Ampacity of Copper Wire by Gauge

Why Insulation Type Matters for Ampacity

The ampacity of a conductor is not determined solely by its gauge (cross-sectional area). The insulation surrounding the conductor has a maximum operating temperature that directly limits how much current the wire can safely carry. Higher-temperature insulation allows the conductor to operate at higher ampacity because the wire can tolerate more heat before the insulation degrades.

NEC Table 310.16 organizes ampacity values by three temperature ratings: 60°C, 75°C, and 90°C. Each insulation type is assigned to one of these categories. Understanding which column applies to your wire—and to the termination points—is essential for proper conductor sizing.

Common Copper Wire Insulation Types

Insulation Type	Temperature Rating	Location Rating	Description
TW	60°C (140°F)	Wet & Dry	Thermoplastic, moisture-resistant. Older style insulation, largely replaced by THWN.
THW	75°C (167°F)	Wet & Dry	Thermoplastic, heat and moisture resistant. Common in commercial wiring.
THHN	90°C (194°F)	Dry Only	Thermoplastic, high heat-resistant nylon jacket. Most common building wire for conduit and raceways.
THWN	75°C (167°F)	Wet & Dry	Thermoplastic, heat and moisture resistant nylon jacket. Common in residential wiring.
THWN-2	90°C (194°F)	Wet & Dry	Upgraded THWN with 90°C rating in both wet and dry locations. Dual-rated THHN/THWN-2 is widely available.
XHHW	90°C (194°F)	Dry Only	Cross-linked polyethylene (XLPE), high heat-resistant. Excellent abrasion and chemical resistance.
XHHW-2	90°C (194°F)	Wet & Dry	Upgraded XHHW rated for wet locations. Popular for commercial and industrial applications.
USE / USE-2	75°C / 90°C	Underground	Underground Service Entrance rated for direct burial. USE-2 has a 90°C rating.

Copper Wire Ampacity by Insulation Type

The following comprehensive table shows copper wire ampacities from 14 AWG through 4/0 AWG for each major insulation type. All values are based on an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway or cable.

Gauge	60°C Insulation		75°C Insulation		90°C Insulation
	TW	UF	THW, THWN, USE	THW-2	THHN, THWN-2, XHHW, XHHW-2, USE-2	Notes
14 AWG	15A	15A	20A	20A	25A	Max breaker: 15A (NEC 240.4D)
12 AWG	20A	20A	25A	25A	30A	Max breaker: 20A (NEC 240.4D)
10 AWG	30A	30A	35A	35A	40A	Max breaker: 30A (NEC 240.4D)
8 AWG	40A	40A	50A	50A	55A	Max breaker: 50A at 75°C
6 AWG	55A	55A	65A	65A	75A	Max breaker: 60A (next standard)
4 AWG	70A	70A	85A	85A	95A	Max breaker: 85A at 75°C
3 AWG	85A	85A	100A	100A	110A	Max breaker: 100A at 75°C
2 AWG	95A	95A	115A	115A	130A	Max breaker: 110A (next standard)
1 AWG	110A	110A	130A	130A	150A	Max breaker: 125A (next standard)
1/0 AWG	125A	125A	150A	150A	170A	Max breaker: 150A at 75°C
2/0 AWG	145A	145A	175A	175A	195A	Max breaker: 175A at 75°C
3/0 AWG	165A	165A	200A	200A	225A	Max breaker: 200A at 75°C
4/0 AWG	195A	195A	230A	230A	260A	Max breaker: 225A (next standard)

Note: For 14 AWG through 10 AWG, the usable ampacity at terminations is limited to the 60°C column per NEC 110.14(C)(1), regardless of insulation rating. For 8 AWG and larger, the 75°C column is typically usable when terminals are rated for 75°C.

Understanding Termination Temperature Limits

NEC 110.14(C)(1) is one of the most important and commonly misunderstood rules in electrical wiring. It states that the ampacity of a conductor must be based on the temperature rating of the termination or connection point—not just the wire insulation.

