Branch-Circuit, Feeder and Service Calculations, Part XXIII
by Charles R. Miller
Published: January 2008
Article 220 – Load Calculations
220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit
A new edition of the National Electrical Code (NEC)
is published every three years. Since its inception in 1897, there have
been 51 editions of the NEC. Starting this month, this column will
reference the 2008 edition.
The purpose
of the Code is the practical safeguarding of persons and property from
hazards arising from the use of electricity (90.1). The NEC is divided
into an introduction and nine chapters. Chapters 1 through 4 apply
generally to all electrical installations. Chapter 5 applies to special
occupancies, such as hazardous (classified) locations, healthcare
facilities, assembly occupancies, manufactured homes and buildings,
marinas and boatyards, and temporary installations. Chapter 6 applies
to special equipment, such as electric signs and outline lighting,
cranes, elevators, welders, information technology equipment,
industrial machinery, swimming pools, and fire pumps. Chapter 7 applies
to special conditions, such as emergency systems; standby systems
(legally required and optional); Class 1, 2 and 3 circuits; fire alarm
systems; and the installation of optical fiber cables and raceways (see
the Table of Contents in the NEC for a complete listing of each
chapter). Chapter 8 covers communications systems and is not subject to
the requirements of Chapters 1 through 7 unless the requirements are
specifically referenced in Chapter 8. Chapter 9 contains tables that
are applicable as referenced. The NEC also contains annexes, labeled A
through H. The annexes are not part of the NEC requirements but are
included for informational purposes only.
Last
month’s Code in Focus covered electric cooking equipment in 220.54.
This month, the discussion continues with calculating loads for
electric ranges and other cooking appliances in dwelling units.
While
household cooking equipment can be added to a feeder or service load
calculation at 100 percent of the equipment’s rating, it is not
required. It is permissible to reduce the rating by applying the demand
factors or by using the maximum demand loads in Table 220.55. For
example, a 12-kW electric range will be installed in a one-family
dwelling. This range can be added to the service load calculation at 12
kW (12,000 watts), or instead of the full rating, this range can be
added to the calculation at a reduced load as specified by Table
220.55. Look in the left column of Table 220.55 for the number of
appliances. This example has only one range; therefore, drop to “1,”
and follow the row across to the appropriate column. Column A is used
where the rating of the appliance is less than 3½ kW. Column B is used
where the rating of the appliances is 3½ kW through 8¾ kW. Column C is
the only column applicable for a 12-kW range. In accordance with Column
C, a 12-kW range has a maximum demand of 8 kW (see Figure 1).
Because household ranges are rarely used at full
potential, the load added to the service or feeder can be less than the
nameplate rating of the appliance. On occasions such as Thanksgiving,
the oven and all four top elements could be on at the same time. But
even then, everything may not be energized at the same time because of
the thermostats.
Table 220.55, along
with the five notes under the table, can be applied to household
electric ranges, wall-mounted ovens, counter-mounted cooking
appliances, and other household cooking appliances individually rated
in excess of 1¾ kW or 1,750 watts. While Columns A and B have a limited
choice of kilowatt ratings, Column C does not. For example, what is the
service demand load for a 9-kW range? Since Column A is for equipment
rated less than 3½ kW and Column B for equipment rated 3½ through 8¾
kW, the demand load can be found only in Column C. The demand loads
shown in Column C are for appliances with ratings of 12 kW or less. The
maximum load required for one 9-kW range when calculating a service or
feeder is 8 kW. Note that this is the same demand as the 12-kW range in
the first example. The maximum demand for one range with a rating
higher than 8¾ kW, but not higher than 12 kW, is in Column C (see
Figure 2).
The procedure is the same when there is more than one
range. Look in the left column for the number of appliances, and follow
the row across to the appropriate column. For example, a 10-unit
apartment building will have a 12-kW range in each apartment. What is
the demand load these ranges will add to a service calculation? Start
by looking in the left column for 10 appliances. Because of the range
ratings, the answer again is in Column C. The maximum demand load
required is only 25 kW (see Figure 3). Although the total nameplate
load is 120 kW (10 × 12), the maximum load at any one time is only 25
kW. As the number of units in a multifamily dwelling increases, the
percentage of ranges likely to be in use simultaneously decreases.
When
there are more than 25 appliances with ratings of more than 8¾ kW each,
the maximum demand must be calculated. If the number of appliances is
between 26 and 40, the calculation is as follows: 15 kW + 1 kW for each
range. Simply add 15 to the number of appliances, and the total is the
kilowatt demand load. For example, what is the service demand load for
40 10-kW ranges? Because the ranges are rated 10 kW each, the demand
load is in Column C. Add 15 to the number of appliances (15 + 40 = 55).
The demand load for 40 10-kW ranges is 55 kW (see Figure 4).
The procedure of adding 15 to the number of
appliances actually starts with 5 appliances. For example, 15 plus 5
appliances equals 20. The maximum demand in Column C for 5 appliances
is 20 kW.
When the number of appliances
(rated more than 8¾ kW) is greater than 40, the calculation method
changes. Starting with 41 ranges, the calculation is as follows: 25 kW
+ ¾ kW for each range. Instead of each appliance counting as 1 kW, each
appliance will count only as ¾ kW (0.75 kW). Multiply the number of
appliances by 0.75, and then add 25 to the product. For example, what
is the service demand load for 60 12-kW ranges? The product of 60
multiplied by 0.75 is 45 (60 × 0.75 = 45). The sum of adding 25 to 45
is 70 (25 + 45 = 70). The maximum demand for 60 ranges rated 12 kW is
70 kW (see Figure 5).
Next month’s article 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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