18 AWG copper wire is not used for house circuits. As a cord it is rated 10 A, and as fixture or control wire it may be protected at no more than 7 A.
18 AWG wire ampacity
18 AWG copper and aluminum ampacity at 60, 75 and 90 °C, up to three current-carrying wires at 30 °C (86 °F) ambient and the rating as an extension cord
Rating
Copper
Aluminum
60 °C, NM-B
—
not made
75 °C
—
not made
90 °C
14 A
not made
As a cord
10 A
—
18 AWG ampacity by temperature rating.
18 AWG copper may be protected by a breaker of no more than 7 A, whatever its ampacity column says.
18 AWG wire size and resistance
18 AWG bare conductor size and copper resistance at operating temperature. The diameter is the nominal AWG value; stranded wire of the same size is slightly larger.
Diameter, AWG nominal
0.0403 in. / 1.02 mm
Area
1,620 cmil / 0.823 mm²
Copper resistance
7.963 Ω per 1,000 ft
What 18 AWG wire is used for
Lamp cords and light-duty extension cords
Thermostat, doorbell and other low-voltage control wiring