BranchCircuit, Feeder and Service Calculations, Part XXVI
by Charles R. Miller
Published: April 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 contains a lot of these requirements, but
calculating branchcircuit, feeder and service loads in accordance with
every one of those requirements is not absolutely necessary. However,
some requirements in Article 220 are mandatory rules.
Mandatory
rules identify actions that are specifically required or prohibited and
are characterized by the use of the terms shall or shall not. An
example of a mandatory rule can be found in 220.12. A unit load of not
less than what Table 220.12 specifies for occupancies therein shall
constitute the minimum lighting load. This requirement states that,
when calculating the lighting load, the load must not be less than the
voltampere unit load per square foot (or square meter) that is
specified in Table 220.12.
Some
requirements in Article 220 are permissive rules. Permissive rules in
the NEC are those that identify actions that are allowed but not
required, are normally used to describe options or alternative methods,
and are identifiable by the use of the terms “shall be permitted” or
“shall not be required.” Requirements in 220.55 are permissive rules.
For example, the load for household electric ranges, wallmounted
ovens, countermounted cooking units and other household cooking
appliances individually rated in excess of 1¾ kW shall be permitted to
be calculated in accordance with Table 220.55.
We
are not required to use Table 220.55. We are permitted to use the
table. For example, a 12kW range will be installed in a dwelling. The
range could be added to the branchcircuit, feeder or service load
calculation at its total rating of 12 kW. Or, in accordance with Column
C in Table 220.55, we are permitted to use 8 kW instead of the total
rating of this range (see Figure 1).
There is one instance when using Table 220.55 is
required. Calculating in accordance with Table 220.55 is required if
you are trying to correctly answer an electrical test question that
pertains to this table.
Including the
total rating of one range in the service load calculation for a
onefamily dwelling does not matter much one way or the other. But,
including the total rating of multiple ranges in a service load
calculation for a multifamily dwelling could make a huge difference in
the size of the service. For example, the total rating for 50 12kW
ranges is 600 kW (600,000W). If the voltage at the service is 208volt,
threephase, the total current rating is 1,667A. By using Table 220.55,
the rating is 62.5 kW (62,500W). At 208V, threephase, the total
current rating is 174A. The difference between using the table and not
using the table is 1,493A. It is not likely that all surface elements
and both elements inside the range will be on simultaneously. If most
of the range is on, it will not be on for any length of time because
thermostats will cycle elements off and on. As the number of ranges
increases, the chances of the ranges being fully loaded decreases. The
table that is now Table 220.55 has been in the Code for more than 45
years.
Up to this point in the series,
household electric cooking equipment ratings have been within the
limits of Column A or Column B, not within the limits of both Columns A
and B. Where the rating of cooking appliances falls under both Columns
A and B, apply the demand factors for each column separately, and add
the results together. This specification is stated in the last sentence
of Note 3, under Table 220.55. For example, what is the service demand
load for 10 3kW, wallmounted ovens and 10 5kW countermounted
cooking units (cooktops)? The ovens have a total rating of 30 kW (10 ×
3 = 30). Because the ovens are rated less than 3½ kW, the demand factor
percent is within the limits of Column A (10 units = 49 percent). The
calculated demand for the ovens is 14.7 kW (30 × 49% = 14.7). The
cooktops have a total rating of 50 kW (10 × 5 = 50). Because the
cooktops are rated within 3½ through 8¾ kW, the demand factor percent
is within the limits of Column B (10 units = 34 percent). The
calculated demand for the cooktops is 17 kW (50 × 34% = 17). Now add
the results together to find the total demand load (14.7 + 17 = 31.7).
The demand for these household cooking appliances is 31.7 kW (31,700W)
(see Figure 2).
Any time Columns A and B can be used to calculate
household cooking appliances, Column C can be used, because Column C is
for household cooking appliances rated 12 kW and under. After
calculating the cooking equipment load using Columns A and B, compare
the load with the demand load in Column C, and then select the lower of
the two loads. For example, what is the service demand load for 15
3kW, wallmounted ovens and 15 6kW, countermounted cooking units?
The ovens have a total rating of 45 kW (15 × 3 = 45). The demand factor
percent in Column A for 15 units is 40 percent. The calculated demand
for the ovens is 18 kW (45 × 40% = 18). The cooktops have a total
rating of 90 kW (15 × 6 = 90). The demand factor percent in Column B
for 15 units is 32 percent. The calculated demand for the cooktops is
28.8 kW (90 × 32% = 28.8). Now add the results together to find the
total demand load (18 + 28.8 = 46.8). The demand for these household
cooking appliances from Columns A and B is 46.8 kW. Since the total
number of units is 30 (15 + 15 = 30), find the demand load in Column C
for 30 units. Where the number of appliances is from 26 to 40, add 15
to the number of appliances and the sum is the kilowatt demand load.
Fifteen added to 30 appliances is 45 (15 + 30 = 45). The demand in
Column C for 30 units is 45 kW. Finally, compare the loads and select
the lower. With this example, the lower number is from Column C (see
Figure 3).
As discussed at the beginning of this month’s Code in
Focus, selecting the lower kilowatt demand load is not required. It is
permitted.
Look back at Figure 2 and
compare the calculated load from Columns A and B with the maximum
demand from Column C. Since there were 10 ovens and 10 cooktops, the
total number of units from the example in Figure 2 was 20. The demand
load from Column C for 20 units is 35 kW. The calculated load for 10
3kW, wallmounted ovens and 10 5kW, countermounted cooking units
(cooktops) from Columns A and B was 31.7 kW. After comparing Column C
with the combined demand load from Columns A and B, the lower of the
two demand loads is from Columns A and B (see Figure 4).
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.
