Wire Gauge Calculator

About the Wire Gauge Calculator

A wire gauge calculator recommends the correct AWG (American Wire Gauge) wire size for any electrical load, accounting for both the current-carrying capacity (ampacity) from NEC Table 310.15 and the voltage drop limitation over the circuit length. Selecting the wrong wire gauge is one of the most common and dangerous electrical mistakes: undersized wire overheats and can cause fires, while oversized wire is wasteful and unnecessarily expensive. In the US and Canada, the National Electrical Code (NEC) governs wire sizing; in the UK, BS 7671 applies with metric mm² conductors. Our calculator covers the NEC framework and shows the complete AWG table from 4/0 (195A) down to 26 AWG (1A), highlighting the recommended gauge for your current load. It also considers voltage drop: NEC recommends no more than 3% voltage drop on branch circuits, which often requires upsizing to the next larger gauge on longer runs. The calculator shows the recommended AWG for both ampacity alone and combined ampacity + voltage drop criteria, ensuring the recommendation meets both requirements.

How It Works

Step 1 — Ampacity requirement: find the minimum AWG rated for at least 125% of the load current (NEC continuous load rule). A 20A circuit requires a conductor rated ≥ 25A: 12 AWG (rated 20A, protected by 20A breaker) is the minimum; 10 AWG (rated 30A) provides more headroom. Step 2 — Voltage drop check (if circuit length is entered): voltage drop = 2 × L × I × R (where L = one-way length in feet, R = resistance in Ω/ft). Maximum acceptable voltage drop = 3% × voltage (NEC recommendation). Minimum wire cross-section = (2 × L × I × ρ) / (Vdrop × A_ref). Example: 20A circuit, 75 feet, 120V, 3% voltage drop allowed. Max voltage drop = 120 × 0.03 = 3.6V. Required resistance ≤ 3.6 / (2 × 75 × 20) = 0.0012 Ω/ft. 12 AWG has 0.00164 Ω/ft — too high. 10 AWG has 0.001038 Ω/ft — within limit. Recommendation: 10 AWG for this run.

Tips & Best Practices

• ✓The NEC 125% continuous load rule: for loads that operate for 3 or more consecutive hours (baseboard heaters, EV chargers, refrigerators, server rooms), the circuit must be rated at 125% of the load. A 20A EV charger requires a 30A circuit with 10 AWG conductors.
• ✓Aluminum versus copper: aluminum wire is lighter and cheaper but has higher resistance (requires one size larger than copper for same ampacity) and needs anti-oxidant compound at connections. Only use aluminum for service entrance feeders and main feeders — not for branch circuits where connection reliability is critical.
• ✓Derating for conduit fill: NEC 310.15(B)(3) requires derating when more than 3 current-carrying conductors are in a conduit. Four conductors = 80% ampacity; seven conductors = 70%; ten conductors = 50%. This can force you up two or three wire sizes for densely filled conduits.
• ✓UK wiring (BS 7671): UK uses metric mm² rather than AWG. The nearest equivalents: 2.5 mm² ≈ 13 AWG (13A circuits); 4 mm² ≈ 11 AWG (20A circuits); 6 mm² ≈ 10 AWG (32A circuits); 10 mm² ≈ 7 AWG (50A circuits). UK wiring regulations also specify maximum voltage drop of 3-5% depending on installation type.
• ✓Temperature rating matters: NEC lists ampacity for 60°C, 75°C, and 90°C rated conductors. Most residential wiring uses 60°C or 75°C rating. Using 90°C rated THHN conductors allows higher ampacity from the same wire size — but the circuit breaker and terminal rating must also be 75°C or 90°C.

Who Uses This Calculator

Homeowners planning DIY electrical projects who need to select correct wire gauge before purchasing materials. Electricians quickly verifying wire size for any current and run length combination. Engineers sizing conductors for control panels, motor branch circuits, and feeder calculations. Building inspectors and project managers verifying that specified wire sizes meet NEC code requirements.