Ampacity Calculator

Calculation steps

1. Base ampacity (90°C)
   30 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
   30 x 1 x 1
   30 A
5. Termination temperature limit (60°C)
   20 A
   Ref: NEC 110.14(C)
6. Final allowable ampacity (lower of the two)
   20 A
7. Termination and OCPD minimum (125% of continuous + 100% of noncontinuous)
   20 A
   Ref: NEC 210.19(A)(1)(a), NEC 210.20(A)
8. Load the corrected ampacity must cover
   20 A
   Ref: NEC 210.19(A)(1)(b)
9. Termination column vs minimum
   20 A meets 20 A
   Ref: NEC 110.14(C)
10. Corrected ampacity vs load
    30 A meets 20 A
    Ref: NEC 210.19(A)(1)(b)
11. Largest overcurrent device this conductor can protect
    20 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.

Ampacity is a chain, not a single number

The value printed in a base ampacity table is a starting point, not an answer. What a conductor may actually carry is that base value after two multipliers, and then whatever the termination it lands on will allow. Skip a link in that chain and the result is optimistic.

The base value depends on the conductor metal and on the temperature rating of its insulation. The two multipliers depend on where the run is and what shares it. The termination limit depends on the equipment at each end, which is usually the part people forget.

The correction chain applied to copper with 75°C insulation

Copper conductors at a 75 degree insulation rating, in 40 degree ambient air, with six current-carrying conductors in the raceway and 75 degree terminations. Both multipliers apply to the base value, and the result is then capped by the termination rating.

Worked examples

Each example is run by the same engine as the calculator above, so the substitution, the result and the code reference come from the same tables.

The four steps, in order

• Start from the base ampacity of the size and metal at the insulation temperature rating (NEC 310.16).
• Multiply by the ambient temperature correction factor for the temperature the raceway actually runs through (NEC Table 310.15(B)(1)(1)).
• Multiply by the adjustment factor for the number of current-carrying conductors sharing the raceway (NEC 310.15(C)(1)).
• Cap the result at the ampacity permitted by the termination temperature rating (NEC 110.14(C)). The lower of the corrected value and the termination value governs.

Why the 90 degree column rarely survives

THHN is a 90 degree conductor, so its 90 degree ampacity is the correct starting point for the correction factors. It is not the number you get to use. Equipment rated 100 A or less generally has terminations rated 60 degrees unless marked otherwise, and equipment above that is usually rated 75 degrees, so the corrected result is capped there.

The practical effect is that the higher temperature column acts as a derating allowance rather than a free upgrade. Reading across to the 90 degree column, taking that value and installing a smaller conductor is the classic version of this mistake.

Continuous loads add a separate factor

The 125% for continuous loads belongs to the circuit, not to the conductor correction chain. Overcurrent devices are sized at not less than 125% of the continuous load plus 100% of the noncontinuous load (NEC 210.20(A)), and the same idea carries into feeder sizing.

NEC 210.19(A)(1) splits the conductor check. The termination-temperature column has to cover the noncontinuous load plus 125% of the continuous load. Separately, the ampacity after ambient and conductor-count corrections has to cover the actual load, not that 125% figure. Applying 125% again to the already corrected ampacity oversizes the wire. The overcurrent device is still sized at 125% of the continuous load under 210.20(A), and that device still has to protect the conductor under 240.4.

The calculator can take the continuous portion of the load separately. If the whole load is continuous, select Yes; if none is, select No; if only part is, choose Part of the load and enter the continuous amperes. Applying 125% to a mixed load as a single number is the usual way one of the two checks ends up wrong.

Where the correction values come from

Ambient correction factors fall as the ambient temperature rises, and their magnitude depends on the insulation rating, so the factor has to come from the column that matches the conductor rather than from a general rule of thumb.

The conductor-count adjustment begins above three current-carrying conductors and gets steeper as the count grows. Rewiring a raceway that was built for three conductors to carry six is not only a fill question; the adjusted ampacity may fall below the load even though the raceway passes the fill test.

Common mistakes

• Using the 90 degree ampacity as the final answer on equipment with 75 degree terminations.
• Counting the equipment grounding conductor as current-carrying. It never counts for the adjustment factor, though it does count for conduit fill.
• Confusing the two factors. The ambient correction applies to the conductor in its environment, while the continuous-load factor applies to the circuit being sized.
• Applying corrections to the termination ampacity instead of the base ampacity. The correction chain starts from the table value for the insulation.

Frequently asked

What is the difference between ampacity and the breaker rating?

Ampacity is what the conductor may carry after corrections; the breaker rating is the rating of the overcurrent device protecting the circuit. The device is chosen to protect the conductor, so it is not larger than the conductor ampacity after correction, with the specific exceptions the code allows for the next standard rating.

Which temperature column should I use?

Use the insulation rating to pick the base value, then take the lower of the corrected result and the termination limit. For equipment rated 100 A or less, terminations are commonly rated 60 degrees unless marked otherwise, and above 100 A they are usually rated 75 degrees, so the 90 degree column often ends up unused for the final answer.

Does a neutral conductor count for the ampacity adjustment?

Only when it is a current-carrying conductor. In a multiwire branch circuit a neutral that carries only the unbalanced current of the other phases is not counted, but a neutral that carries the full current of a nonlinear load, such as many electronic ballasts, is counted. The equipment grounding conductor never counts.

How much does ambient temperature reduce ampacity?

Enough to matter on most industrial work. The factors fall steadily as the ambient climbs above the table baseline, and the reduction is larger for higher insulation ratings. An outdoor run in direct sun or a raceway in a hot mechanical room is exactly where this factor decides whether the conductor is adequate.

Why is my corrected ampacity lower than the termination rating?

Because ambient heat or conductor count reduced it below what the terminations would allow. The lower number governs, so the fix is a larger conductor, a different raceway arrangement, or a lower ambient. Applying only one of the two limits is the usual reason a design looks adequate on paper and is not.

Related guides and references

• Ampacity correction factors
• How to read an ampacity chart
• THHN vs THWN
• Quick reference (printable PDF)