Q. What is the load on the service for 10 office trailers on a construction site? Each unit is identical, featuring 1,548 square foot of usable floor area. The actual number of receptacles is not known. The heating load for each unit is rated at 7kW. The cooling load for each unit is rated at 7.5 hp. Both units are 3-phase, 208VAC. The supply voltage is 208VAC, 3-phase. The construction site for these office trailers is located in southern Texas.

  1. 170,746VA
  2. 85,373VA
  3. 56,637VA
  4. 46,101VA

Answer: A

A mobile home not intended as a dwelling unit (i.e., contractor’s on-site offices) shall not be required to meet the provisions of Art. 550 pertaining to the number or capacity of the circuits required. If the supply voltage is something other than a 120V or 120/240VAC power supply system, Art. 550 does not apply. If Art. 550 does not apply, then the demand factors in Table 550.31 do not apply. In addition, Art. 551, including Table 551.73, does not apply either. And although the lighting load demand factors for dwelling units in Table 220.42 do not apply, the demand factor for “all others” in Table 220.42 does apply in this situation. The demand is at 100% (i.e., no demand).

Let’s run through the calculations required to determine the service load for these 10 mobile on-site office trailers.

(1,548 sq. ft.) X (10 units) X (3.5VA*) X (1.25 continuous load) = 67, 725VA
*Table 220.12 general lighting load for office buildings

Since the actual number of receptacles is not known we must calculate this load as follows:
(1,548 sq. ft.) X (10 units) X 1VA per sq. ft.) = 15,480VA

The cooling load is larger than the heating load so we’ll base our calculations on this figure. As per Table 430.250, the full load current of a 7.5 hp, 3-phase, 208V AC motor is 24.2A.
(24.2A) X (208V) X (1.732) = 8,718.05VA (8,718.05VA) X (10 units) = 87,180.50VA
Round up to 87,181VA

Total load = (67,725VA) + (15,480VA) + (87,181VA) = 170, 386VA

Remember there are no demand factors other than 100% applied.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.