How it works
Required ampacity = 1.25 × continuous load + non-continuous load. Conductor ampacity = base (Table 310.16) × ambient factor × bundling factor, then capped at the terminal-rating column (110.14(C)).
The derating runs as four ordered steps. Step 1 — base ampacity from NEC Table 310.16 in the insulation-temperature column: 90 °C for THHN, but 60 °C for NM-B (Romex) because NEC 334.80 forbids using the higher column. Step 2 — multiply that base by the ambient correction factor from Table 310.15(B)(1) (1.00 at the standard 30 °C, dropping to 0.58 in the 60 °C column at 50 °C). Step 3 — multiply by the adjustment factor from Table 310.15(C)(1) when more than three current-carrying conductors share a raceway or cable (0.80 for 4–6, 0.70 for 7–9, 0.50 for 10–20). Step 4 — cap the derated figure at the ampacity of the temperature column matching your terminations, NEC 110.14(C). The result readout shows only this final terminal-limited step to stay compact, but steps 1–3 are the arithmetic behind it. The terminal cap is the trap that catches first-year apprentices: 90 °C THHN on a 75 °C breaker lug is still limited to the 75 °C column. On the load side, the required ampacity is 125% of any continuous load plus 100% of non-continuous load (NEC 210.20(A) for branch circuits, 215.3 for feeders). The conductor passes when its final derated ampacity meets or exceeds the required ampacity. Once a size is chosen, verify the breaker against the small-conductor cap in NEC 240.4(D) and check voltage drop on long runs.
Code references
- Allowable ampacities (base table) NEC 2023, Table 310.16 — 60/75/90 °C, Cu and Al
- Ambient temperature correction NEC 2023, 310.15(B)(1) and Table 310.15(B)(1)
- Adjustment for >3 current-carrying conductors NEC 2023, 310.15(C)(1) and Table 310.15(C)(1)
- Terminal temperature limitation NEC 2023, 110.14(C)
- NM-B sized from the 60 °C column NEC 2023, 334.80
- Continuous-load 125% multiplier NEC 2023, 210.20(A) (branch) and 215.3 (feeder)
- Small-conductor overcurrent cap NEC 2023, 240.4(D)
- Rooftop conduit ambient adder NEC 2023, 310.15(B)(2)
FAQ
Why does NM-B (Romex) use the 60 °C column?
NEC 334.80 requires that the ampacity of Type NM-B cable be based on the 60 °C conductor temperature rating, even though the individual conductors inside are insulated for 90 °C. So 12 AWG copper NM-B is limited to 20 A (the 60 °C value), not the 25 A you would read in the 75 °C column or the 30 A in the 90 °C column. The 90 °C rating of the conductors may only be used as the starting point for derating calculations, never as the final ampacity.
Can I use 90 °C ampacity with THHN?
Only if every termination in the circuit — breaker lugs, device terminals, splices — is also listed for 90 °C, which is rare. NEC 110.14(C) limits the circuit to the lowest temperature rating in the path. Most breakers and lugs are rated 75 °C (or 60 °C below 100 A), so THHN’s 90 °C column is used only as the base for ambient and bundling derating, then the result is capped at the 75 °C column value.
How many conductors count for bundling?
Count the current-carrying conductors. Hots always count. A neutral that carries only the unbalanced current of a multiwire circuit does not count (NEC 310.15(E)); a neutral that carries harmonic current — common with electronic and LED loads — does count. Equipment grounding conductors never count. When more than three current-carrying conductors share a raceway or cable, the adjustment factors in Table 310.15(C)(1) apply.
What is the 125% continuous-load multiplier?
A continuous load runs for three hours or more — store lighting, an EV charger, a long heating cycle. NEC 210.20(A) (branch circuits) and 215.3 (feeders) require the overcurrent device and conductor to be sized at 125% of the continuous load plus 100% of any non-continuous load. A 16 A continuous load needs a conductor good for at least 20 A. This calculator applies the multiplier automatically and shows the breakdown next to the required ampacity.
What gauge wire do I need for a 20 A circuit?
For a standard 20 A residential branch circuit wired in copper NM-B (Romex), use 12 AWG — its 60 °C ampacity is exactly 20 A per NEC 334.80 and 240.4(D) caps the breaker at 20 A. Use 14 AWG only for 15 A circuits and 10 AWG for 30 A circuits. Aluminum and high-ambient or bundled installations need a closer look — that is what the derating chain above is for.
My attic gets very hot — does that change the wire size?
Yes, often dramatically. NEC 310.15(B)(1) reduces ampacity as ambient temperature rises. At 50 °C (122 °F) the 60 °C-column factor is just 0.58, and conduit on a sunlit roof adds another 17 °C (NEC 310.15(B)(2)). Switch the ambient input to your real attic temperature and the calculator re-derates the chain — you may need to step up a size or two.
This calculator is provided for estimation purposes. Conductor sizing depends on the full installation — terminations, fill, ambient, grouping and load classification. Always verify against the current NEC edition, equipment listings, and local amendments with a licensed electrician or electrical engineer before sizing conductors.