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Wire size

300 Amp Wire Size

Recommended conductor sizes for a 300 A load with 90°C insulation, 75°C terminations, 30°C ambient and three current-carrying conductors. Verify equipment and termination markings; 75°C ratings may be used only where permitted. Adjust the conditions below for your job.

Quick answer

MaterialMinimum sizeAmpacity at that size
Copper350 kcmil310 A
Aluminum500 kcmil310 A

These sizes assume 75°C terminations and a noncontinuous load. Check the actual termination ratings and equipment markings. Type NM cable is subject to the 60°C ampacity limitation under NEC 334.80; check actual manufacturer sizes and permitted applications before choosing cable. The next-higher breaker exception is not assumed in this calculator.

Why not 300 kcmil?

300 kcmil has corrected ampacity 320 A and a 75°C termination-column limit of 285 A. The lower limit is 285 A, below the 300 A load.

In aluminum the next size down is 400 kcmil: 400 kcmil has corrected ampacity 305 A and a 75°C termination-column limit of 270 A. The lower limit is 270 A, below the 300 A load.

Ampacity alone does not settle the size. The check walks the standard sizes from the smallest size with tabulated ampacity data (14 AWG) upward and stops at the first conductor that clears three things at once — the corrected ampacity, the termination temperature column, and the limit on the overcurrent device that can protect it.

Calculator

Inputs

Result

Adequate for the load
Final ampacity
310 A
Conductor area
177.35 mm²
OCPD minimum
300 A
Breaker
300 A

Calculation steps

  1. Base ampacity (90°C)
    350 A
    Ref: NEC 310.16
  2. Ambient temperature correction (30°C)
    1
    Ref: NEC Table 310.15(B)(1)(1)
  3. Adjustment for 3 conductors in raceway
    1
    Ref: NEC 310.15(C)(1)
  4. Adjusted ampacity
    350 x 1 x 1
    350 A
  5. Termination temperature limit (75°C)
    310 A
    Ref: NEC 110.14(C)
  6. Final allowable ampacity (lower of the two)
    310 A
  7. Termination and OCPD minimum (125% of continuous + 100% of noncontinuous)
    300 A
    Ref: NEC 210.19(A)(1)(a), NEC 210.20(A)
  8. Load the corrected ampacity must cover
    300 A
    Ref: NEC 210.19(A)(1)(b)
  9. Termination column vs minimum
    310 A meets 300 A
    Ref: NEC 110.14(C)
  10. Corrected ampacity vs load
    350 A meets 300 A
    Ref: NEC 210.19(A)(1)(b)
  11. Largest overcurrent device this conductor can protect
    300 A
    Ref: NEC 240.4; next-higher exception not assumed

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The PDF contains your inputs, the result, the verdict, the formula with the values substituted, and the code reference for every step. No signup, no watermark, nothing held back.

These calculators are provided for educational, design-planning and general informational purposes only. They do not constitute professional engineering advice or a recommendation for licensed installation. Codes and electrical regulations vary by jurisdiction, and actual installation must be carried out by a licensed electrician in accordance with the code enforced locally. Misuse of these results can cause electrical fire, equipment damage or code violations. The NEC table values on this site reflect NEC 2023; the 2026 edition has not been verified. Verify every value against the edition of the NEC in force in your area. The conditional next-higher breaker exception is not assumed.

Same load, different conditions

300 A is the load, not the answer. Insulation stays at 90°C so ambient correction starts from that column; only the termination rating or the ambient temperature changes:

ConditionsCopperAluminum
90°C insulation, 75°C terminations, 30°C350 kcmil500 kcmil
90°C insulation, 60°C terminations, 30°C500 kcmil750 kcmil
90°C insulation, 75°C terminations, 40°C350 kcmil500 kcmil

Computed by this site's calculation engine. Every row keeps the load at 300 A and changes only the conditions, so the difference is the correction itself.

Frequently asked

What size wire for a 300 amp circuit?
With 90°C insulation such as THHN, 75°C terminations, 30°C ambient and three current-carrying conductors in the raceway, the smallest size that meets the ampacity and termination checks is 350 kcmil copper or 500 kcmil aluminum. Ampacity is only the first check: the termination rating and voltage drop both move the answer.
Does a 300 amp circuit need a larger conductor on a long run?
Often yes. Ampacity sets the minimum and voltage drop then decides the practical size, because a long run can push the conductor one or two sizes up to stay inside the 3% feeder or branch-circuit recommendation. Distance is not part of the ampacity tables, so check the run separately.
Can aluminum be used on a 300 amp circuit?
Yes, where the terminations are listed for aluminum. For this load aluminum lands at 500 kcmil against 350 kcmil in copper, because aluminum carries less current at the same size. Confirm the terminations, and any local restriction on small branch-circuit sizes, before substituting.
Why does the recommended size change with the conditions?
Allowable ampacity depends on the insulation rating, the ambient temperature, how many current-carrying conductors share the raceway and the rating of the terminations. The base table assumes a 30°C ambient with three conductors, so a hotter space or a more crowded raceway is corrected downward and can require the next size up.

Notes for a 300 A circuit

  • The size above assumes 90°C insulation and 75°C terminations. Confirm equipment and termination ratings before choosing the applicable column.
  • Long runs may need a larger conductor to stay within the 3% feeder or branch-circuit voltage drop recommendation. Check with the voltage drop calculator.
  • Conduit sizing for the chosen conductor is on the conduit fill chart.

Cite this page

Use this reference in a report, article, course note or dataset. Calculated examples use this site's engine; reference data and explanatory text have separate sources and review status.

ElecCalcs. “300 Amp Wire Size.” Updated October 29, 2026. https://eleccalcs.com/300-amp-wire-size

Values reflect NEC 2023 and are pending review. Cite the edition you verified against.

Table values reflect NEC 2023 and are pending review. Not professional engineering advice.