Copper AWG Wire Chart: Ampacity, Breaker Sizing, and NEC Derating Rules

For standard residential copper wiring, match your breaker to the 60°C column of the NEC 310.16 wire chart for NM-B (Romex) cables, and the 75°C column for THHN wires in conduit. A 20A breaker requires 12 AWG, a 30A requires 10 AWG, and a 50A requires 6 AWG NM-B or 8 AWG THHN. This guide provides the complete lookup table, derating math, and a concrete decision path so you can pull the right wire without a second trip to the supply house.

How to Read the NEC Copper AWG Wire Chart

The master source for this data is NEC Article 310.16 (formerly 310.15(B)(16)). When you look at a standard ampacity table, you will see three distinct temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F). Picking the wrong column is the most common mistake DIYers make.

• The 60°C Column: Use this for NM-B (Romex) cable and any circuit terminated on a breaker or device rated 100A or less, unless the equipment is explicitly marked otherwise. Most standard residential receptacles and breakers default to this column.
• The 75°C Column: Use this for THHN/THWN-2 wires pulled in conduit, and for equipment/breakers rated over 100A. If you are terminating THHN directly into a modern 75°C-rated panel lug, this is your base ampacity.
• The 90°C Column: Never use this column for final breaker sizing. The 90°C rating exists solely as a starting point for derating calculations (adjusting for ambient heat or wire bundling). Your final derated ampacity can never exceed the 60°C or 75°C limits of the termination points.

The Master AWG Wire Chart (Copper, THHN/THWN-2 & NM-B)

Below is the complete ampacity table for copper conductors. Bookmark this section. The most queried residential sizes (15A, 20A, 30A, 50A, and 100A) are highlighted with quick-jump IDs.

Source: NEC Article 310.16, Copper Conductors, 30°C Ambient Temperature base.

Decision Path: Picking the Right Wire and Breaker

Stop guessing. Use this decision tree to lock in your exact materials for standard 120V/240V residential branch circuits and feeders.

Derating Rules: When the Chart Lies to You

The ampacities in the table above assume two things: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors in a single raceway. When you violate either assumption, you must apply derating factors. I once inspected a DIY job where a maker ran four 12 AWG THHN circuits (8 current-carrying conductors) in a single 3/4-inch EMT conduit and sized the breakers for 30A using the 90°C column. The inspector failed it immediately.

Here is how derating modifies your base value:

1. Count your current-carrying conductors. Hots and neutrals count. Grounds do not count. A standard 120V circuit has 2 current-carrying wires. A 240V circuit with no neutral has 2. A multi-wire branch circuit (MWBC) sharing a neutral counts the neutral as current-carrying if the loads are non-linear (like LED drivers or computers).
2. Apply the bundling multiplier. If you have 4 to 6 current-carrying wires in a conduit, multiply the 90°C column ampacity by 80%. If you have 7 to 9 wires, multiply by 70%.
3. Check ambient temperature. If the conduit runs through an attic that hits 110°F (43°C), you must apply an additional correction factor from NEC Table 310.15(B)(1). For 90°C wire at 110°F, the multiplier is 0.87.
4. The Terminal Limit Rule. After doing the math, your final calculated ampacity cannot exceed the 60°C or 75°C rating of the breaker or device lugs.
   Example: Four 10 AWG THHN wires in a conduit. The 90°C base is 40A. Derate by 80% = 32A. You can safely put this on a 30A breaker. But you cannot use the remaining 2A to justify a 35A breaker, because standard lugs max out at the 75°C column (35A) anyway.

What This Wire Chart Cannot Tell You

An ampacity chart tells you what the wire can handle before the insulation melts. It does not tell you if the circuit will actually work efficiently. Before finalizing your bill of materials, check these three physical limitations:

1. Voltage Drop Over Distance

The NEC recommends a maximum 3% voltage drop for branch circuits and 5% for feeders. If you are running a 50A EV charger 150 feet from the panel, 8 AWG THHN is legally allowed by the ampacity chart, but the voltage drop will exceed 4%, causing the charger to fault or charge slower. The fix: Step up to 6 AWG THHN for runs over 100 feet at 50A, or 4 AWG for runs over 150 feet. Use a dedicated voltage drop calculator rather than relying solely on the ampacity chart.

2. Physical Lug Sizing Limits

Breaker lugs have physical torque and size limits. A standard 100A breaker might only accept wire up to #1 AWG. If your derating math forces you to use 1/0 AWG wire for a 100A subpanel feeder due to high attic heat, that wire physically will not fit into the breaker lug. The fix: You must either use a breaker with a larger lug kit, splice down to a smaller wire inside a junction box (using an appropriately rated terminal block), or switch to aluminum wire which requires a larger AWG but has a smaller physical diameter per ampacity.

3. Aluminum vs. Copper Equivalency

This chart is strictly for copper. If you are buying SER cable for a 200A service entrance or a 100A subpanel feeder, you will likely buy aluminum (XHHW-2) because copper at those sizes is prohibitively expensive. Aluminum requires you to go up two AWG sizes to achieve the same ampacity (e.g., 2 AWG Copper = 115A; 1/0 AWG Aluminum = 120A). Never use a copper chart to size aluminum wire, and always use anti-oxidant paste (like Noalox) on aluminum terminations.

For further reading on conductor applications and advanced derating scenarios, consult the Codes and Standards section of Electrical Contractor Magazine, which regularly publishes NEC code cycle updates and practical jobsite interpretations.