NEC 310.16 Ampacity Chart with Derating Applied

1. Step 1

   Read the column for the insulation

   The insulation temperature rating selects the column: 90 degrees C for THHN and XHHW-2, 75 for THWN and wet-location XHHW, 60 for TW. The termination rating does not select the column.

2. Step 2

   Correct for ambient temperature

   Multiply by the factor for the actual ambient temperature in that column. Above the tabulated range the conductor is not permitted, and this page returns no value rather than extrapolating.

3. Step 3

   Adjust for conductor count

   Multiply by the factor for the current-carrying conductors sharing the raceway: 80 percent for four to six, 70 percent for seven to nine, 50 percent for ten to twenty.

4. Step 4

   Apply the ceilings

   Cap the result at the table value in the termination's column, then at the small conductor overcurrent limit on 14, 12 and 10 AWG. Ceilings apply after the multipliers, never before.

Corrections multiply and ceilings cap. Applying the termination ceiling before the multipliers is the most common arithmetic error in conductor sizing, and it understates the conductor by up to a full size on 90 degree C insulation.

The tabulated 90 degree C value for 8 AWG copper is 55 A. Nothing in the list reaches it: the termination rating alone holds the best case to 50 A, and ten conductors in one raceway leave 27.5 A. A chart that reports 55 A is answering a question nobody is asking.

Aluminum runs one to two sizes larger for the same rating at a fraction of the material cost, which is why it is standard for services and large feeders. Terminations must be listed for aluminum, and the change in size also changes conduit fill and the equipment grounding conductor.

• Match the column to the insulation, not the breaker

  A THHN conductor is read from the 90 degree C column even on 75 degree C lugs. Reading the 75 degree C column from the start and then derating it produces a smaller answer than the code requires and quietly wastes a size on long feeders.

• Assume the lower termination rating unless marked

  Equipment rated 100 A or less is treated as having 60 degree C terminations unless it is marked otherwise, and larger equipment as 75. Set the termination field from the equipment label, because on small conductors this cap governs more often than derating does.

• Use the 60 degree C column for cable assemblies

  Nonmetallic-sheathed cable is limited to the 60 degree C column by its own article regardless of the conductor insulation inside it, so a 12 AWG NM cable is a 20 A conductor. Select TW to read that column here, and confirm against the article covering the cable type.

• Leave the chart when the run is long

  Ampacity is a thermal limit and is independent of length. Beyond roughly 100 feet at branch-circuit voltages, voltage drop governs the size more often than ampacity does, and no ampacity chart will show it.