NEC 310.16 Derating for 8 Current-Carrying Conductors

Why more than three conductors gets derated

Table 310.16 ampacities assume no more than three current-carrying conductors in a raceway or cable and a 30°C (86°F) ambient. Bundle more conductors together and the heat each one sheds has fewer escape paths, so the group runs hotter at the same current. NEC 310.15(C)(1) restores the margin by cutting the allowable ampacity of every conductor in the bundle. The adjustment is keyed to the count of current-carrying conductors, not the raceway size:

Eight conductors sit in the 7-to-9 row, so the factor is 0.70. In NEC 2017 and earlier this table was numbered 310.15(B)(3)(a); the values did not change when it became 310.15(C)(1).

Count the current-carrying conductors correctly

The 8 in this problem is a count of current-carrying conductors, which is not always the same as the number of wires in the pipe. Grounded (neutral) conductors count only when they carry current under NEC 310.15(E): the neutral of a 3-wire circuit fed from a 4-wire wye that carries harmonic current counts, while the neutral of a balanced multiwire circuit does not. Equipment grounding conductors never count. Four 2-wire 120V branch circuits (four ungrounded, four current-carrying neutrals) give eight. So do eight ungrounded conductors from a set of 240V loads with non-current-carrying neutrals. Establish the count first, because it sets the factor.

Start at the 90°C column, then cap at the termination

NEC 110.14(C) lets you use a conductor's highest temperature column for adjustment and correction math, provided the final adjusted ampacity is not used above the temperature rating of the terminations. THHN, THWN-2, and XHHW-2 are 90°C conductors, so the derating starts in the 90°C column of Table 310.16. The derated result is then checked against the 75°C column (most modern breakers and lugs are listed for 75°C) or the 60°C column if the equipment is only rated 60°C. You size the load and overcurrent device to the lower of the two numbers. These are two independent checks: the adjustment for heat, and the termination limit.

Worked examples: eight 90°C copper conductors, 0.70 factor

Each example takes the 90°C copper ampacity from Table 310.16, multiplies by 0.70, and compares against the termination columns for the same size.

A pattern worth internalizing: at a 0.70 factor the adjusted 90°C value lands below the 75°C column for every one of these sizes, so for a bundle of eight the conductor-count adjustment, not the termination, is what governs. The termination limit starts to bite when fewer conductors are bundled or when the terminals are only 60°C.

When the termination check does bind

Drop to four #8 THHN copper conductors and the factor rises to 0.80 (the 4-to-6 row). The adjusted ampacity is 55 × 0.80 = 44 A. Against 75°C terminals (#8 at 50 A) the 44 A still governs. But if the equipment is listed only for 60°C (#8 at 40 A), the termination caps the conductor at 40 A even though the heat math allowed 44 A. That is why you always run both checks and take the smaller result.

Small conductors carry one more limit. NEC 240.4(D) caps overcurrent protection at 30 A for #10 copper, 20 A for #12, and 15 A for #14 regardless of the ampacity you calculate, so a derated #10 good for 28 A of ampacity is still protected at no more than 30 A.

Stacking ambient correction on top

Conductor-count adjustment and ambient-temperature correction are separate multipliers, and when both apply you multiply them together against the 90°C base. Eight #6 THHN copper conductors run through a 45°C attic take the 0.70 count factor and the 0.87 ambient factor (from the 41-to-45°C row of Table 310.15(B)(1), 90°C column): 75 × 0.87 × 0.70 = 45.7 A, still under the 65 A termination limit. Working through the ambient side on its own is covered in ampacity temperature correction for high ambient heat.

Reference table (90°C copper, 8 conductors, 0.70)

Governing ampacity assumes 75°C terminations, the common case; each adjusted value is below its 75°C column figure. Verify the termination rating stamped on your equipment before you rely on the 75°C column.

NEC reference

NEC 2020. Adjustment factors from Table 310.15(C)(1) (formerly Table 310.15(B)(3)(a)). Base ampacities from Table 310.16, 90°C and 75°C columns. Temperature-column and termination rules from 110.14(C) and 310.14(A). Neutral counting from 310.15(E). Small-conductor overcurrent limits from 240.4(D).

Related

• Wire Ampacity Calculator: adjust Table 310.16 ampacity for conductor count, ambient, and termination in one step.
• Ampacity Temperature Correction for High Ambient Heat: the Table 310.15(B)(1) side of derating, worked step by step.
• Wire Size Calculator: size a conductor for ampacity and voltage drop together.