BranchCircuit, Feeder and Service Calculations, Part XXX
by Charles R. Miller
Published: August 2008
Article 220 – Load Calculations
220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit
Knowing where to find load calculation requirements
and understanding how to perform the calculations in accordance with
the National Electrical Code (NEC) is essential for the professional
electrician. Before selecting conductors, overcurrent protection,
panelboards and even raceways, loads must be calculated. Decisions must
be made about which loads will be on each branch circuit and how many
branch circuits will be needed. Decisions must also be made about
feeders and services. Branchcircuit, feeder and service load
calculations are in Article 220 of the NEC. Article 220 is divided into
five parts. General requirements pertaining to branchcircuit, feeder
and service load calculations are in Part I. Part II contains
calculation provisions for branch circuits. Feeder and service
calculation requirements are in Part III. Although not official, Part
III is commonly referred to as the “standard load calculation method.”
Optional feeder and service load calculation provisions are in Part IV
of Article 220. This part 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. As the name implies, the optional calculation method is
an alternative calculation method that is permitted in accordance with
Part IV. Farm loads must be calculated according to the requirements in
Part V.
Last month’s column 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.
Household
electric ranges, wallmounted ovens, countermounted cooking units and
other household cooking appliances can be added to a load calculation
at the full kilowatt rating of the equipment or can be calculated in
accordance with 220.55 and Table 220.55. When using Table 220.55 to
find the cooking equipment’s rating for the service or feeder, the
kilowatt rating from the table will be lower than the equipment’s
nameplate rating. The lower or reduced kilowatt rating is referred to
as the demand load. Column C provides the maximum demand load in
kilowatts (kW) for cooking equipment over 8¾ kW through 12 kW. Either
the first or second note under Table 220.55 applies when one or more
ranges are individually rated more than 12 kW but not more than 27 kW.
Use the first note when all the ranges are rated the same. Use the
second note when the ranges are not rated the same. Where at least one
range is rated over 12 kW through 27 kW and at least one range is rated
more than 8¾ kW but not more than 27 kW, the second note applies. When
using this note, any range with a nameplate rating that is less than 12
kW must be changed to 12 kW before finding the average value of rating.
Find the average value of rating by adding together the ratings of all
ranges to obtain the total connected load, and then divide by the
number of ranges. The maximum demand in Column C shall be increased 5
percent for each kilowatt or major fraction thereof by which this
average value exceeds 12 kW.
The
process of using 12 kW for any range rated less than 12 kW is a
critical step in calculating in accordance with Note 2 under Table
220.55. For example, what is the service demand load for eight 9kW,
six 13kW and six 15kW household electric ranges? Before finding the
average value of rating, change the 9kW ranges to 12kW ranges. Next,
find the total kilowatt rating of all the ranges [(8 × 12) + (6 × 13) +
(6 × 15) = 96 + 78 + 90 = 264]. Now, find the average value by dividing
the total connected load by the total number of ranges (264 ÷ 20 = 13.2
= 13 kW) (see Figure 1).
Now that the
average value for the ranges of unequal rating is known, find the
service demand load for 20 13kW ranges. Because Column C is based on
12kW ranges, subtract 12 from 13 (13 – 12 = 1). Since 13 kW exceeds 12
kW by 1, the maximum demand listed in Column C for 20 ranges must be
increased by 5 percent. The increased amount is 1.75 kW (35 × 5% = 1.75
kW). This increased amount must be added to the Column C demand load
for 20 ranges (35 + 1.75 = 36.75 kW). The service demand load for eight
9kW, six 13kW and six 15kW household electric ranges is 36.75 kW
(see Figure 2).
When calculating the demand load for ranges of
unequal rating in accordance with the second note under Table 220.55,
all ranges rated more than 8¾ kW but less than 12 kW must be changed to
12 kW before finding the average value of rating. For example, what is
the service demand load for five 9kW, five 10kW and five 17kW
household electric ranges? Before finding the average value of rating,
change the 9kW ranges and the 10kW ranges to 12kW ranges. Next, find
the total kilowatt rating of all the ranges [(5 × 12) + (5 × 12) + (5 ×
17) = 60 + 60 + 85 = 205]. Now, find the average value by dividing the
total connected load by the total number of ranges (205 ÷ 15 = 13.67
kW). The average rating of all 15 ranges is 13.67 kW. Notes 1 and 2
specify that the range rating must be increased for each kilowatt of
rating or major fraction thereof by which the rating of the individual
ranges exceeds 12 kW. A “major fraction thereof” is .5 and larger.
Since the .67 is a major fraction, round the average rating of 13.67 up
to 14 kW (see Figure 3).
Now that the
average value for the ranges of unequal rating is known, find the
service demand load for 15 14kW ranges. Because Column C is based on
12kW ranges, subtract 12 from 14 (14 – 12 = 2). Since 14 kW exceeds 12
kW by 2, multiply 2 by 5 percent to find the amount Column C must be
increased (2 × 5% = 10%). The increased amount is 3 kW (30 × 10% = 3
kW). This increased amount must be added to the Column C demand load
for 15 ranges (30 + 3 = 33 kW). The service demand load for five 9kW,
five 10kW and five 17kW household electric ranges is 33 kW (see
Figure 4).
While forgetting to change ranges rated less than 12
kW up to 12 kW may not seem very important, it could be the difference
between a normal condition and an overload. Also, if taking an
electrician’s exam, leaving out this step on a range calculation that
uses Note 2 under Table 220.55 will provide an incorrect answer.
Missing this question could be the difference between pass and fail.
Next month, the discussion of feeder and service load calculations continues.
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.
