Published June 13, 2026

Ampacity and Temperature Correction Explained

Wire comes stamped with an ampacity rating, so it's tempting to treat that number as gospel no matter where or how it's installed. But NEC Table 310.16 — where those numbers come from — is built on a specific set of assumptions. Change those assumptions, and the real-world ampacity changes too.

The Baseline Assumptions Behind Table 310.16

The ampacity values in NEC Table 310.16 assume:

An ambient temperature of 30°C (86°F)
No more than three current-carrying conductors in the raceway or cable

If your installation matches both of those, you can use the table value as-is. If it doesn't — say you're running conductors through a hot attic, or bundling more than three current-carrying conductors in one conduit — you need to apply correction and adjustment factors.

Ambient Temperature Correction — NEC 310.15(B)(1)

This factor accounts for the surrounding air temperature. At the 30°C (78-86°F) baseline, the correction factor is 1.00 — no change. As ambient temperature rises, the factor drops below 1.00, meaning your wire's usable ampacity goes down. As it drops below 30°C, the factor can actually rise above 1.00.

A few example correction factors for 90°C-rated conductors (like THHN):

Ambient Temp	90°C Correction Factor
78-86°F (26-30°C) — baseline	1.00
96-104°F (36-40°C)	0.91
105-113°F (41-45°C)	0.87
114-122°F (46-50°C)	0.82

That last row is realistic for a hot attic on a summer day — and it's a big part of why conductors run through attics sometimes need to be upsized, or why breakers feeding attic equipment trip on hot afternoons even though the load hasn't changed.

Conductor Adjustment Factors — NEC 310.15(C)(1)

This one accounts for how many current-carrying conductors share a raceway or cable. More current-carrying conductors means more combined heat in a confined space, so the allowable ampacity per conductor drops:

Current-Carrying Conductors	Adjustment Factor
1-3	1.00 (100%)
4-6	0.80 (80%)
7-9	0.70 (70%)
10-20	0.50 (50%)

Note this is based on current-carrying conductors specifically — neutrals and grounds aren't always counted, depending on how the circuit is wired. That distinction is its own rabbit hole, but it's worth knowing it exists.

Putting It Together

The final adjusted ampacity is simply:

Base Ampacity × Temperature Correction Factor × Conductor Adjustment Factor

For example: a 10 AWG THHN conductor has a base 90°C ampacity of 40A. In a 105-113°F attic (0.87 factor) with 5 current-carrying conductors in the same raceway (0.80 factor): 40 × 0.87 × 0.80 = 27.84A — a significant drop from the 40A printed on the jacket.

Use our Ampacity Calculator — pick your wire material, type, size, ambient temperature, and conductor count, and it'll apply both correction factors automatically and give you the real adjusted ampacity.

The Bottom Line

The number on the wire jacket is a starting point, not a guarantee. Anytime conductors run through unusually hot spaces or share a raceway with several other current-carrying conductors, run the adjusted ampacity before you size your breaker — it's the difference between a circuit that holds up and one that nuisance-trips (or worse, overheats) the first hot day of summer.

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