Most DIYers looking for an ampacity of wire chart just need three numbers: 14 AWG is 15 amps, 12 AWG is 20 amps, and 10 AWG is 30 amps. But if you are running conduit, sizing a subpanel feeder, or dealing with high ambient heat, those basic numbers will trip your breaker or melt your insulation. The ampacity of a conductor is not a single fixed number; it changes based on the insulation type, the termination temperature rating, and how many wires are bundled together.

Below is the exact data from NEC Table 310.16, a breakdown of how to read the temperature columns, and a concrete decision path to lock in your exact wire gauge and breaker size for your next project.

How to Read the Ampacity of Wire Chart

Before looking at the numbers, you must understand the three temperature columns. The National Electrical Code (NEC) categorizes wire ampacity by the thermal limit of the insulation and the connected terminals. According to NFPA 70 (NEC), here is which column applies to your installation:

  • 60°C Column (NM-B / Romex): Use this column for standard non-metallic sheathed cable used in residential walls. Even though the individual wires inside NM-B often have 90°C insulation, NEC 334.80 legally caps the ampacity at the 60°C column.
  • 75°C Column (THWN / Terminals): Use this column for individual wires in conduit (like THWN) and for sizing the wire to match standard breaker and lug terminations. Per NEC 110.14(C), most equipment rated 100A or less is tested and rated for 75°C terminations.
  • 90°C Column (THHN / Derating): Use this column exclusively as the starting point for calculating derating adjustments (bundling or high ambient heat). You cannot use the 90°C ampacity as your final circuit rating unless every single termination in the circuit is explicitly rated for 90°C, which is virtually nonexistent in residential gear.
Bench Tip: If you are pulling THHN through conduit, you size the breaker based on the 75°C column, but you calculate heat derating starting from the 90°C column. This dual-column math is where most apprentices fail their journeyman exams.

The Master Ampacity of Wire Chart (NEC Table 310.16)

The following table lists the allowable ampacities for insulated copper conductors rated up to 2000 volts, in an ambient temperature of 30°C (86°F). Source: NEC Table 310.16 (2023/2026 Editions).

AWG / kcmil 60°C (NM-B Cable) 75°C (THWN in Conduit) 90°C (THHN Derating Base) Max Standard Breaker (NEC 240.4)
14 AWG15A20A25A15A (Fixed limit)
12 AWG20A25A30A20A (Fixed limit)
10 AWG30A35A40A30A (Fixed limit)
8 AWG40A50A55A50A
6 AWG55A65A75A60A or 70A
4 AWG70A85A95A80A or 90A
3 AWG85A100A110A100A
2 AWG95A115A130A110A or 125A
1 AWG110A130A145A125A or 150A
1/0 AWG125A150A170A150A
2/0 AWG145A175A195A175A or 200A
3/0 AWG165A200A225A200A or 225A
4/0 AWG195A230A260A225A or 250A

Note on Breaker Sizing: NEC 240.4(D) strictly limits 14, 12, and 10 AWG copper to 15A, 20A, and 30A breakers respectively, regardless of the 75°C or 90°C column values. For larger wires, NEC 240.4(B) allows you to round up to the next standard breaker size if the ampacity doesn't match a standard breaker exactly.

Derating: When the Base Chart Values Drop

The chart above assumes two things: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When you violate either assumption, the ampacity drops. This is where the 90°C column earns its keep.

To calculate derated ampacity, you take the 90°C base value and multiply it by the adjustment factor found in NEC Table 310.15(C)(1). The resulting number must still be high enough to handle the breaker size you intend to use.

Worked Example: You are pulling four current-carrying 12 AWG THHN wires through a single conduit to feed two 240V circuits (no neutral).
  • Base 90°C ampacity for 12 AWG = 30A.
  • Four conductors require an 80% derating factor.
  • 30A × 0.80 = 24A derated ampacity.
  • Because 24A is greater than your 20A breaker requirement, 12 AWG THHN is perfectly legal here. If you had pulled 6 conductors (70% factor), 30A × 0.70 = 21A. Still legal for a 20A breaker, but you are getting dangerously close to the limit.

As electrical expert Mike Holt Enterprises frequently highlights, the most common jobsite mistake is applying the derating factor to the 60°C or 75°C column. Always start your derating math at 90°C.

What the Chart Cannot Tell You

The ampacity of wire chart only tells you the thermal limit of the insulation. It tells you absolutely nothing about voltage drop.

If you run a 60A subpanel feeder using 6 AWG copper (rated 65A at 75°C), the chart says you are safe. But if that subpanel is 150 feet away from the main panel, pushing 50A through 150 feet of 6 AWG wire will result in a voltage drop of roughly 4.5%. This exceeds the NEC recommended maximum of 3% for branch circuits and feeders, leading to dimming lights, tripping motor overloads, and inefficient appliance operation.

The Voltage Drop Rule: For any run exceeding 100 feet, run a voltage drop calculation. In the 60A subpanel example above, you must upsize to 4 AWG copper (or 2 AWG aluminum) to keep the voltage drop under 3%, even though the ampacity chart says 6 AWG is thermally sufficient.

Decision Path: Pick Your Exact Wire and Breaker

Stop guessing. Use this decision-tree-table to lock in your materials for high-load circuits. We will use a 48A continuous load (like a hardwired EV charger or large commercial mini-split) as our test case.

Step Rule / Calculation Result for 48A Load
1. Apply Continuous Load Rule NEC 210.20(A): Continuous loads (on for 3+ hours) require the circuit to be sized at 125% of the load. 48A × 1.25 = 60A minimum circuit ampacity.
2. Select the Breaker The breaker must be rated at or above the minimum circuit ampacity. Select a 60A breaker.
3. Check NM-B (Romex) Option Look at the 60°C column. You need a wire with an ampacity of at least 60A. 6 AWG is only 55A (Fails). 4 AWG is 70A (Passes).
4. Check THHN in Conduit Option Look at the 75°C column (assuming standard 75°C breaker lugs). You need at least 60A. 6 AWG is 65A (Passes).

The Final Verdict: What to Buy

If you are wiring this 48A continuous load through finished walls using standard cable, buy 4 AWG NM-B and a 60A breaker. If you are pulling individual wires through PVC or EMT conduit, buy 6 AWG THHN/THWN-2 and a 60A breaker. Do not use 6 AWG NM-B; the 60°C column limits it to 55A, which will cause the 60A breaker to nuisance trip under sustained load and violates NEC 210.20.