Wire Gauge and Ampacity: A Complete Reference Guide

Selecting the correct wire gauge is fundamental to safe electrical work. Undersized wire overheats, melts insulation, and causes fires. Oversized wire wastes money. This guide covers the American Wire Gauge (AWG) system, ampacity tables for common installation methods, and the practical rules electricians use to size conductors correctly.

The AWG System

American Wire Gauge (AWG) is the standard wire sizing system used in the United States. The numbering is counterintuitive: smaller AWG numbers mean larger, heavier wire.

• 14 AWG is thinner than 12 AWG
• 4 AWG is thicker than 8 AWG
• Numbers below 1 AWG continue as 1/0 (one-ought), 2/0, 3/0, 4/0, then 250 kcmil, 300 kcmil, etc. for very large conductors

Wire size directly affects:

• Ampacity: Maximum current the wire can carry safely
• Resistance: Larger wire has lower resistance and less voltage drop
• Flexibility: Smaller wire is more flexible; larger wire is stiffer

Copper vs. Aluminum Conductors

Both copper and aluminum are used for electrical wiring, but with important differences:

Property	Copper	Aluminum
Conductivity	Higher	Lower (~61% of copper)
Ampacity (same gauge)	Higher	~80% of copper
Cost	Higher	Lower
Weight	Heavier	Lighter
Connections	Standard terminals	Requires anti-oxidant compound and compatible terminals
Corrosion	Low	Higher (oxidation)

When aluminum is used: Service entrance conductors, large feeder conductors (100A+), and underground feeder applications. For a 200A service entrance, aluminum 4/0 conductors are standard and far less expensive than copper alternatives.

When copper is preferred: Branch circuit wiring inside walls, connections to devices, and anywhere terminations are frequent. Aluminum branch circuit wiring (common in 1960s–1970s homes) is a known fire hazard when connected with copper-only devices without proper anti-oxidant treatment and CO/ALR-rated devices.

Ampacity Tables

Ampacity depends on the conductor material, insulation type, installation method, and ambient temperature. The following tables cover the most common residential and light commercial scenarios.

Copper Conductors — 60°C and 75°C Insulation (NEC Table 310.12)

Most devices and breakers are rated for 60°C terminations (the limiting factor even if the wire itself is rated higher).

AWG	60°C Ampacity	75°C Ampacity	Common Use
14	15A	20A	General branch circuits (15A max)
12	20A	25A	20A branch circuits
10	30A	35A	Dryers, ranges, EV chargers (30A)
8	40A	50A	EV chargers, ranges, AC units
6	55A	65A	50A circuits, EV chargers
4	70A	85A	60A circuits, feeders
3	85A	100A	80A EV chargers
2	95A	115A	90A–100A feeders
1	110A	130A	Small subpanels
1/0	125A	150A	Subpanels, feeders
2/0	145A	175A	150A feeders
3/0	165A	200A	175A–200A feeders
4/0	195A	230A	200A services

Important: For branch circuits, use the 60°C ampacity column unless you can confirm that both the breaker and all devices are rated for 75°C terminations.

Aluminum Conductors — 75°C Insulation (NEC Table 310.12)

AWG	75°C Ampacity	Common Use
12	20A	— (not recommended for branch circuits)
10	30A	—
8	40A	—
6	50A	—
4	65A	60A circuits
3	75A	—
2	90A	90A feeders
1	100A	—
1/0	120A	100A feeders
2/0	135A	125A feeders
3/0	155A	150A feeders
4/0	180A	175–200A services

Key Ampacity Rules

Rule 1: Size the Breaker to Protect the Wire

The breaker must be sized at or below the wire’s ampacity. A 20A breaker must have at least 12 AWG copper wire. A 15A breaker must have at least 14 AWG copper.

Common circuit configurations:

• 15A circuit: 14 AWG copper, 15A breaker
• 20A circuit: 12 AWG copper, 20A breaker
• 30A circuit: 10 AWG copper, 30A breaker
• 40A circuit: 8 AWG copper, 40A breaker
• 50A circuit: 6 AWG copper, 50A breaker
• 60A circuit: 4 AWG copper, 60A breaker

Rule 2: Continuous Loads at 80%

Any load that operates for 3 hours or more continuously must be calculated at 125% of its rated current (equivalently, the circuit and breaker can only be loaded to 80% of their rating).

For a circuit feeding a 48A EV charger (a continuous load): 48A × 1.25 = 60A minimum circuit rating. You need a 60A breaker and 4 AWG wire.

Rule 3: Conduit Fill Derating

When more than three current-carrying conductors are in the same conduit or cable, their ampacity must be derated:

Conductors	Ampacity Factor
4–6	80%
7–9	70%
10–20	50%
21–30	45%

Example: Six 12 AWG conductors in a conduit — each rated 25A at 75°C — must be derated to 25A × 0.80 = 20A. The 20A breaker still matches, but you’re at the limit.

Note: Grounding conductors do not count as current-carrying for this calculation. Neutrals count only if they carry non-linear current (e.g., with large variable frequency drives or computer equipment).

Rule 4: Voltage Drop

Long wire runs lose voltage due to the wire’s resistance. The NEC recommends (but doesn’t require for most circuits) keeping voltage drop under 3% on branch circuits and under 5% total (feeder + branch combined).

For a 120V, 20A circuit, 3% drop = 3.6V maximum drop. To calculate whether your wire size is adequate for the run length, use the formula:

Voltage Drop = (2 × Length × Current × Resistance per foot)

Or use an online wire sizing calculator — the key takeaway is that long runs often require upsizing the wire to compensate for resistance.

Common rule of thumb: For runs over 100 feet on a 15A or 20A circuit, go up one wire size.

Wire Insulation Types

Different insulations are rated for different temperatures and environments:

Insulation Type	Temp Rating	Wet/Dry	Common Application
NM-B (Romex)	60°C	Dry only	Branch circuits in walls
THHN	90°C (dry) / 75°C (wet)	Dry and wet	Conductors in conduit
THWN-2	90°C wet	Wet	Outdoor conduit, direct burial
XHHW-2	90°C wet	Wet	Service entrance, large feeders
USE-2	90°C	Underground	Direct burial service entrance
UF-B	60°C	Wet	Direct burial branch circuits

For most conduit installations, THHN/THWN-2 is the standard choice — it’s dual-rated for wet and dry locations at 90°C and is readily available at most electrical suppliers.

Practical Wire Selection Guide

Application	Wire	Breaker
General 15A outlets	14 AWG Cu	15A
20A kitchen/bath outlets	12 AWG Cu	20A
Clothes dryer	10 AWG Cu	30A
Electric water heater	10 AWG Cu	30A
Electric range	6 AWG Cu	50A
EV charger (40A EVSE)	8 AWG Cu	50A
EV charger (48A EVSE)	4 AWG Cu	60A
100A subpanel (Cu)	1/0 AWG	100A
100A subpanel (Al)	2/0 AWG	100A
200A service (Al)	4/0 AWG	200A

Use this table as a starting point, but always verify against NEC tables and account for derating, voltage drop, and local amendments before pulling wire on any significant installation.