Wire Gauge Ampacity Calculator
Safe current rating for common copper wire gauges.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Rated ampacity (60°C copper)
- 20 A
- Recommended max continuous load (80%)
- 16 A
- Wire gauge
- 12 AWG
Quick answer
Ampacity is the maximum current a wire can carry continuously without exceeding its insulation's temperature rating. Under the US National Electrical Code (Table 310.16), copper wire in the 60°C column is rated 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG. For continuous loads, keep current to 80% of that: 16 A on 12 AWG.
What the Wire Gauge Ampacity calculator does
This calculator gives the 60°C ampacity of a copper conductor from 14 AWG to 2 AWG, and the 80% figure to use when a load runs for three hours or more. The 60°C column is the conservative one: it applies to NM-B (Romex-type) building cable and to most terminations on breakers and devices rated 100 A or less, so it is the right starting point for typical home branch circuits.
The values assume no more than three current-carrying conductors in a cable or raceway and an ambient temperature of 30°C (86°F). Hot attics, bundled cables, rooftop conduit and aluminum wire all need adjustments that this lookup does not apply. Long runs also need a voltage drop check, which often calls for a larger gauge than ampacity alone.
This is a planning aid, not a substitute for your local electrical code or an inspector. Code editions and local amendments differ, and a licensed electrician should confirm any permanent installation.
The formula
Continuous load limit = 0.80 × rated ampacity (equivalently, ampacity ≥ 1.25 × continuous load)
| Symbol | Meaning | Unit |
|---|---|---|
| rated ampacity | Allowable current from the 60°C copper column of NEC Table 310.16 | A |
| 0.80 | Continuous-load factor for loads expected to run 3 hours or more | — |
Ampacity is a table lookup, not a formula: the values come from NFPA 70, the National Electrical Code, Table 310.16 (copper, not more than three current-carrying conductors, 30°C ambient). The 80% rule reflects the NEC requirement that branch-circuit conductors and overcurrent devices be sized at 125% of continuous load. Separately, NEC 240.4(D) caps the breaker on 14, 12 and 10 AWG copper at 15 A, 20 A and 30 A whatever the insulation rating. Outside the US, IEC 60364 and BS 7671 rate cables by cross-section in mm² and installation method, so these figures do not transfer directly.
Worked examples
14 AWG lighting circuit
A bedroom lighting and outlet circuit wired in 14 AWG NM-B cable.
Inputs
- Wire gauge (AWG)14 AWG
Result
- Rated ampacity (60°C copper)15 A
- Recommended max continuous load (80%)12 A
- Wire gauge14 AWG
14 AWG takes a 15 A breaker at most. A space heater left running for hours should stay under 12 A, which is about 1,440 W at 120 V.
10 AWG for a water heater
A 240 V electric water heater, which the code treats as a continuous load.
Inputs
- Wire gauge (AWG)10 AWG
Result
- Rated ampacity (60°C copper)30 A
- Recommended max continuous load (80%)24 A
- Wire gauge10 AWG
At 24 A continuous, 10 AWG on a 30 A breaker suits a heater of up to about 5,760 W at 240 V. A typical 4,500 W element draws 18.75 A, well inside that.
6 AWG feeder
Choosing wire for a small detached-garage subpanel.
Inputs
- Wire gauge (AWG)6 AWG
Result
- Rated ampacity (60°C copper)55 A
- Recommended max continuous load (80%)44 A
- Wire gauge6 AWG
In the 60°C column 6 AWG copper carries 55 A. If every termination is rated 75°C and the wire is THHN/THWN in conduit, the 75°C column allows 65 A, but check the equipment labels before using it.
