Put in the load and the conditions and read back the smallest conductor Table 310.16 allows, with the correction, the adjustment and the termination limit applied in order. Table values are the 2026 edition as printed, and the device and the ground come with it.
The calculated load on the circuit, before the continuous multiplier.
The arithmetic is the same either way. It changes which section the steps cite.
Ticked, the load is taken at 125 percent for both the conductor and the device.
The column the correction and adjustment factors run on. THHN and THWN-2 are 90 degree wire.
The rating of the lugs at both ends. The default row puts a circuit of 100 amperes or less on the 60 degree column and everything above that on 75.
30 degrees C is the base the table is printed at, so 30 gives a factor of 1.00.
In the one raceway or cable, run together for more than 24 inches. Three or fewer needs no adjustment.
This page holds the table values so you get a size while you are still standing at the panel. The ampacity tools on the calculators page make you type each value in from your own book, because reading the table is the thing the exam is testing.
Use this one on the job. Use that one the week before the test, and read the ampacity derating study page for the order the factors go in and why the termination column is the last word.
Sizing a conductor is two ampacity tests, and the smaller answer wins. The first is 210.19(A)(1) on a branch circuit, or 215.4(A)(1) on a feeder: the conductor carries the noncontinuous load plus 125 percent of the continuous load.
The second is item (2) under 210.19(A), or 215.4(A)(2) for a feeder. After the ambient correction and the conductor count adjustment in 310.15, the conductor still carries the load it actually serves. Section 310.14(A)(2) settles the tie: where more than one ampacity applies, the lowest one is the one you use.
Then 110.14(C), which is where most wrong answers come from. Correction and adjustment may run on the 90 degree column when the insulation is rated 90 degrees, but the finished ampacity may not exceed the ampacity of that same size in the column matching the terminations.
The 100 ampere rule sits in 110.14(C)(1)(a). Terminations on circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG, are used with 60 degree conductors unless the equipment is listed and identified for a higher rating. Above 100 amperes, 110.14(C)(1)(b) puts you on the 75 degree column. Most modern breakers and lugs are marked 75 degrees, and that marking is what lets you use the 75 degree column on a small circuit.
Next comes 240.4(D), the small conductor rule. It caps the overcurrent device at 15 amperes on 14 AWG copper, 20 on 12 AWG copper, 30 on 10 AWG copper, 15 on 12 AWG aluminum and 25 on 10 AWG aluminum, and it applies after the correction factors have been worked. The cap is on the device, not on the conductor, which is why a 20 ampere continuous load cannot sit on 12 AWG copper: the device would have to be 25 amperes and the cap is 20.
The device rating comes out of Table 240.6(A). Under 210.20(A), or 215.5 on a feeder, it is not smaller than the noncontinuous load plus 125 percent of the continuous load. Section 240.4(B) then lets you take the next standard rating above the conductor ampacity when that ampacity does not land on a standard rating, so long as the next rating is 800 amperes or less and the circuit is not a branch circuit feeding more than one receptacle for cord and plug connected portable loads.
The equipment grounding conductor is read from Table 250.122 on the rating of the device ahead of the equipment. Not on the load, and not on the phase conductor. Section 250.122(A) adds one relief: it never has to be larger than the circuit conductors themselves.
Order of operations, and the exam leans on it. Take the load to 125 percent, correct for ambient, adjust for conductor count, hold the result to the termination column, then apply the 240.4(D) cap and pick the device. Running the load against the 90 degree column and stopping there is the trap, and it gives an answer one size too small often enough to be worth a question.
A 40 ampere continuous load, copper, 75 degree terminations, an ambient of 30 degrees C, three current-carrying conductors in the raceway. Under 210.19(A)(1) the conductor has to carry 40 times 1.25, which is 50 amperes.
At 30 degrees C the correction factor is 1.00 and three conductors need no adjustment, so the 90 degree column stands as printed. Table 310.16 gives 8 AWG copper 55 amperes at 90 degrees, and the same row is 50 amperes in the 75 degree column, so 110.14(C) holds it to 50 amperes. That meets the load exactly.
10 AWG copper is 40 amperes at 90 degrees, short of 50, so it is out and 8 AWG is the size. The device is 50 amperes from Table 240.6(A), and Table 250.122 gives 10 AWG copper for a device rated 60 amperes or less. Put those numbers in the calculator above and you get the same three answers back.
The value used for your inputs is read from NEC 2026 240.4(D), Table 240.6(A), Table 250.122, Table 310.15(B)(1)(1), Table 310.15(C)(1) and Table 310.16, the way you would read it in the exam room, and the working above names the row it came from. The tables themselves belong to NFPA and are not printed on this page. Bring your own copy of the 2026 code, which is what the exam requires anyway, and check the row there.
For a 40 ampere continuous load on 75 degree terminations, at 30 degrees C with three current-carrying conductors, the answer is 8 AWG copper. The continuous rule in 210.19(A)(1) takes the load to 50 amperes, and 8 AWG copper is 50 amperes in the 75 degree column of Table 310.16. A 40 ampere noncontinuous load lands on 8 AWG as well, because 10 AWG copper is only 35 amperes at 75 degrees.
A 100 ampere continuous load is 125 amperes of required ampacity, which is 1 AWG copper at 130 amperes in the 75 degree column. A 100 ampere noncontinuous load is 3 AWG copper at 100 amperes in that same column. Single-phase dwelling services and feeders are a separate case with their own table in 310.12, and this page does not use it.
No. Under 210.20(A) the device has to be at least 25 amperes for a 20 ampere continuous load, and 240.4(D) caps the device on 12 AWG copper at 20 amperes. The size that carries it is 10 AWG copper on a 25 ampere device, and this calculator returns that on either 60 or 75 degree terminations.
No. The 90 degree column is a starting point for correction and adjustment when the insulation is rated 90 degrees, and 110.14(C) then holds the finished ampacity to the ampacity of that same size in the column matching the terminations. That is why a 90 degree wire on 75 degree lugs is still a 75 degree wire at the end of the calculation.
Table 310.15(C)(1) starts at four current-carrying conductors bundled or in one raceway for more than 24 inches: four to six is 80 percent, seven to nine is 70 percent, and ten to twenty is 50 percent. A neutral that carries only unbalanced current is not counted under 310.15(E)(2), and 310.15(F) says grounding and bonding conductors are never counted.
Table 250.122 reads on the rating of the overcurrent device ahead of the equipment, not on the load and not on the phase conductor. A 15 ampere device takes 14 AWG copper, 20 amperes takes 12 AWG, up to 60 amperes takes 10 AWG, up to 100 amperes takes 8 AWG and up to 200 amperes takes 6 AWG. Under 250.122(A) it never has to be larger than the circuit conductors.
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