Load Parameters
Load Parameters
Load Parameters
How to Choose the Right Wire Size
Selecting the correct wire gauge involves three checks: ampacity (can the wire carry the current without overheating?), voltage drop (does the cable resistance cause excessive voltage loss over the run length?), and code compliance (NEC, IEC or local regulations).
- Ampacity first: Find the smallest wire whose rated current exceeds your load (×1.25 for continuous loads under NEC).
- Check voltage drop: Long runs require a larger gauge even if ampacity is met. Most codes allow a maximum 3–5% voltage drop.
- Installation derating: Wires in conduit or walls run hotter than wires in free air — ampacity tables are reduced accordingly.
- Temperature derating: Higher ambient temperatures reduce the allowable current. Above 30°C, NEC correction factors apply.
- Material: Aluminium has higher resistance and lower ampacity than copper. Size one or two gauges larger when substituting aluminium.
Wire Ampacity Reference Table (Copper & Aluminium)
| AWG | mm² | Cu Ampacity (conduit) | Al Ampacity (conduit) | Cu Resistance (mΩ/m) |
|---|---|---|---|---|
| 14 | 2.08 | 15 A | — | 8.28 |
| 12 | 3.31 | 20 A | — | 5.21 |
| 10 | 5.26 | 30 A | — | 3.28 |
| 8 | 8.37 | 40 A | 31 A | 2.06 |
| 6 | 13.3 | 55 A | 43 A | 1.30 |
| 4 | 21.2 | 70 A | 55 A | 0.815 |
| 2 | 33.6 | 95 A | 74 A | 0.513 |
| 1 | 42.4 | 110 A | 86 A | 0.407 |
| 1/0 (0) | 53.5 | 125 A | 98 A | 0.322 |
| 2/0 (00) | 67.4 | 145 A | 113 A | 0.256 |
| 3/0 (000) | 85.0 | 165 A | 129 A | 0.203 |
| 4/0 (0000) | 107.2 | 195 A | 152 A | 0.161 |
Based on NEC 310 for 75°C insulation at 30°C ambient. Derate for higher temperatures or bundled conductors. Aluminium not rated below 8 AWG in most residential codes.
Frequently Asked Questions
For a 20-amp circuit you need minimum 12 AWG (3.31 mm²) copper wire in conduit at 30°C ambient. With the NEC 125% safety margin applied to a continuous load, the effective requirement rises to 25 A, which requires 10 AWG (5.26 mm²). Always verify against your actual run length and maximum allowable voltage drop.
Ampacity is the maximum current (in amperes) a conductor can carry continuously without exceeding its temperature rating. It depends on wire material, cross-sectional area, insulation type, installation method and ambient temperature. Exceeding ampacity causes insulation damage and is a fire risk.
NEC Article 210.19(A)(1) requires conductors supplying continuous loads to be sized at 125% of the load current. A continuous load is one expected to remain energised for 3 hours or more. For example: a 16 A continuous load requires a conductor rated for at least 20 A (16 × 1.25 = 20 A).
Aluminium wiring is permitted by most codes for large-gauge feeders (typically 4 AWG and larger) but is not recommended for branch circuits due to oxidation at connections, higher resistance and historical fire-safety concerns with smaller gauges. All connectors, devices and panels must be rated for aluminium (marked AL or AL/CU). Always check local electrical codes.