18 AWG Wire Specs
Ampacity, geometry, resistance, conduit fill and maximum run length for 18 AWG conductors in copper and aluminium.
Quick answer
18 AWG copper carries about 10 A at 75 °C (8 A at 60 °C, 11 A at 90 °C). The bare conductor is 0.0403 in (1.02 mm) and 1,624 circular mils, with 6.3861 Ω per 1000 ft.
Copper 75 °C
10A
Aluminium 75 °C
8A
Diameter
0.0403in
Circular mils
1,624
Full specification
| Size designation | 18 AWG |
|---|---|
| AWG gauge number | 18 |
| Circular mils | 1,624 |
| kcmil | 1.62 |
| Bare diameter (in) | 0.0403 |
| Bare diameter (mm) | 1.024 |
| Cross-section (mm²) | 0.82 |
| THHN/THWN-2 OD (in) | 0.084 |
| Insulated area (sq in) | 0.0055 |
| Copper resistance (Ω/1000 ft) | 6.3861 |
| Aluminium resistance (Ω/1000 ft) | 10.4680 |
| Max standard breaker (Cu, 75 °C) | Below branch-circuit ratings |
Published standard — source: Geometry computed from the ASTM B258 AWG formula; resistance from standard resistivityFormula estimate — source: Ampacity from the site model
Ampacity with derating
Estimated ampacity from the 90 °C column at various ambient temperatures and conductor counts.
| Conductors in raceway | 30 °C ambient | 40 °C ambient | 50 °C ambient | 60 °C ambient |
|---|---|---|---|---|
| 3 | 11 A | 10 A | 9 A | 8 A |
| 6 | 9 A | 8 A | 7 A | 6 A |
| 9 | 8 A | 7 A | 6 A | 5 A |
| 15 | 6 A | 5 A | 4 A | 4 A |
| 25 | 5 A | 5 A | 4 A | 3 A |
Source & method: The governing value is also capped by the termination rating, commonly 75 °C, which for this size is 10 A. Run your own conditions.
Maximum run length at 3% voltage drop
One-way distance for 18 AWG copper before drop exceeds 3%.
| Load | 120 V 1Ø | 240 V 1Ø | 208 V 3Ø | 480 V 3Ø |
|---|---|---|---|---|
| 10 A | 22 ft | 45 ft | 45 ft | 104 ft |
| 15 A | 15 ft | 30 ft | 30 ft | 69 ft |
| 20 A | 11 ft | 22 ft | 22 ft | 52 ft |
| 30 A | 7 ft | 15 ft | 15 ft | 34 ft |
| 40 A | 5 ft | 11 ft | 11 ft | 26 ft |
| 50 A | 4 ft | 9 ft | 9 ft | 20 ft |
How many 18 AWG conductors fit in conduit
Maximum count of this size alone, at the fill percentage matching the resulting count.
| Conduit | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" | 2-1/2" | 3" | 3-1/2" | 4" |
|---|---|---|---|---|---|---|---|---|---|---|
| EMT | 21 | 38 | 62 | 107 | 146 | 242 | 400 | 400 | 400 | 400 |
| IMC | 24 | 42 | 69 | 118 | 160 | 262 | 370 | 400 | 400 | 400 |
| RMC | 22 | 39 | 64 | 110 | 149 | 245 | 351 | 400 | 400 | 400 |
| PVC Sch 40 | 20 | 36 | 60 | 104 | 143 | 237 | 338 | 400 | 400 | 400 |
| PVC Sch 80 | 15 | 29 | 49 | 89 | 123 | 207 | 297 | 400 | 400 | 400 |
Neighbouring sizes
Frequently asked
- How many amps can 18 AWG wire carry?
- 18 AWG copper is estimated at 8 A at 60 °C, 10 A at 75 °C and 11 A at 90 °C, at 30 °C ambient with no more than three current-carrying conductors. Aluminium of the same size is about 8 A at 75 °C.
- What breaker size goes with 18 AWG copper?
- 18 AWG is below the smallest standard branch-circuit device rating and is normally used for control, signal or fixture wiring rather than a protected branch circuit.
- What is the diameter of 18 AWG wire?
- The bare conductor is 0.0403 inches (1.024 mm) in diameter, which is 1,624 circular mils. With THHN/THWN-2 insulation the nominal outside diameter is about 0.084 inches.
- What is the resistance of 18 AWG wire?
- About 6.3861 ohms per 1000 feet for copper and 10.4680 ohms per 1000 feet for aluminium, at 75 °C.
- How far can I run 18 AWG wire?
- Distance depends on load and voltage. At 20 A on a 120 V single-phase circuit, 18 AWG copper stays under 3% voltage drop for about 11 feet one-way. Doubling the voltage roughly doubles the allowable distance.
Materials for this size
Materials for this size
Wire, cable and terminations for 18 AWG.
- THHN/THWN-2 18 AWG copper18 AWG copperBuilding wire in the size this page covers (18 AWG copper).Check price on Amazon →
- NM-B cable 18 AWG18 AWGRomex-type cable for dry indoor branch circuits at this size.Check price on Amazon →
- Wire connectors for 18 AWG18 AWGPush-in or twist-on connectors rated for this conductor size.Check price on Amazon →
- Terminal lugs & anti-oxidantCopper-rated mechanical lugs for panel and disconnect terminations.Check price on Amazon →
Links go to Amazon search results, not a single listing — confirm ratings, insulation type and listing details before buying, and make sure the part is approved for your application.
Related charts & calculators
- Voltage Drop CalculatorPercent drop, end voltage and minimum conductor for any run length.
- Conduit Fill CalculatorFill percentage and maximum conductor count for EMT, RMC, IMC and PVC.
- Breaker Size CalculatorOvercurrent device rating and minimum conductor from load current.
- Wire Ampacity CalculatorDerated ampacity for ambient temperature and bundled conductors.
- Box Fill CalculatorVolume allowances for conductors, clamps, devices and grounds per NEC 314.16.
- Ampacity ChartEstimated allowable ampacity by conductor size, material and insulation rating.