Wire Gauge Chart for Amperage: NEC 310.16 Copper & Aluminum Sizing

When sizing branch circuits or feeders, the direct answer for standard copper residential wiring is: 14 AWG for 15 amps, 12 AWG for 20 amps, 10 AWG for 30 amps, and 8 AWG for 40 amps (or 50 amps with 75°C rated equipment). However, picking the right wire requires more than memorizing four numbers. The allowable ampacity of a conductor changes based on insulation type, termination temperatures, and how many wires are bundled together in a raceway.

This reference guide provides the complete wire gauge chart for amperage based on the National Electrical Code (NEC), explains which temperature column legally applies to your specific installation, and walks through the derating math required for conduit bundles.

How to Read the NEC 310.16 Wire Gauge Chart for Amperage

The table below is derived directly from NFPA 70 (NEC) Table 310.16. It lists the allowable ampacities for insulated conductors rated up to 2000 volts in an ambient temperature of 30°C (86°F).

How to read this table: The columns are split by conductor material (Copper vs. Aluminum) and by the temperature rating of the wire's insulation (60°C, 75°C, and 90°C). To find your baseline ampacity, locate your American Wire Gauge (AWG) size in the first column, then read across to the column that matches your wire's insulation type and your equipment's termination rating.

Source: NFPA 70, National Electrical Code (NEC) Table 310.16. Ambient temperature assumed at 30°C (86°F). Always consult the latest adopted code cycle in your jurisdiction.

Which Temperature Column Applies to Your Installation

The most common mistake DIYers and junior electricians make is looking at the 90°C column for THHN wire and assuming a 12 AWG wire can carry 30 amps. It cannot. The column you are legally permitted to use is dictated by NEC 110.14(C), which governs termination temperatures.

The 60°C Rule for Standard Residential Branch Circuits

For circuits rated 100 amps or less, or for wire sizes 14 AWG through 1 AWG, you must use the 60°C column unless the equipment (breaker, receptacle, or lug) is explicitly tested and marked for 75°C. Most standard residential receptacles and 15A/20A breakers are only rated for 60°C terminations. Furthermore, if you are using NM-B (Romex) cable, NEC 334.80 strictly limits its ampacity to the 60°C column, regardless of the fact that the individual conductors inside the sheath have 90°C insulation.

When You Can Use the 75°C Column

You may use the 75°C column if your equipment is explicitly marked for 75°C terminations. This is standard for larger breakers (typically 30A and above in modern load centers), subpanel feed lugs, and heavy-duty receptacles like NEMA 14-50 ranges or EV chargers. For example, a 4 AWG copper wire in a 75°C rated terminal is good for 85 amps, allowing you to protect it with an 80A breaker.

The 90°C Column is Only for Derating

The 90°C column is almost never used to establish the final allowable ampacity of a circuit. Instead, it serves as the starting baseline for calculating derating (adjustments for heat and bundling), which we cover next. The final calculated ampacity after derating must still be capped by the 60°C or 75°C rating of your breaker lugs.

Derating Factors: How Bundle Size and Ambient Heat Modify Ampacity

The wire gauge chart for amperage above assumes two things: an ambient air temperature of 30°C (86°F), and no more than three current-carrying conductors bundled together. When you pull multiple circuits through a single conduit, the wires heat each other up, reducing their ability to dissipate thermal energy. OSHA electrical safety guidelines and the NEC mandate that you reduce (derate) the wire's ampacity to prevent insulation meltdown.

To calculate derated ampacity, you always start with the 90°C column value, multiply it by the adjustment factor below, and then compare the result to your termination column limit. The lowest of those two numbers is your final allowable ampacity.

Source: NEC Table 310.15(C)(1). Note: Equipment grounding conductors and neutral wires that only carry unbalanced current do not count as current-carrying conductors for derating purposes in standard single-phase multiwire branch circuits.

Worked Derating Example

Imagine you are pulling 12 AWG THHN copper through a conduit that contains 6 current-carrying conductors (three 240V circuits sharing a single neutral, or three standard 120V hot/neutral pairs). You want to protect these wires with standard 20A breakers.

1. Find the 90°C baseline: 12 AWG in the 90°C column = 30 amps.
2. Apply the derating factor: 6 conductors = 80% adjustment. (30A × 0.80 = 24 amps).
3. Check termination limits: Your breaker lugs are rated 60°C/75°C. The 60°C limit for 12 AWG is 20 amps.
4. Final Verdict: The derated wire can handle 24A, but the breaker lug is limited to 20A. Since 24A > 20A, the wire is sufficiently protected. You can safely use a 20A breaker.

However, if you added more circuits and had 10 conductors in the pipe (50% derating), the math changes: 30A × 0.50 = 15 amps. Your wire is now only legally allowed to carry 15 amps. You must either downgrade to a 15A breaker or upsize the wire to 10 AWG THHN (40A × 0.50 = 20A).

What This Wire Gauge Chart Cannot Tell You

While NEC Table 310.16 is the bible for thermal ampacity, it does not account for the physics of long wire runs or physical space constraints. Before finalizing your materials list, you must verify three additional factors:

1. Voltage Drop Over Distance

Ampacity charts assume the wire can handle the heat of the current, but they ignore the resistance that causes voltage to sag over long distances. The NEC (Informational Note 210.19(A)(4)) recommends a maximum 3% voltage drop for branch circuits and 5% total for feeder plus branch. If you are running a 20A, 120V circuit using 12 AWG copper to a detached garage 120 feet away, you will experience roughly a 4.1% voltage drop. To fix this, you must upsize to 10 AWG or 8 AWG, even though the ampacity chart says 12 AWG is sufficient for 20 amps.

2. Conduit Fill Capacity

You might mathematically prove that eight 12 AWG THHN wires can safely carry your load after derating, but they might not physically fit in your conduit. NEC Chapter 9, Table 1 limits conduit fill to 40% of the raceway's internal cross-sectional area when pulling three or more wires. Always check a conduit fill calculator before buying wire; pulling eight 12 AWG wires through a half-inch EMT conduit is a physical nightmare and violates code.

3. Local AHJ Amendments and Continuous Loads

Finally, the chart provides raw ampacity, not breaker sizing rules for continuous loads. If a load will run for 3 hours or more (like an EV charger, baseboard heater, or commercial lighting), NEC 210.20(A) requires you to multiply the continuous load by 125% to size the breaker and wire. A 32A continuous EV charger requires a 40A breaker (32 × 1.25 = 40), which mandates a minimum of 8 AWG copper wire in a 60°C termination environment. Always defer to your local Authority Having Jurisdiction (AHJ) for final sign-off, as local inspectors may have regional amendments that supersede baseline NEC tables.