NEC 310.15: Conductor Ampacity Derating (Ambient and Bundling Factors)

The rule, in plain terms

NEC 310.15 sets the allowable ampacity of a conductor. You start with a base value from a table (almost always Table 310.16), then you correct it for two conditions the table did not account for: ambient temperature that is not 30 C (86 F), and bundling more than three current-carrying conductors together. Two separate factors, multiplied against the same base.

• Base ampacity: Table 310.16, picked from the conductor's insulation column (60, 75, or 90 C).
• Ambient correction: 310.15(B), using the correction factors in Table 310.15(B)(1) or (2). Equals 1.00 at 30 C and drops as ambient rises.
• Adjustment factor: 310.15(C)(1), using Table 310.15(C)(1), when MORE THAN three current-carrying conductors are bundled or share a raceway longer than 24 inches.
• Termination limit: 110.14(C). The final installed ampacity can never exceed the 60 or 75 C column value at the lowest-rated termination, conductor, or device on the circuit.

The formula you carry into the exam: Final ampacity = base ampacity x ambient correction factor x adjustment factor. Then take that result and compare it to the 110.14(C) termination ampacity. Whichever is lower is your answer.

Why derating exists

Conductors make heat when they carry current. The insulation has a temperature it can live with (60, 75, or 90 C) and Table 310.16 is built around getting rid of that heat into a 30 C room with three conductors in the pipe. Raise the room temperature and the conductor cannot shed as much heat, so it can carry less current safely. Pack more current-carrying conductors into the same raceway and they all heat each other, so each one can carry less. Derating is not red tape. It is the code keeping the insulation under its rating so the wire does not cook inside the wall.

Ambient temperature correction (310.15(B))

Table 310.16 is rated at 30 C (86 F). When the ambient is hotter, you multiply the base by a correction factor below 1.00 from Table 310.15(B)(1) for the general case, or Table 310.15(B)(2) for the Fahrenheit version. The factor depends on both the ambient temperature band and the insulation column you started in. A few common rows for the 90 C column, as reference points only (pull the exact factor from your own codebook on test day):

• 36 to 40 C (97 to 104 F): 0.91 for the 90 C column
• 41 to 45 C (105 to 113 F): 0.87 for the 90 C column
• 46 to 50 C (114 to 122 F): 0.82 for the 90 C column

The 60 C and 75 C columns have their own factors in the same table, and they drop faster than the 90 C factors. That is the practical argument for starting derating from the 90 C column when you have 90 C insulation like THHN: you get more headroom to absorb the corrections before the termination limit catches you. For attic and rooftop runs, also check 310.15(B)(2) and the rooftop conditions, because the effective ambient there can run well above the air temperature.

Adjustment factors for bundling (310.15(C)(1))

This is the more-than-three rule. When MORE THAN three current-carrying conductors are bundled together, or run together in a raceway longer than 24 inches, you reduce ampacity by the percentage in Table 310.15(C)(1):

• 4 to 6 current-carrying conductors: 80% (multiply by 0.80)
• 7 to 9 conductors: 70% (0.70)
• 10 to 20 conductors: 50% (0.50)
• 21 to 30 conductors: 45% (0.45)
• 31 to 40 conductors: 40% (0.40)
• 41 or more conductors: 35% (0.35)

Two thresholds matter here. The count has to be MORE THAN three before any adjustment applies, so exactly three current-carrying conductors get no adjustment. And the raceway or bundle has to run longer than 24 inches; nipples and short sleeves under 24 inches are exempt under 310.15(C)(1).

Counting current-carrying conductors (310.15(E))

This is the step that wrecks more answers than the math. The adjustment factor depends entirely on getting the count right. The rules:

• Every ungrounded (hot) conductor that carries current counts.
• The neutral usually does NOT count. On a balanced multiwire circuit the neutral only carries the unbalanced current, so 310.15(E)(1) says it is not counted.
• The neutral DOES count when it carries harmonic or nonlinear-load current. On a 3-phase, 4-wire wye system supplying nonlinear loads, the neutral carries current even when the phases are balanced, so 310.15(E)(3) makes you count it.
• The equipment grounding conductor (EGC) NEVER counts. It normally carries no current. 310.15(F) is explicit.
• A conductor used only for equipment grounding or bonding does not count.

