Understanding Ampacity Charts: Choosing the Right Wire for the Job

Understanding ampacity is one of the most important skills for anyone working with electrical systems. 

Whether you are a contractor pulling wire on a job site, an electrician sizing a service panel, or an HVAC technician wiring up a new unit, you need to know how much current a wire can safely carry. 

Take a look at the ampacity charts below for reference.

One of the most common questions in the field is whether to use copper or aluminum wire. Both have their place in electrical work. Here is a comparison table showing how they stack up:

When you need to substitute aluminum for copper, you generally need to go up two AWG sizes. Here are some common equivalents based on similar ampacity ratings:

Note: Always verify exact ampacity requirements for your specific application and installation conditions.

The standard ampacity values assume an ambient temperature of 86°F (30°C). If you are working in hotter conditions, you need to reduce the ampacity. If the area is cooler, you can sometimes increase it. Use this correction factor chart to adjust your calculations:

Source: NEC Table 310.16

How to use this chart: Multiply the base ampacity by the correction factor. For example, if you have 10 AWG THHN copper wire (90°C rating, 40 amp base) and the ambient temperature is 40°C (104°F), multiply 40 amps by 0.91 to get 36.4 amps. Round down for safety.

When you run more than three current-carrying conductors in a single raceway or cable, heat builds up faster. The wires cannot cool off as easily when they are packed together. The NEC requires you to derate the ampacity based on how many conductors are in the raceway:

Source: NEC 310.15(B)(3)(a)

Note that this derating does not apply to nipples (short conduit runs) that are 24 inches or less in length. Also, neutral conductors that carry only unbalanced current and equipment grounding conductors do not count toward the total.

Different wire types have different insulation materials and temperature ratings. Knowing your wire type helps you pick the right column on the ampacity chart:

Source: NEC Table 310.13

For dwelling units, the NEC allows slightly different ampacity values for service entrance conductors, service laterals, and main feeders. This table shows the minimum conductor sizes for common residential service ratings:

Source: NEC 310.15(B)(7), Based on 75°C rated conductors.

Even though a wire might have a higher ampacity based on its insulation rating, the NEC limits the overcurrent protection (breaker or fuse size) for smaller conductors. These limits apply unless the code specifically permits otherwise:

Source: NEC 240.4(D)

This means that even though 12 AWG THHN copper wire has an ampacity of 30 amps at 90°C, you can only protect it with a 20-amp breaker in most applications. The higher temperature rating is useful when you need to apply correction factors for heat or conductor fill.

Here are some real-world tips to help you use these charts correctly on the job:

• Always calculate the total load first. Add up the wattages of all devices on the circuit, then divide by the voltage to get the required current in amps.
• Check your terminal ratings. Most equipment terminals are rated for 60°C or 75°C wire. Even if your wire is rated for 90°C, you might need to use the lower column for your ampacity calculation.
• Consider voltage drop. On long runs, you might need to upsize the wire to prevent voltage drop, even if a smaller wire would handle the ampacity.
• Use aluminum connections correctly. If you are using aluminum wire, make sure all terminals are rated for aluminum (marked AL or AL/CU). Apply an antioxidant compound to prevent corrosion at the connections.
• Stack your derating factors. If you have both high ambient temperature and multiple conductors in a raceway, apply both correction factors to get the final ampacity.
• Check local codes. Your local jurisdiction might have amendments to the NEC that affect ampacity requirements. Always verify with your local building inspector or authority having jurisdiction.

Ampacity charts are essential tools for anyone working with electrical systems. They help you choose the right wire size to keep installations safe and code-compliant. Remember that the base ampacity values assume standard conditions. Actual installations often require you to apply correction factors for temperature and conductor fill.

Whether you are pulling wire for a new service upgrade, running conduit for an industrial installation, or just wiring up a new outlet, these charts will help you make the right choice. When in doubt, go up a wire size. The cost of slightly larger wire is nothing compared to the cost of a failed installation or, worse, a fire.

Always consult the current edition of the National Electrical Code and your local building codes for the most up-to-date requirements. The values in these charts are based on the NEC and are provided as a general reference.