The Master Wiring Chart for Amperage (NEC Table 310.16)

The definitive wiring chart for amperage in the United States is dictated by the National Electrical Code (NEC) Table 310.16. This table establishes the allowable ampacities for insulated copper conductors rated up to 2000 volts. Before pulling wire or sizing a breaker, you must understand how to read this chart: the ampacity of a wire is not a single fixed number, but a variable dependent on the insulation's temperature rating and the termination limits of your equipment.

How to read this table: The columns represent the temperature rating of the wire insulation (60°C, 75°C, 90°C). The rows represent the American Wire Gauge (AWG) or kcmil size. You must select the column that matches the lowest temperature rating in your entire circuit—usually the breaker or receptacle termination lug, not just the wire jacket.
Table 310.16: Allowable Ampacities of Insulated Copper Conductors (Rated 0-2000V, 30°C Ambient)
Wire Size (AWG/kcmil) 60°C Column (140°F)
NM-B, TW, UF		 75°C Column (167°F)
THWN, XHHW, THHN*		 90°C Column (194°F)
THHN, THWN-2*
14 AWG15A20A25A
12 AWG20A25A30A
10 AWG30A35A40A
8 AWG40A50A55A
6 AWG55A65A75A
4 AWG70A85A95A
3 AWG85A100A115A
2 AWG95A115A130A
1 AWG110A130A145A
1/0 AWG125A150A170A
2/0 AWG145A175A195A
3/0 AWG165A200A225A
4/0 AWG195A230A260A

Source: NFPA 70 National Electrical Code, Table 310.16. *Note: While THHN is rated 90°C in dry locations, its ampacity for termination sizing is almost always limited to the 75°C column due to equipment lug ratings per NEC 110.14(C).

Quick-Jump Ampacity Rows for Common Circuits

For bookmark-friendly reference, here are the most queried residential and light-commercial circuit sizes. These values assume standard copper wire, an ambient temperature of 86°F (30°C) or less, and no more than three current-carrying conductors in a raceway.

  • 15A Circuit (Lighting/General Receptacles): 14 AWG (60°C column). Breaker: 15A max.
  • 20A Circuit (Kitchen/Bath/Laundry): 12 AWG (60°C column). Breaker: 20A max.
  • 30A Circuit (Dryer/Water Heater): 10 AWG (60°C column). Breaker: 30A max.
  • 50A Circuit (Range/EV Charger): 6 AWG (60°C column for NM-B) or 8 AWG (75°C column for THHN in conduit). Breaker: 50A max.
  • 100A Subpanel Feeder: 3 AWG (75°C column for THHN) or 2 AWG (60°C column for NM-B/UF). Breaker: 100A max.
  • 200A Service Entrance: 2/0 AWG Copper (75°C column). Breaker: 200A max.

Decision Tree: Which Temperature Column Applies to You?

The most common mistake DIYers and junior apprentices make is looking at the 90°C column because the wire jacket says "THHN-90°C." You cannot use the 90°C column to size your breaker unless every single termination point in the circuit is also rated for 90°C—which is virtually nonexistent in residential gear. Use this decision path to lock in your concrete column pick.

Installation Scenario Condition Final Column Pick
Running NM-B (Romex) or UF-B cable inside walls/trenches. NEC 334.80 mandates NM-B ampacity is limited to 60°C, regardless of the 90°C rating of the internal THHN conductors. 60°C Column
Running THHN/THWN-2 in conduit to a standard residential breaker panel. Standard residential breakers and lugs are tested and rated for 75°C terminations per NEC 110.14(C)(1)(a)(4). 75°C Column
Running THHN in conduit to a disconnect switch or motor starter. Verify the lug stamp. If stamped 75°C, use 75°C. If unmarked and under 100A, default to 60°C. 75°C or 60°C (Check stamp)
Calculating derating adjustments for high ambient heat or bundling. NEC 310.15 allows you to use the 90°C column only as a starting baseline to apply derating math, provided the final derated ampacity still supports the breaker size. 90°C Column (For math only)
The Golden Rule of Terminations: Per NEC 110.14(C), the ampacity of your wire cannot exceed the temperature rating of the termination lug. If your wire is 90°C but your breaker lug is 75°C, you must size the wire using the 75°C column.

Derating Factors: When Base Ampacity Drops

The numbers in the master chart assume ideal conditions: an ambient temperature of 86°F (30°C) and no more than three current-carrying conductors (CCCs) bundled together. When you deviate from these baselines, the physical insulation begins to trap heat, and you must apply derating factors to the base ampacity.

1. Bundling (More than 3 CCCs in a Conduit)

When you pull multiple circuits through a single conduit, the heat from adjacent wires compounds. NEC Table 310.15(C)(1) dictates the following adjustment factors for the 90°C column baseline:

  • 4 to 6 CCCs: Multiply base ampacity by 80%.
  • 7 to 9 CCCs: Multiply base ampacity by 50%.
  • 10 to 20 CCCs: Multiply base ampacity by 50% (with specific engineering exceptions).

Worked Example: You are pulling four 12 AWG THHN current-carrying conductors (two 120V circuits sharing a neutral is not applicable here; these are four distinct hot wires) through a single EMT conduit. The 90°C base ampacity for 12 AWG is 30A. Because you have 4 CCCs, you multiply 30A by 0.80, yielding a derated ampacity of 24A. Since 24A is greater than your 20A breaker requirement, 12 AWG is legally compliant and safe.

2. High Ambient Temperature

If your conduit runs through an attic in a southern climate where temperatures routinely exceed 86°F (30°C), you must apply temperature correction factors from NEC Table 310.15(B)(1). For example, in a 110°F (43°C) attic, you must multiply the 90°C base ampacity by 0.87. If you are running 10 AWG THHN (base 40A at 90°C), the corrected ampacity drops to 34.8A, which is still sufficient for a 30A breaker.

What This Wiring Chart for Amperage Cannot Tell You

While NEC Table 310.16 is the ultimate authority on thermal limits, relying on it blindly will lead to failed inspections and poor system performance. The wiring chart for amperage has three critical blind spots you must calculate separately:

  1. Voltage Drop: The NEC does not strictly mandate voltage drop limits for branch circuits (it is a "fine print note" recommendation), but standard engineering practice dictates a maximum 3% drop for branch circuits and 5% total from service to load. If you are running a 50A EV charger 150 feet from the panel, 6 AWG copper will carry the amperage thermally, but the voltage drop will exceed 4%. You must upsize to 4 AWG or 3 AWG to maintain voltage integrity over distance.
  2. Conduit Fill Capacity: Table 310.16 tells you how hot the wire gets, but it does not tell you if the wire will physically fit in the pipe. NEC Chapter 9, Table 1 limits conduit fill to 40% for three or more wires. If you try to pull four 2 AWG THHN wires into a 1-inch PVC conduit to feed a subpanel, it will physically jam and damage the insulation, regardless of what the ampacity chart says. Always cross-reference with Chapter 9, Table 5.
  3. Short-Circuit and Ground-Fault Let-Through Current: Ampacity charts assume steady-state continuous loads. They do not account for the magnetic and thermal forces of a dead short. Your wire must possess an adequate Short-Circuit Current Rating (SCCR) and be protected by a breaker with the correct AIC (Ampere Interrupting Capacity) rating to ensure the wire does not vaporize before the breaker trips.

By anchoring your sizing decisions to the 60°C or 75°C columns of Table 310.16, applying derating math only from the 90°C baseline, and verifying voltage drop for long runs, you will design circuits that are both code-compliant and thermally bulletproof.