CON-3 — Adjustment, Correction, and Special Ampacity Conditions

Count current-carrying conductors first

Under 310.15(B)(3)(a), adjustment applies where more than three current-carrying conductors are installed in a raceway or cable, or where single conductors or multiconductor cables are installed without maintaining spacing for more than 24 in. The table factors are:

• 4–6 current-carrying conductors: 80 percent;
• 7–9: 70 percent;
• 10–20: 50 percent;
• 21–30: 45 percent;
• 31–40: 40 percent; and
• 41 and above: 35 percent.

Count conductors by electrical function, not by the number of insulated wires visible. Ungrounded conductors carrying load count. Equipment grounding and bonding conductors do not carry normal load and are not counted. Each current-carrying conductor in a paralleled set counts. Spare conductors generally count because they can be energized, while conductors connected only to components that cannot be simultaneously energized are treated under the rule's condition.

Section 310.15(B)(5) controls neutral treatment. A neutral carrying only imbalance current from other conductors of the same circuit need not be counted. A 120/240 V single-phase, three-wire circuit with line-to-neutral loads therefore generally has two current-carrying ungrounded conductors; its neutral carries only their imbalance.

Do not turn that into “neutrals never count.” In a three-wire circuit consisting of two phase conductors and the neutral of a four-wire, three-phase wye system, the neutral carries current comparable to the line-to-neutral load and counts. On a four-wire, three-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic current makes the neutral current-carrying for this calculation. Describe the system and load before deciding.

The raceway-nipple exception is precise: adjustment factors do not apply to conductors in a raceway not over 24 in. long. It does not excuse a long bundled run because only one crowded portion is visible. Type AC and MC cable bundling also has specific 2017 conditions and a 60 percent rule above its stated conductor count; use that exception's exact cable, jacket, conductor-size, and support conditions rather than generalizing it.

Correct for ambient temperature

Table 310.15(B)(2)(a) corrects ampacity for ambient temperatures other than the 30°C basis of Table 310.15(B)(16). In the 90°C column, selected factors are 0.96 at 31–35°C, 0.91 at 36–40°C, 0.87 at 41–45°C, 0.82 at 46–50°C, 0.71 at 56–60°C, and 0.65 at 61–65°C. The 60°C and 75°C columns have different factors; use the column matching the conductor's permitted insulation rating in that location.

When both conductor-count adjustment and ambient correction apply, multiply both by the permitted insulation-column value. Then compare the product with terminal, cable, and equipment limitations. Do not apply the factor to load current, do not subtract a percentage, and do not first cap the table value at 75°C if a 90°C conductor is permitted to use its higher rating for adjustment.

Example: nine current-carrying 8 AWG copper THWN-2 conductors share a raceway in a 40°C ambient and terminate on 75°C equipment. The 90°C table value is 55 A. Nine conductors use 70 percent, and 40°C uses the 0.91 factor from the 90°C column:

55 A × 0.70 × 0.91 = 35.035 A

The 75°C termination cap is 50 A. The adjusted and corrected value is lower, so allowable ampacity is about 35 A. This is not automatic permission for a particular standard overcurrent-device size; apply Article 240 and the load rules separately.

Apply the 2017 rooftop rule

For the 2017 edition, 310.15(B)(3)(c) does not use the older multi-row rooftop height-adder table. Where a raceway or cable is exposed to direct sunlight on or above a rooftop and the distance from the roof to its bottom is less than 7/8 in., add 33°C (60°F) to the outdoor ambient temperature before selecting the Table 310.15(B)(2)(a) correction factor. Type XHHW-2 insulated conductors are excepted from this specific rooftop adder. At 7/8 in. or more, the special 33°C adder does not apply, but correction for the actual ambient still can.

State the assumptions. Example: six current-carrying 3 AWG copper THWN-2 conductors are in a raceway exposed to direct sun, 1/2 in. above a roof. Outdoor design ambient is assumed to be 30°C; no XHHW-2 exception applies. Rooftop ambient for the calculation is 30°C + 33°C = 63°C. The 90°C correction factor for 61–65°C is 0.65, the six-conductor adjustment is 0.80, and the 90°C copper ampacity is 115 A:

115 A × 0.65 × 0.80 = 59.8 A

With 75°C terminals, the independent terminal cap is 100 A, so 59.8 A controls. If the raceway were shaded, at least 7/8 in. above the roof, or contained XHHW-2, the special rooftop premise would change; actual ambient, other heat sources, and all remaining installation rules would still require evaluation.

Enforce special wiring-method caps

For Type NM cable, 334.80 uses the 60°C ampacity even though 90°C insulation values can be used as the adjustment or correction starting point when the final value does not exceed the 60°C limit. For example, 12 AWG copper conductors in bundled NM-B cables total eight counted conductors and have a 90°C value of 30 A and a 70 percent adjustment: 30 A × 0.70 = 21 A. The final NM cap is the 20 A 60°C value, and 240.4(D) also generally limits 12 AWG copper protection to 20 A. With ten counted conductors, 30 A × 0.50 = 15 A, so the adjusted value—not the 20 A cap—controls.

Other special conditions include cable tray rules, thermal insulation around cable, ambient temperature at terminations, raceway spacing, conductor bundling, motor and HVAC articles, and dwelling-service allowances. Never apply the dwelling-service percentage to a feeder that does not carry the entire dwelling load described by 310.15(B)(7). Record the edition, table basis, conductor count, neutral decision, ambient source, rooftop height and sunlight condition, insulation rating, factors, terminal ratings, and final limiting value so another electrician can reproduce the calculation.