How to read NEC Table 310.16, the wire ampacity chart

Table 310.16 gives the allowable ampacity of insulated copper and aluminum conductors in a raceway, cable or earth. It is the starting point for almost every wire size, and the number printed in it is rarely the number you get to use. This guide reads the table the way the Code applies it, and every figure here is pulled from the same data the wire size calculator uses. The values are the same in NEC 2023 and 2026.

What Table 310.16 assumes

The table is written for no more than 3 current-carrying conductors and a 30 °C ambient. Rows are conductor sizes. Columns are the temperature rating of the conductor insulation (60, 75 and 90 °C), with a copper side and an aluminum side. Conditions that differ from the table basis change the number, covered below. This is an excerpt, not the whole table; see the ampacity chart for the full 14 AWG to 500 kcmil listing.

Aluminum has no 14 AWG row in this table: the 14 AWG listing applies to copper-clad aluminum only. Aluminum also carries less current per size, so a given load usually needs a larger size than copper. Read across the row for your size, pick the column that matches the limiting temperature, and treat that number as the starting point rather than the answer.

Which column to use: termination temperature decides

Insulation rating and termination rating are two separate limits. The insulation column tells you how hot the conductor itself may run, so a 90 °C conductor such as THHN can start from the 90 °C column when you apply derating. The terminals on the breaker, panel or device set a second limit under 110.14(C), and the ampacity you use can never exceed the lesser of the two.

In practice, terminations of circuits rated 100 A or less, and terminations for 14 through 1 AWG conductors, are treated as 60 °C unless the equipment is listed and marked for 75 °C. Larger circuits use the 75 °C column. That is why #12 THHN on a standard breaker is limited by its 20 A 60 °C figure even though the 90 °C column shows 30 A. The 90 °C column is useful mainly as the starting number when ambient temperature or conductor count forces a derate.

Small conductors: the 240.4(D) overcurrent cap

Table 310.16 ampacity is not the same as the breaker you may use. Under 240.4(D), the smallest conductors have a hard cap on overcurrent protection: 15 A for 14 AWG copper, 20 A for 12 AWG copper and 30 A for 10 AWG copper (aluminum and copper-clad aluminum are capped lower on the smaller sizes). The cap applies regardless of which column you used, unless another section for the specific equipment applies (motors, air conditioning and refrigeration equipment, and taps each have their own rules), so #10 copper with a 40 A 90 °C figure still stops at a 30 A breaker.

Correcting for ambient temperature and conductor count

Two corrections apply on top of the table. Ambient temperature uses Table 310.15(B)(1)(1), based on 30 °C. At 36-40 °C, for example, the factors are x0.82 for the 60 °C column, x0.88 for 75 °C and x0.91 for 90 °C. Hotter ambients drop factors quickly, and some columns have no listed factor at all. A reading between whole degrees is rounded up to the next row.

More than 3 current-carrying conductors need the adjustment in Table 310.15(C)(1), applied as a percentage of the ampacity:

Corrected ampacity = table ampacity x ambient factor x adjustment percent
Usable ampacity = lesser of (corrected ampacity, termination column ampacity)

Continuous loads add 125%

A continuous load runs for 3 hours or more. For these, the conductor must carry 125% of the load (210.19(A)(1) for branch circuits, 215.2(A)(1) in NEC 2023 and 215.4(A)(1) in NEC 2026 for feeders), and the breaker is sized from the same 125% figure under 210.20(A). The 125% check uses the ampacity before ambient and conductor-count factors; the ordinary check against the load itself then uses the corrected ampacity. A wire must pass both. The breaker comes from Table 240.6(A) standard ratings. The calculator does not apply the 240.4(B) allowance to round up to the next standard size, so its breaker is the smallest standard rating at or above the design load.

Two worked examples

Both examples use copper THHN (90 °C insulation) on 60 °C terminals. The results below come from the calculator's own functions.

Example 1: a 40 A continuous load at 30 °C with 3 conductors.

Required = 40 A x 125% = 50 A, checked in the 60 °C column
Smallest copper size = #6 AWG (60 °C column 55 A)
Breaker = 50 A

Example 2: a 30 A noncontinuous load in a 40 °C attic with 6 current-carrying conductors in one raceway.

Table value for #8 AWG, 90 °C column = 55 A
After ambient correction = 50.05 A
After conductor adjustment = 40.04 A
Corrected value is above the 60 °C termination column, so the usable ampacity = 40 A
Smallest copper size = #8 AWG, breaker = 30 A

Here the derating, not the load, picked the wire: with 30 °C and 3 conductors the same load needs only #10 AWG. For quick answers on common circuits see the pre-worked pages for a 20-amp circuit, a 30-amp circuit or a 50-amp circuit. Voltage drop is separate and is not part of ampacity; long runs may need a larger wire.

The order matters. Pick the column, apply the ambient and conductor-count factors, compare against the termination column, then check the 240.4(D) cap and the standard breaker rating. A result that skips one of these steps looks fine on paper and fails inspection, so run the whole sequence every time.