BranchCircuit, Feeder and Service Calculations, Part XVI
by Charles R. Miller
Published: June 2007
Article 220 – Load Calculations
220.52 Small Appliance and Laundry Loads—Dwelling Unit
The National Electrical Code (NEC) contains an introduction, nine chapters and annexes. An outline showing Code
arrangement is in 90.3. Chapters 1 through 4 apply generally; Chapters
5 though 7 apply to special occupancies, special equipment or other
special conditions. Chapters 5 though 7 supplement or modify the
general rules in the first four chapters. Chapters 1 through 4 apply
except as amended by Chapters 5, 6 and 7 for the particular conditions.
For the most part, Chapter 8 is an article unto itself. It covers
communications systems and is not subject to the requirements of
Chapters 1 through 7 except where the requirements are specifically
referenced in Chapter 8. Chapter 9 consists of tables and is followed
by annexes.
Annexes are not part of the requirements of the NEC,
but are included for informational purposes. Annex C and D can be
especially helpful. Annex C contains conduit and tubing fill tables for
conductors and fixture wires of the same size. Tables 4 and 5 in
Chapter 9 can be used to determine the maximum number of conductors
permitted in raceways or to determine the minimum size raceway needed
for certain conductors. If all the conductors are the same size and
same type of insulation, simply look at Annex C. No calculation is
necessary. Annex D is a great resource because it contains examples of
calculation methods from the Code.
Loadcalculation requirements are in article 220 of the National Electrical Code.
Last month’s Code in Focus concluded by covering demand factors for
nondwelling receptacle loads in 220.44 and Table 220.44. This month,
the topic continues with motor loads as specified in 220.50.
Motorload
calculations are referenced twice in part II of Article 220. Both of
these references contain requirements for calculating branch circuits
supplying motor loads. Part III, titled “Feeder and Service Load
Calculations,” also contains requirements for motor loads. Motor loads shall be calculated
in accordance with 430.24, 430.25 and 430.26 and with 440.6 for
hermetic refrigerant motor compressors [220.50].Unless calculating the
motor overload protection (heaters and overloads), do not use the
actual current rating marked on the nameplate. When calculating motor
loads, use the values given in Tables 430.247 through 430.250. The
values given in those tables shall be used to determine the ampacity of
conductors or ampere ratings of switches, branchcircuit shortcircuit
and groundfault protection, instead of the actual current rating
marked on the motor nameplate [430.6(A)(1)]. This provision does not
apply to motors built for low speeds (less than 1,200 rpm) or high
torques for multispeed motors. Separate motor overload protection shall
be based on the motor nameplate current rating [430.6(A)(1)] (see
Figure 1).
Motoroverload protection requirements are in Part III (430.31 through 430.44) of Article 430.
Conductors
supplying several motors, or a motor(s) and other load(s), shall have
an ampacity not less than 125 percent of the fullload current rating
of the highest rated motor plus the sum of the fullload current
ratings of all the other motors in the group, as determined by
430.6(A), plus the ampacity required for the other loads [430.24]. When
performing load calculations for feeders and services, if there is more
than one motor, start by finding the fullload current of each motor.
Next, multiply the highest fullload current by 125 percent. Finally,
add the fullload currents of the other motor(s) into the calculation.
For example, what is the minimum rating in amperes for conductors
supplying a 5 hp, 230volt, 3phase motor; a 10 hp, 230volt, 3phase
motor; a 15 hp, 230volt, 3phase motor; and a 10 hp, 230volt,
singlephase motor? Fullload currents for 3phase motors are in Table
430.250, and fullload currents for singlephase motors are in 430.248.
The fullload current for a 5 hp, 230volt, 3phase motor is 15.2
amperes; the fullload current for a 10 hp, 230volt, 3phase motor is
28 amperes; the fullload current for a 15 hp, 230volt, 3phase motor
is 42 amperes; and the fullload current for a 10 hp, 230volt,
singlephase motor is 50 amperes (see Figure 2).
Although the 15 hp motor has the largest amount
of horsepower, the motor with the highest fullload current rating is
the singlephase motor. Multiply 50 amperes by 125 percent (50 × 125%
= 62.5). Now add to this value to the fullload currents of the other
motors (62.5 + 15.2 + 28 + 42 = 147.7 = 148). The minimum rating in
amperes for conductors supplying these motors is 148 amperes (see
Figure 3).
As specified in 430.24, conductors supplying two
or more motors must have an ampacity not less than 125 percent of the
fullload current rating of the highest rated motor plus the sum of the
fullload current ratings of all the other motors in the group or on
the same phase. It may not be necessary to include all the motors into
the calculation. It is permissible to balance the motors as evenly as
possible between phases before performing motorload calculations. For
example, what is the minimum rating in amperes for conductors supplying
a 10 hp, 208volt, 3phase motor and three 3 hp, 120volt, singlephase
motors? First, find the fullload currents for the motors. The
fullload current for a 10 hp, 208volt, 3phase motor is 30.8 amperes
[Table 430.250]. Fullload currents for singlephase motors are in
430.248. Note that the currents listed in the 115volt column are
permitted for system voltage ranges of 110 to 120 volts. The fullload
current for a 3 hp, 120volt motor is 34 amperes. Next, balance the
motors as evenly as possible between phases. Connect the 3phase motor
to each of three ungrounded (hot) conductors. Because the 3 hp motors
are 120volts, connect each motor to the grounded conductor and one
ungrounded conductor. One motor will be on phase A, one motor on B and
one on phase C. Although there are four motors total, there are only
two motors on each phase (see Figure 4).
Because the motors are balanced between phases,
the fullload current on each phase is 64.8 amperes (30.8 + 34 = 88.8).
Multiply 34 amperes by 125 percent (34 × 125% = 42.5).
Now add to this value the fullload currents of the other motor on the
same phase (42.5 + 30.8 = 73.3 = 73). The minimum rating in amperes for
conductors supplying these motors is 73 amperes (see Figure 5).
The next motor reference in 220.50 pertains to
multimotor and combinationload equipment. The ampacity of the
conductors supplying multimotor and combinationload equipment shall
not be less than the minimum circuit ampacity marked on the equipment
in accordance with 430.7(D). Where the equipment is not factorywired
and the individual nameplates are visible in accordance with
430.7(D)(2), the conductor ampacity shall be determined in accordance
with 430.24 [430.25]. Hermetic refrigerant motor compressor loads on
feeders, and services must be calculated in accordance with 440.6.
Next month’s article 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.
