Cable Ampacity Calculator

What the cable ampacity calculator does

Ampacity is the current a conductor can carry continuously without overheating its insulation. The table value assumes 30°C air and no more than three current-carrying conductors together; hotter rooms and full raceways reduce it. The cable ampacity calculator starts from the table, applies both corrections, and returns the derated ampacity, plus the smallest conductor for a load if you enter one.

Ampacity derating formula

The calculator follows the NEC method:

I(table) comes from Table 310.16 for the insulation column. The temperature factor comes from Table 310.15(B)(1): at 40°C it is 0.82 for 60°C wire, 0.88 for 75°C, and 0.91 for 90°C. The adjustment factor from Table 310.15(C)(1) is 1.0 for up to three conductors, 0.8 for four to six, 0.7 for seven to nine, and 0.5 for ten to twenty.

Cable ampacity calculator example

An 8 AWG copper THHN conductor (90°C) in a 40°C space with six current-carrying conductors in the conduit:

Enter a 40 A load with the same conditions and the cable ampacity calculator confirms 8 AWG (8.37 mm²) as the smallest conductor. At the default 30°C with three conductors, 12 AWG copper at 75°C carries 25 A, though its breaker is still limited to 20 A.

How to use the cable ampacity calculator

Choose copper or aluminum and the insulation temperature rating printed on the cable, such as THHN at 90°C. Pick the wire size, then enter the hottest ambient temperature along the run and the number of current-carrying conductors in the same raceway or cable. Neutrals that carry only unbalanced current and ground wires do not count. Add the load current to have the calculator choose the size for you.

Reading wire ampacity calculator results

The derated ampacity is what the conductor can carry in your conditions. Two more rules apply before you pick a breaker. First, terminations: most breakers and lugs are rated 75°C, so the final ampacity cannot exceed the 75°C column even with 90°C wire, and circuits of 100 A or less often use the 60°C column. Second, the small conductor rule limits 14, 12, and 10 AWG copper to 15, 20, and 30 A protection. Use the 90°C value only as the starting point for derating.

When to use a current carrying capacity calculator

• Checking a conductor in a hot attic or rooftop run
• Derating when many circuits share one conduit
• Picking the wire size for a feeder or branch circuit
• Comparing copper and aluminum for the same load
• Reviewing an existing installation before adding load

Common cable ampacity calculator mistakes

• Counting neutrals and grounds. Only current-carrying conductors count toward bundling.
• Ignoring terminal ratings. The final value cannot exceed the terminal temperature column.
• Using room temperature on a roof. Use the hottest air the cable passes through.
• Skipping the 125% rule. Size continuous loads at 125%.

Tips for ampacity derating

• Split large groups of circuits into separate raceways to keep factors high.
• Use 90°C wire so derating starts from a higher value.
• Check local amendments, since some codes change the tables.

Related cable sizing tools

Ampacity is one of three checks on a conductor. The wire size calculator sizes cable for voltage drop, the breaker size calculator picks the protection, and the conduit fill calculator checks the raceway. Take the largest conductor any of them asks for.

Cable size and current rating in practice

A conductor is sized by the strictest of several limits. Ampacity keeps the insulation cool; voltage drop keeps the far end of a long run at a usable voltage; short circuit withstand keeps the cable alive until the breaker trips. On short runs ampacity usually wins, and on long runs voltage drop takes over, which is why a 20 A circuit 200 ft away often needs 10 AWG rather than 12. Continuous loads, those running three hours or more, need conductors and breakers sized at 125% of the load. Heat is the common thread: cables bundled tightly, run through hot attics, or buried in insulation cannot shed heat, so their safe current drops. That is what the derating factors model. After you pick a size here, check the drop with the wire size calculator.

Wire ampacity chart for aluminum versus copper

Aluminum conducts about 61% as well as copper, so an aluminum conductor needs to be roughly two sizes larger for the same current: 4/0 aluminum at 75°C carries 180 A, close to 3/0 copper at 200 A. Aluminum costs less and weighs much less, which is why it dominates service entrances and large feeders, but it needs terminals rated AL/CU and the correct torque. Small aluminum branch circuits are rare today. Whatever the metal, the conductor area in mm² shown by the calculator lets you compare with metric data sheets.

Wire ampacity chart

Table ampacity in amps at 30°C with three or fewer current-carrying conductors, from NEC Table 310.16:

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