Copper ampacity by insulation temperature rating
Values from NEC Table 310.16 for copper conductors, 30°C ambient, no more than three current-carrying conductors together. You may only use a higher column if the wire's insulation and every terminal it connects to are rated for that temperature.
| AWG | Area (mm²) | 60°C (NM-B, UF) | 75°C (THWN, XHHW) | 90°C (THHN, XHHW-2) |
|---|---|---|---|---|
| 14 | 2.08 | 15 | 20 | 25 |
| 12 | 3.31 | 20 | 25 | 30 |
| 10 | 5.26 | 30 | 35 | 40 |
| 8 | 8.37 | 40 | 50 | 55 |
| 6 | 13.3 | 55 | 65 | 75 |
| 4 | 21.2 | 70 | 85 | 95 |
| 2 | 33.6 | 95 | 115 | 130 |
Why the 60°C column usually governs
A conductor is only as good as its weakest connection. NEC 110.14(C) says that for circuits rated 100 A or less, or conductors 14 through 1 AWG, you use the 60°C ampacity unless the equipment is marked for 75°C. Most modern breakers and panels are marked 60/75°C, which allows the 75°C column for individual THHN/THWN conductors, but NM-B cable is limited to its 60°C value by NEC 334.80 even though its individual wires are 90°C rated.
The 90°C column is mainly a starting point for derating. When you correct for a hot location or bundle many conductors, you can begin from the 90°C value, then the final answer must still not exceed the 60°C or 75°C limit of the terminations.
Situations that reduce ampacity
- Hot locations: above 30°C (86°F) ambient, multiply by a correction factor from NEC Table 310.15(B)(1). Conduit on a sunny rooftop runs much hotter than the air around it.
- More than three current-carrying conductors in one raceway or bundled cable: NEC Table 310.15(C)(1) reduces ampacity to 80% for 4–6 conductors and 70% for 7–9.
- Aluminum wire: carries less current than copper of the same gauge, typically needing one or two sizes larger.
- Long runs: ampacity says nothing about voltage drop. A 20 A, 120 V circuit run 100 ft on 12 AWG loses about 5% of its voltage at full load, so long runs often step up one or two sizes. Size the conductor with the wire gauge calculator.
- Motor and HVAC circuits: follow the nameplate minimum circuit ampacity and the code's motor rules, which differ from ordinary branch circuits.
How to use the Wire Gauge Ampacity Calculator
- 1Find the gauge printed on the cable jacket or wire insulation (for example "12 AWG" or "12-2 NM-B").
- 2Select that copper gauge in the calculator.
- 3Read the rated 60°C ampacity; this is the largest breaker the wire normally supports for general circuits.
- 4Use the 80% figure for loads that run three hours or more, such as heaters, EV chargers and water heaters.
- 5Apply any temperature, bundling or voltage-drop adjustments for your installation, then confirm against local code.
Frequently asked questions
How many amps can 12 gauge wire carry?
12 AWG copper is rated 20 A in the 60°C column and goes on a 20 A breaker. For continuous loads, keep the draw to 16 A.
Can I put 14 gauge wire on a 20 amp breaker?
No. NEC 240.4(D) limits 14 AWG copper to a 15 A overcurrent device. A 20 A breaker on 14 AWG lets the wire overheat before the breaker trips.
What size wire do I need for 30 amps?
10 AWG copper is the usual choice for a 30 A circuit such as a dryer or water heater. Long runs may need 8 AWG to limit voltage drop.
What is the 80% rule for breakers and wire?
A load that runs continuously for three hours or more should use no more than 80% of the circuit rating. On a 50 A EV charger circuit that means a maximum of 40 A of charging current.
Why does THHN show higher ampacity than Romex of the same size?
THHN insulation is rated 90°C, but NM-B cable (Romex) is limited to its 60°C ampacity by code because heat builds up inside the cable jacket and in walls.
Does this apply to aluminum wire?
No. Aluminum has higher resistance, so the code assigns it lower ampacities than copper of the same gauge. Use the aluminum columns of NEC Table 310.16.
References
- NFPA 70: National Electrical Code, Table 310.16 — National Fire Protection Association
- IEC 60364-5-52: Low-voltage electrical installations – Wiring systems — International Electrotechnical Commission
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
- Report an error