The standard wire sizing table for US residential and commercial copper wiring is based on NFPA 70 (NEC) Table 310.16. For a standard 15A branch circuit, use 14 AWG (rated 15A at 60°C); for a 20A circuit, use 12 AWG (rated 20A at 60°C); and for a 30A circuit, use 10 AWG (rated 30A at 60°C). You must always size the wire based on the lowest temperature rating of the cable, the termination lugs, or the connected device, regardless of the wire's actual insulation rating.

The Core NEC Wire Sizing Table (Copper, 60°C to 90°C)

This table is extracted directly from NFPA 70 (National Electrical Code) Table 310.16. Before using it, understand how to read the columns: the AWG column lists the physical wire size. The three temperature columns (60°C, 75°C, and 90°C) represent the maximum allowable ampacity (continuous current) based on the temperature rating of the weakest link in your circuit. The values below assume an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in a raceway or cable.

Bookmark Quick-Jump: The most frequently queried rows for home wiring are highlighted in bold below. If you are wiring standard 120V/240V receptacles, lighting, or dryers, you will almost exclusively use 14, 12, 10, 8, and 6 AWG.
NEC Table 310.16 - Allowable Ampacities of Insulated Copper Conductors (Up to 3 Conductors in Raceway, 30°C Ambient)
AWG Size 60°C (140°F) Column 75°C (167°F) Column 90°C (194°F) Column
14 AWG15A20A25A
12 AWG20A25A30A
10 AWG30A35A40A
8 AWG40A50A55A
6 AWG55A65A75A
4 AWG70A85A95A
3 AWG85A100A115A
2 AWG95A115A130A
1 AWG110A130A145A
1/0 AWG125A150A170A

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is looking at the 90°C column because they are pulling 90°C-rated THHN wire in conduit, and then sizing the breaker to that higher number. This violates NEC 110.14(C) and creates a severe fire hazard. The correct column is dictated by your cable type and termination hardware.

Cable Type vs. Required Ampacity Column
Cable / Wire Type Insulation Rating Ampacity Column to Use Why?
NM-B (Romex) 90°C 60°C Column NEC 334.80 explicitly restricts NM-B ampacity to the 60°C column, regardless of its 90°C insulation.
UF-B (Underground) 90°C 60°C Column Similar to NM-B, UF-B ampacity is capped at the 60°C column per NEC 339.5.
THHN / THWN-2 90°C 75°C Column While the wire is 90°C, standard breakers, lugs, and receptacles are only rated for 75°C terminations.
XHHW-2 (Wet/Dry) 90°C 75°C Column Same termination limitations apply as THHN for standard commercial/residential panels.
The 90°C Column Exception: You are only allowed to use the 90°C column for derating calculations (adjusting for high ambient heat or bundling multiple wires in a conduit). Once you apply the derating math to the 90°C base number, the final adjusted ampacity must still be equal to or greater than the breaker size, and you must verify it against the 75°C or 60°C termination limits. For a deep dive into termination rules, reference the wire sizing guidelines at All About Circuits.

Derating Factors and What the Table Cannot Tell You

The base wire sizing table above assumes ideal conditions: exactly 30°C (86°F) ambient air, and no more than three current-carrying conductors bundled together. When real-world jobsite conditions deviate from this, the base ampacity must be modified.

How Derating Rows Modify the Base Value

When you pull more than three current-carrying conductors through a single conduit (like a multi-wire branch circuit or a subpanel feeder), the wires heat each other up. You must multiply the base ampacity by an adjustment factor. Note: Equipment grounding conductors (bare copper or green) never count toward this total. Neutral conductors only count if they carry unbalanced current from non-linear loads or specific multi-wire setups.

NEC Table 310.15(C)(1) - Adjustment Factors for More Than Three Conductors
Number of Current-Carrying Conductors Adjustment Factor (Percent) Example (12 AWG THHN at 90°C base = 30A)
1 - 3 100% 30A * 1.0 = 30A
4 - 6 80% 30A * 0.8 = 24A
7 - 9 70% 30A * 0.7 = 21A
10 - 20 50% 30A * 0.5 = 15A

If you pull six 12 AWG THHN wires in a conduit, your derated ampacity drops to 24A. Because 24A is still above the standard 20A breaker size, you can still protect the circuit with a 20A breaker. However, if you pull 10 wires in that same conduit, the ampacity drops to 15A, meaning you must downsize your breaker to 15A or pull larger (10 AWG) wire to compensate.

What the Wire Sizing Table Cannot Tell You

Relying solely on Table 310.16 leaves three critical engineering gaps in your installation:

  • Voltage Drop: The table assumes the wire can handle the heat, but it ignores distance. A 12 AWG wire on a 20A breaker is perfectly legal for a 150-foot run according to the ampacity table, but it will suffer a severe voltage drop (over 5%), causing motors to overheat and lights to dim. For runs over 50 feet, always calculate voltage drop and consider upsizing one AWG step (e.g., using 10 AWG for a 20A long-run circuit).
  • Conduit Fill Capacity: The table tells you the wire can carry the current, but not whether it physically fits in the pipe. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. Pulling four 6 AWG THHN wires into a 1/2-inch EMT conduit will result in jammed wires, damaged insulation, and a failed inspection.
  • Short-Circuit Withstand: Ampacity measures continuous thermal loading. It does not guarantee the wire can survive the massive magnetic and thermal forces of a 10,000A short circuit before the breaker trips. This is why proper breaker coordination and torqueing lugs to manufacturer specifications (using an inch-pound torque screwdriver) is mandatory to prevent arc faults.

Always treat the wire sizing table as your starting baseline, not your final checklist. Verify your termination temperatures, calculate your derating factors for bundled wires, and run a voltage drop calculation for any circuit exceeding 50 feet in length.