NEC Ampacity Chart: Wire Sizes, Breaker Limits, and Temperature Columns Explained

The ampacity chart (officially NEC Table 310.16) defines the maximum continuous current a wire can carry before its insulation degrades or melts. For standard home branch circuits, you must use the 60°C column for 14, 12, and 10 AWG copper wire, and the 75°C column for 8 AWG and larger, assuming standard residential terminations. The 90°C column is strictly for derating calculations, not for final breaker sizing.

How to Read the NEC Ampacity Chart

Before pulling wire, you need to understand how the National Electrical Code (NEC) structures ampacity limits. The chart is divided into three temperature columns: 60°C (140°F), 75°C (167°F), and 90°C (194°F). These columns correspond to the thermal rating of the wire's insulation and the equipment it connects to.

Which column applies to your installation? This is the most common point of failure for DIYers and junior electricians. According to NEC 110.14(C), the termination limits of your breakers, lugs, and receptacles dictate the column you must use, regardless of how thick the wire's insulation is.

• For circuits rated 100A or less (or 14 AWG through 1 AWG): You must use the 60°C column to determine your maximum ampacity, even if you are pulling 90°C-rated THHN wire in conduit. This is because standard residential receptacles and older breakers are typically only rated for 60°C terminations.
• For circuits rated over 100A (or wire larger than 1 AWG): You may use the 75°C column, as heavy-duty panels, subpanels, and large disconnects are universally rated for 75°C terminations.
• The 90°C column: You can never use this column to size your final breaker. It is only used as a mathematical starting point when calculating derating factors for bundled wires or high ambient temperatures.

The Complete Copper Wire Ampacity Chart (NEC Table 310.16)

Below is the complete reference table for copper conductors, sourced directly from the 2023/2026 NFPA 70 (NEC) Table 310.16. The 'Max Standard Breaker' column applies the NEC 240.4(B) 'next-size-up' rule, while strictly enforcing the hard limits of NEC 240.4(D) for small conductors.

What the Ampacity Chart Cannot Tell You (Derating & Limits)

Looking up the base number in the chart above is only step one. The ampacity chart assumes ideal conditions: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors bundled together. When real-world jobsite conditions deviate, you must modify the base value.

How Derating Rows Modify the Base Value

When you pull more than three current-carrying conductors through a single raceway or conduit, the wires heat each other up. NEC Table 310.15(C)(1) requires you to apply a correction factor to the 90°C column base value, then compare that result to your termination column limit.

Worked Example: You are pulling four 12 AWG THHN current-carrying conductors in a single conduit for a 20A circuit.
1. The 90°C base ampacity for 12 AWG is 30A.
2. The derating factor for 4-6 conductors is 80%.
3. 30A × 0.80 = 24A.
4. Because 24A is still greater than the 60°C termination limit of 20A, you can safely use a 20A breaker. However, if you bundled 10 conductors (50% derating), 30A × 0.50 = 15A. You would be forced to upsize to 10 AWG wire to maintain the 20A circuit safely.

Voltage Drop: The Invisible Limitation

The ampacity chart tells you how to prevent a fire; it tells you absolutely nothing about voltage drop. If you run a 12 AWG wire on a 20A breaker for 150 feet to a remote garage outlet, the wire will not melt, but the voltage at the receptacle will sag below 114V under load, potentially damaging power tools or electronics. For runs exceeding 50 feet, always cross-reference your wire size with a voltage drop calculator to ensure you stay within the recommended 3% branch circuit limit. Upsizing the wire gauge is the only fix for long-distance voltage drop.

Frequently Asked Questions

What Size Wire Do I Need for a 50 Amp Breaker?

For a 50-amp breaker, you need 6 AWG copper wire if your terminations are rated for 75°C (which most modern 50A breakers and EV charger lugs are). If you are using NM-B (Romex) cable, or if the equipment specifically states 60°C terminations, you must use the 60°C column, which limits 6 AWG to 55A. Since 55A allows a 'next-size-up' to 60A, 6 AWG NM-B is still legally sufficient for a 50A breaker, but 4 AWG is often preferred by inspectors for heavy continuous loads like EV chargers.

Can I Use the 90°C Column to Size My Breaker?

No. A common and dangerous mistake is looking at the 90°C column for 10 AWG THHN (40A) and installing a 40A breaker. NEC 110.14(C) strictly limits circuits rated 100A or less to the 60°C termination column. Therefore, 10 AWG is hard-limited to 30A, regardless of the fact that the THHN insulation itself can physically withstand 40A without melting. The 90°C column is exclusively a mathematical tool for derating calculations.

Does the Ground Wire Count Towards Ampacity Derating?

No. When calculating how many current-carrying conductors are in a conduit for derating purposes, equipment grounding conductors (bare copper or green) are excluded per NEC 310.15(C)(1). They do not carry continuous load current under normal operation. However, if you are running a multi-wire branch circuit (MWBC) with a shared neutral, the neutral does count as a current-carrying conductor if it carries unbalanced load from non-linear loads, though in standard residential split-phase MWBCs, the neutral is often excluded from the derating count.

Why Is My 8 AWG Wire Limited to 40 Amps When the Chart Says 50?

This is the classic 110.14(C) trap. If you look at the 75°C column, 8 AWG is rated for 50A. However, if you are using 8 AWG NM-B cable (Romex), NEC 334.80 forces you to use the 60°C column. In the 60°C column, 8 AWG is rated for exactly 40A. Therefore, an 8/2 NM-B cable feeding a hot tub or heavy appliance must be protected by a 40A breaker, not a 50A breaker. To get a full 50A out of 8 AWG, you must pull individual THHN wires in conduit and land them on 75°C-rated lugs.