Article 220 of the National Electrical Code (NEC) provides requirements for calculating branch-circuit, service and feeder loads. Definitions for the three types of loads that Article 220 covers are in Article 100. A branch circuit is the circuit conductor between the final overcurrent device protecting the circuit and the outlet(s). Branch circuits are divided into four categories: appliance, general purpose, individual and multiwire (see Figure 1).
With the exception of branch circuits that supply only motor loads, requirements for branch circuits are in Article 210.
Feeders, as defined in Article 100, are all circuit conductors between the service equipment, the source of a separately derived system, or other power supply source, and the final branch-circuit overcurrent device. Article 215 covers the installation requirements, overcurrent protection requirements, minimum size and ampacity of conductors for feeders that supply branch-circuit loads (see Figure 2).
Article 100 defines service as the conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served. Article 230 covers service conductors and equipment for control and protection of services and their installation requirements (see Figure 3).
Last month’s column concluded by covering the calculated load for appliance and laundry branch circuits in 220.82(B)(2). This month, the discussion continues with optional feeder or service load calculations as specified in 220.82.
The optional calculation for a one-family dwelling starts out the same as the standard method calculation specified in Part III of Article 220. General lighting loads, small-appliance branch circuit loads and laundry loads are calculated the same regardless of the method (standard or optional). The two methods are not the same when it comes to appliances. With the standard method load calculation, when there are four or more fastened-in-place appliances, it is permissible to apply a 75 percent demand factor [220.53]. With the optional method, do not apply the 75 percent demand factor to fastened-in-place appliances. Include the nameplate rating of all appliances that are fastened in place, permanently connected or located to be on a specific circuit [220.82(B)(3)(a)]. All fastened-in-place appliances, regardless of the number, must be included at 100 percent. For example, what is the optional method service load calculation (before applying the demand factor) for a one-family dwelling with the following fastened-in-place appliances: a dishwasher rated 10 amperes (A) at 120 volts (V); a horsepower, 120V kitchen waste disposer; a trash compactor rated 7.5A at 120V; and a 4.5 kilowatts (kW), 240V water heater? The dishwasher has a load of 1,200 watts (W) (10 120 = 1,200). The load of the kitchen waste disposer is 1,176W (9.8 120 = 1,176). The compactor has a load of 900W (7.5 120 = 900). The load of the water heater is 4,500W (4.5 1,000 = 4,500). Although there are four fastened-in-place appliances, do not apply the 75-percent demand factor. Before applying the demand factor in 220.82(B), the calculated load for these four appliances is 7,776W (1,200 + 1,176 + 900 + 4,500 = 7,776) (see Figure 4).
Another difference between the standard and optional method is how household-cooking equipment is calculated. With the standard method, it is permissible to apply Table 220.55 demand factors to household electric ranges, wall-mounted ovens, counter-mounted cooking units and other household cooking appliances individually rated in excess of 1 kW. With the optional method, ranges, wall-mounted ovens and counter-mounted cooking units are added to the calculation at 100 percent of the nameplate rating [220.82(B)(3)(b)]. For example, what is the optional method service load calculation (before applying the demand factor) for a one-family dwelling with one 8-kW, 240V cooktop and two 5-kW, 240V ovens? These household cooking appliances will be supplied from a single branch circuit and located in the same room. The combined rating of the cooktop and ovens is 18 kW (8 + 5 + 5 = 18). Since ranges, wall-mounted ovens and counter-mounted cooking units must be added to the calculation at nameplate rating, add 18 kW to the general loads covered in 220.82(B) (see Figure 5).
Likewise, clothes dryers must be added to the optional method load calculation at nameplate rating [220.82(B)(3)(c)]. This provision does not apply to clothes dryers that are connected to the 20A laundry branch circuit covered in 220.82(B)(2). With the standard method load calculation for a one-family dwelling with an electric clothes dryer, the Table 220.54 demand factor is 100 percent. Therefore, regardless of the load calculation method, if a one-family dwelling has one clothes dryer, it must be included at 100 percent of the nameplate rating. There is one substantial difference when it comes to clothes dryers. With the standard method, the clothes dryer load must be at least 5,000W (volt-amperes) for each dryer served. With the optional method, there is no minimum rating. For example, what is the optional method service load calculation (before applying the demand factor) for a one-family dwelling with one 4 kW, 240V clothes dryer? Since clothes dryers must be added to the calculation at nameplate rating and because there is not a minimum nameplate rating, add 4 kW to the general loads covered in 220.82(B) (see Figure 6).
It is not required that a dwelling have a receptacle outlet installed for an electric clothes dryer.
Some items on this list fall into more than one category; a water heater is a good example. A water heater must be added to the optional method load calculation at nameplate rating [220.82(B)(3)(d)]. Note that a water heater is listed separately in 220.82(B)(3). But, a water heater is also fastened in place, permanently connected, and located on a specific circuit, which is covered in 220.82(B)(3)(a). If the dwelling has a water heater and it was included with the fastened-in-place appliances, do not include it here. If the water heater was not included with the fastened-in-place appliances, add the nameplate rating of the water heater to the general loads covered in 220.82(B).
Do not include heating and air conditioning equipment in the list of fastened-in-place appliances. Heating and air conditioning equipment is covered in 220.82(C). Depending on the type of system, demand factors may or may not apply to that equipment.
Next month’s column will continue the discussion of feeder and service load calculations.
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, charles@charlesRmiller.com and www.charlesRmiller.com.