Ampacity of Insulated Conductors

ConductorsNEC 2023CEC Equivalent: CEC Table 2

Think of this table as the speed limit for electrical current through a wire. Just like a road has a posted maximum speed, every wire has a maximum amount of current it can safely handle before it starts overheating. The table is organized into three temperature columns (60, 75, and 90 degrees Celsius) for both copper and aluminum conductors. The values assume a comfortable 86-degree-Fahrenheit ambient temperature and no more than three current-carrying conductors bundled together. If your job conditions differ from those assumptions, you need to apply correction and adjustment factors before relying on the number you looked up. The 75-degree column is the workhorse for most commercial work because the vast majority of breakers and terminal lugs are rated for 75 degrees.

When You Need This

Sizing branch-circuit conductors for any residential or commercial installation
Confirming that existing wiring can handle an increased load, such as adding an EV charger or upgrading a panel
Selecting a feeder wire size for a new sub-panel run
Deciding between copper and aluminum for a cost-sensitive large feeder installation
Studying for a journeyman or master electrician exam — this table shows up on almost every test

Key Points

1Values assume 30 degrees C (86 degrees F) ambient — derate for hotter environments using 310.15(B)

2Values assume 3 or fewer current-carrying conductors — derate for more using 310.15(C)

3The 75-degree C column is used most often because most equipment terminals are rated 75 degrees C

4The 90-degree C column is primarily used as a starting point for derating calculations, not as a final ampacity

5For circuits 100 A or less with standard equipment, you default to the 60-degree C column unless the terminal is marked 75 degrees C

6Aluminum conductors have lower ampacity than copper for the same wire size — always check the right column

7This table only covers conductors in raceways, cables, or direct-buried — free-air ampacities are in Table 310.17

Common Mistakes

Using the 90-degree C column as the final ampacity when the equipment terminals are only rated for 75 degrees C

Forgetting to apply ambient temperature correction factors when working in attics, rooftops, or hot industrial spaces

Ignoring conduit-fill adjustment when stuffing more than 3 current-carrying conductors into a single raceway

Confusing the copper column with the aluminum column and under-sizing aluminum feeders

Applying the table values to free-air installations — Table 310.17 is the correct reference for those situations

Exam Tip

On the exam, always read carefully whether the question asks for the 60, 75, or 90 degree column. The default for circuits 100 A or less is 60 degrees C unless the question states that the equipment is listed for 75 degrees C terminations.

Frequently Asked Questions

The wire insulation might handle 90 degrees C, but the breaker or lug it connects to probably cannot. Section 110.14(C) requires you to match the ampacity column to the lowest temperature rating in the circuit — usually the terminal. The 90-degree column is mostly used as a higher starting point before applying derating factors.

Table 310.16 covers conductors installed in raceways, cables, or directly buried in earth — bundled conditions. Table 310.17 covers conductors in free air where heat can dissipate more easily, so the ampacities are higher for the same wire size.

No. The table gives raw ampacity. For continuous loads (running 3 hours or more), you still need to multiply the load by 125 percent per NEC 210.19(A) and then confirm the conductor ampacity from this table can handle that increased value.

Inline Tools

Wire Size Calculator

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Related Code Sections

310.15(B)Ambient Temperature Correction Factors 310.15(C)Conduit Fill Adjustment Factors 110.14(C)Temperature Limitations at Terminations

This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.