Branch-Circuit, Feeder and Service Calculations, Part XVII

by Charles R. Miller
Published: July 2007

Article 220 – Load Calculations

220.52 Small Appliance and Laundry Loads—Dwelling Unit

Requirements for calculating branch-circuit, feeder and service loads are in Article 220 of the National Electrical Code (NEC). Article 220’s requirements are divided into five parts. General requirements for load-calculation procedures are in Part I (220.1 through 220.5). Part II (220.10 through 220.18) provides calculation provisions for branch circuits. Feeder and service load calculation requirements (sometimes referred to as the standard method) are in Part III of Article 220. Sections in Part III include 220.40 through 220.61. Optional feeder and service load calculation provisions are in Part IV (220.80 through 220.88). Part V (220.100 through 220.103) provides calculation specifications for farm loads. Figure 220.1 (in the NEC) shows information on the organization of Article 220. Results from load calculations in Article 220, along with provisions from other article, can be used to find conductor sizes and ampere ratings for overcurrent protection.

Last month’s column concluded by covering motor load calculations in 220.50. This section in Article 220 references 430.24, 430.25 and 430.26 for motor loads and 440.6 for hermetic refrigerant motor compressor loads. This month, the discussion continues with fixed electric space heating loads in 220.51.

Fixed electric space heating loads shall be calculated at 100 percent of the total connected load. However, in no case shall a feeder or service load current rating be less than the rating of the largest branch circuit supplied [220.51]. Unlike motor and motor-compressor loads that must be calculated at 125 percent, fixed electric space heating loads must only be calculated at 100 percent. For example, floor plans for a small office are specifying the installation of seven wall heaters; each heater is rated 3,000 watts at 240 volts. How much load will these heaters add to a 240-volt, single-phase service? The air conditioning load will be less than the heating load. In accordance with 220.51, calculate the heaters at 100 percent (7 × 3,000 = 21,000 watts). Since the service voltage is known, the total current draw of the heaters can be calculated. Simply divide the total watts by 240 volts (21,000 ÷ 240 = 87.5 = 88 amperes). The calculated load in amperes for seven 3,000 watt, 240-volt heaters on a 240-volt, single-phase service is 88 amperes (see Figure 1).

If the ½ hp blower motor is the largest motor in the calculation for the service, the ampacity must not be less than 125 percent of the full-load current rating plus the calculated load of the electric heat. For example, the blower motor in the heating/cooling package unit in the last example will be the largest motor in the one-family dwelling.

How much load will this package unit add to a 240-volt, single-phase service? Multiply the motor’s full-load current by 125 percent before adding it to the electric heat (4.9 × 125% = 6.125 + 40 = 46.125 = 46 amperes). Because the blower motor is the largest motor in the service load calculation, the calculated load is now 46 amperes (see Figure 3).

While fixed electric space heating loads must be calculated in accordance with 220.51 when calculating feeder and service loads, branch-circuit loads must be calculated in accordance with Article 424. See the reference to fixed electric space heating equipment in Table 220.3. Branch-circuit sizing requirements for fixed electric space heating equipment are in 424.3. Where sizing branch circuits, fixed electric space heating equipment shall be considered continuous load [424.3(B)]. As stipulated in 210.19(A)(1), branch-circuit conductors must have an ampacity of not less than 125 percent of the continuous load. For example, what minimum size 75°C branch-circuit conductors are required to feed the heating/cooling package unit in Figure 3? While the service load for this unit was calculated at 100 percent, the branch-circuit load must be calculated at 125 percent because it is considered a continuous load. In accordance with 430.22(A), the blower motor must also be calculated at 125 percent. The blower motor and heating element have a combined current of 44.9 amperes (4.9 + 40 = 44.9 amperes). After multiplying the combined current by 125 percent, the 75°C branch-circuit conductors must have an allowable ampacity of at least 56 amperes (44.9 × 125% = 56.125 = 56 amperes). The minimum size 75°C conductors required to feed this heating/cooling package unit are 6 AWG copper conductors (see Figure 4).

The examples of this month’s Code in Focus were calculated on the larger of the heating or cooling loads. See 220.60 for using only the largest of noncoincident loads when calculating the total load of a feeder or service.

Where meeting the exception to 220.51, it is permissible to calculate the fixed electric space heating load at less than 100 percent. Where reduced loading of the conductors results from units operating on duty-cycle, intermittently, or from all units not operating at the same time, the authority having jurisdiction may grant permission for feeder and service conductors to have an ampacity less than 100 percent, provided the conductors have an ampacity for the load so determined [220.51 Exception].

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 NFPA’s “Electrical Reference.” He can be reached at 615.333.3336, charles@charlesRmiller.com or www.charlesRmiller.com.