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Wire Size Calculator

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

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:

  1. Ampacity requirement: Wire ampacity ≥ load current (NEC Table 310.16, 75°C column)
  2. 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.

  1. Ampacity check: 30A load → 10 AWG copper (35A at 75°C) ✓
  2. Voltage drop check (10 AWG, R = 1.24):
    VD = (2 × 150 × 1.24 × 30) / 1000 = 11.16V → 4.65% ✗ (exceeds 3%)
  3. Try 8 AWG (R = 0.778):
    VD = (2 × 150 × 0.778 × 30) / 1000 = 7.00V → 2.92% ✓
  4. 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

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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.