Ampacity Derating Practice Questions

B. A 30 degree C ambient and not more than three current carrying conductors

The conditions are printed right above the table: conductors rated 0 through 2000 volts, insulation rated 60, 75 or 90 degrees C, a 30 degree C ambient, and not more than three current carrying conductors. Every correction and adjustment factor exists to move a number off those base conditions. The 40 degree base belongs to the free air and higher temperature tables, which is why it reads plausibly here.

NEC 2026 310.16

D. The one built on a 30 degree C base

Article 310 carries two correction tables and they print different factors for the same ambient. The 30 degree base table is the one whose own note names Table 310.16 and Table 310.17, and Table 310.16 points back at 310.15(B) in return. Reading the wrong table produces a wrong conductor size with nothing absurd about the number, which is the worst kind of error to carry into a timed paper.

NEC 2026 310.15(B)(1), Table 310.15(B)(1)(1)

A. Three quarters of an inch or more

The adder attaches only where the distance from the roof to the bottom of the raceway or cable is less than three quarters of an inch. Three quarters of an inch of standoff removes it entirely, which is why raceways get run on strut over flat roofs. The trigger is small enough that a stem will give you the dimension and expect you to compare it.

NEC 2026 310.15(B)(2)

C. 68 degrees C

The adder is 33 degrees C, and it goes onto the outdoor temperature before the lookup, so 35 plus 33 gives 68 degrees C. Order matters here in a way it does not between the two multiplications, because reading the table at 35 and then applying the adder afterward is not a defined operation at all. The 60 in the choices is the Fahrenheit form of the same adder, sitting there to be grabbed by mistake.

NEC 2026 310.15(B)(2)

D. XHHW-2

The exception under the rooftop subsection names XHHW-2 and nothing else. An item that puts a specific insulation type on a roof is checking whether you know the exception is there rather than testing the arithmetic. The other three are ordinary 90 degree types with no exception written for them.

NEC 2026 310.15(B)(2)

B. 24 inches

The threshold is 24 inches, and it applies both to conductors in a raceway and to single conductors or multiconductor cables bundled outside one. Under that length the bundle does not hold enough heat for the factors to attach. The same 24 inches turns up in the conduit fill notes for a nipple, and an item that stuffs a nipple between two cabinets is usually reaching for both rules at once.

NEC 2026 310.15(C)(1)

C. 45 percent

The adjustment table steps in bands, and 21 through 30 conductors sits at 45 percent. Ten through 20 is the band above it at 50 percent and 31 through 40 is the band below at 40 percent, so a miscount by one or two conductors near a band edge moves the answer. Read the count first and the band second.

NEC 2026 Table 310.15(C)(1)

A. They count, because the table footnote includes spare conductors

The footnote under the adjustment table defines the count as the total number of conductors in the raceway or cable, spares included, then adjusts it only for the neutral rule and the grounding rule. A spare is copper in the pipe taking up thermal room whether or not anything is landed on it. The footnote does carve out conductors connected to components that cannot be energized at the same time, which is a different situation from an unterminated spare.

NEC 2026 Table 310.15(C)(1)

B. Only the power and lighting conductors

The subsection says the adjustment factors apply only to the number of power and lighting conductors where different systems occupy a common raceway. Counting every conductor in the pipe over-derates the circuit and gives a conductor a size larger than the code asks for. The last choice inverts the rule and switches adjustment off entirely, which the section never does.

NEC 2026 310.15(C)(1)

D. Rigid metal, intermediate metal, PVC or RTRC, not over 10 feet, not more than four conductors

Three things have to be true together: the physical protection is one of those four raceway types, the protected length is not over 10 feet, and the number of conductors is not over four. Miss any one and the exclusion does not apply. The third choice borrows the 24 inch figure from the short raceway exclusion, which is a different item in the same list.

NEC 2026 310.15(C)(1)

A. Not exceeding 24 inches

A raceway not over 24 inches long is too short to build the heat the factors were written for, so the adjustment is switched off. That figure is the same 24 inches that starts the adjustment when a bundle is longer than it, read from the other direction. The 10 foot number belongs to the trench exclusion in the same list.

NEC 2026 310.15(C)(1)

C. A 3-wire circuit made from two ungrounded conductors and the neutral of a 4-wire 3-phase wye system

The positive list names three cases: a 2-wire circuit, a 3-wire circuit built from two ungrounded conductors and the neutral of a 4-wire 3-phase wye system, and a 4-wire 3-phase wye circuit where the major portion of the load is nonlinear. In that second case the neutral carries roughly the same current as the phase conductors rather than a difference between them. The wye circuit with a linear load falls out of the list and lands in the exclusion instead.

NEC 2026 310.15(E)(1)

C. Three conductors, and no adjustment applies

The load is linear, so the neutral carries only unbalanced current and the exclusion keeps it out of the count. The grounding conductor is kept out by its own subsection. That leaves three current carrying conductors, and three is not more than three, so no adjustment attaches at all.

