Branch-Circuit, Feeder and Service Calculations, Part XXIV
by Charles R. Miller
Published: February 2008
Article 220 – Load Calculations
220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit
Requirements for calculating branch-circuit, feeder
and service loads are in Article 220 of the National Electrical Code
(NEC). Article 220 is divided into five parts. General requirements for
load-calculation procedures are in Part I. Part II covers requirements
for branch-circuit load calculations. Part III contains specifications
for feeder and service load calculations. This part sometimes is
referred to as the standard method for feeder and service load
calculations. Part IV, “Optional Feeder and Service Load Calculations,”
contains optional load-calculation procedures for a single dwelling
unit, an existing dwelling unit, a multifamily dwelling, two dwelling
units, a school, an existing installation and a new restaurant. Part V
provides calculation specifications for farm loads. Figure 220.1 (in
the NEC) shows information on the organization of Article 220. Results
from load calculations in Article 220, along with provisions from other
articles, can be used to find conductor sizes and ampere ratings for
overcurrent protection.
Last month’s Code
in Focus covered electric cooking equipment in 220.55. This month, the
discussion continues with calculating loads for electric ranges and
other cooking appliances in dwelling units.
Table
220.55, along with 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 kilowatts (kW) or 1,750 watts (W). Up to this point, the
discussion has included only ranges with ratings above 8¾ kW. While
Column C contains maximum demands in kilowatts, Columns A and B contain
demand factor percentages. These percentages are multiplied by the
kilowatt ratings to determine the demand load in kilowatts. Column A is
for household cooking appliances rated less than 3½ kW. For example, in
accordance with Article 220, what is the service demand load for a
3-kW, wall-mounted oven? The demand factor percent for one piece of
equipment from Column A is 80 percent. Therefore, multiply the kilowatt
rating of the appliance by 80 percent (3 × 80% = 2.4 kW). The service
demand load for a 3-kW, wall-mounted oven is 2.4 kW (see Figure 1).
Note that since the value is in kilowatts, numbers to
the right of the decimal are not usually rounded up or dropped. In this
example, the number to the right of the decimal represents 400W (2.4 kW
= 2,400W).
When there is more than one
appliance with a rating less than 3½ kW, look in the left column of
Table 220.55 for the number, and follow the row across to the
percentage in Column A. For example, what is the service demand load
for 25 3-kW, wall-mounted ovens? First, find the demand factor percent
in Column A for 25 appliances (30%). Now, calculate the total kilowatts
of the wall-mounted ovens (3 × 25 = 75). Finally, multiply the total
kilowatt load by the demand factor percent (75 × 30% = 22.5). The
service demand load for 25 3-kW, wall-mounted ovens is 22.5 kW (see
Figure 2).
Column B is for household
cooking appliances rated at least 3½ kW, but not more than 8¾ kW. For
example, what load should be added to a feeder calculation for a
counter-mounted cooking unit (cooktop) that is rated 5 kW? The demand
factor percent for one piece of equipment from Column B is 80 percent.
Multiply the equipment’s kilowatt rating by 80 percent (5 × 80% = 4
kW). The feeder demand load for a 5-kW, counter-mounted cooking unit is
4 kW (see Figure 3).
Although the demand factor percent for one appliance
in Column A is the same as it is for one appliance in Column B, the
percentages are not the same when there is more than one appliance. For
example, what is the service demand load for 25 5-kW, counter-mounted
cooking units? First, find the demand factor percent in Column B for 25
appliances (26%). Now, calculate the total kilowatts of the
counter-mounted cooking units (5 × 25 = 125). Finally, multiply the
total kilowatt load by the demand factor percent (125 × 26% = 32.5).
The service demand load for 25 5-kW, counter-mounted cooking units is
32.5 kW (see Figure 4).
It would seem,
because of the progression of Columns A and B, that Column C would be
for household cooking appliances rated more than 8¾ kW through 12 kW.
However, Column C is not limited to appliances rated more 8¾ kW. As
stated in the heading, Column C is for household cooking appliances
“not over 12 kW rating.” Therefore, any time Columns A or B can be used
to calculate household cooking appliances, Column C also can be used.
If the load for some cooking equipment can be found in two columns,
then which column should be used? Note 3 under Table 220.55 answers
this question. In lieu of the method provided in Column C, it shall be
permissible to add the nameplate ratings of all household cooking
appliances rated more than 1¾ kW but not more than 8¾ kW and multiply
the sum by the demand factors specified in Column A or B for the given
number of appliances. If the load for cooking equipment can be found in
Column A and Column C, it is permissible to use the lowest rating. If
the load for cooking equipment can be found in Column B and C, it also
is permissible to use the lowest rating.
If
the equipment is rated from 3½ to 8¾ kW, start by calculating the
demand load from Column B. Then compare the calculated load from Column
B with the demand load from Column C and select the lower of the two.
For example, what is the service demand load for six 8-kW ranges?
First, calculate the load for six 8-kW ranges from Column B (6 × 8 ×
43% = 20.64 kW). Now, because 8 kW is not over 12 kW, find the load for
six ranges from Column C (21 kW). Finally, compare the two, and select
the lower number. The service demand load for six 8-kW ranges is 20.64
kW or 20,640W. For this example, the lower of the two numbers is from
Column B (see Figure 5).
Finally, don’t make the mistake of thinking Columns A
and B (when compared to Column C) will always be the lower of the two
numbers. If the load for cooking equipment can be found in Columns A or
B, compare it with Column C, and then select the lower of the two. In
the last example, if one more range is added, the column having the
lowest demand load changes. For example, what is the service demand
load for seven 8-kW ranges? First, calculate the load for seven 8-kW
ranges from Column B (7 × 8 × 40% = 22.4 kW). Next, look in Column C
for the maximum demand (22 kW). Finally, compare the two, and select
the lower number. The service demand load for seven 8-kW ranges is 22
kW or 22,000W. For this example, the lower of the two numbers is from
Column C (see Figure 6).
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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