Branch-Circuit, Feeder and Service Calculations, Part XXI
by Charles R. Miller
Published: November 2007
Article 220 – Load Calculations
220.54 Electric Clothes Dryers—Dwelling Unit
Load calculation requirements are in Article 220 of
the National Electrical Code (NEC). It provides requirements for
calculating branch-circuit, feeder and service loads. The Code divides
requirements in Article 220 into five parts. Part I covers general
requirements for calculation procedures. Part II provides calculation
provisions for branch circuits. Part III contains feeder and service
calculation requirements (sometimes referred to as the standard
method). Part IV specifies optional feeder and service load
calculations are specified. Part V regulates the calculation of farm
loads. Results from calculations in Article 220 are used in conjunction
with requirements from other articles to find conductor sizes and
ampere rating for overcurrent protection. While Article 220 does not
contain provisions for determining the minimum number of branch
circuits, results from this article’s calculations together with rules
from other articles can be used to find the minimum number of branch
circuits. As stipulated in 210.11, the minimum number of branch
circuits shall be determined from the total calculated load and the
size or rating of the circuits used. In all installations, the number
of circuits shall be sufficient to supply the load served. In no case
shall the load on any circuit exceed the maximum specified by 220.18.
Last month’s Code in Focus covered electric clothes dryers in 220.54.
This month, the discussion continues with calculating loads for
electric clothes dryers in dwelling units.
The load for household electric clothes dryers in a dwelling unit(s)
shall be either 5,000 watts (volt-amperes) or the nameplate rating,
whichever is larger, for each dryer served [220.54]. As discussed last
month, an electric clothes dryer is not a required item in a dwelling
unit load calculation, but if an electric clothes dryer will be
installed, the load added to the calculation must be at least 5,000
volt-amperes. For example, what is the minimum one-family dwelling
service load for a 4.7-kW combination clothes washer and dryer? While
there is no mention of a combination clothes washer and dryer in
Article 220, an example in Annex D shows a load calculation with a
washer/dryer. In the load calculation of Example D2(b), a combination
clothes washer and dryer is calculated exactly the same as a clothes
dryer (see Example D2(b) in Annex D). Since the washer/dryer is only
4,700 volt-amperes (4.7 × 1,000) and does not meet the required
minimum, change this load to 5,000 volt-amperes. The minimum service
load for this combination clothes washer and dryer is 5,000
volt-amperes (see Figure 1).
This first example illustrates a washer/dryer load
calculation in a dwelling unit. Having a combination washer/dryer load
in the calculation does not mean it is permissible to omit the laundry
circuit load required by 220.52(B). A load of not less than 1,500
volt-amperes shall be included for each two-wire laundry branch circuit
installed as required by 210.11(C)(2) [220.52(B)]. Likewise, a 125-volt
receptacle outlet is still required for the laundry area even if one is
not required for the washer/dryer unit [210.11(C)(2), 210.52 and
210.52(F)] (see Figure 2).
It is permissible to apply an additional demand factor
of 70 percent to the calculated feeder or service load of household
electric clothes dryers [220.61(B)]. For example, a clothes dryer will
be installed in a single-family dwelling. What is the minimum neutral
load on the service for a 5,000-watt clothes dryer? When calculating
feeder or service neutral loads for clothes dryers, start by finding
the feeder or service load. Because there is only one dryer, the
minimum service load (at 100 percent) is 5,000 volt-amperes. Now,
multiply the service load by 70 percent (5,000 × 70% = 3,500). The
minimum neutral load on the service for a 5,000 watt clothes dryer is
3,500 volt-amperes (see Figure 3).
Where there are five or more electric clothes dryers on
a feeder or service, the load can be derated or reduced. In accordance
with 220.54, it is permissible to apply the demand factors in Table
220.54 to household electric clothes dryers. Where the number of dryers
is one through four, no derating is permissible. The demand factors
change with every unit where the number of clothes dryers is five
through 42. Regardless of the number of dryers over 42, the demand
factor remains the same. Where there are 43 or more electric clothes
dryers on a feeder or service, the total dryer load can be multiplied
by 25 percent. For example, a multifamily dwelling will have a
5,500-watt electric clothes dryer in each of 100 units. What is the
minimum service load for these clothes dryers? If the nameplate is more
than 5,000 volt-amperes, the load calculation must be based on the
nameplate. First, multiply the nameplate rating (because it is more
than the required minimum) by the number of dryers (5,500 × 100 =
550,000). Next, because the number of dryers is more than 42, multiply
the total dryer load by 25 percent (550,000 × 25% = 137,500). The
minimum clothes dryer load for this multifamily dwelling is 137,500
volt-amperes, or 135.5 kVA (see Figure 4).
The demand factor of 70 percent for dryer neutral loads
can be applied to the calculated load even after demand factors in
Table 220.54 have been applied. For example, what is the minimum
neutral load on the service for a 10-unit multifamily dwelling with
each unit having a 5.5-kW clothes dryer? First, find the total load
before the application of demand factors (5.5 × 1,000 × 10 = 55,000).
Now, find the demand factor percent across from 10 dryers in Table
220.54 (50 percent). Next, multiply the total dryer load by the demand
factor (55,000 × 50% = 27,500). Finally, apply the demand factor
permitted for household electric clothes dryers (27,500 × 70% =
19,250). The minimum neutral load on the service for 10 5.5-kW clothes
dryers in this multifamily dwelling is 19,250 volt-amperes (see Figure
5).
Although this section provides requirements for feeder
and service loads, these calculations also apply to branch-circuit
loads. Load calculations shall be permitted as specified in 220.54 for
electric dryers and in 220.55 for electric ranges and other cooking
appliances [220.14(B)].
Next month’s Code in Focus 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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