Wire Size Calculator (NEC)
Find the correct wire size (AWG) for any circuit under the National Electrical Code (NEC). Based on NEC Table 310.16 conductor ampacity ratings at 60 °C, 75 °C, and 90 °C, with temperature and conduit-fill derating. Covers copper and aluminium conductors.
Minimum wire size
8 AWG
Base ampacity (75 °C)
50A
Derated ampacity
50A
8 AWG copper carries the load after derating
Ambient factor 1 × raceway factor 1 applied to the 75 °C column of NEC Table 310.16.
Remember NEC 240.4(D): 14 AWG is limited to 15 A, 12 AWG to 20 A, and 10 AWG to 30 A regardless of ampacity.
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How NEC wire sizing works
Conductor selection starts with NEC Table 310.16, which lists the allowable ampacity of each AWG/kcmil size at three insulation temperature ratings. You then apply two adjustments:
- Ambient temperature correction(Table 310.15(B)) - for ambient above 30 °C the ampacity is reduced.
- Conductor bundling adjustment (Table 310.15(C)) - when more than three current-carrying conductors share a raceway, ampacity is reduced to 80%, 70%, 50% or 45%.
The terminal rule still applies: even if the 90 °C column is used for derating, the final conductor must be protected at no more than the ampacity of its terminal rating (usually 75 °C).
Worked example
A 40 A continuous load on 75 °C copper terminals, four conductors in a raceway at 30 °C ambient:
- Conduit factor for 4–6 conductors = 0.80
- 8 AWG copper (50 A at 75 °C) × 0.80 = 40 A - exactly at the load
- For a continuous load, size at 125% → 50 A, requiring 6 AWG copper
Frequently Asked Questions
- What is the minimum wire size for a 20-amp circuit (NEC)?
- Per NEC Table 310.16, 12 AWG copper is rated 20 A at 60 °C and 25 A at 75 °C. For a 20-amp breaker, 12 AWG copper is the minimum.
- Why does the NEC limit 14 AWG to 15 amps when the wire is rated for more?
- NEC 240.4(D) applies the 'small conductor rule', limiting 14 AWG to 15 A overcurrent protection, 12 AWG to 20 A and 10 AWG to 30 A, regardless of the wire's temperature rating.
- When should I use the 75 °C column vs the 60 °C column?
- Use 75 °C for conductors connected to terminals rated 75 °C or higher (most modern breakers and panelboards). Use 60 °C for older equipment or when terminal ratings are unknown.
- Does wire size affect voltage drop?
- Yes. Larger wire has lower resistance and lower voltage drop. Use our Voltage Drop Calculator to check both ampacity and voltage drop for long runs.
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