📉 CEC Derating Table
The short answer
Four or more countable conductors in the same raceway or cable and you have to derate: you may only use part of the conductor's ampacity. CEC Table 5C sets how much — 80% at four to six conductors, 70% from seven to twenty-four, and less again above that. The practical answer for the most common case: 12 AWG copper on a 20 A breaker holds 20 A through 7–24 conductors.
The adjustment table (CEC Table 5C)
| Current-carrying conductors | 1–3 | 4–6 | 7–24 | 25–42 | 43+ |
|---|---|---|---|---|---|
| Use this much of the ampacity | 100% | 80% | 70% | 60% | 50% |
Source-verified against two independent reproductions of CEC Table 5C 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 | 7–24 | 25–42 | 43+ |
|---|---|---|---|---|---|
| 14 AWG | 15 A | 15 A | 15 A | 15 A | 12 A |
| 12 AWG | 20 A | 20 A | 20 A | 18 A | 15 A |
| 10 AWG | 30 A | 30 A | 28 A | 24 A | 20 A |
| 8 AWG | 40 A | 40 A | 38 A | 33 A | 27 A |
| 6 AWG | 55 A | 55 A | 52 A | 45 A | 37 A |
These are the amps left after the adjustment factor, the 60°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
Rule 4-004 decides what counts. Count insulated power and lighting conductors. A bonding conductor never counts, and neither do control or signal conductors. A neutral carrying only unbalanced current in a normally balanced circuit does not count; a neutral carrying current comparable to the phases, or significant harmonic current, does.
Heat counts too (CEC Table 5A)
| Ambient temperature | 60°C | 75°C | 90°C |
|---|---|---|---|
| 35°C | 0.91 | 0.94 | 0.96 |
| 40°C | 0.82 | 0.88 | 0.91 |
| 45°C | 0.71 | 0.82 | 0.87 |
| 50°C | 0.58 | 0.75 | 0.82 |
| 55°C | 0.41 | 0.67 | 0.76 |
| 60°C | — | 0.58 | 0.71 |
| 65°C | — | 0.47 | 0.65 |
| 70°C | — | 0.33 | 0.58 |
| 75°C | — | — | 0.50 |
| 80°C | — | — | 0.41 |
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.
The United States does this differently
The United States is genuinely different here, and it is the widest gap between the two codes on this page. The NEC drops to 50% at ten conductors, while CEC Table 5C stays at 70% all the way to twenty-four. Do not carry a US number onto a Canadian 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, Rule 4-006 terminations, and Rule 8-104 continuous loads to your own conductor.
Related
Common questions
- How many wires can I put in a conduit before derating?
- Derating starts at the fourth countable conductor, not at a raceway 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. Rule 4-004 does not count bonding conductors for the adjustment factor. They still take up room in the raceway 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 CEC Table 5A and the conductor adjustment, multiplied together.