For standard residential 120V/240V branch circuits using copper THHN/THWN-2 wire in a raceway, you must use the 75°C column of NEC Table 310.16 to determine your final ampacity. The baseline capacities for the most common sizes are: 14 AWG = 15A, 12 AWG = 20A, 10 AWG = 30A, 8 AWG = 40A, and 6 AWG = 55A. If you are using non-metallic sheathed cable (NM-B / Romex), you are strictly limited to the 60°C column regardless of the wire's internal insulation rating.

How to Read This Electric Wire Capacity Chart

The most common mistake DIYers and junior apprentices make is looking at the 90°C column for THHN wire and sizing their breaker to that higher number. This violates NEC 110.14(C) termination provisions. Here is how to read the columns correctly:

  • 60°C Column: Use this for NM-B (Romex) cable, UF-B cable, and any equipment where the termination temperature is unmarked or explicitly rated for 60°C. This is your hard limit for most standard residential cable runs.
  • 75°C Column: Use this for THHN/THWN-2 wire in conduit when terminating on modern breakers and lugs (like Square D QO or Siemens EQ series), which are almost universally rated for 75°C. This column dictates your maximum breaker size.
  • 90°C Column: You only use this column as your starting baseline for calculating derating adjustments (ambient temperature or conduit bundling). The final derated ampacity cannot exceed the 75°C or 60°C termination limit.
Bench Rule of Thumb: If you are pulling individual THHN wires in EMT or PVC conduit to a subpanel, start your math in the 90°C column for derating, but cap your final breaker size using the 75°C column. If you are running NM-B cable through studs, ignore the 90°C and 75°C columns entirely; use the 60°C column.

NEC Table 310.16: Copper and Aluminum Ampacity

The following data is sourced directly from NEC (NFPA 70) Table 310.16. It assumes an ambient temperature of 30°C (86°F) and not more than three current-carrying conductors in a raceway or cable.

AWG / kcmil Copper 60°C (NM-B) Copper 75°C (THHN Term) Copper 90°C (THHN Base) Aluminum 75°C (XHHW)
1415A20A25A-
1220A25A30A-
1030A35A40A-
840A50A55A40A
655A65A75A50A
470A85A95A65A
385A100A115A75A
295A115A130A90A
1110A130A145A100A
1/0125A150A170A120A
2/0145A175A195A135A
3/0165A200A225A155A
4/0195A230A260A180A

Quick-Jump: Most Queried Wire Sizes and Breakers

Bookmarks and field queries overwhelmingly focus on the small conductor sizes used for standard receptacle and lighting circuits. Here are the hard limits governed by NEC 240.4(D), which overrides the table above for small copper conductors:

  • 14 AWG Copper: Maximum breaker size is 15 Amps. Even if your 14 AWG THHN is rated 25A at 90°C, NEC 240.4(D) strictly caps the overcurrent protection at 15A. Use for 15A lighting and receptacle circuits.
  • 12 AWG Copper: Maximum breaker size is 20 Amps. The 90°C column says 30A, but the code caps it at 20A. Use for 20A kitchen, bathroom, and general receptacle circuits.
  • 10 AWG Copper: Maximum breaker size is 30 Amps. Use for standard electric dryers, water heaters, and heavy window AC units.
  • 8 AWG Copper: Maximum breaker size is 40 Amps (using the 75°C column). Use for EV Level 2 chargers (32A continuous load requires a 40A breaker) and older electric ranges.
  • 6 AWG Copper: Maximum breaker size is 60 Amps (Standard breaker size above the 55A 75°C rating, permitted by NEC 240.4(B) next-size-up rule). Use for modern electric ranges and 60A subpanel feeders.

Derating Rules: When Base Ampacity Drops

The chart above assumes perfect conditions: 30°C (86°F) ambient air and no more than three current-carrying conductors in a conduit. When you deviate from this, you must apply derating factors using the 90°C column as your starting baseline. According to the Copper Development Association's ampacity guidelines, failing to derate leads to insulation degradation and premature failure.

