Quick answer: Size wire in two steps — (1) its ampacity (after derating) must be ≥ the design current, and (2) the voltage drop must stay within 3% over the run. As a rough guide: 15 A → 14 AWG (1.5 mm²), 20 A → 12 AWG (2.5 mm²), 30 A → 10 AWG (6 mm²), 50 A → 8 AWG (10 mm²). Get the exact answer with the wire size calculator.
Why the "right" wire size matters
Undersize the wire and it runs hot — insulation ages, breakers nuisance-trip, and in the worst case it becomes a fire risk. Oversize it and you burn money on copper you don't need and struggle to land fat conductors on small terminals. The correct size is the smallest conductor that safely carries the current and delivers usable voltage at the far end.
The two rules of wire sizing
- Ampacity — the wire's rated current, after derating for heat and grouping, must be at least the load current. For continuous loads (on 3 h+), size at 125% of the load.
- Voltage drop — keep it to 3% or less from the board to the load. On long runs this rule, not ampacity, decides the size (see how to calculate & fix voltage drop).
Wire size chart (copper, by amps)
| AWG | mm² (nearest) | Ampacity (A) | Typical circuit |
|---|---|---|---|
| 14 | 1.5 | 15 – 20 | Lighting |
| 12 | 2.5 | 20 – 25 | Receptacles |
| 10 | 6 | 30 – 35 | Water heater, small AC |
| 8 | 10 | 40 – 50 | Cooktop, EV charger |
| 6 | 16 | 55 – 65 | Sub-feeder |
| 4 | 25 | 70 – 85 | Feeder |
| 2 | 35 | 95 – 115 | Large feeder |
| 1/0 | 50 | 125 – 150 | Service |
| 2/0 | 70 | 145 – 175 | Service |
| 4/0 | 120 | 195 – 230 | Service entrance |
Indicative copper ampacities; the exact value depends on insulation temperature rating and installation method (NEC 310.16 / IS 732). Note NEC 240.4(D) caps overcurrent protection at 15 A (14 AWG), 20 A (12 AWG) and 30 A (10 AWG).
AWG or mm²?
AWG (American Wire Gauge) is a gauge number where smaller number = bigger wire; mm² is the actual metric cross-sectional area. India and IEC countries specify mm²; North America uses AWG. The chart above pairs the nearest standard sizes so you can work in either.
Don't forget derating
Table ampacity assumes one circuit at a benign temperature. Reduce it for high ambient temperature (India often 40–45 °C), for bundling (more than three current-carrying conductors together), and for conduit fill. Effective ampacity = table value × temperature factor × grouping factor.
Worked example — what size wire for 40 A?
Load = 40 A continuous → design at 125% = 50 A Ampacity: 8 AWG (10 mm²) copper ≈ 50 A ✓ Short run (< 30 m): 8 AWG is fine. Long run (60 m, 240 V): re-check voltage drop — 8 AWG may exceed 3%, so upsize to 6 AWG (16 mm²).
So the answer isn't just "8 AWG" — it's "8 AWG if the run is short, 6 AWG if it's long." The cable size calculator checks both automatically.
FAQ
What size wire for 30 amps? 10 AWG copper (about 6 mm²) for a 30 A circuit; NEC caps 10 AWG at 30 A. Upsize on long runs for voltage drop.
What size wire for 50 amps? 8 AWG copper (10 mm²); use 6 AWG if terminals are 60 °C-rated or the run is long.
Does wire size depend on distance? Yes — longer runs need a larger wire to keep voltage drop within 3%, even when ampacity is satisfied.
Is a bigger wire ever a problem? Only cost and termination — oversizing is electrically safe but wastes copper and may not fit the lug.