Branch-Circuit, Feeder and Service Calculations, Part XX
by Charles R. Miller
Published: October 2007
Article 220 – Load Calculations
220.54 Electric Clothes Dryers—Dwelling Unit
Understanding how to perform load calculations in accordance with the National Electrical Code (NEC)
is an important part of an electrician’s professional career. Loads
must be calculated before installing branch circuits, feeders and
services. Branch-circuit, feeder and service load calculation
requirements are in Article 220 of Chapter 2. The NEC
contains an introduction, nine chapters and annexes. Chapters 1 through
4 apply generally to all electrical installations. Chapters 5 through 7
apply to special occupancies, special equipment or other special
conditions. These three chapters supplement or modify the general rules
specified in the first four chapters. Chapter 8 covers communications
systems and is not subject to the requirements of Chapters 1 through 7
unless the requirements are specifically referenced therein. Chapter 9
consists of tables that are applicable as referenced. Annexes follow
the tables in Chapter 9. Annexes are not part of the requirements of
the NEC. They but are included for informational purposes
only. While the 2005 edition contains seven annexes (A through G), the
2008 edition contains eight (A through H).
Last
month’s Code in Focus concluded by covering fastened-in-place appliance
calculations that are specified in 220.53. 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]. Unlike the required minimum
load of 1,500 volt-amperes for a laundry branch circuit, there is no
required minimum load if an electric clothes dryer will not be
installed. Also, a gas dryer could be installed and, therefore, not
draw the type of load specified in 220.54. When calculating the load on
a feeder or service, if an electric clothes dryer will be installed,
the calculation must include a load of at least 5,000 watts
(volt-amperes). For example, what is the minimum service load for a
clothes dryer that has a nameplate rating of 4.5 kilowatts (kW)? Assume
dryer kilowatt ratings are equivalent to kilovolt-ampere (kVa) ratings
for these load calculations. First, convert kilowatts to watts (or
volt-amperes) by multiplying the kilowatts by 1,000 (4.5 × 1,000 =
4,500). Since this dryer is only 4,500 volt-amperes, it does not meet
the minimum load specified for a clothes dryer. The minimum service
load for this clothes dryer is 5,000 volt-amperes (see Figure 1).
If the nameplate rating exceeds the required minimum
rating of 5,000 volt-amperes, use the nameplate rating. For example,
what is the minimum feeder load for a clothes dryer that has a
nameplate rating of 5.6 kW? First, convert kilowatts to watts (or
volt-amperes) by multiplying kilowatts by 1,000 (5.6 × 1,000 = 5,600).
Since the load of this clothes dryer is more than the required minimum,
use the nameplate rating. The minimum feeder load for this clothes
dryer is 5,600 volt-amperes (see Figure 2).
Where there are five or more electric clothes
dryers on a feeder or service, the load can be derated or reduced. The
demand factor for one, two, three or four dryers is 100 percent;
therefore, no derating is permissible. In accordance with 220.54, it is
permissible to apply the demand factors in Table 220.54 to household
electric clothes dryers.
For
example, what is the calculated dryer load for an eight-unit
multifamily dwelling with each unit having a 5.2 kW clothes dryer?
First, multiply the nameplate rating (because it is more than the
required minimum) by 1,000 and then by the number of dryers (5.2 ×
1,000 × 8 = 41,600). Next, find the demand factor percent across from
eight dryers in Table 220.54 (60 percent). Finally, multiply the total
dryer load by the demand factor (41,600 × 60 percent = 24,960). The
minimum clothes dryer load for this multifamily dwelling is 24,960
volt-amperes (see Figure 3). Do not apply Table 220.54 demand factors
to the laundry facility (laundromat) of a multifamily dwelling. It is
possible that all dryers will be operating at one time in a laundry
facility.
Where the number of dryers is 12 or more, a calculation
may be necessary to find the demand factor percent. For example, what
is the calculated dryer load for a 20-unit multifamily dwelling with
each unit having a 4.8-kW clothes dryer? Since each dryer is only 4,800
volt-amperes (4.8 × 1,000) and does not meet the required minimum,
change each to 5,000 volt-amperes and multiply by the number of dryers
(5,000 × 20 = 100,000). Now, find the demand factor percentage from the
formula across from 12 to 22 dryers [% = 47 – (number of dryers – 11)].
Perform the part of the formula in the parentheses first [(number of
dryers – 11) = (20 – 11) = (9)]. Now subtract 9 from 47 to find the
demand factor percentage (% = 47 – 9 = 38%). Finally, apply the demand
factor to the total dryer load (100,000 × 38% = 38,000). The minimum
clothes dryer load for this multifamily dwelling is 38,000 volt-amperes
(see Figure 4). In the 2008 NEC, the formula is stated as 47
minus 1% for each dryer exceeding 11. Both formulas will yield the same
demand factor percentages when calculating 12 to 23 dryers. The number
of dryers exceeding 11 is 9 (20 – 11 = 9). The demand factor is 38
percent after subtracting 9 from 47 (47 – 9 = 38%).
The demand factor for 23 dryers is simply 35 percent.
The formula is a little different when the number of dryers is 24 to
42. For example, what is the calculated dryer load for a 40-unit
multifamily dwelling with each unit having a 5,500-watt electric
clothes dryer? First, multiply 5,500 volt-amperes by the number of
dryers (5,500 × 40 = 220,000). Now, find the demand factor percentage
from the formula across from 24 to 42 dryers {% = 35 – [0.5 × (number
of dryers – 23)]}. Start by solving the equation inside the parentheses
(number of dryers – 23) = (40 – 23) = (17). Next, solve the equation
within the brackets [0.5 × 17] = [8.5]. Now subtract 8.5 from 35 (35 –
8.5 = 26.5). Finally, apply the demand factor to the total dryer load
(220,000 × 26.5% = 58,300). The minimum clothes dryer load for this
multifamily dwelling is 58,300 volt-amperes (see Figure 5). In the 2008
NEC, the formula is stated as 35% minus 0.5% for each dryer
exceeding 23. Since the number of dryers exceeding 23 is 17 (40 – 23 =
17), multiply 17 by 0.5% (17 × 0.5% = 8.5). The demand factor is 26.5
percent after subtracting 8.5 from 35 (35 – 8.5 = 26.5%).
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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