BranchCircuit, Feeder and Service Calculations, Part XXXI
by Charles R. Miller
Published: September 2008
Article 220 – Load Calculations
220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit
Article 220 of the National Electrical Code (NEC)
contains requirements for calculating branchcircuit, feeder and
service loads. Article 220 is divided into five parts. General
requirements for loadcalculation procedures are in Part I. Part II
covers requirements for branchcircuit load calculations. Feeder and
service load calculations are in Part III. Part III is sometimes
referred to as the standard method for feeder and service load
calculations. Part IV is titled “Optional Feeder and Service Load
Calculations,” and it 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. Feeder and service loads can be calculated in
accordance with the requirements in either Part III or Part IV. Whether
calculating by the standard method or optional method, the neutral load
for feeders and services must be calculated in accordance with the
requirements in 220.61. Part V provides calculation specifications for
farm loads. Figure 220.1 (in the NEC) shows a diagram on the
organization of Article 220. Loads calculated in accordance with the
provisions in Article 220 are used with requirements from other
articles to find conductor sizes and ampere ratings for overcurrent
protection. For example, results from Part II in Article 220 are used
with the provisions in 210.19 to size branchcircuit conductors and
210.20 to determine ratings for overcurrent protective devices (fuses
and breakers).
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.
Calculating
the neutral load for feeders and services supplying household electric
ranges, wallmounted ovens and countermounted cooking units is
accomplished by using Table 220.55 and 220.61(B).
First,
calculate the feeder or service load by applying the appropriate Table
220.55 demand factors or obtaining the maximum demand in kilowatts.
After the maximum unbalanced load has been determined, apply an
additional demand factor of 70 percent to determine the neutral load
[220.61(B)]. For example, what is the neutral demand load for a feeder
supplying one 12kW range? The maximum demand in Column C of Table
220.55 for one 12kW range is 8 kW. Therefore, the feeder load for this
range is 8 kW. Now apply the additional demand factor of 70 percent (8
× 70% = 5.6 kW). Instead of 12,000 watts or even 8,000 watts, the
neutral load for a feeder supplying one 12kW range is 5.6 kW or 5,600
watts (see Figure 1).
Regardless of the
mixture of ranges, wallmounted ovens and countermounted cooking
units, after finding the Table 220.55 demand load for the feeder or
service, multiply the demand load by 70 percent to obtain the neutral
demand load. For example, what is the service neutral demand load for
five 3½kW wallmounted ovens, five 5kW countermounted cooking units
and 10 8kW ranges? Because none of the appliances are rated less than
3½ kW or more than 8¾ kW, the demand factor will come from Column B.
First,
find the total number of units (5 + 5 + 10 = 20). Next, find the demand
factor percent in Column B across from 20 units (28 percent). Now find
the total kilowatts of all the appliances. The ovens have a total
rating of 17.5 kW (5 × 3.5 = 17.5). The cooktops have a total rating of
25 kW (5 × 5 = 25). The ranges have a total rating of 80 kW (10 × 8 =
80). The combined rating of all the appliances is 122.5 kW (17.5 + 25 +
80 = 122.5).
Next, multiply the total
kilowatt load by the demand factor percent (122.5 × 28% = 34.3). The
service demand load for these 20 units is 34.3 kW. Any time Column A or
B can be used to calculate household cooking appliances, Column C can
be used because it is for household cooking appliances “not over 12 kW
rating.”
With this example, the
calculated demand in Column B will be used because it is lower than the
maximum demand in Column C. Finally, multiply the service demand load
of 34.3 kW by 70 percent (34.3 × 70% = 24.01 kW). The service neutral
demand load for five 3½kW wallmounted ovens, five 5kW
countermounted cooking units and 10 8kW ranges is 24.01 kW (see
Figure 2).
When finding the service or feeder neutral demand
load for ranges individually rated more than 12 kW (or more than 8¾ kW
for ranges of unequal ratings) but not more than 27 kW, start by
finding the service or feeder demand load by using either Note 1 or
Note 2 under Table 220.55. Then, apply the additional demand factor of
70 percent, as permitted by 220.61(B), to determine the neutral load.
For example, what is the service neutral demand load for five 14kW,
five 16kW and five 17kW household electric ranges? Because these
ranges are rated more than 12 kW and are of unequal ratings, use Note 2
under Table 220.55 to calculate the service demand load. Find an
average value of rating by adding together the ratings of all ranges to
obtain the total connected load. The total connected load is 235 kW [(5
× 14) + (5 × 16) + (5 × 17) = 70 + 80 + 85 = 235]. Now divide the total
connected load by the number of ranges for the average value of rating
(235 ÷ 15 = 15.67 kW). The average rating of all 15 ranges is 15.67 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. Since the .67 is a major fraction,
round the average rating of 15.67 up to 16 kW (see Figure 3).
Because
the average range rating is 16 kW, find the service demand load for 15
16kW ranges. Since Column C is based on 12kW ranges, the maximum
demand in Column C must be increased 5 percent for each additional
kilowatt or rating by which the rating of individual ranges exceeds 12
kW. Subtract 12 from 16 (16 – 12 = 4). Since 16 kW exceeds 12 kW by 4,
multiply 4 by 5 percent to find the amount Column C must be increased
(4 × 5 = 20%). The maximum demand listed in Column C for 15 ranges must
be increased by 20 percent. The increased amount is 6 kW (30 × 20% = 6
kW). This increased amount must be added to the Column C demand load
for 15 ranges (30 + 6 = 36 kW). The service demand load for these 15
ranges after applying Note 2 to Table 220.55 is 36 kW. Finally,
multiply the service demand load of 36 kW by 70 percent (36 × 70% =
25.2 kW). The service neutral demand load for five 14kW, five 16kW
and five 17kW household electric ranges is 25.2 kW (see Figure 4).
The permitted reduction for a feeder or service
neutral load calculation also applies to household electric clothes
dryers that are calculated in accordance with Table 220.54.
Next month’s Code in Focus will continue 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 “Electrician's Exam Prep.” He can be reached at
615.333.3336, charles@charlesRmiller.com or www.charlesRmiller.com.
