Load calculations—Art. 220 (part 1)
Refer to the drawing, and give the best answer:
1. In doing a load calculation, assume you have to sum the following loads: 14.75A, 25.49A, and 20.24A. What is the result for the purposes of applying other code rules relative to conductor ampacity, etc?
2. The drawing shows various types of receptacle outlets. In a residence, how many VA do you need to allow in a general-purpose branch-circuit calculation for an outlet with the device at the bottom?
d. not specified
3. In a store, how many VA do you have to allow for two duplex receptacles installed side-by-side in a two-gang box when you make a branch-circuit load calculation?
a. 90 VA
d. not specified
4. In the same store, which of the answers (from Question 3) would be correct for the device at the bottom?
5. Again in the same store, how many triplex receptacles (the top center device) could you put on a 20A branch circuit? Would the same answer apply to single receptacles (as at the right) if they were located in groups of threes in three-gang boxes? That is, could you install the same number of outlets comprised of three single receptacles in three-gang boxes as you could triplex receptacles in single gang boxes?
a. 13, no
b. 13, yes
c. 8, no
d. 8, yes
e. 26, no
f. 26, yes
6. If you place five quadruplex receptacle devices, like the one at the bottom of the drawing, above a 20-ft show window for lighting, how much load does this add to the branch circuit involved? How much load do you need to allow for the feeder?
a. 900VA, 900VA
b. 900VA, 4000VA
c. 1800VA, 1800VA
d. 1800VA, 4000VA
Answers and Discussion
1. d, Sec. 220-2(b). Only major fractions, defined as those equaling one half (0.5) or greater, must be rounded up. Minor fractions may be disregarded. This number, 0.5, is a pure number. This means you compare your result, however precise, to the decimal equivalent of one half. In the example, the fractional result (0.48) is less than one half. It doesn’t equal one half, unless first rounded to one decimal place. Even then, the result (actual current) never increased the extra 0.02A to make it equal to one half, and thereby justifying treatment as the higher number. Don’t make the mistake of rounding your calculations twice.
2. d, Sec. 220-3(b)(10). Dwelling occupancies have their general purpose receptacle outlets figured into the square-footage calculations, and you don’t have to make any specific allowances for receptacles, whether multiple (and multiple to any degree) or not. Although periodically controversial, the rule has stood the test of time.
3. and 4. c, Sec. 220-3(b)(9). Each strap counts independently as 180VA, until you get to the case where four receptacles are a part of one device. Only at that point do you begin counting actual receptacles, at 90VA each.
5. b, Sec. 220-3(b)(9). Here again, the triplex receptacle positions its three receptacles on a single strap, and the 180VA rule applies once, to the single strap supporting three receptacles. The rule applies to the strap, however, and not to the outlet. A three gang box supporting three separate receptacle devices, even single receptacles, is charged per strap, or 540VA per outlet in this case.
6. d, Sec. 220-3(b)(7); 220-3(b)(9), 220-12(a). The quad device counts as 90VA per receptacle, for a total of 360VA each. The branch circuit load to the show window follows the actual outlet loading, although you do have the option of using the 200VA per foot calculation. When you get to the level of the feeder, however, the 200VA per foot allowance becomes mandatory. This follows the same principle as in Sec. 210-11(b), where a feeder has to be sized on the full load as calculated on a sq-ft basis, but branch circuits need only be installed to serve the connected load.