Wire Size Calculator

Determine minimum wire size (AWG) for your circuit based on load current, distance, and voltage drop per NEC.

By TradeCalc, Electrician Calculators — Code-Referenced — NEC 310.16, NEC 210.19, NEC 215.2

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How to Determine Wire Size

Understanding Wire Sizing (AWG)

Wire size in North America is measured in American Wire Gauge (AWG). Counterintuitively, a smaller AWG number means a larger wire. For example, 10 AWG is larger than 12 AWG and can carry more current. Above 1 AWG, sizes continue as 1/0 (one-ought), 2/0, 3/0, and 4/0, then into kcmil (thousands of circular mils).

Properly sizing wire requires checking two things: (1) the wire's ampacity must handle the load, and (2) the voltage drop over the distance must stay within NEC limits.

NEC Table 310.16 — Ampacity Reference

Wire size is determined by the larger of:

Ampacity requirement: Wire ampacity ≥ load current (NEC Table 310.16, 75°C column)
Voltage drop requirement: Voltage drop ≤ 3% for branch circuits, ≤ 5% total

NEC Table 310.16 lists allowable ampacities for copper and aluminum conductors at 60°C, 75°C, and 90°C temperature ratings. For most modern installations, the 75°C column is used because most equipment terminals are rated for 75°C per NEC 110.14(C).

Worked Example

Scenario: Size wire for a 30A, 240V single-phase load, 150 ft run, copper, max 3% voltage drop.

Ampacity check: 30A load → 10 AWG copper (35A at 75°C) ✓
Voltage drop check (10 AWG, R = 1.24):
VD = (2 × 150 × 1.24 × 30) / 1000 = 11.16V → 4.65% ✗ (exceeds 3%)
Try 8 AWG (R = 0.778):
VD = (2 × 150 × 0.778 × 30) / 1000 = 7.00V → 2.92% ✓
Result: Use 8 AWG copper (satisfies both ampacity and voltage drop).

Practical Tips

Always use the 75°C column for termination ratings unless the equipment is specifically marked for a higher temperature.
Derate for ambient temperatures above 30°C and for more than 3 current-carrying conductors in a conduit (see NEC 310.15).
The small conductor rule (NEC 240.4(D)) limits breaker sizes: 14 AWG → 15A, 12 AWG → 20A, 10 AWG → 30A, regardless of ampacity.
When in doubt, go one size larger — the material cost difference is minimal compared to the safety benefit.

Code References

NEC 310.16, NEC 210.19, NEC 215.2

Frequently Asked Questions

How do I know what wire size to use for my circuit?

Wire size is determined by two factors: (1) the ampacity must be sufficient for the load current, per NEC Table 310.16, and (2) the voltage drop must not exceed 3% for branch circuits. Calculate both and use the larger wire size. For example, a 30A load at 100ft on 240V single-phase requires at least 10 AWG copper for ampacity, but voltage drop may require 8 AWG.

What is the difference between 60°C, 75°C, and 90°C wire ratings?

The temperature rating indicates the maximum operating temperature of the wire insulation. Most general wiring uses 75°C rated terminals (THWN, THHN). 60°C (TW, UF) is used for older installations. 90°C (THHN, XHHW) is common but you must use the 75°C column for terminations per NEC 110.14(C), unless the equipment is rated for 90°C.

When should I increase wire size beyond what ampacity requires?

Increase wire size when: (1) the voltage drop exceeds 3% for branch circuits or 5% total, (2) the wire runs through high-ambient-temperature areas requiring derating, (3) there are more than 3 current-carrying conductors in a conduit requiring adjustment factors, or (4) for motor circuits that may have high starting currents.