So a standard 3-phase, 4-wire feeder with three hots, one neutral on linear loads, and one EGC has three current-carrying conductors: the three hots. No adjustment. Swap to nonlinear loads and the neutral counts, making four, which drops you to the 80% band.

Worked example: 8 current-carrying 12 AWG THHN in a hot raceway

Eight 12 AWG THHN current-carrying conductors plus an EGC, all in one raceway, run longer than 24 inches, through a space at 40 C (104 F) ambient. Terminations are 75 C rated. Find the final ampacity per conductor.

• Base: 12 AWG copper, 90 C column of Table 310.16 (THHN is 90 C), = 30 A. Start at 90 C because the insulation allows it and it gives the most headroom.
• Ambient correction at 40 C: this falls in the 36 to 40 C band, so for the 90 C column the factor is 0.91 (310.15(B), Table 310.15(B)(1)).
• Count current-carrying conductors: 8 hots count; the EGC does not. 8 conductors lands in the 7 to 9 band, so the adjustment factor is 0.70 (310.15(C)(1)).
• Apply both factors to the base: 30 A x 0.91 x 0.70 = 19.11 A. Round to about 19.1 A.
• Now the 110.14(C) termination check. With 75 C terminations, the cap is the 75 C column value for 12 AWG = 25 A. Our derated 19.1 A is already below that, so the termination limit does not pull it down further.
• Final ampacity: about 19.1 A per conductor. That is the number you size the circuit and protect the conductor against, not the 30 A you started with.

Note what happened: the 90 C base of 30 A got cut to roughly 19 A by the two corrections, and the 75 C termination limit (25 A) was not the controlling number this time because derating took it below 25 A first. On a cooler or less-crowded run, the termination limit is often what controls. You have to compute both and take the lower.

The key exception: short raceways and small bundles

The 310.15(C)(1) adjustment factors do not apply when the bundle or raceway is 24 inches or shorter (the nipple rule), or when fewer than four current-carrying conductors are present. There are also conditions in 310.15(C)(1) for cables stacked or bundled without maintaining spacing. If a question hands you a 12-inch nipple between two boxes with six conductors through it, the answer is no adjustment, because the run is under 24 inches. The exam loves this one.

How derating shows up on the exam

Derating questions are rarely a single lookup. They stack conditions and watch whether you apply them in the right order. Expect these shapes:

Pure count: a list of conductors in a raceway, and you have to report how many are current-carrying before any factor applies. The trap is a neutral or an EGC slipped into the list to be miscounted.

Single correction: only ambient, or only bundling, with the answer being base times one factor. Easy points if you read which condition is present.

Stacked corrections: both ambient and bundling apply, and you multiply both factors against the 90 C base, then check the 75 C termination. This is the canonical multi-step problem and the most common derating question on the journeyman exam.

Reverse sizing: they give you a required load and the conditions, and you work backward to find the smallest conductor whose derated ampacity still covers the load. Same factors, applied in reverse.

The traps that cost points

• Forgetting to derate at all and answering with the raw Table 310.16 value.
• Counting the EGC as a current-carrying conductor. It never counts (310.15(F)).
• Counting a neutral on a balanced linear-load circuit. It does not count unless it carries harmonic or nonlinear current (310.15(E)).
• Reporting the 90 C value as the final answer instead of respecting the 60 or 75 C termination limit under 110.14(C).
• Applying the adjustment factor when only three current-carrying conductors are present. The rule is MORE THAN three.
• Adjusting for a raceway 24 inches or shorter. The nipple exception exempts it.
• Confusing this derating step with the separate 125% continuous-load sizing step from 210.19 and 215.2. Those are two different calculations done at two different points; derating sets what the conductor can carry, the 125% rule sets how big the conductor and breaker must be for a continuous load.