For standard 120V/240V residential branch circuits, use 14 AWG copper for 15A, 12 AWG for 20A, 10 AWG for 30A, 8 AWG for 40A, and 6 AWG for 55A (routinely used on 60A breakers with 75°C terminations). This copper wire gauge table is based on the National Electrical Code (NEC) Table 310.16, which dictates the maximum allowable ampacity for copper conductors based on their insulation temperature rating and installation method. Sizing wire correctly prevents insulation meltdown, voltage drop, and nuisance breaker trips.

How to Read the Copper Wire Gauge Table (and Which Column Applies)

The most common mistake DIYers and junior electricians make is looking at the 90°C column because modern THHN wire is rated for 90°C, and assuming they can use that higher ampacity. You cannot. The column you must use is dictated by the weakest link in your circuit, which is almost always the termination point (the breaker lug or receptacle screw).

NEC Article 110.14(C) outlines the rules for termination temperatures:

  • 60°C Column: You must use this column for 14, 12, and 10 AWG conductors, regardless of the wire's insulation rating. Most standard residential NM-B (Romex) cable is also limited to the 60°C column.
  • 75°C Column: You may use this column for 8 AWG and larger conductors, provided the equipment (breakers, lugs, disconnects) is explicitly rated for 75°C. Nearly all modern residential panelboards and breakers are 75°C rated.
  • 90°C Column: This column is only used as the starting base ampacity for derating calculations (like bundling wires in a hot attic). It is almost never used for final overcurrent protection sizing because standard terminations are not rated for 90°C.
Bench Tip: If you are pulling individual THHN wires in conduit for a 60A subpanel feeder, you size the wire using the 75°C column (which allows 6 AWG copper at 65A). If you are running NM-B cable for a 60A range circuit, you must use the 60°C column, which requires 4 AWG copper (rated 70A) because NM-B is locked to the 60°C column.

The Complete NEC Table 310.16 Copper Wire Gauge Table

Below is the complete ampacity chart for copper conductors rated 0-2000 volts, based on an ambient temperature of 30°C (86°F). Bookmark this section. The most frequently queried residential sizes are highlighted with anchor IDs. 14 AWG12 AWG10 AWG8 AWG6 AWG4 AWG3 AWG2 AWG1 AWG1/0 AWG2/0 AWG3/0 AWG4/0 AWG
AWG / kcmil 60°C (140°F)
NM-B / Romex		 75°C (167°F)
THHN Terminations		 90°C (194°F)
THHN Wire Limit / Derating Base
15A20A *25A *
20A25A *30A *
30A35A *40A *
40A50A55A
55A65A75A
70A85A95A
85A100A115A
95A115A130A
110A130A145A
125A150A170A
145A175A195A
165A200A225A
195A230A260A

* Note: While the 75°C and 90°C columns show higher numbers for 14, 12, and 10 AWG, NEC 110.14(C) forces you to use the 60°C column for final breaker sizing on these small gauges.

Derating: How Installation Conditions Modify Base Ampacity

The ampacities in the table above assume you are in a 30°C (86°F) environment and have no more than three current-carrying conductors in a single raceway or conduit. When conditions change, the wire's ability to shed heat drops, and you must derate (reduce) the ampacity.

Ambient Temperature Derating: If your conduit runs through a 110°F attic, you cannot use the base table values. You must multiply the 90°C column value by the temperature correction factor from NEC Table 310.15(B)(1). For 110°F (43°C), the factor is 0.87. A 6 AWG THHN wire (90°C base = 75A) becomes 65.25A (75 x 0.87). You then check if 65.25A is sufficient for your 75°C termination requirement (65A for a 60A breaker). It is, so 6 AWG is still acceptable.

Conductor Bundling Derating: When you pull more than three current-carrying conductors in a single conduit (e.g., two multi-wire branch circuits sharing a neutral), they heat each other up. For 4-6 conductors, you multiply the 90°C base ampacity by 80%. For 7-9 conductors, multiply by 70%. For 10-20 conductors, multiply by 50%.

War Story: I once saw an apprentice pull four 10 AWG THHN wires in a conduit for two 30A circuits, assuming 10 AWG was good for 30A. Because there were 4 current-carrying wires, the 90°C base of 40A had to be derated by 80%, yielding 32A. While 32A technically still covers a 30A breaker, if he had added a fifth wire (derating to 70%, or 28A), the wire would be undersized for the 30A breaker, creating a fire hazard masked by a correctly sized breaker.

Decision Path: Picking the Exact Wire for Your Circuit

Use this decision-tree-table to lock in your exact wire gauge. Do not guess; follow the row that matches your installation.
Scenario Base Column Rule Derating Check Final Concrete Pick
Standard 20A bedroom receptacle circuit using NM-B (Romex) cable in a standard wall cavity. 60°C column (NEC 110.14(C) for ≤10 AWG and NM-B limits). None (ambient <86°F, not bundled in conduit). 12 AWG NM-B on a 20A breaker.
50A hot tub feeder using individual THHN wires in PVC conduit, 4 current-carrying wires, 80°F ambient. 75°C column (THHN terminations on 50A breaker are 75°C rated). 4 wires = 80% derating on 90°C base. 8 AWG (55A x 0.8 = 44A) is too small. 6 AWG (75A x 0.8 = 60A) is sufficient. 6 AWG THHN on a 50A breaker.
100A subpanel feeder using individual THHN wires in conduit, normal conditions. 75°C column (Panel lugs and 100A main breaker are 75°C rated). None (3 or fewer current-carrying conductors, normal temp). 3 AWG THHN (Rated 100A at 75°C) on a 100A breaker.
30A dryer outlet using NM-B cable. 60°C column (NM-B cable limitation). None. 10 AWG NM-B on a 30A breaker.

What This Table Cannot Tell You (and Where to Look Next)

The copper wire gauge table is the starting point for ampacity, but it is not a complete design tool. It will not warn you about the following critical failure modes:
  1. Voltage Drop: NEC Table 310.16 does not account for distance. If you are running a 12 AWG wire 150 feet to a shed for a 15A load, the wire will not melt (ampacity is fine), but the voltage drop will exceed the recommended 3-5% limit, causing motors to overheat and lights to dim. For runs over 100 feet, consult NEC Chapter 9, Table 8 for conductor resistance and calculate voltage drop. You will likely need to upsize to 10 AWG or 8 AWG.
  2. Conduit Fill Capacity: Just because you can derate 10 wires in a conduit doesn't mean they will physically fit. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires to prevent jamming and insulation damage during the pull.
  3. Physical Lug Constraints: A 100A breaker might have an ampacity rating that accepts 3 AWG wire, but the physical lug tunnel might only be rated to accept a maximum of 1/0 AWG. Always check the manufacturer's spec sheet for the specific breaker or panelboard to ensure the physical wire size fits the termination hardware.

For 95% of residential DIY projects using NM-B cable, default strictly to the 60°C column: 14 AWG for 15A, 12 AWG for 20A, and 10 AWG for 30A. When pulling individual THHN wires in conduit for feeders, size the wire using the 75°C column to match standard breaker and panel lug ratings, apply derating factors for bundling, and always verify voltage drop for runs over 100 feet. When in doubt, your local Authority Having Jurisdiction (AHJ) has the final say on code compliance.