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Wire Ampacity Calculator (NEC Table 310.16)

Allowable ampacity by conductor size, material and termination column, with the adjustment factors you applied.

Inputs

The conductor

Sizes run from 14 AWG to 500 kcmil, the range of the table used here.

Copper or aluminum. Aluminum needs roughly two sizes larger for the same ampacity and is not made at 14 AWG.

The usable column is set by the weakest termination, not by the insulation (NEC 110.14(C)). 60 C for older equipment, 75 C for most modern equipment rated 100 A and below, 90 C only when every termination is 90 C rated.

Percent of the table ampacity after the 310.15(B) ambient correction and the 310.15(C)(1) conductor-count adjustment. Leave 100 for free air or a circuit that is already accounted for.

The load

The current the circuit will carry, so the ampacity can be compared with something.

Result

Ampacity after adjustment
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Table 310.16 ampacity at 30 C
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Headroom over the load
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240.4(D) device limit for this size
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Load as a share of ampacity
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Overcurrent limit to remember
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Where this stops applying
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Calculation details
Circular mils
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Adjustment applied
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What to check

  • Conductor size, material and the termination temperature rating
  • Ambient correction from 310.15(B) and the conductor-count adjustment from 310.15(C)(1)
  • Circuit load, to compare the ampacity against

Standards referenced

  • NFPA 70 National Electrical Code (NEC) — 2023 edition · Branch circuits, conductors, protection, raceways, and services. Local AHJ adoptions vary; the enforcing edition is set by the jurisdiction.

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