NEC 310.16 Amperage Cable Chart: Copper Wire Sizes & Ampacity

For standard US residential copper branch circuits using NM-B (Romex) cable, the baseline amperage cable chart values are: 14 AWG = 15 amps, 12 AWG = 20 amps, and 10 AWG = 30 amps. These values assume a 60°C temperature rating and an ambient temperature of 30°C (86°F). If you are pulling individual THHN conductors in conduit to a 75°C rated breaker, you can often step up to the 75°C column, but termination rules strictly govern your final allowable ampacity.

The NEC 310.16 Amperage Cable Chart (Copper)

The table below is sourced directly from NFPA 70 (NEC) Table 310.16. It lists the allowable ampacities for insulated copper conductors rated up to 2000 volts. The columns represent the temperature rating of the wire's insulation (60°C, 75°C, 90°C). How to read this table: Find your American Wire Gauge (AWG) size in the left column, then read across to the temperature column that matches the lowest-rated component in your circuit (wire insulation, breaker lug, or terminal block). Always assume an ambient air temperature of 30°C (86°F) unless applying derating factors.

*Note: Per NEC 240.4(D), small conductors (14, 12, and 10 AWG copper) are strictly limited to 15A, 20A, and 30A overcurrent protection respectively, regardless of the higher ampacities shown in the 75°C and 90°C columns, unless specific exceptions apply (like motor circuits).

Which Column Applies to Your Installation?

The most common mistake DIYers and junior apprentices make is blindly using the 90°C column because THHN wire is printed with a 90°C rating. Under NEC 110.14(C) termination rules, the allowable ampacity of a circuit is dictated by the lowest temperature rating of any connected component, termination, or conductor.

• The 60°C Column: Use this for standard non-metallic sheathed cable (NM-B / Romex). Even though the internal conductors might be rated 90°C, the outer PVC jacket limits the assembly to 60°C per NEC 334.80. Also use this column for older homes with breakers manufactured before 1995, which are typically only rated for 60°C terminations.
• The 75°C Column: Use this when pulling individual THHN/THWN-2 conductors in conduit to modern residential and commercial breakers. Almost all standard Square D, Eaton, and Siemens breakers rated 100A or less are factory-listed for 75°C terminations.
• The 90°C Column: You almost never use this column to determine your final breaker size. Its primary legal use is as the starting point for derating calculations before applying the termination temperature cap.

How Derating Rows Modify the Base Value

The amperage cable chart assumes two ideal conditions: an ambient air temperature of exactly 30°C (86°F), and no more than three current-carrying conductors bundled together. When real-world jobsite conditions deviate from this, you must apply derating factors.

Scenario: Bundled Conductors in Conduit
Imagine you are pulling four current-carrying 12 AWG THHN wires through a single EMT conduit to feed a multi-wire branch circuit. NEC Table 310.15(C)(1) requires an 80% derating factor for 4-6 conductors.

1. Start with the 90°C column for 12 AWG THHN: 30A.
2. Apply the 80% bundling derating: 30A × 0.80 = 24A.
3. Check termination limits: Your breaker is rated 75°C. The 75°C column for 12 AWG is 25A.
4. Final allowable ampacity is the lower of the derated value (24A) and the termination limit (25A). Therefore, your wire is good for 24A, which safely allows you to use a standard 20A breaker.

If you are working in an attic where ambient temperatures reach 50°C (122°F), you must also apply the ambient temperature correction factor (0.82 for 90°C wire) before or in addition to the bundling factor, which can rapidly shrink your wire's capacity.

What the Amperage Cable Chart Cannot Tell You

The amperage cable chart is purely a thermal limit table. It tells you the maximum current a wire can carry before its insulation begins to degrade or melt. It completely ignores voltage drop.

If you run a 12 AWG copper wire on a 20A breaker to a workshop outlet 150 feet away, the NEC 310.16 chart says this is perfectly legal and safe from a fire perspective. However, using the resistance values from NEC Chapter 9, Table 8 (1.98 ohms per 1000 feet for 12 AWG uncoated copper), a 16A continuous load will result in a voltage drop of roughly 9.5 volts. That is nearly an 8% drop on a 120V circuit, which will cause motors to overheat, lights to dim, and electronics to brown out.

The Rule of Thumb: The amperage chart dictates your minimum wire size for safety. Voltage drop calculations dictate your practical wire size for performance. For runs exceeding 100 feet, always calculate voltage drop and expect to upsize your wire by at least one or two AWG sizes.

Amperage Cable Chart FAQ

What size wire do I need for a 50-amp breaker?

For a standard 50-amp breaker (like an electric range or EV charger), you need 6 AWG copper wire if your terminations are rated 75°C, which is standard for modern breakers and receptacles. If you are using aluminum wire (like SER cable for a subpanel), you must upsize to 4 AWG aluminum, as aluminum has a higher electrical resistance and lower ampacity per AWG size.

Can I use the 90°C column for standard residential breakers?

No, not for determining your final overcurrent protection size. While the wire itself can physically handle the heat at 90°C, the mechanical lugs inside standard residential breakers (100A and below) are only tested and listed for 75°C. If you push 90°C-rated current into a 75°C lug, the lug will overheat, potentially causing a thermal failure or fire at the panel bus bar. You only use the 90°C column as the baseline math for derating adjustments.

How does aluminum wire change the amperage cable chart values?

Aluminum conducts electricity less efficiently than copper, so you must move down the chart to find an equivalent ampacity. As a general rule, aluminum wire requires a cross-sectional area roughly one to two AWG sizes larger than copper for the same current. For example, to carry 100 amps at 75°C, you need 3 AWG copper, but you must use 1 AWG aluminum. Always ensure your panel lugs are rated for aluminum (marked ALR or CU/AL) and apply an anti-oxidant compound like Noalox to prevent galvanic corrosion.

Does the amperage cable chart account for voltage drop over long distances?

No. The chart only addresses thermal heating limits. The NEC recommends (but does not strictly mandate for all branch circuits) a maximum voltage drop of 3% for branch circuits and 5% for the combined feeder and branch circuit. If your run is longer than 75-100 feet, you must perform a separate voltage drop calculation using the formula: VD = (2 × L × I × R) / 1000, where L is one-way length in feet, I is current in amps, and R is the resistance per 1000 feet from NEC Chapter 9, Table 8.