NEC 2026 310.15(C)(1), 310.15(E)(2), 310.15(F)

A. The lowest rating of the insulated conductors it runs with

A bare or covered conductor takes the lowest temperature rating among the insulated conductors it is installed with, because they all share the same heat. Fixing it at 60 or at 90 turns a relative rule into an absolute one and gets the answer wrong in both directions. The termination rating is a separate test that happens later.

NEC 2026 310.15(D)

D. 36.1 amperes

Start in the 90 degree column at 55 amperes, correct for the 46 to 50 degree row at 0.82 to get 45.1, then adjust for four conductors at 0.80 to get 36.1 amperes. The termination check is run separately on the untouched 75 degree value of 50 amperes, and since 36.1 is the smaller of the two it governs. Reporting the 50 amperes is what happens when the termination number is mistaken for the answer instead of a second test.

NEC 2026 Table 310.16, Table 310.15(B)(1)(1), Table 310.15(C)(1), 110.14(C)

C. 77.3 amperes

The 90 degree column gives 115 amperes, the 31 to 35 degree row gives 0.96, and nine conductors falls in the 7 through 9 band at 0.70, so 115 times 0.96 times 0.70 comes to 77.3 amperes. The 75 degree column reads 100 amperes untouched, which is the termination limit and is not binding here. Applying only the adjustment and skipping the correction produces the 80.5.

NEC 2026 Table 310.16, Table 310.15(B)(1)(1), Table 310.15(C)(1), 110.14(C)

B. 117.5 amperes

The 90 degree aluminum column gives 135 amperes and the 41 to 45 degree row gives 0.87, so the corrected value is 117.5 amperes. Three conductors means no adjustment. The termination limit is the untouched 75 degree value of 120 amperes, so the corrected figure is the smaller of the two and it governs; the 104.4 comes from correcting the 75 degree number, which is the one number in this problem that never gets multiplied.

NEC 2026 Table 310.16, Table 310.15(B)(1)(1), 110.14(C)

D. 20 amperes

The derating runs 30 amperes in the 90 degree column, times 0.91 for the 36 to 40 degree row, times 0.80 for six conductors, which lands at 21.8 amperes. The small conductor cap then holds 12 AWG copper to 20 amperes, and the section places that cap after the correction and adjustment rather than before them. A derated ampacity above the cap does not lift it, which is exactly why an item walks you through the arithmetic and then asks for the breaker.

NEC 2026 240.4(D), Table 310.16, Table 310.15(B)(1)(1), Table 310.15(C)(1)

C. 47.5 amperes

At the base ambient the correction factor is 1.00, so only the adjustment bites: 95 amperes in the 90 degree column times 0.50 for the 10 through 20 conductor band gives 47.5 amperes. The 85 amperes in the 75 degree column is the termination limit and it is not what the load has to fit under here. Starting from the 75 degree column and adjusting it produces the 42.5.

NEC 2026 Table 310.16, Table 310.15(C)(1), 110.14(C)

A. The column matching the conductor insulation rating, checked against the ampacity at the termination rating

The factors are applied to the ampacity for the conductor temperature rating, and the corrected and adjusted result is not permitted to exceed the ampacity at the termination rating. That is why the 90 degree column is the right place to start even when the lugs are 75 degree: there is more ampacity to spend before the factors have finished. The termination number is read fresh from its own column and never gets multiplied.

NEC 2026 310.15(A), 110.14(C)

B. Use it for ampacity adjustment, correction, or both

The section says outright that a conductor rated higher than the termination may be used for adjustment, correction or both. That single sentence is the whole argument for beginning in the 90 degree column. It does not raise the ceiling, because the corrected and adjusted result still has to sit under the ampacity at the termination rating.

NEC 2026 110.14(C)

C. After both the correction and the adjustment have been applied

The opening sentence of the small conductor rule places the cap after any correction for ambient and any adjustment for conductor count. That fixes its position at the end of the sequence rather than anywhere in the middle. The cap applies to 14, 12 and 10 AWG in both copper and aluminum, so restricting it to one material is wrong on its face.

NEC 2026 240.4(D)

D. No, because the limit on this run works out to 6 feet

The exception permits the higher ampacity where the lower ampacity portion does not exceed the lesser of 10 feet or 10 percent of the total circuit. Ten percent of 60 feet is 6 feet, and 6 is less than 10, so 6 feet is the limit and the 8 foot section blows through it. Reading only the 10 foot half of the exception is precisely the mistake the word lesser is there to catch.

NEC 2026 310.14(A)(2)

A. The lowest one

The selection rule takes the lowest applicable value over the circuit length, because the hottest stretch is what decides whether the insulation survives. An exception lets a short transition keep the higher figure, but the general answer is the low one. Averaging is not an operation the code performs anywhere in this article.

NEC 2026 310.14(A)(2)

B. 95

The three copper columns for 4 AWG read 70, 85 and 95 amperes, so the 90 degree figure is 95. That number is a starting point rather than an answer, since ambient, conductor count and the termination rating all still have to be settled. Landing one column to the left gives you the 85, which is the most common slip on a table lookup.

NEC 2026 Table 310.16