Copper Wire Amp Rating Chart: How to Read NEC Table 310.16

When sizing conductors for a branch circuit or feeder, the definitive copper wire amp rating chart is dictated by NFPA 70, specifically NEC Table 310.16. The short answer for standard residential branch circuits using non-metallic sheathed cable (NM-B/Romex) is straightforward: 14 AWG is rated for 15 amps, 12 AWG for 20 amps, and 10 AWG for 30 amps. However, if you are pulling individual THHN conductors in conduit for a subpanel, wiring a heavy-duty 240V welder receptacle, or routing wires through a hot attic, relying on those basic baseline numbers will result in failed inspections, tripped breakers, or melted terminal lugs.

This guide provides the exact ampacity values, explains the critical temperature column rules that dictate your final wire size, and breaks down the derating math you need for real-world installations.

The Master Copper Wire Amp Rating Chart (NEC Table 310.16)

The table below reproduces the allowable ampacities for insulated copper conductors rated up to 2000 volts, based on an ambient temperature of 30°C (86°F). This data is sourced directly from the National Electrical Code (NEC) Table 310.16 (formerly 310.15(B)(16) in older code cycles).

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• 14 AWG: 15A max overcurrent protection (NEC 240.4(D) strictly limits this, regardless of insulation temperature).
• 12 AWG: 20A max for standard receptacles; 25A allowed for specific motor/HVAC applications.
• 10 AWG: 30A max for dryers and standard water heaters.
• 6 AWG: 55A (NM-B cable) or 65A (THHN in conduit) for 50A/60A subpanel feeders and EV chargers.
• 4 AWG: 70A (NM-B) or 85A (THHN) for larger subpanels.
• 2 AWG: 95A (NM-B) or 115A (THHN) for 100A residential subpanel feeders.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at the 90°C column because it offers the highest ampacity, allowing them to use smaller, cheaper wire. This is a violation of NEC 110.14(C), which enforces the 'weakest link' rule for terminations.

To determine which column you must use for your final wire sizing, you have to evaluate both the wire insulation and the equipment terminals:

1. The Wire Insulation: Modern THHN/THWN-2 wire is rated for 90°C. NM-B (Romex) is also technically 90°C wire, but the NEC specifically restricts its ampacity to the 60°C column.
2. The Terminations (Lugs/Breakers): Standard residential breakers, receptacles, and panel lugs are typically rated for 75°C. Older equipment (pre-1990s) may only be rated for 60°C.

The Rule: You must use the lowest temperature rating in the entire circuit path to determine your base ampacity. If you pull 90°C THHN wire into a panel with 75°C rated lugs, your base ampacity is dictated by the 75°C column. If you use NM-B cable, you are locked into the 60°C column, even if the breaker lugs are rated 75°C.

Derating Factors and What the Chart Cannot Tell You

NEC Table 310.16 assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. Real-world jobsites rarely match these assumptions. Here is how derating modifies the base values, followed by the critical limitations of the chart.

How Derating Modifies Base Ampacity

When you pull more than three current-carrying conductors in a single raceway (conduit), the wires heat each other up. You must apply a correction factor from NEC Table 310.15(C)(1).

Worked Example: You are pulling four current-carrying 12 AWG THHN wires through EMT conduit for two 20A multi-wire branch circuits.
1. Base ampacity from the 90°C column for 12 AWG = 30A.
2. Derating factor for 4-6 conductors = 80%.
3. 30A × 0.80 = 24A.
4. Compare 24A to the 75°C termination limit (25A). The lower value is 24A.
Since 24A is greater than your 20A breaker, 12 AWG THHN is perfectly legal and safe here. If you had pulled six current-carrying wires (derating drops to 70%), the math would be 30A × 0.70 = 21A. You would still be fine for a 20A breaker, but you are getting dangerously close to the limit.

What the Amp Rating Chart Cannot Tell You

Table 310.16 is strictly a thermal limit chart for wire insulation. It does not account for electrical physics or load duration. Never size a feeder based solely on this table without checking these three factors:

• Voltage Drop: The NEC recommends (and many local AHJs mandate) a maximum 3% voltage drop on branch circuits and 5% total for feeders. Table 310.16 will tell you that 10 AWG is legal for a 30A load, but if that 30A load is a heavy motor 150 feet away from the panel, the voltage drop will exceed 3%, causing the motor to overheat and fail. You must consult manufacturer voltage drop calculators or NEC Chapter 9, Table 8 for DC resistance to upsize the wire.
• Continuous vs. Non-Continuous Loads: If a load will run for 3 hours or more (like an EV charger, commercial lighting, or a space heater), NEC Article 210.20 requires you to multiply the load by 125%. A 30A continuous EV charger requires a circuit rated for 37.5A (30 × 1.25), meaning you must step up to 8 AWG wire on a 40A breaker, even though the chart says 10 AWG handles 30A.
• Local AHJ Amendments: The National Electrical Code is a baseline standard. Your local Authority Having Jurisdiction (AHJ) or municipal inspector may have local amendments that require larger minimum wire sizes for specific applications, such as mandating 12 AWG minimum for all 15A lighting circuits to reduce voltage drop. Always verify local code before pulling wire.