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NEC Cable Size Calculator

Size the conductors of a multi-conductor cable when more than three current-carrying conductors are bundled together. Base ampacity comes from NEC Table 310.16; the more-than-three-conductor adjustment from Table 310.15(C)(1) and ambient correction from Table 310.15(B)(1) derate it, and the result is capped at the termination rating per NEC 110.14(C). NEC 2020.

Cable and load conditions

Sizes the smallest conductor whose derated ampacity carries the load. Base ampacity from NEC Table 310.16; more-than-three-conductor adjustment from Table 310.15(C)(1); ambient correction from Table 310.15(B)(1); result capped at the termination column per NEC 110.14(C). Voltage drop is not included; use the Wire Size Calculator for distance-driven sizing.

How to use this calculator

  1. Enter the load current each conductor carries, and check "continuous" for loads that run three hours or more (applies the 125% factor).
  2. Enter the number of current-carrying conductors in the cable or raceway (Table 310.15(C)(1) derating starts above three).
  3. Select the conductor material, insulation rating, and the termination rating of the lowest-rated terminal per NEC 110.14(C).
  4. Set the ambient temperature. The result is the smallest conductor whose derated, termination-capped ampacity carries the load.

NEC reference

This calculator uses NEC Table 310.16 (base allowable ampacities), NEC Table 310.15(C)(1) (adjustment for more than three current-carrying conductors), NEC Table 310.15(B)(1) (ambient temperature correction), and NEC 110.14(C) (termination temperature limitation), NEC 2020. Table 310.16 was designated Table 310.15(B)(16), and Table 310.15(C)(1) was Table 310.15(B)(3)(a), in NEC 2017; the ampacity and adjustment values are unchanged across the 2017, 2020, and 2023 editions. NEC 240.4(D) small-conductor overcurrent limits are applied as a sizing safeguard and shown as a note where they apply.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

How multi-conductor cable is sized

Conductor sizing for a bundled cable is a derated-ampacity problem, not a formula. The base ampacity is a Table 310.16 lookup by material and insulation column (60/75/90°C), assuming 30°C (86°F) ambient and not more than three current-carrying conductors. Two derating steps and one cap modify that base, and the tool returns the smallest size that still covers the load: 

Final usable ampacity = min( base × ambient × conductor-count , termination limit )
  • Conductor-count adjustment: Table 310.15(C)(1), the headline factor: 0.80 for 4-6 conductors, 0.70 for 7-9, 0.50 for 10-20, and lower beyond.
  • Ambient correction: Table 310.15(B)(1), applied when the ambient differs from 30°C, read from the same insulation column.
  • Termination cap: NEC 110.14(C). Derating may start from the 90°C column, but the final ampacity can’t exceed the Table 310.16 value at the lowest-rated termination, typically 75°C.

The two derating factors multiply; they are not alternatives. For a continuous load the required ampacity is 125% of the load current per NEC 210.19(A)(1).

Worked example

A cable with six current-carrying THHN copper conductors (90°C insulation), each carrying 40 A, landing on 75°C terminations at 86°F (30°C) ambient: 

Try 8 AWG Cu:
Base (90°C col, Table 310.16)        55 A
Ambient (26-30°C, ×)               1.00
Conductors 4-6 (×, 310.15(C)(1))   0.80
Derated  55 × 1.00 × 0.80   =     44 A
Termination cap (75°C, 110.14(C))    50 A
Final  min(44, 50)             =     44 A   ≥ 40 A  →  OK

8 AWG copper works: 44 A of usable ampacity covers the 40 A load. 10 AWG would derate to 40 × 0.80 = 32 A, short of 40 A. Drop the bundle to three conductors and the count factor disappears; 8 AWG is then good to its 50 A termination limit.

Common mistakes

  • Skipping the conductor-count adjustment. The 4-6 conductor factor of 0.80 alone can push a cable up a full size. It applies whenever more than three current-carrying conductors share the cable or raceway (NEC Table 310.15(C)(1)).
  • Mis-counting current-carrying conductors. The equipment grounding conductor never counts. A neutral carrying only the unbalanced current of a multiwire circuit doesn’t count; a neutral with significant nonlinear load does (NEC 310.15(E)).
  • Forgetting the 110.14(C) termination cap. Derating from the 90°C column is allowed, but the final ampacity can’t exceed the 75°C column on standard equipment.
  • Using these values for NM cable directly. NEC 334.80 bases Type NM ampacity on the 60°C column. Select the 60°C column for NM; the 75/90°C columns are for conductors in conduit and cable types rated to those terminations.

Frequently asked questions

How does bundling more than three conductors affect cable size?

Once a raceway or cable carries more than three current-carrying conductors, NEC Table 310.15(C)(1) requires an adjustment factor because the bundled conductors cannot shed heat as easily. Four to six conductors derate to 80 percent, seven to nine to 70 percent, ten to twenty to 50 percent, and it keeps dropping. The factor multiplies the Table 310.16 base ampacity, so a heavily loaded multi-conductor cable often needs a larger conductor than the same load in a two or three wire circuit.

What counts as a current-carrying conductor?

Per NEC 310.15(E), the ungrounded (hot) conductors always count. A neutral that carries only the unbalanced current of the other conductors, such as the neutral of a balanced multiwire branch circuit or a balanced 3-phase wye, does not count. A neutral carrying significant harmonic current from nonlinear loads does count. The equipment grounding conductor never counts. Enter only the conductors that actually count so the Table 310.15(C)(1) factor is right.

Is this different from the Wire Size Calculator?

Yes. The Wire Size Calculator sizes a single circuit for a load over a distance against a target voltage drop, then picks a breaker. This Cable Size Calculator focuses on multi-conductor cable where the governing factor is the more-than-three-conductor derating from Table 310.15(C)(1). It does not include voltage drop or a distance input. For a long run where voltage drop may govern, size here for ampacity first, then confirm the run with the Voltage Drop Calculator.

Do I derate from the 90 degree C column?

For a 90 degree C conductor such as THHN or THWN-2, NEC 310.15 lets you start the derating from the 90 degree C column of Table 310.16, then apply the ambient and conductor-count factors. The final result is still capped at the termination temperature, usually 75 degree C, per NEC 110.14(C). This calculator does exactly that: it derates from the insulation column you select, then caps at the termination column, which is the conservative, never-undersize order.

Does the small-conductor rule apply to cable sizing?

Yes. NEC 240.4(D) caps overcurrent protection for 14, 12, and 10 AWG copper at 15, 20, and 30 amps regardless of ampacity. This tool will not recommend a small conductor that has the derated ampacity for the load but cannot be legally protected for it, and it notes when a smaller size was rejected for that reason. The cap is an overcurrent-protection limit, not the conductor ampacity.

Does NM cable (Romex) use these same values?

Not exactly. NEC 334.80 requires Type NM cable ampacity to be based on the 60 degree C column even though the conductors are 90 degree C rated, and the 90 degree C rating is used only as the starting point for derating. This calculator uses the general Table 310.16 ampacities and lets you pick the column, so for NM cable select the 60 degree C column as the basis. For MC, TC, and conductors in conduit, the column follows the conductor insulation and terminations.

Related tools

Studying for the licensing exam? Our own NEC Code Quickstart works all twelve exam calculations start to finish, each with the code reference. Also in paperback.

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For deeper NEC training on multi-conductor cable sizing and derating, Mike Holt's NEC courses are the industry standard.

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