Wire Ampacity Chart

Wire ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. This chart lists the allowable ampacities for copper and aluminum building wire in the familiar 60 °C, 75 °C and 90 °C columns, with the correction and adjustment factors that reduce ampacity in hot locations and crowded conduits — in a printable and spreadsheet format.

Copper wire ampacity chart

Values are for not more than three current-carrying conductors in a raceway, cable or earth (directly buried), based on an ambient temperature of 30 °C (86 °F), as tabulated in NEC Table 310.16. Always confirm with the edition of the code adopted in your area.

Aluminum wire ampacity chart

Aluminum has higher resistance than copper, so a larger aluminum conductor is needed for the same current — roughly two AWG sizes larger in the common range. Aluminum terminations must be rated for aluminum (marked AL or AL/CU).

Which temperature column to use

The ampacity you may use depends on the lowest temperature rating of anything in the circuit — the wire insulation, and the terminals of the breaker and equipment it connects to. Many breakers and devices are rated 60 °C or 75 °C at their terminals. As a general practice, circuits rated 100 A or less (or with conductors 14–1 AWG) are commonly limited to the 60 °C column unless the equipment is listed for 75 °C, and larger circuits to the 75 °C column. The 90 °C column is mainly used as the starting point for correction and adjustment calculations with 90 °C-rated wire such as THHN.

Small conductor breaker limits

The NEC limits breaker sizes for small conductors regardless of the higher table values, with specific exceptions for motors, air conditioning and other equipment. This is why 12 AWG copper is used on 20 A circuits even though its 90 °C ampacity is 30 A.

Ambient temperature correction

Multiply the table ampacity by the factor for the ambient temperature where the wire runs — attics and rooftops can be much hotter than the room below.

Adjustment for more than three conductors

Bundled conductors cannot shed heat as easily. Neutrals that carry only unbalanced current and equipment grounding conductors are generally not counted.

Worked example

A circuit uses 10 AWG THHN copper (90 °C) in a conduit that runs through an attic at up to 50 °C, with six current-carrying conductors in the conduit. Start with the 90 °C ampacity, 40 A. Apply the temperature factor 0.82 and the bundling factor 0.80: 40 × 0.82 × 0.80 = 26.2 A. The result must also not exceed the ampacity allowed by the terminal rating (35 A at 75 °C or 30 A at 60 °C), and the small-conductor rule limits the breaker to 30 A. So the conductor’s corrected ampacity is about 26 A, and a 25 A breaker (next standard size down) would be appropriate in this situation — subject to the code and an electrician’s judgement.

What is in the template pack

• Printable ampacity chart — copper and aluminum on one page for the panel or van.
• Correction and adjustment factor sheet.
• Ampacity calculator workbook (Excel) — select a table value, enter temperature and bundling factors and get the corrected ampacity.
• Voltage drop worksheet — estimate drop for long runs.
• Circuit schedule template for panel documentation.

Safety and code compliance

This chart is a reference for planning and education. Electrical work must follow the code adopted in your jurisdiction, the listings and instructions of the equipment, and often requires a permit and a licensed electrician. Conditions such as conductors in sunlight on rooftops, continuous loads (sized at 125%), motor circuits and voltage drop on long runs require additional calculations.

Formats

Charts are print-ready PDFs; the calculator and voltage drop worksheet are Excel workbooks with formulas; the circuit schedule is editable in Word and Excel.