| Standard | Wire Size | Ampacity | Volt Drop | Drop % |
|---|---|---|---|---|
| NEC (US) | AWG 3 (26.7 mm²) | 100 A | 2.87 V | 2.39% |
| IEC (EU) | 25 mm² | 101 A | 3.07 V | 2.56% |
| BS (UK) | 16 mm² | 76 A | 4.79 V | 3.99% |
| AS/NZS | 16 mm² | 76 A | 4.79 V | 3.99% |
Always verify local code requirements. Wire sizing depends on insulation type, ambient temperature, and conduit fill.
| Current | AWG | mm² | Ampacity | Drop % |
|---|---|---|---|---|
| 50 A | AWG 4 | 21.2 mm² | 85 A | 2.51% |
| 60 A | AWG 3 | 26.7 mm² | 100 A | 2.39% |
| 100 A | AWG 1 | 42.4 mm² | 130 A | 2.51% |
| Distance | AWG | mm² | Ampacity | Drop % |
|---|---|---|---|---|
| 75 ft | AWG 4 | 21.2 mm² | 85 A | 2.26% |
| 100 ft | AWG 3 | 26.7 mm² | 100 A | 2.39% |
| 150 ft | AWG 2 | 33.6 mm² | 115 A | 2.85% |
Choosing the correct wire gauge is a critical safety decision. An undersized wire heats up under load, degrades insulation over time, and can cause an electrical fire. An oversized wire is safe but wasteful — larger copper conductors cost more and are harder to terminate. The goal is the smallest wire that satisfies both the ampacity requirement (the wire can carry the load continuously without overheating) and the voltage drop limit (the load receives sufficient voltage to operate correctly).
NEC Table 310.16 lists the base ampacity for copper conductors at 75 °C. Two correction factors reduce that number in real installations. Temperature derating (NEC 310.15(B)(2)) applies when ambient temperature exceeds 30 °C — a 40 °C attic can cut ampacity by roughly 13 %. Bundling derating (NEC 310.15(C)(1)) applies when four or more current-carrying conductors share a conduit — nine conductors together require a 70 % derating factor. Always apply both corrections before selecting a breaker.
Voltage drop is calculated as VD = (2 × L × I × ρ) / A, where L is the one-way wire length in metres, I is the current in amps, ρ is the copper resistivity (0.0172 Ω·mm²/m at 20 °C), and A is the conductor cross-section in mm². The factor of 2 accounts for the round-trip path through hot and neutral conductors. NEC recommends keeping branch circuit voltage drop at or below 3 % and the combined feeder-plus-branch drop at or below 5 %. A higher drop means motors run hotter, LED drivers flicker, and sensitive electronics malfunction.
Different regions use different wire sizing systems. NEC (used in the US and Canada) measures conductors in AWG (American Wire Gauge) — a counter-intuitive scale where a larger AWG number means a smaller wire. IEC 60364 (Europe and most of the world) and BS 7671 (United Kingdom) specify conductors in mm² cross-sectional area, making size comparisons straightforward. AS/NZS 3000 (Australia and New Zealand) also uses mm². IEC and BS limit voltage drop to 3 % for lighting and 5 % for other circuits; NEC recommends 3 % for branch circuits. This calculator shows the recommended size under all four standards simultaneously so you can compare across jurisdictions.
Each row in the results table represents one wiring standard. The Wire Size column shows the recommended conductor — AWG number for NEC, mm² cross-section for IEC, BS, and AS/NZS. Ampacity is the continuous current rating of that conductor at the standard operating temperature. Volt Drop shows the actual voltage lost across the full round-trip length at your specified current; Drop % expresses that loss as a percentage of the supply voltage. A green percentage means the circuit is within limits; amber means it is approaching the limit; red means it exceeds the standard's allowance and you should consider a larger conductor, a higher voltage, or a shorter run.
These results are calculated references based on published code tables and idealized copper resistivity. Real-world installations involve additional factors: conduit type, number of conductors, ambient temperature, conductor insulation class, and local code amendments. Always verify your wire selection with a licensed electrician before making permanent electrical installations. For life-safety feeders or services above 100 A, consult a licensed electrical engineer.