Conductor ampacity

Updated November 8, 2026 · Written for the 2023 NEC

What ampacity is and why it matters on the exam

Ampacity is a heat limit. Current through a conductor's resistance makes heat, and insulation breaks down if it gets too hot. Article 100 defines ampacity as the current a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

"Conditions of use" is why derating exists. A conductor in a hot room or a crowded raceway sheds heat poorly, so it must carry less current. Exams test this heavily because one question combines a table lookup, two multiplications, and a termination check. For more calculation practice, see the exam calculations guide.

How Table 310.16 is organized

Table 310.16 gives allowable ampacities for not more than three current-carrying conductors in a raceway, cable, or directly buried in earth, at an ambient temperature of 30°C (86°F). If your installation matches those conditions, the table value needs no correction or adjustment.

The table has a copper half and an aluminum or copper-clad aluminum half. Each half has three temperature columns: 60°C, 75°C, and 90°C. The insulation type tells you which column the conductor is rated for:

• TW and UF are 60°C insulations.
• THW and THWN are 75°C insulations, and so is XHHW when it is in a wet location.
• THHN, THWN-2, and XHHW-2 are 90°C insulations (THHN in dry and damp locations only).

Insulation ratings come from Table 310.4(1). Single conductors in free air use Table 310.17, which gives higher values because air cools them better.

The key rules (with NEC sections)

• Ambient temperature correction, 310.15(B). When the ambient is not 30°C, multiply by the correction factor from Table 310.15(B)(1)(1), taken from the same temperature column you started in. Ambients below 30°C give factors above 1.
• Rooftop adder, 310.15(B)(2). Raceways or cables in direct sunlight less than 3/4 in above a roof get 33°C (60°F) added to the outdoor ambient. Type XHHW-2 is exempt.
• More than three current-carrying conductors, 310.15(C)(1). Multiply by the adjustment factor from Table 310.15(C)(1). This also applies to cables bundled without spacing for more than 24 in. Raceway nipples 24 in or shorter are exempt.
• What counts, 310.15(E) and (F). Equipment grounding and bonding conductors never count. A neutral carrying only the unbalanced current of a single-phase 3-wire circuit does not count. A neutral counts when it serves two phases of a 4-wire wye system, or when most of a 4-wire wye load is nonlinear.
• Termination limits, 110.14(C)(1). For circuits of 100 A or less, or conductors 14 AWG through 1 AWG, treat terminations as 60°C unless the equipment is listed and marked for 75°C. For circuits over 100 A or conductors larger than 1 AWG, use 75°C.
• Using the 90°C column, 110.14(C). You may derate from a conductor's higher 90°C rating, but the final ampacity cannot exceed the table value for the termination temperature.
• Cable limits. NM cable has 90°C conductors, but its final ampacity cannot exceed the 60°C value (334.80). UF cable is also limited to 60°C (340.80).
• Small conductors, 240.4(D). 14, 12, and 10 AWG copper are generally limited to 15 A, 20 A, and 30 A overcurrent protection, whatever the derated number.

How to calculate derated ampacity

Work the steps in the same order every time:

1. Find the table ampacity in the column that matches the insulation rating.
2. Multiply by the ambient temperature correction factor.
3. Multiply by the adjustment factor for the number of current-carrying conductors.
4. Look up the ampacity in the termination temperature column.
5. Use the lower of the results from step 3 and step 4.

If the ambient is given in Fahrenheit, convert it first so you land in the right row.

Worked examples

Example 1: correction and adjustment both apply

Six current-carrying 3 AWG THHN copper conductors are in one EMT run through a mechanical room where the ambient is 104°F. The equipment terminations are rated 75°C. What is the usable ampacity?

• Convert the ambient: (104 − 32) × 5 ÷ 9 = 40°C.
• THHN is 90°C, so start in the 90°C column: Table 310.16 gives 115 A for 3 AWG copper.
• The 90°C correction factor for 40°C is 0.91.
• Six current-carrying conductors take an 80% adjustment factor.
• Derated ampacity: 115 × 0.91 × 0.80 = 83.7 A.
• The 75°C column gives 100 A for 3 AWG copper. That is the termination limit.
• The lower value governs, so the usable ampacity is 83.7 A.

This conductor could serve an 80 A noncontinuous load. Because 83.7 A is not a standard overcurrent device size, 240.4(B) may allow the next standard size up, 90 A, if all of its conditions are met.

Example 2: the termination limit governs

Three 1/0 THHN copper conductors are in a raceway where the ambient is 35°C. Terminations are 75°C.

• Start at the 90°C value: 170 A.
• The 90°C correction factor for 35°C is 0.96.
• Only three current-carrying conductors, so no adjustment applies.
• Corrected ampacity: 170 × 0.96 = 163.2 A.
• The 75°C value for 1/0 copper is 150 A.
• The usable ampacity is 150 A, because the lugs, not the wire, are the weak point.

The pattern: light derating usually leaves the termination limit in control, and heavy derating usually leaves the derated number in control.

Common exam mistakes

• Answering from the 90°C column. It is a starting point for derating. Check the termination column before you answer.
• Mixing columns. If you start at 90°C, use the 90°C correction factor. THWN uses the 75°C column and the 75°C factor.
• Counting the equipment grounding conductor. It is never current-carrying for adjustment.
• Skipping the Fahrenheit conversion. Picking a row by the Fahrenheit number gives the wrong factor.
• Ignoring 240.4(D). A 12 AWG copper conductor derated to 21 A still gets a 20 A breaker.
• Treating NM cable like THHN. Its final ampacity is the 60°C value.

Practice questions

Frequently asked questions