BranchCircuit, Feeder and Service Calculations, Part XXXIV
by Charles R. Miller
Published: December 2008
Article 220 – Load Calculations
220.55 Electric Ranges and Other Cooking Appliances—Dwelling Unit
The National Electrical Code (NEC) contains a large
number of calculations. Understanding how to perform NEC calculations
is essential to electricians, engineers, inspectors and others involved
in the electrical industry. Knowing how to perform calculations could
even be the difference between pass and fail on a journeyman or master
electrician’s exam.
Article 220
contains a number of calculations. Requirements for calculating
branchcircuit, feeder and service loads are in Article 220. It may be
necessary to calculate the loads by using the provisions in Article 220
before selecting conductors and overcurrent devices. For example,
results from calculations in Article 220 are used with provisions in
210.19(A) to find the minimum branchcircuit conductor size.
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.
Examples
in last month’s column covered demandload calculations for ranges and
also the selection of the branchcircuit conductors supplying power to
those ranges. Although there are many conductor types to choose from,
last month’s examples specifically asked for copper conductors with a
60°C temperature rating. In sizing conductors, another factor that must
be considered is the temperature ratings of the conductor
terminations. Table 310.16 provides allowable ampacities for conductors
with temperature ratings of 60°C (140°F), 75°C (167°F) and 90°C
(194°F). The temperature ratings at the top of Table 310.16 are the
maximum operating temperatures for the conductor types listed in the
column. See Table 310.13(A) for conductor applications and insulations.
Different conductor types have different allowable ampacities. In most
cases, the allowable ampacity increases as the temperature rating of
the conductor increases. For example, look at the ampacities for a 6
AWG copper conductor. A 6 AWG copper conductor in the 60°C column has
an allowable ampacity of 55 amperes; the same conductor in the 75°C
column has an allowable ampacity of 65 amperes; and the same 6 AWG
copper conductor in the 90°C column has an allowable ampacity of 75
amperes (see Figure 1).
There is an asterisk adjacent to some of the
conductors in Table 310.16. A footnote at the bottom of the table
references the smallconductor rule in 240.4(D). The smallconductor
rule stipulates that unless specifically permitted in 240.4(E) or (G),
overcurrent protection shall not exceed 15 amperes for 14 AWG, 20
amperes for 12 AWG and 30 amperes for 10 AWG copper conductors. See
240.4(D) for limitations on other conductors.
Although
a conductor is listed in the 90°C column, the ampacity will be limited
by the temperature rating of the termination. As specified in
110.14(C), the temperature rating associated with the ampacity of a
conductor shall be selected and coordinated so as not to exceed the
lowest temperature rating of any connected termination, conductor or
device. Equipment termination provisions are divided into two
categories. The first, in 110.14(C)(1)(a), covers equipment for
circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG
conductors. The second, in 110.14(C)(1)(b), covers equipment for
circuits rated over 100 amperes or marked for conductors larger than 1
AWG. Branch circuits calculated in accordance with Table 220.55 will
fall under equipment for circuits rated 100 amperes or less, or marked
for 14 AWG through 1 AWG conductors. Except for certain motors, there
are only three stipulations in 110.14(C)(1)(a). Compliance with one of
the three provisions is required. The first provision states conductors
from the 60°C must be installed. The second provision states conductors
with higher temperature ratings can be installed provided the ampacity
of those conductors are not greater than the ampacity listed in the
60°C column. The third provision states that conductors with higher
temperature ratings can be installed, and the ampacity from the
conductor’s column can be used if the equipment is listed and
identified for use with such conductors.
When
selecting branchcircuit conductors from the 75°C and 90°C columns, the
temperature ratings of the conductor terminations must be considered.
For example, what minimum size THHN copper conductors are required for
a household range rated 17 kW at 120/240 volts? In this example, the
temperature ratings of the conductor terminations are unknown. 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. 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
(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 (10 × 1,000 = 10,000 watts). 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 17kW range is 42 amperes. The conductor must have an ampacity of
at least 42 amperes (see Figure 2).
THHN conductors are in the 90°C column. In that column,
8 AWG conductors have an allowable ampacity of 55 amperes. But,
because of 110.14(C), an 8 AWG conductor is not adequate for this
42ampere load. Section 110.14(C)(1)(a)(2) states that conductors with
higher temperature ratings (such as this THHN conductor) can be
installed, but the conductor’s ampacity must be determined based on the
60°C ampacity of the conductor size used. The ampacity out of the 60°C
column for an 8 AWG copper conductor is only 40 amperes. The smallest
size copper conductor in the 60°C column with an allowable ampacity of
at least 42 amperes is 6 AWG copper. The minimum size THHN copper
conductor required for the household electric range rated 17 kW at
120/240 volts (in this example) is 6 AWG (see Figure 3).
In the last example, the temperature ratings of the conductor terminations were unknown.
If
the terminations are listed and identified for use with conductors with
higher temperature ratings, it may be permissible to use a higher
ampacity. Do not exceed the rating of the lowest rated termination. For
example, what minimum size THHN copper conductors are required for a
household range rated 19 kW at 120/240 volts? In this example, the
temperature ratings of the conductor terminations are rated 75°C.
Subtract 12 from 19 (19 – 12 = 7). Since 19 kW exceeds 12 kW by 7,
multiply 7 by 5 to find the percentage Column C must be increased (7 ×
5 = 35%). The maximum demand listed in Column C for one range must be
increased 35 percent. The increased amount is 2.8 kW (8 × 35% = 2.8
kW). This increased amount must be added to the Column C demand load
for one range (8 + 2.8 = 10.8 kW). Convert kilowatts to watts by
multiplying 10.8 by 1,000 (10.8 × 1,000 = 10,800 watts). To find
amperes, divide the load by the rated voltage (10,800 ÷ 240 = 45
amperes). The branchcircuit conductor for this range must have an
ampacity of at least 45 amperes (see Figure 4).
An
8 AWG, THHN, copper conductor (in the 90°C column) has an allowable
ampacity of 55 amperes. Because the temperature ratings of the
conductor terminations are not rated 90°C, the ampacity from the 90°C
column is not permitted. The temperature ratings of the conductor
terminations are rated 75°C. Therefore, because of 110.14(C)(1)(a)(3),
it is permissible to use the allowable ampacity in the 75°C column. The
smallest size copper conductor in the 75°C column with an allowable
ampacity of at least 45 amperes is 8 AWG copper. The minimum size THHN
copper conductor required for the household electric range rated 19 kW
at 120/240 volts (in this example) is 8 AWG (see Figure 5).
The feeder and service load calculations discussion 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.
