A photovoltaic source circuit consists of one string of modules whose combined rated short-circuit current is 10 amperes. The Code sets the maximum circuit current at 125 percent of that rated short-circuit current, and then requires the circuit conductors to have an ampacity of at least 125 percent of the maximum circuit current before any temperature correction or conductor adjustment is applied. What minimum conductor ampacity results?
- 15.6 amperes
- B12.5 amperes
This applies only the first 125 percent factor and stops. The conductor sizing step is a second, separate multiplication that this answer omits.
- C10 amperes
This uses the rated short-circuit current with no factors at all, which ignores both the sustained output of an array in bright cold conditions and the continuous duty allowance.
- D19.5 amperes
This applies the 125 percent factor three times. Only two multiplications are called for, one to establish maximum circuit current and one to size the conductor.
Why A is correct
The calculation stacks two separate 125 percent factors. The first converts rated short-circuit current into maximum circuit current, and the second sizes the conductor for continuous duty. Ten amperes becomes 12.5 amperes and then 15.6 amperes of required ampacity, all before temperature correction and conductor adjustment.
What this question is testing
This tests whether you apply both required factors, in the right order, and whether you know that the result is a pre-correction value rather than a final conductor selection.
On the job
On a real rooftop the correction that follows this calculation dominates. Conductors in conduit on a hot roof see ambient temperatures well above the table basis, and the correction factor can cut usable ampacity by a third or more, which is why photovoltaic feeders are routinely a size or two larger than the naive arithmetic suggests. Installers who size from the 15.6 ampere figure alone find their conductors undersized once the roof temperature adder is applied, and the correction is checked on every plan review.
Memory technique
One twenty-five for the sun, one twenty-five for continuous.
Exam tip
Photovoltaic circuits multiply by 125 percent twice for different reasons. Write both steps out so you do not stop after the first.
Where to look it up
Article 690 Part II, the circuit sizing and current sections. The temperature correction that follows comes from the ampacity tables and correction factors in Article 310.