Wire Size Calculator

Why this size

1. Base ampacity (90°C)
   95 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
   95 x 1 x 1
   95 A
5. Termination temperature limit (60°C)
   70 A
   Ref: NEC 110.14(C)
6. Final allowable ampacity (lower of the two)
   70 A
7. Termination and OCPD minimum (125% of continuous + 100% of noncontinuous)
   60 A
   Ref: NEC 210.19(A)(1)(a), NEC 210.20(A)
8. Load the corrected ampacity must cover
   60 A
   Ref: NEC 210.19(A)(1)(b)
9. Termination column vs minimum
   70 A meets 60 A
   Ref: NEC 110.14(C)
10. Corrected ampacity vs load
    95 A meets 60 A
    Ref: NEC 210.19(A)(1)(b)
11. Largest overcurrent device this conductor can protect
    70 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.

What this calculator solves for

It finds the smallest standard conductor that still carries the load once the corrections are applied. That is a different question from what the ampacity table says about a size, and it is the one that matters when material is being ordered.

The answer is the first size, in ascending order, that passes both conductor checks and can still be protected by the required overcurrent device. The termination column has to cover a continuous load at 125%. The corrected ampacity has to cover the actual load. The device sized at 125% still has to fit 240.4, including the small-conductor limits.

The order that matters

• Work out the actual load, and the 125% figure used for the overcurrent device and the termination column (NEC 210.20(A) and 210.19(A)(1)(a)).
• Decide the conductor metal and the insulation temperature rating you intend to install.
• Apply the ambient correction and the conductor-count adjustment to that insulation column (NEC Table 310.15(B)(1)(1) and NEC 310.15(C)(1)). The corrected value has to cover the actual load, not the 125% figure.
• Cap the usable ampacity at the termination temperature rating (NEC 110.14(C)). That column has to cover the 125% figure.
• Take the smallest standard size that passes both checks and can be protected under 240.4, then check voltage drop on the real run length.

Why the answer is often bigger than expected

Two limits are easy to miss. Terminations rated 60 degrees cap the usable ampacity below the insulation column, and a continuous load raises the overcurrent device and that termination check by 25%. The corrected ampacity only has to cover the actual load. A conductor picked only from the high temperature column usually fails the termination check.

Voltage drop is the third factor and it is a separate consideration. A long run can force another size increase even after the ampacity chain is satisfied, which is why the voltage drop calculator is worth running before cable is ordered.

Frequently asked

What size wire do I need for a 100 amp sub panel?

It depends on the termination rating, the ambient temperature and how many current-carrying conductors share the raceway, which is why a single number is not reliable. Run the load through this calculator with your actual conditions, and check the voltage drop on the real run length as well, since a sub panel feeder is often long enough for drop to decide the size.

Does the calculator use the high temperature ampacity column?

It uses the base ampacity for the insulation rating you select, then applies the correction factors, then caps the result at the termination rating. The higher column therefore acts as a starting point for derating rather than as a usable value, which matches how the code treats it.

Why does the number of conductors in the raceway change the wire size?

More than three current-carrying conductors in one raceway reduces their allowable ampacity through the adjustment factors in NEC 310.15(C)(1). The reduction can push the smallest adequate size up by one or two steps, and it applies on top of the ambient correction rather than instead of it.

Should I size the wire to the breaker or to the load?

Size the conductor to the load after corrections, then choose the overcurrent device so that it protects that conductor. Working in the other direction, starting from a device rating and picking the conductor to match, is how a circuit ends up with a device that is too large for its conductor.

Related guides and references

• Wire size for a 100 A sub panel
• Copper vs aluminum wire
• The voltage drop 3% rule
• Quick reference (printable PDF)