For circuits rated 100A or less (which covers 14 AWG through 1 AWG in most cases), the termination temperature limit is typically:

• 60°C for equipment marked only for use with 60°C-rated wire
• 75°C for equipment marked for use with 75°C-rated wire (most modern breakers, panels, and lugs)

This means that even if you install THHN wire (rated 90°C), the usable ampacity at the breaker or panel termination is limited to the 75°C column value for 8 AWG and larger, or the 60°C column value for 14–10 AWG. The 90°C rating is only useful for derating calculations—it provides "headroom" when applying ambient temperature or bundling correction factors.

When Each Insulation Type Is Used

THHN/THWN-2 (Dual-Rated)

The most common wire type for residential and commercial conduit wiring. The dual-rated THHN/THWN-2 designation means it meets both the THHN standard (90°C, dry) and the THWN-2 standard (90°C, wet and dry). This versatile wire can be used in virtually any indoor or outdoor conduit application. It is available in sizes 14 AWG through 4/0 AWG and is the default wire pulled through EMT, PVC, and rigid conduit.

XHHW-2

XHHW-2 uses cross-linked polyethylene (XLPE) insulation instead of the PVC insulation used in THHN/THWN. XLPE offers superior abrasion resistance, chemical resistance, and slightly better performance at high temperatures. XHHW-2 is commonly used in commercial and industrial installations, particularly where conduit runs pass through areas with oils, solvents, or extreme temperatures. It is also commonly used in the conductors of MC (metal-clad) cable and some types of service entrance cable.

USE / USE-2

USE (Underground Service Entrance) cable is rated for direct burial without conduit. USE-2 is the upgraded version with a 90°C rating. USE cable is typically found in underground service entrance runs from the utility transformer to the meter base, and from the meter base to the main panel. Individual USE-2 conductors are also used in underground conduit runs where moisture is present.

TW and UF

TW (Thermoplastic, Water-resistant) is an older 60°C-rated insulation that has been largely replaced by THWN and THWN-2. UF (Underground Feeder) is a flat gray cable rated for direct burial, used for outdoor circuits to detached buildings, pumps, and landscape lighting. UF cable carries the same ampacity as TW at the 60°C rating.

How to Choose the Right Copper Wire

Follow this decision process to select the correct copper wire for your application:

1. Calculate the circuit load in amperes. For continuous loads (3+ hours), multiply by 1.25 per NEC 210.19.
2. Determine the installation environment. Indoor dry location? Outdoor wet location? Underground? This determines the required insulation type.
3. Check termination ratings. Look at the breaker or equipment nameplate. Most residential breakers are rated 75°C. For 14–10 AWG, you are limited to the 60°C column regardless.
4. Apply derating factors if applicable. Check ambient temperature corrections (NEC 310.15(B)(2)) and bundling corrections (NEC 310.15(B)(3)(a)). Use the 90°C column of the table as the base for derating calculations.
5. Select the wire gauge. Choose a gauge whose ampacity at the applicable temperature column equals or exceeds the circuit load. Verify the breaker size does not exceed the ampacity per NEC 240.4.

Ampacity Comparison: Copper vs. Aluminum

Copper has approximately 61% higher conductivity than aluminum. This means that for the same gauge, copper wire carries significantly more current. To match the ampacity of copper, aluminum wire must be one to two sizes larger:

Application	Copper Gauge (75°C)	Aluminum Gauge (75°C)	Ampacity
15A circuit	14 AWG	12 AWG	15–20A
20A circuit	12 AWG	10 AWG	20–25A
30A circuit	10 AWG	8 AWG	30–40A
50A circuit	6 AWG	4 AWG	50–65A
100A service	2 AWG	1/0 AWG	100–120A
200A service	3/0 AWG	4/0 AWG	200–230A

For a complete comparison, see our Wire Ampacity Chart.

When to Use a Calculator

Selecting copper wire involves balancing ampacity, insulation type, termination ratings, derating factors, and voltage drop. Our Wire Size Calculator handles all of these variables automatically. Enter your circuit load, distance, ambient temperature, and conductor count to get an NEC-compliant wire recommendation instantly.

Frequently Asked Questions

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