How to Size Wire for Your Electrical Project
A practical guide to selecting the correct wire gauge for any circuit
Choosing the right wire gauge is one of the most critical decisions in any electrical project. Use a wire that is too small and you risk overheating, voltage drop, or even fire. Use one that is too large and you waste money. This guide walks you through the NEC and IEC methods so you can size wire confidently.
Why Wire Size Matters
Every wire has resistance. When current flows through resistance, heat is produced. A wire sized too small for its load will overheat under full current, degrading insulation and creating a fire hazard. Wire size also affects voltage drop: the longer the run and the higher the current, the more voltage is lost along the wire before it reaches the load.
The Two Constraints: Ampacity and Voltage Drop
Wire selection must satisfy two independent constraints. First, ampacity: the wire must be capable of carrying the full load current without exceeding its temperature rating. Second, voltage drop: the wire must be sized so the voltage at the load end stays within acceptable limits — NEC recommends no more than 3% drop for branch circuits, 5% combined for feeder and branch.
How to Calculate Wire Size (NEC Method)
Start with the circuit current in amps. Convert the one-way distance to meters. Apply the formula: minimum cross-section (mm²) = 2 × L × I × ρ / (V × drop%). For copper, ρ = 0.0172 Ω·mm²/m. Then look up the next larger standard AWG size that also meets ampacity. NEC Table 310.15 provides ampacity ratings for common insulation types at 60°C, 75°C, and 90°C.
AWG vs mm²: Which System to Use
The American Wire Gauge (AWG) system is used in the US and Canada under NEC. The metric mm² system is used in Europe (IEC 60364), UK (BS 7671), and Australia/New Zealand (AS/NZS 3000). Higher AWG numbers mean smaller wires: AWG 14 (2.08 mm²) is smaller than AWG 10 (5.26 mm²). In metric, larger numbers always mean larger wires. When working across standards, always verify both the cross-section area and the ampacity from the applicable table.
Common Wire Sizes for Residential Circuits
For a 15 A branch circuit at 120V with a 50-foot run, NEC recommends AWG 14 (2.08 mm²). For a 20 A circuit, AWG 12 (3.31 mm²). For a 30 A circuit such as a dryer or water heater, AWG 10 (5.26 mm²). For a 50 A range circuit, AWG 6 (13.3 mm²). Always add 20–25% margin for continuous loads (loads on more than 3 hours continuously).
FAQ
Can I use a larger wire than required?
Yes. Using a larger wire than the minimum calculated size is always safe and reduces voltage drop. The only downside is cost. You cannot use a smaller wire than calculated — that creates a safety hazard.
Does wire length affect which size I need?
Yes. Longer wire runs have more resistance, causing more voltage drop. For runs over 50 feet, you often need to go one AWG size larger than ampacity alone would require, just to keep voltage drop within NEC limits.
What is the difference between 60°C and 75°C ampacity ratings?
Ampacity depends on the maximum operating temperature of the insulation. THWN-2 and THHN are rated 75°C or 90°C and carry more current than older 60°C types. For residential work connecting to 60°C-rated breakers and terminals, NEC 110.14 requires you to use the 60°C column even if the wire itself is rated higher.