BranchCircuit, Feeder and Service Calculations, Part XXXIII
by Charles R. Miller
Published: November 2008
Article 220 – Load Calculations
220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit
The National Electrical Code (NEC) is divided into an
introduction and nine chapters. Some items covered in the introduction
include purpose and scope of the Code; arrangement of the Code; an
explanation of mandatory rules, permissive rules and explanatory
material; formal interpretations; and units of measurement. Chapters 1
through 4 apply generally to all electrical installations. Chapter 1,
titled General, contains definitions and requirements for electrical
installations. Chapter 2 contains requirements pertaining to wiring and
protection. Chapter 3 covers wiring methods and materials.
The
last of the chapters that apply generally is Chapter 4. This chapter
contains requirements for general use equipment. Chapters 5 through 7
supplement or modify the general rules found in the first four
chapters. 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 eight annexes (A through H) that are not part of NEC
requirements. They are included for informational purposes only.
Article
220, one of the articles that applies generally to all electrical
installations, provides requirements for calculating branchcircuit,
feeder and service loads. This article has been the topic of a detailed
study in this column.
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.
Requirements
for calculating branchcircuit loads for household cooking appliances
are in the fourth note under Table 220.55. Results from the
calculations are used to size the branchcircuit conductors and
overcurrent protection. As discussed last month, Note 4 (under Table
220.55) can be divided into three parts. The first part covers a
branchcircuit load calculation for one range. Branch circuits are
similar to feeders and services in that the size of the branch circuit
can be based on the nameplate rating of the range, or the branch
circuit can be sized based on the demand factors in Table 220.55.
Determining the load, whether full load or demand load, is the first
step (see last month’s column). Determining the conductor size and the
overcurrent protective device are the next steps. Requirements for
sizing branchcircuit conductors and overcurrent devices are in Article
210.
As specified in 210.19(A)(3),
branchcircuit conductors supplying household ranges, wallmounted
ovens, countermounted cooking units and other household cooking
appliances shall have an ampacity not less than the rating of the
branch circuit and not less than the maximum load to be served. The
minimum rating of the branch circuit for one range can be determined by
applying the demand factors in Table 220.55 [220.18(C)]. For example,
what minimum size 60°C copper conductor is required for a household
electric range rated 8 kW at 120/240 volts? The demand factor percent
from Column B for one unit is 80 percent. Multiply the rating of the
range by 80 percent (8 × 80% = 6.4 kW). Convert kilowatts to watts or
voltamperes (VA) by multiplying 6.4 by 1,000 (6.4 × 1,000 = 6,400 VA).
To find amperes, divide the load by the rated voltage (6,400 ÷ 240 =
26.7 = 27 amperes). The minimum rating of the branch circuit supplying
power to this 8kW range is 27 amperes, and therefore, the conductor
must have an ampacity of at least 27 amperes (see Figure 1).
Now that the minimum ampacity is known, the conductor
can be selected. Table 310.16 provides the allowable ampacities of
insulated conductors rated 0 through 2,000 volts. Because this example
is asking for a 60°C copper conductor, the conductor must be selected
from the 60°C column, under copper, in Table 310.16. The ampacities in
Table 310.16 are based on three or fewer currentcarrying conductors
and an ambient temperature of 30°C or 86°F. Copper conductors are on
the left side, and aluminum or copperclad aluminum conductors are on
the right side. Look on the left side of Table 310.16, and follow the
60°C column down until a conductor has an ampacity of at least 27
amperes. The smallest size conductor with an allowable ampacity of at
least 27 amperes is 10 AWG copper. The minimum size 60°C copper
conductor required for a household electric range rated 8 kW at 120/240
volts is 10 AWG (see Figure 2).
When calculating branch circuits for ranges over 12 kW,
use the first note under Table 220.55. For example, what minimum size
60°C copper conductor is required for a household range rated 16.5 kW
at 120/240 volts? 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 or major fraction thereof by which the rating of
individual ranges exceeds 12 kW. Since the .5 is a major fraction,
round the rating of 16.5 up to 17 kW. Subtract 12 from 17 (17 – 12 =
5). Since 17 kW exceeds 12 kW by 5, multiply 5 by 5 percent to find the
amount Column C must be increased by (5 × 5% = 25%). The maximum demand
listed in Column C for one range must be increased by 25 percent. The
increased amount is 2 kW (8 × 25% = 2 kW). This increased amount must
be added to the Column C demand load for one range (8 + 2 = 10 kW).
Convert kilowatts to watts by multiplying 10 by 1,000. To find amperes,
divide the load by the rated voltage (10,000 ÷ 240 = 41.7 = 42
amperes). The minimum rating of the branch circuit supplying power to
this 16.5 kW range is 42 amperes. The conductor must have an ampacity
of at least 42 amperes (see Figure 3).
Because this example also is asking for a 60°C copper
conductor, the conductor must be selected from the 60°C column, under
copper, in Table 310.16. Look on the left side of Table 310.16 and
follow the 60°C column down until a conductor has an ampacity of at
least 42 amperes. The smallest size conductor with an allowable
ampacity of at least 42 amperes is 6 AWG copper. The minimum size 60°C
copper conductor required for a household electric range rated 16.5 kW
at 120/240 volts is 6 AWG (see Figure 4).
While cooktops and ovens are quite often permanently
connected (hardwired), ranges are usually cordandplug connected. In
accordance with 210.21(B)(4), the ampere rating of a range receptacle
can be based on a single range demand load as specified in Table 220.55.
Discussion of feeder and service load calculations continues next month.
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.
