Branch-Circuit, Feeder and Service Calculations, Part XIX
by Charles R. Miller
Published: September 2007
Article 220 – Load Calculations
220.53 Appliance Load—Dwelling Unit
While knowing how to perform load calculations from Article 220 of the National Electrical Code (NEC)
could be the difference between pass and fail on a journeyman or master
electrician’s exam, it actually is important in an electrician’s daily
life. Before sizing conductors and overcurrent protective devices, the
loads must be calculated. It could be as simple as looking on a
nameplate and finding the minimum circuit amperes or the maximum
overcurrent protection. Then again, it may be necessary to calculate
the load in accordance with Article 220 before selecting conductors and
overcurrent protective devices.
Requirements
in Article 220 are divided into five parts. Part I covers general
requirements for load calculation procedures. Branch-circuit load
calculation procedures are in Part II. Feeder and service load
calculation requirements (sometimes referred to as the standard method)
are in Part III. Part IV contains optional feeder and service load
calculation provisions. Farm loads must be calculated in accordance
with Part V of Article 220.
Last
month’s column concluded by covering small appliance and laundry load
calculations in 220.52. This month, the discussion continues with
appliance loads in dwelling units.
If
there are four or more fastened-in-place appliances on the same feeder
or service in a dwelling unit, it is permissible to derate or reduce
the load. It shall be permissible to apply a demand factor of 75
percent to the nameplate rating load of four or more appliances
fastened in place—other than electric ranges, clothes dryers,
space-heating equipment or air conditioning equipment—that are served
by the same feeder or service in a one-family, two-family or
multifamily dwelling [220.53]. Appliance is defined as utilization
equipment (generally not industrial) that is normally built in
standardized sizes or types and is installed or connected as a unit to
perform one or more functions such as clothes washing, air
conditioning, food mixing, deep frying, etc. [Article 100]. Some
examples of appliances that are fastened in place include dishwashers,
kitchen-waste disposers, trash compactors, attic fans and water heaters
(see Figure 1).
Certain appliances and equipment must not
be included when determining the number of fastened-in-place
appliances. In accordance with 220.53, electric ranges, clothes dryers,
space-heating equipment or air conditioning equipment must not be
included with the number of appliances that are fastened in place (see
Figure 2). Provisions for these appliances are in other sections of
Article 220’s Part III. Electric clothes dryers are covered in 220.54
and Table 220.54. Electric ranges are covered in 220.55 and Table
220.55. Space heating load calculation provisions are in 220.51. As
specified in 220.50, loads for air conditioning equipment must be
calculated in accordance with 440.6.
Applying the demand factor to less than
four fastened-in-place appliances in a dwelling is not permitted. For
example, the following appliances will be installed in a one-family
dwelling: a dishwasher rated 10 amperes at 120 volts; a ½ hp, 120-volt
kitchen-waste disposer; and a 4.5 kW, 240-volt water heater.
Kilovolt-amperes (kVA) shall be considered equivalent to kilowatts (kW)
for loads calculated in this article. The load for these three
appliances must be calculated at 100 percent. The dishwasher has a load
of 1,200 watts (10 × 120 = 1,200). Since the kitchen-waste (or garbage)
disposer is a motor, the load must be determined by multiplying the
full-load current in Table 430.248 by the system voltage. In accordance
with Table 430.248, the full-load current of a ½-hp, 120-volt motor is
9.8 amperes. Although the current value of 9.8 amperes is in the column
titled “115 volts,” the currents listed in this column are permitted
for system voltage ranges of 110 to 120 volts (see the last sentence of
the text in Table 430.248). The disposer has a load of 1,176 watts (9.8
× 120 = 1,176). The load of the water heater is 4,500 watts (4.5 ×
1,000 = 4,500). Since derating is not permissible for only three
appliances, the calculated load at 100 percent is 6,876 watts (1,200 +
1,176 + 4,500 = 6,876) (see Figure 3).
With the addition of one more
fastened-in-place appliance, applying a demand factor of 75 percent to
the nameplate-rating load is permissible. For example, what is the
service load calculation for a one-family dwelling with the following
appliances: a dishwasher rated 10 amperes at 120 volts; a ½-hp,
120-volt kitchen-waste disposer; a trash compactor rated 7.5 amperes at
120 volts; and a 4.5-kW, 240-volt water heater? The dishwasher has a
load of 1,200 watts (10 × 120 = 1,200). The load of the kitchen-waste
disposer is 1,176 watts (9.8 × 120 = 1,176). The compactor has a load
of 900 watts (7.5 × 120 = 900). The load of the water heater is 4,500
watts (4.5 × 1,000 = 4,500). The total before applying the demand
factor is 7,776 watts (1,200 + 1,176 + 900 + 4,500 = 7,776). Next,
because there are four appliances, multiply the load by the 75 percent
demand factor. The calculated service load for these appliances is
5,832 watts (7,776 × 0.75 = 5,832). Note that three of the appliances
in this example were the same as the three appliances in the previous
example. By adding one more appliance, the load calculation is now less
than the load calculation in Figure 3 (see Figure 4).
The fastened-in-place appliance-demand
factor also can be applied when calculating a feeder. It is permissible
to apply the 75 percent demand factor to each feeder with four or more
fastened-in-place appliances. There may be an occasion when there will
be at least four appliances on the service but less than four
appliances on each feeder. For example, a one-family dwelling will have
two feeders supplied by the service. One feeder will provide power to a
water heater and two attic ventilation fans. The other feeder will
provide power to a dishwasher, a kitchen-waste disposer and a trash
compactor. Since only three appliances are on each feeder, it is not
permissible to apply the 75 percent demand factor to either feeder load
calculation. The demand factor can be applied to the service load
calculation because it will supply power to six fastened-in-place
appliances (see Figure 5).
Next month’s column continues 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 NFPA’s “Electrical Reference.” He can be reached at 615.333.3336, charles@charlesRmiller.com or www.charlesRmiller.com.
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