NEC Ampacity Tables (310.16): How to Read Them

Table 310.16 is the most referenced table in the National Electrical Code, and for good reason: it sets the baseline ampacity for nearly every copper and aluminum conductor installed in a typical building. Reading it correctly takes about five minutes to learn, but misreading it can mean oversized breakers, overheated wire, or a failed inspection. Here is how the table is structured and how to use it properly.

What Table 310.16 Actually Shows

The full NEC title is "Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60°C Through 90°C, Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth." That title contains most of what you need to know.

The table applies to conductors installed in conduit, cable assemblies, or direct burial with no more than three current-carrying conductors. The moment you add a fourth or more current-carrying conductors, you leave 310.16 territory and must apply adjustment factors from 310.15(C)(1) before you can use those numbers.

The table is split into two major sections: copper conductors on the left and aluminum or copper-clad aluminum on the right. Within each section, three temperature columns appear: 60°C, 75°C, and 90°C. These refer to the conductor's insulation temperature rating, not ambient air temperature.

A simplified excerpt for copper conductors looks like this:

Note that a 12 AWG copper conductor jumps from 20 A at 60°C to 30 A at 90°C. That difference matters, but only under specific conditions.

The Termination Temperature Rule (NEC 110.14(C))

Here is where many people go wrong. The 90°C column gives the highest number, so the temptation is to use it by default. Most of the time you cannot.

NEC Section 110.14(C) requires that the ampacity of a conductor be limited by the temperature rating of the termination point, meaning the lugs, breakers, connectors, and terminals the wire lands on. The rule breaks down like this:

• Equipment rated for 60°C only: use the 60°C column.
• Equipment rated for 75°C: use the 75°C column (most common for residential and commercial panels).
• Equipment rated for both 75°C and 90°C: still limited to the 75°C column unless the conductors, terminations, and equipment are all listed for 90°C operation.

In practice, the large majority of residential and light commercial breaker panels, disconnects, and outlet boxes have 75°C-rated terminals. So most installers size wire to the 75°C column of Table 310.16. The 90°C column still has a role: it is used as the starting point when applying correction or adjustment factors, then the result is compared against the 75°C value to confirm it does not exceed that limit.

For a deeper look at how ampacity is defined and what affects it, that article covers the physics behind these ratings.

Correction Factors for Ambient Temperature

The ampacity values in 310.16 assume an ambient air temperature of 30°C (86°F). Install conductors in an attic that regularly hits 50°C in summer, and those values no longer apply without adjustment.

NEC Table 310.15(B)(1) provides correction factors. At 50°C ambient, the correction factor for 90°C-rated insulation is 0.82. For 75°C insulation it drops to 0.71. Apply the factor by multiplying the table ampacity by the correction factor.

Example: a 10 AWG copper conductor with THHN insulation (90°C rated) in a 50°C attic space.

1. Start with the 90°C column: 40 A.
2. Apply the ambient correction: 40 × 0.82 = 32.8 A.
3. Compare against the 75°C column (which is your termination limit): 35 A.
4. 32.8 A < 35 A, so the corrected value governs. Round down to the nearest standard breaker size: 30 A.

If the same wire were in a 30°C space, you would use the 75°C column directly: 35 A, protected by a 35 A breaker (if standard sizes allow) or the next size down.

Adjustment Factors for Bundled Conductors

The three-conductor limit in the table title is the other common trip point. Run six current-carrying conductors through a single conduit and the heat they generate compounds. NEC 310.15(C)(1) requires an adjustment factor to account for this.

For four to six current-carrying conductors: multiply by 0.80. For seven to nine conductors: multiply by 0.70. For ten to twenty conductors: multiply by 0.50.

These factors apply to the 90°C column value before comparing against the termination-limited column. Grounding conductors and neutral conductors in balanced three-phase circuits typically do not count as current-carrying conductors for this calculation, but a neutral that carries harmonic current from non-linear loads does count.

The wire derating guide for temperature and conduit fill walks through the combined-factor math in more detail.

A Worked Lookup from Start to Finish

Say you need to size wire for a 40 A, 240 V residential clothes dryer circuit. The panel has 75°C-rated terminals. Ambient temperature in the utility room is a normal 25°C. One circuit, no bundling issues.

1. Start at the 75°C column (termination limit governs, ambient is below 30°C so no derating needed).
2. You need at least 40 A. Looking at the copper side of 310.16, 8 AWG at 75°C gives 50 A. That exceeds 40 A, so 8 AWG copper works.
3. Confirm with the AWG wire size chart that 8 AWG is within the physical range expected for a dryer circuit.
4. Breaker size: the 40 A load is 100% of the 40 A overcurrent device, acceptable for continuous and non-continuous loads here.

Result: 8 AWG copper, 40 A two-pole breaker, 75°C terminations. Clean and straightforward.

If you want to see this kind of lookup built into a repeatable process, the full cable sizing procedure covers load calculation, voltage drop, and breaker selection together.

One note before you proceed with any real installation: always verify ampacity selections against the current edition of the NEC adopted in your jurisdiction, and have a licensed electrician review the design. Code editions vary by state and municipality, and field conditions introduce variables that a table lookup alone cannot capture.

Frequently asked questions

Can I use the 90°C column if my wire is THHN?

THHN is rated for 90°C, so you can use the 90°C column as a starting point for correction and adjustment calculations. The final ampacity you apply, however, cannot exceed the value in the 75°C column if your terminals are 75°C-rated, which they usually are. The 90°C rating lets you start with a higher number before derating, which can allow you to use a smaller conductor in high-temperature or high-fill conduit runs.

Does the neutral conductor count toward the three-conductor limit?

In a standard single-phase 120/240 V circuit, the neutral carries only the unbalanced current and does not count as a current-carrying conductor. In a three-phase, four-wire circuit where the neutral carries significant third-harmonic current (common with switching power supplies and variable-frequency drives), the neutral does count. If you are unsure, count it and apply the adjustment factor conservatively.

Is Table 310.16 the same in every state?

The table itself is published by NFPA and does not change between printings of the same NEC edition, but states and municipalities adopt different NEC editions on different schedules. California, for example, adopted the 2022 NEC after some states were still enforcing the 2017 edition. Always check which edition your authority having jurisdiction (AHJ) has adopted before pulling a permit.

What is the difference between ampacity for copper versus aluminum at the same AWG?

Aluminum conductors have higher resistance per unit length than copper, so their ampacity ratings are lower for the same wire gauge. A 2 AWG aluminum conductor at 75°C is rated for 90 A; a 2 AWG copper conductor at 75°C is rated for 115 A. For a full comparison of the trade-offs, see the copper vs aluminum wire overview.