Wire Ampacity Ratings

Direct Answer

National Electrical Code (NEC) Table 310.16 establishes allowable conductor ampacities based on conductor material (Copper vs Aluminum), ambient temperature, and insulation thermal ratings (60°C TW/NM-B Romex, 75°C THWN, 90°C THHN).

Source: NFPA 70: National Electrical Code (NEC) Table 310.16 & Section 240.4(D) · Last reviewed Aug 28, 2026

Specs

Governing Code	NFPA 70: National Electrical Code (NEC) Table 310.16
Small Conductor Overcurrent Rule (NEC 240.4D)	14 AWG Cu: Max 15A \| 12 AWG Cu: Max 20A \| 10 AWG Cu: Max 30A
14 AWG Copper (NM-B / Romex)	15 Amps (60°C column applied per NEC 334.80)
12 AWG Copper (NM-B / Romex)	20 Amps (Standard residential branch circuit)
10 AWG Copper (NM-B / Romex)	30 Amps (Clothes dryers, 30A water heaters)
8 AWG Copper (75°C THWN / NM-B)	40A (NM-B 60°C) / 50A (THWN 75°C conduit)
6 AWG Copper (75°C THWN / NM-B)	55A (NM-B 60°C) / 65A (THWN 75°C conduit)
4 AWG Copper (75°C THWN)	85 Amps (Residential subpanel feeder)
2 AWG Copper (75°C THWN)	115 Amps (100A subpanel / main feeder)
4/0 Aluminum (75°C XHHW / SE)	180 Amps under standard NEC Table 310.16 ampacity; commonly used as the minimum conductor for a 200A dwelling service under NEC's separate service-conductor allowance (Table 310.12, formerly 310.15(B)(7))

NEC Table 310.16 Allowable Conductor Ampacities (Copper & Aluminum)

Conductor Size	Copper 60°C (NM-B/Romex)	Copper 75°C (THWN/Conduit)	Copper 90°C (THHN Derating)	Aluminum 75°C (Service/Sub)	Max Breaker Size (NEC 240.4D)
14 AWG	15 A	20 A	25 A	—	15 Amps
12 AWG	20 A	25 A	30 A	20 A	20 Amps
10 AWG	30 A	35 A	40 A	30 A	30 Amps
8 AWG	40 A	50 A	55 A	40 A	40A (NM-B) / 50A (THWN)
6 AWG	55 A	65 A	75 A	50 A	60 Amps
4 AWG	70 A	85 A	95 A	65 A	85 Amps
3 AWG	85 A	100 A	115 A	75 A	100 Amps
2 AWG	95 A	115 A	130 A	90 A	100A (Al) / 115A (Cu)
1/0 AWG	125 A	150 A	170 A	120 A	150 Amps
2/0 AWG	145 A	175 A	195 A	135 A	175 Amps
4/0 AWG	195 A	230 A	260 A	180 A	200 Amps (Residential Service, via NEC 310.12 dwelling allowance)

The 60°C NM-B Rule vs 90°C THHN Derating

Understanding terminal and insulation temperature ratings prevents dangerous overheating and code violations:

NEC 334.80 NM-B (Romex) Limitation: Although modern nonmetallic-sheathed cable uses 90°C rated conductors (THHN), the NEC mandates that the final ampacity must not exceed the 60°C column. Thus, 10 AWG Romex is limited to 30 A, even though 10 AWG THHN in conduit carries 35 A at 75°C.
Conduit Bundling Derating: When more than 3 current-carrying conductors are installed in a single raceway, ampacity must be derated (4–6 wires = 80%, 7–9 wires = 70%). The calculation starts from the 90°C table rating, allowing THHN to absorb derating penalties without dropping below breaker trip ratings.
The 4/0 Aluminum / 200A Service Exception: Under the general Table 310.16 ampacity schedule, 4/0 aluminum at 75°C is only rated for 180 A — not 200 A. It is still the standard minimum conductor for a 200A single-family dwelling service because NEC's separate service/feeder table for dwellings (Table 310.12, formerly 310.15(B)(7)) permits smaller conductors than Table 310.16 would otherwise require, based on dwelling load diversity. Sizing a 200A service from Table 310.16 alone (misreading it as if 4/0 Al were rated 200 A there) is a common but incorrect shortcut.