The definitive amperage chart for copper wire in the United States is governed by NEC Table 310.16 (formerly 310.15(B)(16)). For standard residential branch circuits (15A to 50A), you will almost always use the 60°C column because standard non-metallic sheathed cable (NM-B/Romex) is limited to that temperature rating. For feeders, subpanels, and individual THHN conductors in conduit, the 75°C column typically applies due to the temperature limits of standard breaker and lug terminations.

This reference provides the complete allowable ampacities for copper conductors, assuming an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway. Always verify your local Authority Having Jurisdiction (AHJ) amendments, as local code has final authority over any national standard.

How to Read the NEC Copper Wire Ampacity Table

Before pulling wire, you must understand which temperature column applies to your specific installation. The NEC rates wire insulation (like THHN or XHHW) separately from the equipment terminations (like breakers and lugs). The rule of thumb is that the lowest temperature rating in the entire circuit dictates the allowable ampacity.

  • The 60°C Column: Use this for NM-B (Romex), UF-B, and any circuit rated 100 amps or less where the termination temperature is not explicitly marked. If you are wiring standard 15A, 20A, or 30A receptacles and lighting circuits in a home, this is your column.
  • The 75°C Column: Use this for THHN/THWN-2 wires pulled in conduit, as well as feeders and service entrance conductors. Most modern breakers, lugs, and subpanel busbars are rated for 75°C. Even if your THHN wire is rated for 90°C, you must cap your final ampacity at the 75°C value because the breaker lug will melt or degrade at higher temperatures.
  • The 90°C Column: This column is almost never used for final sizing. It is strictly used as the starting baseline for derating calculations (adjusting for high ambient heat or bundling multiple wires in a single conduit).
Callout Tip: Never size a breaker larger than the wire's ampacity, with very specific exceptions for motor starting currents (NEC Article 430) and overcurrent device standard sizes (NEC 240.4(B)). For standard residential loads, the breaker must be equal to or less than the wire's ampacity.

Complete Amperage Chart for Copper Wire (NEC Table 310.16)

Below is the complete data table for copper wire ampacities sourced directly from the National Fire Protection Association (NFPA) NEC 2023 Edition, Table 310.16. Use the quick-jump links below to find the most commonly queried residential sizes.

Quick-Jump Common Sizes: 14 AWG (15A) | 12 AWG (20A) | 10 AWG (30A) | 8 AWG (40A) | 6 AWG (55A/65A) | 2 AWG (115A) | 2/0 AWG (200A Service)

Copper AWG Size 60°C (NM-B / Romex) 75°C (THHN Terminations) 90°C (THHN/THWN-2 Derating Base)
14 AWG15A20A25A
12 AWG20A25A30A
10 AWG30A35A40A
8 AWG40A50A55A
6 AWG55A65A75A
4 AWG70A85A95A
3 AWG85A100A110A
2 AWG95A115A130A
1 AWG110A130A145A
1/0 AWG125A150A170A
2/0 AWG145A175A195A
3/0 AWG165A200A225A
4/0 AWG195A230A260A

What This Chart Cannot Tell You (Derating & Physical Limits)

Looking up the base ampacity is only the first step. The table above assumes ideal conditions: an ambient temperature of 30°C (86°F) and a maximum of three current-carrying conductors in a single raceway. When real-world conditions deviate, you must apply derating factors.

How Derating Modifies the Base Value

If you are pulling wire through a hot attic or bundling multiple circuits in one conduit, the wires cannot dissipate heat effectively. You must adjust the ampacity using two specific NEC tables:

  1. Ambient Temperature Correction (NEC Table 310.15(B)(1)): If your attic reaches 45°C (113°F), you must multiply the 90°C column ampacity by 0.87. For example, 10 AWG THHN (40A at 90°C) derates to 34.8A. You then compare this to your termination limits.
  2. Adjustment Factors for Bundling (NEC Table 310.15(C)(1)): If you pull four to six current-carrying conductors in one conduit, you multiply the 90°C ampacity by 80%. If you pull seven to nine, you multiply by 70%.

Note: Equipment grounding conductors (bare copper or green) do not count as current-carrying conductors for bundling derating. However, a neutral wire that carries unbalanced current from a multi-wire branch circuit or non-linear loads (like LED drivers) DOES count.

Physical and Electrical Limitations

The ampacity chart tells you what the wire can handle thermally, but it cannot tell you:

  • Voltage Drop: A 10 AWG wire can safely carry 30A, but if you run it 150 feet to a detached garage, the voltage drop will exceed the recommended 3% limit. For long runs, you must size up based on resistance (NEC Chapter 9, Table 8), not just thermal ampacity.
  • Lug Physical Fit: A standard 100-amp breaker lug might only physically accept up to #1 AWG wire. If your derating calculations force you to use 1/0 AWG, you cannot physically land it on the breaker. You must either use a breaker with larger lugs or install a gutter/splice block to step down to a smaller pigtail.
Safety Warning: Always de-energize the panel and verify zero voltage with a tested multimeter before terminating any conductors. Use a calibrated torque screwdriver to tighten lugs to the manufacturer's specified inch-pound rating. Loose connections cause high resistance, leading to arcing and fires, regardless of how perfectly you sized the wire.

Frequently Asked Questions

What size copper wire do I need for a 50-amp breaker?

If you are using THHN/THWN-2 individual conductors in conduit, you need 6 AWG copper, which is rated 65A in the 75°C column. However, if you are using NM-B (Romex) cable, you must use the 60°C column, where 6 AWG is only rated for 55A. Because 55A is not a standard breaker size, you must step up to 4 AWG NM-B (rated 70A at 60°C) to legally protect the circuit with a 50-amp breaker.

Can I use the 90°C column for THHN wire to get a higher ampacity?

No. While THHN insulation is rated for 90°C, the breakers, lugs, and busbars it connects to are almost universally rated for a maximum of 75°C. NEC 110.14(C) dictates that the lowest temperature rating in the circuit governs. You only use the 90°C column as a mathematical baseline to apply derating factors for heat or bundling; the final adjusted ampacity still cannot exceed the 75°C termination limit.

Why does a 200-amp residential service use 2/0 AWG copper instead of 3/0 or 4/0?

Looking strictly at Table 310.16, 2/0 AWG copper is rated 175A at 75°C, which seems too small for a 200A main breaker. However, NEC 310.12 provides a specific exception for single-family dwelling service entrance conductors. Because residential loads are highly diversified (not everything runs at peak draw simultaneously), the code explicitly allows 2/0 AWG copper (or 4/0 AWG aluminum) for a 200-amp residential service.

Does the ground wire count towards the amperage chart limits?

No. The equipment grounding conductor (EGC) only carries current during a fault condition to trip the breaker. Under normal operation, it carries zero current. Therefore, it is completely excluded from the ampacity chart sizing and is not counted as a current-carrying conductor when calculating bundling derating factors.

Where can I find the official NEC tables and updates?

The National Fire Protection Association (NFPA) publishes the National Electrical Code. You can view the standard and track local adoptions via the official NFPA NEC portal. For practical field applications and code interpretations, industry resources like EC&M Magazine provide excellent breakdowns of complex derating scenarios.