutral carrying only unbalanced current doesn’t count for bundling, but one serving nonlinear loads does.
- Aluminum needs AL-rated lugs and anti-oxidant. Never land it on a copper-only terminal.