Conduit Bundling (NEC Table 310.15(C)(1)):
If you pull 4 to 6 current-carrying conductors in a single raceway, you must multiply the base ampacity by 80%.
Worked Example: You are pulling six 10 AWG THHN wires (two 120V circuits sharing a neutral, plus a 240V circuit) through a single 3/4-inch EMT conduit. 1. Base 90°C ampacity for 10 AWG = 40A. 2. Bundling adjustment for 6 wires = 80%. 3. 40A x 0.80 = 32A derated ampacity. 4. Check termination limit (75°C column) = 35A. 5. The lower of the two governs: 32A. You must protect this wire with a 30A breaker, not a 35A or 40A breaker.

Ambient Temperature (NEC Table 310.15(B)(1)):
If your conduit runs through a 110°F (43°C) attic, the correction factor for 90°C wire is 0.87. A 10 AWG wire (40A base) drops to 34.8A. It can still safely be used on a 30A breaker, but an 8 AWG wire running the same route would drop from 55A to 47.8A, meaning you could no longer use it on a 50A breaker.

Decision Path: Picking the Right Wire and Breaker

Use this decision-tree-table to terminate your sizing process with a single, code-compliant pick. Do not skip steps.

Step Action Rule / Calculation
1. Calculate Load Sum the nameplate amperage of all devices on the circuit. If the load runs for 3 hours or more (continuous), multiply the total load by 1.25 (125%).
2. Select Wire Type Choose NM-B for stud cavities, THHN for conduit. Lock in your termination column: 60°C for NM-B, 75°C for THHN in standard breakers.
3. Apply Derating Count conductors and check ambient temp. If >3 wires or >86°F, multiply 90°C base ampacity by the NEC adjustment factor.
4. Find Governing Ampacity Compare Derated Ampacity vs. Termination Ampacity. The lower of these two numbers is your maximum allowable circuit ampacity.
5. Size the Breaker Select standard breaker size (15, 20, 30, 40, 50, 60). Breaker must be ≤ Governing Ampacity. (Exception: NEC 240.4(B) allows next-size-up if ampacity falls between standard sizes, e.g., 55A wire on a 60A breaker).

What This Chart Cannot Tell You

An ampacity chart is only one-third of the wire sizing equation. Before you buy your spools and breakers, you must account for these three physical and electrical realities:

  1. Voltage Drop: NEC 310.15 dictates ampacity (heat), but it does not guarantee performance over distance. If you are running a 6 AWG copper feeder 150 feet to a detached garage subpanel, the wire will handle 55A without melting, but the voltage at the far end will drop below 114V under heavy load. Use a voltage drop calculator and upsize your wire (e.g., to 4 AWG or 2 AWG aluminum) to keep drop under 3% for branch circuits and 5% total for feeders.
  2. Conduit Fill Capacity: Just because six 8 AWG THHN wires are electrically permitted 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. Six 8 AWG THHN wires require a minimum of 1-inch EMT or Schedule 40 PVC; they will not legally fit in 3/4-inch conduit.
  3. Lug Physical Limits: Standard 15A and 20A residential breakers (like the Eaton BR or Square D Homeline series) physically cannot accept wire larger than 8 AWG or 6 AWG depending on the specific model. If your voltage drop calculation demands 4 AWG wire for a 20A circuit, you cannot land it directly on the breaker. You must pigtail it to a smaller wire using an insulated terminal block or a Polaris connector inside a junction box.
The Default Recommendation: When in doubt for standard residential branch circuits under 100 feet, buy copper THHN/THWN-2 for conduit runs and NM-B for interior walls. Always use the 75°C column for your final breaker sizing on THHN, use the 60°C column for NM-B, and never exceed the hard caps of NEC 240.4(D) for 14, 12, and 10 AWG wire. Your local AHJ (Authority Having Jurisdiction) has final say on all installations, so always pull the required permits.