Wire Size Calculator — NEC, IEC, BS, AS/NZS | WireStandard

The wire size calculator determines the minimum conductor size for a circuit given load current, one-way run length, and system voltage. It checks both NEC 310.16 ampacity limits and the NEC 215.2(A)(1)(b) voltage-drop guidance (3% branch, 5% combined) simultaneously, then outputs recommended AWG for North America and mm² for IEC, BS, and AS/NZS jurisdictions. Electricians, electrical engineers, and inspectors use it to confirm compliance before pulling wire.

Formula

Single-phase: VD = 2 × L × I × ρ / A. Three-phase: VD = √3 × L × I × ρ / A. L = one-way length (m), I = current (A), ρ = copper resistivity 0.0172 Ω·mm²/m (IACS, 20 °C), A = conductor cross-section (mm²). Ampacity is checked against NEC Table 310.16 (60 °C or 75 °C column). NEC 215.2(A)(1)(b) informational note recommends keeping branch-circuit voltage drop within 3%.

Worked Example

20 A breaker, 100 ft (30.48 m) one-way run, 120 V, single-phase, copper. NEC 310.16 (60 °C column) permits AWG 12 for 20 A. Check voltage drop: VD = 2 × 30.48 × 20 × 0.0172 / 3.31 = 6.34 V = 5.3% — exceeds the 3% branch-circuit guideline. Upsize to AWG 10 (5.26 mm²): VD = 2 × 30.48 × 20 × 0.0172 / 5.26 = 3.99 V = 3.3%. Verify both ampacity AND voltage drop before pulling wire.

Frequently Asked Questions

Why does my breaker size not match the wire size in the result?

NEC 240.4(D) restricts small conductors: AWG 14 is protected at 15 A maximum, AWG 12 at 20 A, AWG 10 at 30 A — regardless of a larger breaker rating. Additionally, the 125% continuous-load rule (NEC 210.19) means a 20 A continuous load requires a circuit rated at 25 A minimum, which may push the wire size up even though the breaker stays at 20 A.

When should I size for voltage drop instead of ampacity?

Ampacity controls wire sizing on short runs. Once the one-way distance exceeds roughly 50 ft (15 m) at typical residential currents, voltage drop begins to influence the selection. Beyond 150 ft (45 m), voltage drop almost always dictates a larger conductor than ampacity alone would require. Always check both criteria; the calculator picks whichever requirement demands the larger wire.

Why do NEC and IEC give different recommended sizes?

NEC sizing is based on ampacity tables in NEC Table 310.16 using 60 °C or 75 °C termination temperature columns, with separate conductor sizing for aluminum vs. copper. IEC 60364-5-52 defines reference installation methods (A1, B2, C, E, F) with correction factors for ambient temperature, grouping, and soil thermal resistivity. Both approaches are technically sound but calibrated for different wiring systems, insulation types, and installation conditions within their respective jurisdictions.

Browse common wire sizes

Quick reference for popular amp and distance combinations.