Using the optional dwelling method on a 2,000 square foot house at 3 volt-amperes per square foot, with two small-appliance circuits and one laundry circuit at 1,500 volt-amperes each, a 12-kilowatt range, a 5-kilowatt dryer, a 4.5-kilowatt water heater, a 1.2-kilowatt dishwasher, and an 0.8-kilowatt disposal, plus a 5-kilowatt air conditioner and 9 kilowatts of fixed electric space heating that cannot run together, what is the calculated service load at 240 volts? Take the first 10 kilovolt-amperes of general load at 100 percent and the remainder at 40 percent, and take the space heating at 65 percent.
- A127 amperes
This adds both the air conditioner and the space heating. Loads that cannot operate at the same time are compared, and only the larger of the two is carried into the total.
- 106 amperes
- C103 amperes
This carries the air conditioner and drops the heating. The two must be compared after their own percentages are applied, and heating at 65 percent comes out higher than the air conditioner at 100 percent.
- D119 amperes
This takes the space heating at its full 9 kilowatts. The optional method allows fixed electric space heating with fewer than four separately controlled units to be figured at 65 percent.
Why B is correct
The optional method lumps every general load at nameplate value, discounts everything past the first 10 kilovolt-amperes to 40 percent, and then adds back only the larger of the heating or cooling load. Here heating at 65 percent works out to 5,850 VA, which beats the 5,000 VA air conditioner, so 19,600 + 5,850 = 25,450 VA divided by 240 volts gives 106 amperes.
What this question is testing
Tests a complete optional-method dwelling calculation end to end: assembling general loads at nameplate, applying the 10 kilovolt-ampere and 40 percent split, applying the correct percentage to fixed electric space heating, comparing heating against cooling and carrying only the larger, and converting volt-amperes to amperes at 240 volts.
On the job
The optional method is what lets a fully loaded modern house land on a 200-ampere service, and most residential shops use it exclusively. The heating versus cooling comparison is where money is made or lost: apply the 65 percent factor before comparing, and a heat-dominant house in a cold climate often calculates smaller than the contractor expected. Plan reviewers check that comparison line first.
Memory technique
Ten at full fare, the rest at forty percent, then add whichever of heat or cool is bigger.
Exam tip
Apply the heating percentage first, then compare it against the air conditioning value; the comparison is between the adjusted numbers, not the nameplates.
Where to look it up
Article 220, Part IV, optional calculation for a dwelling unit. The heating and air conditioning list is a separate subsection from the general load subsection, so read both before you total anything.