What is the amp rating for 18 gauge wire?

Use 10 A for 18 AWG copper on equipment with 75 °C terminations, which covers most breakers and panel lugs. Read 8 A instead when the conductor is inside NM-B cable, and treat 11 A as the derating base only. 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.

18 AWG Wire — Amp Rating, Ampacity & Diameter

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

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 standardFormula estimate

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

Calculate your exact run

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

Mix conductor sizes in the fill calculator

18 AWG copper ampacity vs aluminium ampacity

Ampacity by conductor material and insulation temperature rating, at 30 °C ambient with no more than three current-carrying conductors.

Material	60 °C (TW, NM-B)	75 °C (THW, THWN, XHHW)	90 °C (THHN, THWN-2, XHHW-2)
Copper	8 A	10 A	11 A
Aluminium	6 A	8 A	9 A

The 18 AWG copper ampacity figure you use on a real circuit is the 75 °C column, 10 A, because breakers and lugs on most equipment are listed for 75 °C even when the conductor insulation is rated 90 °C. The 90 °C value, 11 A, is the starting point for ambient and conductor-count derating only — you derate from 90 °C, then cap the answer at the termination rating.

Aluminium of the same size carries roughly 80% of the copper value (8 A at 75 °C), so an aluminium feeder is normally stepped up one to two sizes to match a copper circuit. NM-B cable is always read from the 60 °C column regardless of the temperature stamped on the conductor insulation.

Source & method: Values are model estimates from conductor geometry and resistivity — see methodology and verify against the NEC edition enforced locally.

Neighbouring sizes

Compare 18 AWG with the sizes immediately above and below it before sizing a circuit.

Larger16 AWG13 A copper at 75 °C · 0.0508" diameter

Size	Cu 75 °C	Al 75 °C	Diameter (in)	Circular mils
18 AWG (this page)	10 A	8 A	0.0403	1,624
16 AWG	13 A	10 A	0.0508	2,583
14 AWG	18 A	14 A	0.0641	4,107

18 AWG amp rating, diameter and metric equivalent

The amp rating for 18 AWG wire depends on the insulation temperature rating, not on the conductor alone. Copper ampacity is about 8 A on a 60 °C conductor, 10 A at 75 °C and 11 A at 90 °C, all at 30 °C ambient with no more than three current-carrying conductors in the raceway. The carrying capacity drops from there in hot spaces or crowded conduit, and terminations rated 75 °C cap what you can actually use.

Geometrically, 18 AWG is 0.0403 in (1.02 mm) in bare diameter, which is 1,624 circular mils. The cross sectional area works out to 0.82 mm², so the nearest IEC 60228 metric conductor is the next standard size at or above that figure — AWG sizes and mm² sizes never line up exactly, so round up rather than to the closest value.

American Wire Gauge (AWG) is a standard specification for the sizes of solid round wires used as electrical conductors, and the AWG number counts the drawing operations used to reach the diameter. That is why larger gauges are thinner wire and why standardized wire steps follow a fixed ratio: one AWG step changes diameter by about 12.2% and area by about 26%. Above 1 AWG the American standard continues 1/0 to 4/0, then switches to kcmil for anything larger.

Where 18 AWG actually gets used

18 AWG is not branch-circuit wire. It appears as fixture wire inside luminaires, as flexible cord conductors on small appliances, as control and signal conductors, and in class 2 low-voltage circuits such as thermostats and doorbells, where the power source itself limits the available current.

Because the conductor is small, mechanical damage rather than heating is the usual failure: broken strands at a crimp, a nicked conductor in a fixture canopy, a strand bridging a terminal. Supports, strain relief and the correct crimp barrel matter more here than any ampacity calculation.

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 is the amp rating for 18 gauge wire?: Use 10 A for 18 AWG copper on equipment with 75 °C terminations, which covers most breakers and panel lugs. Read 8 A instead when the conductor is inside NM-B cable, and treat 11 A as the derating base only. 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.