Electrical Wire Diameter Chart: AWG Dimensions, Ampacity, and Conduit Fill

Most makers and DIY electricians searching for an electrical wire diameter chart actually need two distinct data sets mashed together: the physical dimensions (required for conduit fill calculations and bending radius) and the ampacity ratings (required for breaker sizing). Before scrolling to the master table, here are the bookmark-friendly quick-jump values for the most common residential branch circuits (assuming copper conductors, 60°C/75°C terminations, and standard NEC overcurrent rules):

• 14 AWG: 0.0641" (1.628 mm) diameter | 15A max breaker
• 12 AWG: 0.0808" (2.053 mm) diameter | 20A max breaker
• 10 AWG: 0.1019" (2.588 mm) diameter | 30A max breaker
• 8 AWG: 0.1285" (3.264 mm) diameter | 40A max breaker

The Master Electrical Wire Diameter and Ampacity Chart

How to read this table: The physical diameter and cross-sectional area data are sourced from NEC Chapter 9, Table 8. The ampacity (current-carrying capacity) values are sourced from NEC Table 310.16 for copper conductors with up to three current-carrying conductors in a raceway, at an ambient temperature of 30°C (86°F). The temperature columns (60°C, 75°C, 90°C) represent the insulation rating of the wire and the termination limits of your connected devices. For standard residential NM-B (Romex) cable, you are generally restricted to the 60°C column. For THHN/THWN-2 wire in conduit, you use the 75°C or 90°C column depending on the equipment lugs.

*Note on small conductors: Per NEC 240.4(D), the overcurrent protection (breaker size) for 14, 12, and 10 AWG copper wire is hard-capped at 15A, 20A, and 30A respectively, regardless of the insulation's 75°C or 90°C rating, unless specific motor or welder exceptions apply.

Which Temperature Column Applies to Your Installation?

The most common mistake DIYers make with an ampacity chart is blindly using the 90°C column because they bought THHN wire. You cannot do this. The National Electrical Code enforces a "weakest link" rule under NEC 110.14(C).

To determine your column, you must look at the termination temperature rating of the equipment the wire connects to (breakers, lugs, receptacles).

• The 60°C Column: Use this for NM-B (Romex) cable, as the overall jacket limits the internal conductors to 60°C. You also use this column for any equipment rated 100A or less that does not explicitly state a 75°C rating on its label.
• The 75°C Column: This is the standard for most modern residential breakers, subpanel lugs, and heavy-duty receptacles (like a 50A range outlet). If you pull THHN wire in conduit to a 75°C rated breaker, you use this column for your final breaker sizing.
• The 90°C Column: You almost never use this column for final ampacity/breaker sizing because standard terminations aren't rated for it. The 90°C column is used exclusively as the starting baseline for derating calculations (explained below).

How Derating Factors Modify the Base Ampacity

The baseline chart above assumes you have no more than three current-carrying conductors in a raceway and an ambient temperature of 30°C (86°F). Real-world jobsites rarely cooperate. When conditions change, you must apply derating factors to the 90°C column (for THHN/THWN-2) to find your adjusted ampacity.

1. Conduit Fill Derating (NEC 310.15(C)(1))

When wires are bundled in a conduit, they cannot dissipate heat effectively. If you pull 4 to 6 current-carrying conductors through a single EMT or PVC conduit, you must multiply the base ampacity by 80%. For 7 to 9 conductors, the multiplier drops to 70%.

Worked Example: You are pulling four 8 AWG THHN current-carrying conductors through a conduit to a subpanel.
1. Look at the 90°C column for 8 AWG: 55A.
2. Apply the 80% derating factor (4 conductors): 55A × 0.80 = 44A.
3. The adjusted ampacity is 44A. Per NEC 240.4(B), you can round up to the next standard breaker size, which is 45A (if available) or safely use a 40A breaker.

2. Ambient Temperature Derating

If your conduit runs through a hot attic in the summer where temperatures hit 110°F (43°C), you must apply a temperature correction factor from NEC Table 310.15(B)(1). For 90°C wire at 41-45°C ambient, the multiplier is 0.87. You multiply this in addition to the conduit fill factor if both conditions exist.

What This Chart Cannot Tell You (And How to Calculate It)

A diameter and ampacity chart is a thermal limit reference. It tells you how much current the wire can handle before the insulation melts or the breaker trips. It does not account for power quality or physical installation limits. Here is what the table leaves out:

Voltage Drop Over Distance

Ampacity charts assume negligible wire length. In reality, wire has resistance. According to industry technical resources and NEC informational notes, you should limit voltage drop to 3% on branch circuits and 5% total from the service entrance to the furthest outlet.

For a 120V circuit, a 3% drop is 3.6V. Using NEC Chapter 9 Table 8, uncoated 12 AWG copper has a resistance of 1.93 ohms per 1,000 feet. If you run a 12 AWG circuit 100 feet to a 15A space heater (200 feet total round-trip), the voltage drop is calculated as:

Voltage Drop = Current × (Resistance per ft × Total Length)
VD = 15A × (0.00193 Ω/ft × 200 ft) = 5.79V (4.8% drop)

In this scenario, 12 AWG is thermally safe (ampacity-wise), but electrically poor. You must step up to 10 AWG to keep the voltage drop under 3%.

Physical Conduit Fill Limits

The diameter column tells you the bare wire size, but for conduit fill calculations (NEC Chapter 9, Table 1), you must use the overall diameter including insulation. For example, a bare 12 AWG wire is 0.0808", but 12 AWG THHN has an overall diameter of roughly 0.115". You cannot exceed 40% conduit fill for three or more wires. Always use the manufacturer's specific insulation dimensions for conduit math, not the bare copper diameter.

Short Circuit Withstand Rating

The chart does not tell you how the wire behaves under a massive, instantaneous short-circuit fault. Smaller wires (like 14 AWG) can vaporize if protected by an oversized breaker or if the available fault current at the panel is exceptionally high. This is why NEC 240.4(D) strictly limits small conductor breaker sizes, and why you must never install a 30A breaker on 14 AWG wire, even for a brief "test."