A school building has 60,000 square feet of floor area and a connected load of 12 volt-amperes per square foot. The optional method for schools applies 100 percent to the first 3 volt-amperes per square foot, 75 percent to the portion from 3 up through 20 volt-amperes per square foot, and 25 percent to anything above that. What is the calculated load?

Why D is correct

The method brackets the connected load by volt-amperes per square foot rather than by total volt-amperes, which trips people who try to apply the percentages to the raw total. Splitting 12 volt-amperes per square foot into 3 at full value and 9 at three quarters, then multiplying each by the floor area, gives 180,000 plus 405,000, or 585,000 volt-amperes.

What this question is testing

The item tests whether the candidate can apply demand brackets stated in volt-amperes per square foot, keeping the unit conversion straight and adding the brackets rather than choosing between them.

On the job

Schools carry heavy lighting, kitchen, and mechanical loads that never run together, which is why the optional method exists for them. On an addition or a renovation the calculation decides whether the existing service can absorb the new wing. Because the brackets are stated per square foot, the area figure has to be right, and disputes usually come down to whether covered walkways and mechanical penthouses are counted in the floor area.

Memory technique

Three at full, the next seventeen at three quarters, the rest at a quarter.

Exam tip

When brackets are expressed per square foot, do the bracketing first and multiply by area last. It halves the arithmetic.

Where to look it up

Look in the load calculation article, in the optional calculations part, at the section covering schools and its demand factor table.