Load Calculations — Art. 220 (part 2)

Give the best answer.

1. Which of the following 15A 125V duplex receptacles on a two-family house counts as part of the general lighting load for the dwelling unit they’re connected to, instead of being taken as additional load? Assume they’re not in the actual dwelling unit living space.

a. The one for the garage door opener

b. The one in the siding on the outside wall, for occasional use as necessary

c. The one in the basement next to the panel, for general use

d. Two of the above

e. All of the above

f. None of the above

2. How many 20A branch circuits must you install to feed 80 ft of lighting track at a museum? There will be 50W pin spots every 4 ft (starting 2 ft from the beginning) for a long-term display of a series of 20 small paintings.

a. One

b. Two

c. Three

d. Four (feed at both ends)

3. For the load in Question 2, how much additional load must you allow for the feeder?

a. 7500VA

b. 6000VA

c. 1250VA

d. 1000VA

4. If you connect a receptacle located behind the refrigerator to its own 15A branch circuit, how much load must you include for the refrigerator in the service calculation for a new house?

a. None

b. 1200VA

c. 1500VA

d. No can do; it must be a 20A small appliance circuit.

5. A 16-unit condominium, with two bathrooms in each, will be wired with separate 20A branch circuits to each bathroom. How much additional load does this design decision impose?

a. 48kVA

b. 24kVA

c. 5760VA

d. None

6. If you install a heat pump at a new house with 18kVA of backup resistance heat to supplement a 30A compressor, what does that represent in terms of 240V feeder load in the optional calculation? Assume the heat is internally staged, but the compressor runs whenever called.

a. 68A

b. 75A

c. 105A

d. None of the above

Answers and Discussion

1. d, Sec. 220-3(b)(10). Due to the restructuring of this section, the general-purpose receptacles have a “home” for sure, even if they aren’t on the side of the door with the conditioned air. They’re taken as part of the general lighting load, and you don’t have to make any allowances for them in your feeder calculation. The receptacle for the garage door opener, however, has to reflect the actual load.

2-3. a,a, Sec. 220-12(b). The load allowances you need to make for lighting track installations only apply to the feeder. You install the track and branch circuits as required to serve the actual load, and then you apply the calculation to the feeder. This concept is well established: Sec. 210-11(b) requires full feeder capacity for a calculated load, but you only have to install branch circuiting for the actual load. Here, a continuous branch circuit load would need to accommodate 1250VA—well within the capacity of a single circuit. On the other hand, the feeder needs to accommodate 150VA for every 2 ft of track installed, or 6000VA, which figures at 7500VA due to continuous loading.

4. a, Sec. 220-16 Ex. This individual branch circuit serves an appliance, but it isn’t a small-appliance branch circuit per se. This differs from the usual requirement of 1500VA for each circuit actually installed, even if you install more than the minimum. Although only two circuits are “required by Sec. 210-11(c)(1),” that referenced paragraph clearly allows for additional circuits. Furthermore, Sec. 220-16 would not need the new exception for the refrigerator circuit if only two such circuits would be charged against the feeder load, regardless of the number actually installed. All convenience receptacles in these areas must be on small-appliance branch circuits, and that’s the proper context for the word “required.”

5. d, Sec. 220-16. This section pointedly does not mention any required allowance for bathroom circuits. The 20A branch-circuit requirement accommodates large loads that are extremely transient in character, with negligible effect on the feeder. See also Example D2a, which makes this point explicitly.

6. c, Sec. 220-30(c)(2). Since the controller doesn’t prevent the compressor from running at the same time as the heat, the loads need to be combined and then taken at 100%. The 1996 NEC allowed you to figure the combination of the compressor and supplementary heat at 65%—no more. In this case, the 30A compressor load adds to the 18kVA resistance load.