Which temperature column should I use?

Use the column matching your conductor insulation: 60 C for TW/UF, 75 C for THW/THWN/XHHW, and 90 C for THHN/THWN-2/XHHW-2. The 90 C column is mainly used as a starting point for derating, not as the final terminal rating.

What is the 110.14(C) termination rule?

Equipment terminals are rated 60 C or 75 C, and NEC 110.14(C) requires you to size the conductor from the column matching the lowest-rated termination in the circuit. You may still derate from the 90 C column, but the final ampacity cannot exceed the termination column value.

How is the ambient correction applied?

Table 310.16 is based on a 30 C ambient. NEC Table 310.15(B)(1) gives a multiplier for other ambients, lower than one above 30 C and higher below it. This tool multiplies the base ampacity by that factor for the selected column.

Does this handle four or more conductors in a raceway?

No, the base table assumes three or fewer current-carrying conductors. With four or more you must apply an additional NEC 310.15(C)(1) adjustment factor, such as 80 percent for four to six conductors, on top of the ambient correction.

Why do copper and aluminum differ so much?

Aluminum has higher resistance per area than copper, so it carries less current at the same size and heats up faster. A given aluminum conductor is typically rated one to two sizes lower in ampacity than the same copper size.

What is the Wire Ampacity Calculator (NEC 310.16)?

Selects the correct ampacity for copper or aluminum conductors from the NEC Table 310.16 by wire gauge, insulation temperature column (60/75/90 C), and applies the ambient temperature correction factor. For licensed electricians sizing branch circuits and feeders. It runs free in your browser on Gera Tools, with nothing uploaded.

Wire Ampacity Calculator (NEC 310.16)

Name: Wire Ampacity Calculator (NEC 310.16)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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NEC Table 310.16 is the foundational ampacity table for conductors in raceways and cables, and reading the wrong column or skipping the ambient correction is a common sizing error. This tool returns the exact published value and the temperature-corrected ampacity.

How it works

The tool looks up the published Table 310.16 ampacity for your material, size, and insulation column, then applies the Table 310.15(B)(1) ambient factor:

base      = Table 310.16[material][size][temp column]
factor    = Table 310.15(B)(1)[temp column][ambient band]
corrected = base × factor

The base table assumes a 30 C ambient and not more than three current-carrying conductors in a raceway. The temperature column must match the conductor’s insulation type.

Insulation type to temperature column mapping

This is the step most errors start with. Choosing the wrong column means either undersizing (a fire risk) or oversizing (unnecessary cost). Common insulation types and their columns:

Insulation	Column	Typical use
TW, UF	60 C	Older wiring, direct burial
THW, THWN, XHHW	75 C	Most commercial and residential branch circuits
THHN, THWN-2, XHHW-2	90 C	High-temp or as derating starting point

Most modern branch circuit wire is THHN, which has a 90 C insulation rating. But the 90 C column is often the starting point for derating, not the final permitted ampacity, because NEC 110.14(C) limits you to the temperature rating of the lowest-rated terminal in the circuit — which is usually 60 C or 75 C on standard equipment.

The termination rule (NEC 110.14(C))

This rule trips up many sizing calculations. Even if you use 90 C rated THHN, if the breaker or device terminal is rated only 75 C, you must use the 75 C column value as your maximum ampacity. The 90 C starting point only helps when you need to derate from it (for example, for high ambient or conductor bundling) and the derated result still falls below the 75 C column value.

Worked example — 12 AWG THHN copper

Table 310.16 base ampacity at 90 C column: 30 A
Terminal rating on typical residential breaker: 75 C — so cap at 75 C column value: 25 A
NEC 240.4(D) small conductor rule limits 12 AWG overcurrent protection to: 20 A
At a 36–40 C ambient, Table 310.15(B)(1) gives a 90 C correction factor of 0.91 → 30 × 0.91 = 27.3 A derated, still above the 20 A protection limit.
Final result: 12 AWG THHN copper on a 20 A breaker — correctly sized.

Conductor count adjustment

The base table assumes three or fewer current-carrying conductors in a raceway. If there are four or more, NEC 310.15(C)(1) requires an additional adjustment factor applied on top of the ambient correction:

Conductors in raceway	Adjustment factor
4–6	0.80 (80%)
7–9	0.70 (70%)
10–20	0.50 (50%)

This tool does not apply the conductor-count factor automatically. You must apply it as a separate multiplier when bundling more than three current-carrying conductors.

Notes

This tool is for estimation and plan-check use. The current edition of the NEC is the authoritative source for all ampacity decisions, and all electrical work must comply with the applicable local code and be performed by or under the supervision of a licensed electrician where required.