Conductor Ampacity

What conductor ampacity is

Ampacity is the maximum current, in amperes, that a conductor can carry continuously without exceeding its temperature rating. It is not a fixed property of the wire alone — it depends on the conductor size and material, the temperature rating of the insulation, and the conditions around the conductor, especially ambient heat and how many other current-carrying conductors share the raceway or cable. Table 310.16 is the workhorse for the exam: ampacities for insulated conductors run in a raceway, cable, or earth, with no more than three current-carrying conductors, based on a 30°C ambient.

Temperature ratings and terminations

Table 310.16 lists each conductor size in three insulation-temperature columns — 60°C, 75°C, and 90°C. The trap is that you rarely get to keep the 90°C value as your final ampacity. Section 110.14(C) ties the usable ampacity to the lowest temperature rating in the circuit, which is usually the equipment terminations. As a rule of thumb, terminations on equipment rated 100 amps or less default to 60°C unless the equipment and conductors are listed for 75°C, while equipment above 100 amps is generally rated 75°C.

So the 90°C column is mostly a starting point for math, not the answer. You apply correction and adjustment factors to the 90°C ampacity, but the final result can never exceed the ampacity shown in the column that matches your termination rating.

Correction and adjustment factors

Two derating steps come up constantly:

• Ambient temperature correction. The table assumes 30°C. In a hotter space, multiply the conductor's ampacity by the temperature correction factor from 310.15(B); the hotter the ambient, the smaller the factor and the lower the ampacity.
• Conductor bundling adjustment. When more than three current-carrying conductors share a raceway or cable, multiply by an adjustment factor: about 80 percent for 4 to 6 conductors, 70 percent for 7 to 9, and 50 percent for 10 to 20. Neutral conductors that carry only balanced return current generally are not counted.

When both apply, you multiply the 90°C ampacity by both factors.

Counting current-carrying conductors correctly is half the battle. The grounded (neutral) conductor of a balanced circuit, and the equipment grounding conductor, normally do not count. A neutral that carries only unbalanced current is not counted, but a neutral carrying significant harmonic current from nonlinear loads is treated as current-carrying. There is also a useful exemption: the adjustment factors do not apply to short raceway nipples — runs not exceeding 24 inches — so a packed but short connection between two enclosures escapes the bundling penalty.

A worked example

Take an 8 AWG copper THHN conductor, which is 90°C rated. Its 90°C ampacity is 55 amps and its 75°C ampacity is 50 amps. Suppose six current-carrying conductors share the raceway (80 percent adjustment) in a 40°C space (about a 0.91 correction factor). Starting from the 90°C value: 55 × 0.80 × 0.91 ≈ 40 amps. Because 40 amps is below the 50-amp termination limit, the corrected 40 amps governs. This two-step pattern — derate from the 90°C column, then cap at the termination column — is one of the most heavily tested calculations on the journeyman exam.

In most commercial work the 75°C column ends up being the practical sizing column, because that is the common termination rating, so the corrected 90°C value is compared against it and the smaller number wins. Read the question carefully for the termination rating and the conditions of use; those two details decide which column governs and which factors you have to apply.