Wire Ampacity ChartWire Ampacity Chart

NEC 2023 · Copper & aluminum · AWG / kcmil

Ampacity calculator

Calculate copper or aluminum wire ampacity using NEC Table 310.16. The calculator applies ambient temperature correction, conductor-count adjustment, and terminal limits. The result shows each factor and the allowable current.

Compare sizes in the wire ampacity chart for copper and aluminum at 60°C, 75°C, and 90°C.

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01. Calculate wire ampacity

Conductor and installation details

Change an input to update the result. Current values are in amperes for one conductor.

Conductor material
Insulation temperature rating

Check the wire rating for the wet or dry installation location. THHN is typically rated for 90°C in dry locations.

Equipment termination rating

Check the rating marked on the equipment terminals. It may be lower than the wire insulation rating.

Count within the same raceway or cable. Exclude equipment grounding conductors.

Allowable ampacity after terminal limit
65 A

6 AWG Copper · 90°C insulation · 75°C terminals

Base ampacity · Table 310.16
75 A
Ambient correction factor
× 1
Conductor-count adjustment
× 1
Corrected and adjusted ampacity
75 A
Terminal and insulation limit
65 A

75 A × 1 × 1 = 75 A. The allowable ampacity is 65 A.

NEC 240.4(D) has no small-conductor limit for this size. Use the applicable rules of NEC 240.4 to select overcurrent protection. The ampacity result is not a recommended breaker size.

This calculator uses NEC 2023 Table 310.16 for insulated conductors in raceway, cable, or earth, rated up to 2,000 V. Check additional rules for NM-B or UF cable, free-air installations, parallel runs, and load sizing.

How to use the ampacity calculator

  1. Select the conductor material and size printed on your wire, in AWG or kcmil.
  2. Select the insulation rating for the installation location. Check the temperature rating marked on each termination.
  3. Set the ambient temperature around the wiring and count the current-carrying conductors sharing the raceway or cable.
  4. Read the allowable ampacity and calculation breakdown. Check the separate small-conductor protection limit before choosing an overcurrent protective device.

Multiply the base ampacity by the ambient correction factor and conductor-count adjustment factor. Limit the result to the terminal and insulation column ratings. The homepage has the ambient correction and conductor adjustment tables.

Example with 6 AWG copper at 40°C and six conductors

The base ampacity of 6 AWG copper with 90°C insulation is 75 A. At 40°C ambient, or 104°F, the correction factor is 0.91. Six current-carrying conductors require an adjustment factor of 0.80. The equipment in this example has 75°C terminals.

75 A × 0.91 × 0.80 = 54.6 A

The 75°C terminal column allows 65 A. The adjusted ampacity is lower, so the result is 54.6 A. At 30°C ambient with three current-carrying conductors, the adjusted ampacity is 75 A. The terminal limit then controls, giving a result of 65 A.

NEC sources and calculation limits

Base values come from NFPA 70, National Electrical Code, 2023 edition, Table 310.16. Table 310.15(B)(1)(1) supplies the ambient correction factors. Table 310.15(C)(1) supplies the conductor-count adjustment factors. NEC 110.14(C) sets terminal limits. NEC 240.4(D) sets small-conductor protection limits.

The calculator uses Table 310.16 for insulated conductors in raceway, cable, or earth. It does not apply NM-B or UF cable limits, free-air tables, parallel-conductor rules, medium-voltage thermal models, or local amendments. Check physical conduit fill separately from conductor-count adjustment.

The calculator truncates displayed results to two decimal places. It uses unrounded values to calculate limits. If the table has no correction factor for your selection, it shows no result. A small-conductor protection ceiling is a limit, not a recommended standard breaker size.

Have a qualified electrical professional check the NEC edition adopted locally, installation rules, and equipment markings before using these results for an installation. Read about our data and methods.

Ampacity calculator FAQ

What is ampacity?
Ampacity is the maximum current a conductor can carry under specific installation conditions without exceeding its temperature rating. It depends on wire size, material, insulation, ambient temperature, and conductor grouping.
How do I calculate wire ampacity with derating?
Find the base ampacity in NEC Table 310.16 for the material, wire size, and insulation rating. Multiply it by the ambient correction factor and conductor-count adjustment factor. Apply the terminal and insulation limits to that result. Check cable-specific restrictions and overcurrent protection separately.
Can I use 90°C ampacity with 75°C terminals?
Use the 90°C column for correction and adjustment if the conductor is rated for 90°C in the installation location. With 75°C terminals, the final ampacity cannot exceed the 75°C column value. Check the equipment markings and NEC 110.14(C).
What is the ampacity of 12 AWG copper wire?
NEC Table 310.16 lists 20 A at 60°C, 25 A at 75°C, and 30 A at 90°C. NEC 240.4(D) limits overcurrent protection to 20 A unless a stated exception applies. Ambient temperature and conductor grouping can reduce the usable ampacity.
Do grounding and neutral wires count for derating?
Exclude equipment grounding conductors from the current-carrying count. They still occupy space when you calculate conduit fill. A neutral carrying only unbalanced current may be excluded from the count. Some neutrals serving nonlinear loads must be counted. Check the rules for your circuit in NEC 310.15(E).
Does this calculator account for voltage drop or continuous loads?
This calculator gives the ampacity of the wire size you select. It does not calculate voltage drop or select a wire size for a load. It also does not apply the usual 125% continuous-load sizing requirement. Check those requirements separately, as they may call for a larger conductor.