BranchCircuit, Feeder and Service Calculations, Part XVII
by Charles R. Miller
Published: July 2007
Article 220 – Load Calculations
220.52 Small Appliance and Laundry Loads—Dwelling Unit
Requirements for calculating branchcircuit, feeder
and service loads are in Article 220 of the National Electrical Code
(NEC). Article 220’s requirements are divided into five parts. General
requirements for loadcalculation procedures are in Part I (220.1
through 220.5). Part II (220.10 through 220.18) provides calculation
provisions for branch circuits. Feeder and service load calculation
requirements (sometimes referred to as the standard method) are in Part
III of Article 220. Sections in Part III include 220.40 through 220.61.
Optional feeder and service load calculation provisions are in Part IV
(220.80 through 220.88). Part V (220.100 through 220.103) provides
calculation specifications for farm loads. Figure 220.1 (in the NEC)
shows information on the organization of Article 220. Results from load
calculations in Article 220, along with provisions from other article,
can be used to find conductor sizes and ampere ratings for overcurrent
protection.
Last month’s column concluded
by covering motor load calculations in 220.50. This section in Article
220 references 430.24, 430.25 and 430.26 for motor loads and 440.6 for
hermetic refrigerant motor compressor loads. This month, the discussion
continues with fixed electric space heating loads in 220.51.
Fixed
electric space heating loads shall be calculated at 100 percent of the
total connected load. However, in no case shall a feeder or service
load current rating be less than the rating of the largest branch
circuit supplied [220.51]. Unlike motor and motorcompressor loads that
must be calculated at 125 percent, fixed electric space heating loads
must only be calculated at 100 percent. For example, floor plans for a
small office are specifying the installation of seven wall heaters;
each heater is rated 3,000 watts at 240 volts. How much load will these
heaters add to a 240volt, singlephase service? The air conditioning
load will be less than the heating load. In accordance with 220.51,
calculate the heaters at 100 percent (7 × 3,000 = 21,000 watts). Since
the service voltage is known, the total current draw of the heaters can
be calculated. Simply divide the total watts by 240 volts (21,000 ÷ 240
= 87.5 = 88 amperes). The calculated load in amperes for seven 3,000
watt, 240volt heaters on a 240volt, singlephase service is 88
amperes (see Figure 1).
Heating units equipped with blower motors must be
calculated in accordance with 220.51 and 220.50. For example, a
heating/cooling package unit will be installed in a onefamily
dwelling. The electric heater is rated 9.6 kW at 240 volts. The blower
motor inside the package unit that circulates the air will be a ½
horsepower, 240volt motor. How much load will this package unit add to
a 240volt, singlephase service? The air conditioning load will be
less than the heating load. The heat is calculated at 9,600 watts (9.6
× 1,000 = 9,600 watts). The fullload current in amperes of a ½ hp,
singlephase, 240volt motor (from Table 430.248) is 4.9 amperes. The
fullload current of the motor can be converted to watts (or
voltamperes) and added to the heat load (4.9 × 240 = 1,176 + 9,600 =
10,776 watts). Or the heater watts could be converted to amperes and
added to the motor’s fullload current rating (9,600 ÷ 240 = 40 + 4.9 =
44.9 = 45 amperes). The calculated load in amperes for this
heating/cooling package unit (including the blower motor) on a
240volt, singlephase service is 45 amperes (see
Figure 2). Note, unless it is the largest motor in the feeder or
service load calculation, do not multiply the fullload current of the
motor by 125 percent.
If the ½ hp blower motor is the largest motor in the
calculation for the service, the ampacity must not be less than 125
percent of the fullload current rating plus the calculated load of the
electric heat. For example, the blower motor in the heating/cooling
package unit in the last example will be the largest motor in the
onefamily dwelling.
How much load will
this package unit add to a 240volt, singlephase service? Multiply the
motor’s fullload current by 125 percent before adding it to the
electric heat (4.9 × 125% = 6.125 + 40 = 46.125 = 46 amperes). Because
the blower motor is the largest motor in the service load calculation,
the calculated load is now 46 amperes (see Figure 3).
While fixed electric space heating loads must be
calculated in accordance with 220.51 when calculating feeder and
service loads, branchcircuit loads must be calculated in accordance
with Article 424. See the reference to fixed electric space heating
equipment in Table 220.3. Branchcircuit sizing requirements for fixed
electric space heating equipment are in 424.3. Where sizing branch
circuits, fixed electric space heating equipment shall be considered
continuous load [424.3(B)]. As stipulated in 210.19(A)(1),
branchcircuit conductors must have an ampacity of not less than 125
percent of the continuous load. For example, what minimum size 75°C
branchcircuit conductors are required to feed the heating/cooling
package unit in Figure 3? While the service load for this unit was
calculated at 100 percent, the branchcircuit load must be calculated
at 125 percent because it is considered a continuous load. In
accordance with 430.22(A), the blower motor must also be calculated at
125 percent. The blower motor and heating element have a combined
current of 44.9 amperes (4.9 + 40 = 44.9 amperes). After multiplying
the combined current by 125 percent, the 75°C branchcircuit conductors
must have an allowable ampacity of at least 56 amperes (44.9 × 125% =
56.125 = 56 amperes). The minimum size 75°C conductors required to feed
this heating/cooling package unit are 6 AWG copper conductors (see Figure 4).
No calculation is necessary if the heating/cooling unit
provides minimum circuit ampacity and maximum branchcircuit
overcurrent protection on the nameplate. For example, a heating/cooling
package unit has electric heat with a rating of 14.4 kW at 208 volts.
The minimum/maximum fuse or HACRtype breaker shown on the nameplate is
100 amperes. Since the rating for the overcurrent protective device is
provided, no branchcircuit calculation is necessary. This
branchcircuit load must be protected by a 100ampere fuse or HACRtype
circuit breaker (see Figure 5).
The examples of this month’s Code in Focus were
calculated on the larger of the heating or cooling loads. See 220.60
for using only the largest of noncoincident loads when calculating the
total load of a feeder or service.
Where
meeting the exception to 220.51, it is permissible to calculate the
fixed electric space heating load at less than 100 percent. Where
reduced loading of the conductors results from units operating on
dutycycle, intermittently, or from all units not operating at the same
time, the authority having jurisdiction may grant permission for feeder
and service conductors to have an ampacity less than 100 percent,
provided the conductors have an ampacity for the load so determined
[220.51 Exception].
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.
