Wire Ampacity Calculator

NEC Table 310.16 Base Ampacities

NEC Table 310.16 lists allowable ampacities for insulated conductors rated up to 2000 V with not more than three current-carrying conductors in raceway or cable, at a 30 °C (86 °F) ambient. The table has three columns by insulation temperature rating. You use the column matching your conductor insulation type, then apply any adjustment or correction factors required by 310.15.

Common Copper Ampacities (NEC Table 310.16)

Critical: 14/12/10 small-conductor rule. NEC 240.4(D) limits overcurrent protection to 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG regardless of what Table 310.16 says. You cannot put a 12 AWG wire on a 25 or 30 A breaker even if you upgrade the insulation. This rule prevents well-meaning upgrades from creating fire hazards in old work.

Derating in Hot Attics

Hot attics can reach 50-60 °C (120-140 °F) during summer in southern US climates. NEC 310.15(B)(3)(c) requires an additional rooftop adder above that. A 30 A conductor at 50 °C ambient and 90 °C insulation derates to 30 × 0.82 = 24.6 A. At 55 °C it drops to 22.8 A. For long runs through hot attics, upsize one wire gauge to stay above the required ampacity after derating.

Termination Temperature Rule

NEC 110.14(C) says you must use the column matching the termination temperature, not the conductor insulation. Most residential breakers and receptacles are rated 60 °C for 14/12/10 AWG and 75 °C for 8+ AWG. Even if you run 90 °C THHN, the ampacity is limited to the terminal rating. The 90 °C column is only useful when you're stacking derates — the 90 °C base is higher, so more derate headroom before hitting the 75 °C limit.

Derating Factors Quick Reference

NEC Table 310.15(B)(1) ambient temperature correction factors applied to Table 310.16 base ampacities:

NEC 310.15(C)(1) bundle (more than 3 current-carrying conductors) adjustments:

• 4-6 wires: ×0.80
• 7-9 wires: ×0.70
• 10-20 wires: ×0.50
• 21-30 wires: ×0.45
• 31-40 wires: ×0.40
• 41+ wires: ×0.35

Frequently Asked Questions

What is the difference between NM-B and THHN?

NM-B is the Romex-type in-wall cable with an outer jacket. THHN/THWN-2 is individual conductors for conduit. NM-B internally is 90 °C rated but NEC limits it to 60 °C ampacity per 334.80.

Can I use the 90 C column for Romex?

No. NM-B is rated 60 °C for ampacity calculations per NEC 334.80. Use the 60 °C column.

Do 3 current-carrying wires in NM-B need derating?

No — up to 3 wires is the table base. Derating starts at 4 current-carrying conductors.

What is the 83% rule?

NEC 310.12 lets residential service-entrance conductors carry 83% of the service rating (e.g., 2 AWG Cu at 115 A × 0.83 → not allowed; actually 4 AWG Cu at 85 A × 1.17 ≈ 100 A for a 100 A service). Table 310.12 gives the specific sizes.

How does bundled wires cause derating?

Heat from each current-carrying wire cannot escape when wires are packed together. The derating factor (NEC 310.15(C)(1)) accounts for this heat trapping. Conduit fill and bundle derate are separate requirements — you must check both.