Ampacity is the maximum continuous current a conductor can carry under specified conditions without exceeding its temperature rating. NEC Table 310.15(B)(16) defines standard values for copper and aluminum wires.

When do I need to derate for conduit fill?

NEC 310.15(B)(3)(a) requires derating when more than 3 current-carrying conductors share a raceway or cable. The derating factor reduces ampacity to account for heat buildup from adjacent conductors.

Which insulation rating should I use?

Use the temperature rating of your specific wire insulation (printed on the cable jacket). Common types: THHN/THWN-2 = 90°C, THWN = 75°C, TW = 60°C. The termination temperature rating at the breaker or device may also limit you to 60°C or 75°C — use the lower of the two.

Why does aluminum wire have lower ampacity than copper?

Aluminum has higher electrical resistivity than copper, so it generates more heat per ampere. NEC tables reflect this with lower ampacity values for the same gauge size.

Are these values for 60 Hz AC at 30°C ambient?

Yes. NEC Table 310.15(B)(16) assumes 30°C (86°F) ambient air temperature and not more than 3 current-carrying conductors. Additional correction factors apply for higher ambient temperatures or underground installations.

Wire Ampacity Calculator

Name: Wire Ampacity Calculator
Availability: InStock
Author: Nham Vu

Select wire material, AWG size, insulation rating, and number of conductors in conduit to get the derated NEC ampacity.

Wire Parameters

Wire Material

AWG Gauge

Insulation Temperature Rating

Current-Carrying Conductors in Conduit (for derating)

Neutral conductors that carry only unbalanced current are not counted.

Ampacity Result

Select parameters and click Calculate.

NEC 310.15(B)(16) Reference Table

Base ampacity (no derating) at 30°C ambient, ≤3 conductors.

AWG	60°C (A)	75°C (A)	90°C (A)

Conduit Fill Derating Factors

NEC 310.15(B)(3)(a) — applied when >3 current-carrying conductors share a raceway.

Conductors in Conduit	Derating Factor
1–3	1.00 (none)
4–6	0.80
7–9	0.70
10–20	0.50
21–30	0.45
31–40	0.40
41+	0.35

Disclaimer: These values are based on NEC Table 310.15(B)(16) for general use. Actual wiring installations must comply with applicable local codes (NEC, IBC, local amendments) and must be designed and performed by a licensed electrician. Additional correction factors may apply for ambient temperatures above 30°C, underground/direct-burial installations, or specific conduit fill scenarios not covered here.

Summary