Understanding NEC Derating: Temperature and Conductor Bundling

Why heat is the whole story

Current flowing through a conductor generates heat (I²R losses), and every wire insulation type has a maximum safe operating temperature — 75°C for the THHN/THWN-2 conductors this site's tables are built on. The base NEC ampacity table already assumes that heat is being generated and dissipated under two specific baseline conditions: 30°C ambient air, and no more than three current-carrying conductors bundled together. Both derating factors below exist because real installations routinely violate one or both of those assumptions, and when they do, the wire can't safely carry as much current as the base table says — not because the table is wrong, but because the table's assumptions no longer apply.

Ambient temperature correction, in full

Hotter surrounding air gives a conductor less margin before it reaches its own maximum safe temperature — the wire is generating the same heat, but starting from a higher baseline with less room before the insulation's limit. NEC Table 310.15(B)(1) sets correction factors for 75°C-rated conductors:

Ambient temperature	Correction factor
10°C or less	1.20
11-15°C	1.15
16-20°C	1.11
21-25°C	1.05
26-30°C	1.00 (baseline)
31-35°C	0.94
36-40°C	0.88
41-45°C	0.82
46-50°C	0.75
51-55°C	0.67
56-60°C	0.58
61-65°C	0.47
66-70°C	0.33
71-75°C	0.16

Notice it works both directions — a cooler-than-baseline installation actually gets a bonus above 1.00, not just a penalty when it's hot. Most people only ever encounter the penalty side (attics, rooftops, equipment rooms), which is why that's the direction that gets attention.

Conductor bundling adjustment, in full

More current-carrying conductors packed into one raceway or cable means more combined heat with proportionally less surface area per conductor to shed it into the surrounding air. NEC Table 310.15(C)(1) sets adjustment factors by conductor count:

Current-carrying conductors	Adjustment factor
1-3	1.00 (baseline)
4-6	0.80
7-9	0.70
10-20	0.50
21-30	0.45
31-40	0.40
41+	0.35

This is a steep curve — going from 3 to 4 conductors drops the factor straight to 0.80, a 20% cut, for adding just one more circuit to the raceway. It's a common mistake to add "just one more" circuit to an existing conduit without re-checking this table.

The two factors multiply — they don't add

This is the part most often gotten wrong: apply both factors by multiplying them together with the base ampacity, not by adding percentages. 10 AWG copper is rated 35A at baseline. In a 40°C attic (0.88) with six bundled conductors (0.80): the real ampacity is 35 × 0.88 × 0.80 = 24.64A — not 35A minus (12% + 20% = 32%), which would give a different (and wrong) 23.8A. The difference is small at this example's numbers, but multiplicative stacking of several derating conditions can diverge substantially from additive shortcuts at more extreme combinations. Use the ampacity calculator rather than doing this arithmetic by hand for anything beyond a quick sanity check.

Frequently asked questions

Why does temperature affect wire ampacity?

A conductor generates heat proportional to the current squared (I²R). Insulation has a maximum safe operating temperature, and hotter ambient air leaves less margin between the wire's operating temperature and that limit before the current has to be reduced to compensate.

How many conductors before I need to derate for bundling?

Four or more current-carrying conductors in the same raceway or cable triggers the first adjustment tier (0.80) under NEC Table 310.15(C)(1). Three or fewer needs no bundling adjustment.

Do grounding conductors count toward the bundling adjustment?

Generally no — grounding conductors don't normally carry current under normal operation and aren't counted as current-carrying for this adjustment. A properly balanced multiwire branch circuit's neutral is also often excluded under NEC rules. Confirm which conductors in your specific raceway count with a licensed electrician rather than assuming.