Wire Size Calculator — NEC Ampacity

How NEC wire sizing actually works

Picking a conductor is a two-rule check. The conductor has to (1) carry the load without overheating, and (2) be no smaller than the lowest-rated termination on the circuit. Get both right and you've satisfied NEC 110.14(C) and NEC 310.15.

Step 1 — Required ampacity

For continuous loads (anything drawing current for 3 hours or more), the conductor and overcurrent device must be sized at 125% of the load per NEC 210.19(A) and 215.2(A). Non-continuous loads use the load value directly.

Step 2 — Termination column

Look up the conductor in NEC Table 310.16 using the column matching the lowest termination rating in the circuit. For most equipment up to 100 A, that's the 60°C column. For commercial equipment marked "75°C only" or "60/75°C," use 75°C. The 90°C column exists only as a starting ampacity for derating — you can never terminate to it.

Step 3 — Ambient and conduit derating

When the ambient or the conduit fill is harsher than the 30°C baseline / 1–3 conductors baseline, apply correction factors to the 90°C ampacity, then make sure that derated value still meets the load.

Your final conductor pick is the smaller of: (a) the size whose 60/75°C ampacity meets the required ampacity, and (b) the size whose derated 90°C ampacity meets the required ampacity. The bigger of those two minimums is what you install.

Step 4 — Voltage drop (separate check)

NEC 210.19 informational note recommends total branch + feeder voltage drop ≤ 5%. Long runs may need to upsize beyond what ampacity alone requires. Use the Voltage Drop Calculator to verify.

NEC Table 310.16 — common ampacities

Insulated conductors rated up to 2000 V, not more than three current-carrying in a raceway, ambient 30°C (86°F).

Per NEC 2023 Table 310.16. Verify against the NEC edition adopted in your jurisdiction. This page is a decision aid — confirm with the AHJ before energizing.

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