Three current-carrying 6 AWG THWN-2 copper conductors are installed in a raceway in an ambient of 55 degrees Celsius. The 90-degree correction factor for that ambient is 0.76. Table 310.16 gives 6 AWG copper as 55 amperes at 60 degrees, 65 amperes at 75 degrees and 75 amperes at 90 degrees. Terminations at both ends are rated 75 degrees Celsius. What is the maximum load these conductors may serve?
- A65 amperes
This is the termination limit with no correction applied at all. In a 55-degree ambient the conductor cannot carry its full rated current, so the correction step cannot be skipped.
- B49 amperes
This applies the 0.76 factor to the 75-degree column value of 65 amperes. Correction factors belong on the conductor insulation rating, and the termination limit is applied afterward as a ceiling, not as a starting point.
- 57 amperes
- D42 amperes
This applies the correction to the 60-degree column. Nothing in the problem points to 60 degrees; the insulation is rated 90 degrees and the terminations are rated 75 degrees.
Why C is correct
Correction factors are always applied to the ampacity column matching the conductor insulation, which is 90 degrees for THWN-2. Seventy-five amperes times 0.76 gives 57 amperes. That corrected value is then checked against the 65-ampere termination limit, and because 57 is the smaller number it becomes the maximum permitted load.
What this question is testing
Tests whether you know the correct order of operations for conductor sizing: start at the insulation rating, apply ambient correction and any conductor-count adjustment, then compare the result to the termination temperature limit and take the lower value. It also checks that you can recognize when correction rather than termination controls.
On the job
Boiler rooms, rooftop chases, attics above insulation and enclosures next to furnaces all run hot enough to matter, and the ambient is measured at the conductors, not at the thermostat down the hall. Most of the time the 75-degree termination is the binding constraint and electricians get in the habit of stopping there. In a genuinely hot space the corrected 90-degree value drops below the termination value and the whole circuit has to be upsized, which is the situation this problem is built around.
Memory technique
Start high, correct down, then let the lugs cap it.
Exam tip
Correct from the 90-degree column, then cap at the termination column. Whichever number ends up smaller is your ampacity.
Where to look it up
Table 310.16 for the three columns, the ambient correction tables in Article 310, and the termination temperature rules in Article 110.