NEC BRANCH CIRCUIT CALCULATIONS
PDH Enterprises, LLC
PO Box 942
Morrisville, NC 27560
(919)208-5296
NEC Branch Circuit Calculations
Course #201
This course provides a review of the calculations required when dealing with
branch circuits associated with Article 220 of the National Electrical Code.
Students successfully completing this course will be able to demonstrate
knowledge of the most common topics and calculations within Article 220.
To receive credit for this course, each student must pass an online multiple
choice exam of ten (10) questions. A passing score is 70% or better.
Completion of this course and successfully passing the exam will qualify the
student for one (1) hour of continuing education credit. All information
necessary to complete the examination will be presented within this course
document and within the 2008 version of the National Electrical Code or
NEC.
Course Author:
Chris Barrow, PE
In this section we will cover the most common types of calculations required for
branch circuits and dwellings.
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NEC BRANCH CIRCUIT CALCULATIONS
The following NEC Articles should be studied carefully as they are very important to
this section.
1. Article 210.8(A) Dwelling units. All 125-volt, 15- and 20-amp, single-phase
receptacles shall have ground-fault circuit protection for personnel when installed
in the following locations (see exceptions for each location):
(1) Bathrooms
(2) Garages
(3) Outdoors
(4) Crawl spaces
(5) Unfinished basements
(6) Kitchens
(7) Laundry, utility, and wet bar sinks
(8) Boathouses
2. Article 210.12 Arc-Fault Circuit Interrupter Protection. All 120-volt, single phase,
15 and 20 amp branch circuits supplying outlets installed in dwelling unit family
rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms,
recreation rooms, closets, hallways, or similar shall be protected by a listed arcfault circuit interrupter.
3. Article 210.20(A) Overcurrent Protection for branch circuits of continuous and
non-continuous loads. If a branch circuit supplies continuous or any combination
on continuous and non-continuous loads, the rating of the overcurrent device shall
not be less than the non-continuous load plus 125% of the continuous load.
(Remember from Article 310, this also applies to the conductor of the branch
circuit)
4. