NEC 310.15 Ampacity Derating

The order that keeps you out of trouble

Ampacity derating questions look hard because the exam stacks two different conditions on top of the base table value. Do not start with the calculator. Name the conditions first.

1. Base ampacity: Table 310.16.
2. Ambient correction: hot spaces lower the allowable ampacity.
3. Adjustment factor: more than three current-carrying conductors lower the allowable ampacity.
4. Termination limit: the final answer cannot ignore the rated termination column.

If you want the arithmetic checked after the setup, use the free ampacity calculator. If you are studying for the exam, learn the sequence first.

What counts as current-carrying

The adjustment factor depends on the count. This is where the exam tries to take the point from you.

• Ungrounded conductors count.
• An equipment grounding conductor does not count.
• A neutral on a balanced linear circuit usually does not count.
• A neutral carrying nonlinear or harmonic current can count.
• The raceway or bundle must run longer than 24 inches before the adjustment factor applies.

Worked example: 8 current-carrying 12 AWG THHN

Eight 12 AWG copper THHN current-carrying conductors and one equipment grounding conductor are run in EMT longer than 24 inches. The ambient temperature is 40 C. Terminations are rated 75 C. What ampacity can each conductor carry after derating?

1. Base ampacity: 12 AWG copper THHN uses the 90 C insulation column for derating headroom. Table 310.16 gives 30 A.
2. Ambient correction: at 40 C, use the 90 C column correction factor from the ambient table. Common exam value: 0.91.
3. Conductor count: 8 current-carrying conductors. The equipment grounding conductor does not count.
4. Adjustment factor: 8 conductors falls in the 7 to 9 band. Common exam value: 70%, or 0.70.
5. Derated ampacity: 30 A x 0.91 x 0.70 = 19.11 A.
6. Termination check: 75 C termination limit for 12 AWG copper is higher than 19.11 A, so the derated value controls.
7. Answer: about 19.1 A per conductor.

Wrong answers the exam wants you to pick

Using the raw Table 310.16 value

The raw table value is only the starting point. If heat or conductor count applies, the final ampacity is lower.

Counting the equipment grounding conductor

The EGC is not part of the current-carrying count. Counting it can push the problem into the wrong adjustment band.

Skipping the termination check

Starting from the 90 C insulation column is common for derating, but the installed conductor still has to respect the termination rating. Compute the derated value, then compare.

How to drill this for the exam

Write this order on scratch paper: table, ambient, count, termination. Then run five short problems where only one condition changes. That teaches the pattern faster than rereading Article 310 for an hour.