Pick the conductor, the ambient temperature and the number of current-carrying conductors, and read what ampacity is left after correction, adjustment and the termination limit. The values are NEC 2026 Table 310.16, Table 310.15(B)(1)(1) and Table 310.15(C)(1) as printed.
This is the rating printed on the insulation, and it is the column the correction and adjustment factors start from. The type letters are the common ones Table 310.16 lists over each column.
Read the marking on the breaker, the lug or the equipment. Under 110.14(C)(1)(a) equipment rated 100 amperes or less is 60 C unless it is listed and marked for higher, and 110.14(C)(1)(b) puts equipment over 100 amperes at 75 C. Terminations listed for 90 C are unusual.
The table is built on a 30 C ambient, which is 86 F, so 30 C returns a factor of 1.00 and changes nothing.
Count the ungrounded conductors, and the neutral only where 310.15(E)(1) makes it current-carrying. A grounding or bonding conductor is never counted, under 310.15(F). Three or fewer means no adjustment at all.
Section 310.15(B)(2) adds 33 C, or 60 F, to the outdoor temperature for a raceway or cable exposed to direct sunlight on or above a roof where the gap from the roof to the bottom of it is less than 3/4 inch. Hold it higher than that and there is no adder.
This page holds the three tables, so you get a number while you are still standing at the panel. The derating tool on the calculators page makes you type each factor in from your own book, because finding the right cell is the thing the exam is testing.
Use this one on the job, and read the ampacity derating study page for the order of operations and where candidates lose the point.
Ampacity is four numbers in a fixed order. Table 310.16 gives the starting value for the size, the metal and the conductor temperature column. Section 310.15(B) corrects it for ambient temperature, 310.15(C)(1) adjusts it for the number of current-carrying conductors, and 110.14(C) caps the result at the terminations.
Correction comes from Table 310.15(B)(1)(1), which is built on a 30 C ambient. Read down to the row that holds your ambient and across to the column matching the conductor rating, and multiply. Where a raceway or cable sits in direct sunlight less than 3/4 inch above a roof, 310.15(B)(2) adds 33 C to the outdoor temperature first, and XHHW-2 is excepted from that adder.
Adjustment comes from Table 310.15(C)(1) and starts at the fourth current-carrying conductor. Four through six is 80 percent, seven through nine is 70 percent, ten through twenty is 50 percent. Neutrals count only where 310.15(E)(1) says they carry current, and grounding conductors never count, under 310.15(F).
The termination cap is where most of the marks are lost. Section 310.15(A) lets you correct and adjust from the conductor rating, but only if the corrected and adjusted value does not exceed the ampacity at the termination temperature rating under 110.14(C). So a 90 C conductor starts at the 90 C column and finishes against the 75 C or 60 C column, and 310.14(A)(2) says the lowest value governs when more than one applies.
The trap is doing this out of order, or stopping early. Correcting and adjusting from the 60 C column when the wire is 90 C throws away ampacity you are entitled to. Reporting the 90 C column as the answer, without bringing it back to the terminations, gives a number no inspector accepts.
One last check sits outside the ampacity arithmetic. Section 240.4(D) caps the overcurrent device on 14, 12 and 10 AWG, and the section says the cap applies after the correction and adjustment factors, so it is the final line rather than the first. It limits the device, not the ampacity, which is why this page prints it separately.
Take 6 AWG copper THHN in a raceway with six current-carrying conductors, an ambient of 40 C, landing on equipment marked for 75 C terminations. Table 310.16 gives 6 AWG copper in the 90 C column as 75 amperes, and that is where a 90 C conductor starts.
The 36 to 40 C row of Table 310.15(B)(1)(1) reads 0.91 in the 90 C column. Six current-carrying conductors put you on the 4 to 6 row of Table 310.15(C)(1), which is 80 percent. So 75 times 0.91 times 0.80 is 54.6 amperes.
Now the cap. The 75 C column for 6 AWG copper is 65 amperes, and 54.6 is under it, so the terminations do not bite and the answer stands at 54.6 amperes. Change the terminations to 60 C and the cap becomes 55 amperes, which still does not bite. Put the same wire in a 50 C ambient instead and the factor falls to 0.82, the answer falls to 49.2 amperes, and derating is what limits the run either way.
The value used for your inputs is read from NEC 2026 240.4(D), 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.
Start at the Table 310.16 value for the size, the metal and the conductor temperature column. Multiply it by the ambient correction factor from Table 310.15(B)(1)(1), then by the adjustment factor from Table 310.15(C)(1) for the number of current-carrying conductors. Compare what is left against the Table 310.16 value at the termination temperature rating under 110.14(C), and the lower of those two is the ampacity you may use.
In Table 310.15(B)(1)(1) it is 0.82 in the 60 degree column, 0.88 in the 75 degree column and 0.91 in the 90 degree column. Those sit on the 36 to 40 degree C row, which is 96 to 104 degrees F. The correction chart further down this page prints all sixteen rows.
Section 310.15(C)(1) applies once more than three current-carrying conductors share a raceway or a cable without spacing for longer than 24 inches. Four through six conductors take 80 percent, seven through nine take 70 percent, and ten through twenty take 50 percent. A run of 24 inches or less is outside the rule, and so is a grounding or bonding conductor under 310.15(F), which is never counted.
Only if the terminations at both ends are rated for it, which they almost never are. Section 110.14(C) holds the conductor to the lowest temperature rating in the connection, and 110.14(C)(1)(a) starts equipment rated 100 amperes or less at 60 degrees unless it is listed and marked otherwise. The 90 degree column earns its keep as the starting number for correction and adjustment, and the answer still has to come back down to the termination column.
It caps the overcurrent device on the smaller sizes at 15 amperes for 14 AWG copper, 20 for 12 AWG copper and 30 for 10 AWG copper, with 15 and 25 for 12 and 10 AWG aluminum. The section says the cap applies after the ambient and conductor count factors have been applied, so it is the last thing you check. It limits the device rather than the ampacity, which is why this page prints it on its own line.
Section 310.15(B)(2) adds 33 degrees C, which is 60 degrees F, to the outdoor temperature where a raceway or cable sits in direct sunlight on or above a roof and the gap from the roof to the bottom of it is under 3/4 inch. Raise the raceway above that gap and the adder goes away. The section also excepts XHHW-2, and both the checkbox and the exception are on the calculator above.
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