# Branch-Circuit, Feeder and Service Calculations, Part LVI

By | Oct 15, 2010

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Article 220 of the National Electrical Code (NEC) contains requirements for calculating branch-circuit, feeder and service loads. While understanding load calculation requirements is essential to the professional electrician, knowing what to do with the results from the load calculations is just as important.

Fixed electric space heating is not limited to space heaters and electric strip heat that is part of a package unit or furnace. Fixed electric space heating could also be a heat pump with supplementary heat. With a heat pump, the compressor (and accompanying motors) and some or all of the electric heat can be energized at the same time. The load contribution of a heat pump is the air conditioning system load plus the maximum amount of heat that can be on while the air conditioner compressor is energized. For example, what is the optional method service load calculation (before applying the Table 220.84 demand factor) for a six-unit multifamily dwelling that will have an identical heat pump for each dwelling unit? Each unit’s compressor draws 26.8A at 240V, the blower motor draws 5.8A at 240V, and the condenser fan motor draws 2.6A at 240V. The electric heat in this unit has a rating of 15 kW. The compressor and all of the heat in this heat pump can be energized at the same time. Assume the heating kilowatt rating is equivalent to kilovolt-amperes. The load of the air conditioner compressor is 6,432 VA (26.8 240 = 6,432). The load of the blower motor is 1,392 VA (5.8 240 = 1,392). The load of the condenser fan motor is 624 VA (2.6 240 = 624). The air conditioning load for each unit is 8,448 VA (6,432 + 1,392 + 624 = 8,448). Since the air conditioning system and all of the heat can be on at the same time, add the two together (8,448 + 15,000 = 23,448 VA). The total load for six units is 140,688 VA (23,448 6 = 140,688). The optional method service load calculation (before applying the Table 220.84 demand factor) for the heating and air conditioning system in this six-unit multifamily dwelling is 140,688 VA (see Figure 4).

MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and “The Electrician’s Exam Prep Manual.” He can be reached at 615.333.3336, [email protected] and www.charlesRmiller.com.

## Charles R. Miller

Charles R. Miller, owner of Lighthouse Educational Services, teaches custom-tailored seminars on the National Electrical Code and NFPA 70E. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician's Exam Prep Manual.” He can be reached at 615.333.3336 and [email protected]. Connect with him on LinkedIn.

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