How to Determine the Right Wire Conductor Size for THWN and THHN?

Updated: May 6

THHN stands for Thermoplastic High Heat-Resistant Nylon-coated wire. It's typically used in dry locations.
THWN is similar, standing for Thermoplastic Heat and Water-Resistant Nylon-coated. It’s rated for both wet and dry locations.

Most modern wires are dual-rated as THHN/THWN-2, meaning they meet both standards and can be used in nearly any environment.

The size of the conductor depends on several factors:

Here’s a quick reference chart based on the NEC ampacity guidelines (90°C rating in conduit, THHN/THWN-2 copper conductors):

Wire Size (AWG or kcmil)	Conductor Diameter (inches)	Ampacity (90°C / 194°F)	Typical Breaker Size	Common Use
14 AWG	0.0641	25 Amps	15 Amps	Lighting, receptacles (residential)
12 AWG	0.0808	30 Amps	20 Amps	General circuits
10 AWG	0.1019	40 Amps	30 Amps	Water heaters, AC units
8 AWG	0.1285	55 Amps	40-50 Amps	Ovens, small subpanels
6 AWG	0.1620	75 Amps	60 Amps	Subpanels, car chargers
4 AWG	0.2043	95 Amps	70-90 Amps	Subfeeds, large appliances
3 AWG	0.2294	115 Amps	100 Amps	Larger subpanels
2 AWG	0.2576	130 Amps	125 Amps	Main feeders, heavy loads
1 AWG	0.2893	150 Amps	150 Amps	Main panel feeds
1/0 AWG	0.3249	170 Amps	175 Amps	Residential 200A service
2/0 AWG	0.3648	195 Amps	200 Amps	200A service entrance
3/0 AWG	0.4096	225 Amps	225 Amps	Large residential/commercial feeds
4/0 AWG	0.4600	260 Amps	250-300 Amps	Commercial panel feeds
250 kcmil	0.5085	290 Amps	300 Amps	Large industrial loads
300 kcmil	0.5491	320 Amps	350 Amps	High current feeders
350 kcmil	0.5873	350 Amps	400 Amps	Big service entrances
400 kcmil	0.6241	380 Amps	400-450 Amps	Large industrial feeders
500 kcmil	0.6981	430 Amps	450-500 Amps	Long distance high loads
600 kcmil	0.7677	475 Amps	500-600 Amps	High-demand facilities
750 kcmil	0.8620	535 Amps	600 Amps	Large-scale industrial power

Recent Posts