How many wires can I put in a conduit before derating?

Derating starts at the fourth current-carrying conductor, not at a conduit size. Three or fewer and the base ampacity stands.

Do I have to derate 12 AWG for six wires in a pipe?

You apply the factor, but the answer does not change: 12 AWG copper still comes out at 20 A, because the small-conductor rule was already the lower limit.

Does the ground wire count?

No. Equipment grounding conductors are not current-carrying, so they are not counted for the adjustment factor. They still take up room in the conduit for fill purposes.

What if the conduit is in a hot attic as well?

Both factors apply to the same starting ampacity: the ambient correction from NEC Table 310.15(B)(1) and the conductor adjustment, multiplied together.

📉 NEC Derating Table

Canadian answer (CEC Table 5C)

The short answer

Four or more current-carrying conductors in the same conduit or cable and you have to derate: you may only use part of the wire's ampacity. NEC Table 310.15(C)(1) sets how much — 80% at four to six conductors, 70% at seven to nine, 50% from ten to twenty, and less again above that. The practical answer for the most common case: 12 AWG THHN copper on a 20 A breaker holds 20 A through 7–9 conductors, and drops to 15 A at 10–20.

The adjustment table (NEC Table 310.15(C)(1))

Current-carrying conductors	1–3	4–6	7–9	10–20	21–30	31–40	41+
Use this much of the ampacity	100%	80%	70%	50%	45%	40%	35%

Source-verified against two independent reproductions of NEC Table 310.15(C)(1) and sealed, so a changed digit fails our own checks before anything ships. How we verify.

What that does to real wire

Copper, 90°C insulation	1–3	4–6	7–9	10–20	21–30	31–40	41+
14 AWG	15 A	15 A	15 A	12 A	11 A	10 A	8 A
12 AWG	20 A	20 A	20 A	15 A	13 A	12 A	10 A
10 AWG	30 A	30 A	28 A	20 A	18 A	16 A	14 A
8 AWG	50 A	44 A	38 A	27 A	24 A	22 A	19 A
6 AWG	65 A	60 A	52 A	37 A	33 A	30 A	26 A

These are the amps left after the adjustment factor, the 75°C termination limit, and the small-conductor rule are all applied — the same order the calculator uses. That is why 12 AWG still reads 20 A at 4–6 conductors: the factor takes 90°C ampacity down, but the small-conductor rule was already the lower limit. Ampacity Check.

What counts as a conductor

Count ungrounded conductors and any neutral that carries current under normal conditions, including neutrals carrying harmonic current from non-linear loads. Count spare conductors. Do not count equipment grounding conductors, grounding electrode conductors, or conductors that cannot be energized at the same time.

Heat counts too (NEC Table 310.15(B)(1))

Ambient temperature	60°C	75°C	90°C
≤10°C	1.29	1.20	1.15
11-15°C	1.22	1.15	1.12
16-20°C	1.15	1.11	1.08
21-25°C	1.08	1.05	1.04
26-30°C	1.00	1.00	1.00
31-35°C	0.91	0.94	0.96
36-40°C	0.82	0.88	0.91
41-45°C	0.71	0.82	0.87
46-50°C	0.58	0.75	0.82
51-55°C	0.41	0.67	0.76
56-60°C	—	0.58	0.71
61-65°C	—	0.47	0.65
66-70°C	—	0.33	0.58
71-75°C	—	—	0.50
76-80°C	—	—	0.41
81-85°C	—	—	0.29

Heat from the air and heat from the bundle are separate problems, and both factors apply to the same starting ampacity. Wire in a hot attic inside a crowded conduit takes both.

Canada does this differently

Canada is genuinely different here, and it is the widest gap between the two codes on this page. Where the NEC drops to 50% at ten conductors, CEC Table 5C stays at 70% all the way to twenty-four. Do not carry a Canadian number onto a US job, or the other way round.

What this page does not do

This page is the table and what it does to ordinary copper. It does not count your conductors for you and it does not size the circuit. Once you know the count, the Ampacity Check applies these factors, the termination limit, and continuous loads to your own conductor.

Ampacity Check · Wire ampacity chart · Conduit Fill

Common questions

How many wires can I put in a conduit before derating?: Derating starts at the fourth current-carrying conductor, not at a conduit size. Three or fewer and the base ampacity stands.
Do I have to derate 12 AWG for six wires in a pipe?: You apply the factor, but the answer does not change: 12 AWG copper still comes out at 20 A, because the small-conductor rule was already the lower limit.
Does the ground wire count?: No. Equipment grounding conductors are not current-carrying, so they are not counted for the adjustment factor. They still take up room in the conduit for fill purposes.
What if the conduit is in a hot attic as well?: Both factors apply to the same starting ampacity: the ambient correction from NEC Table 310.15(B)(1) and the conductor adjustment, multiplied together.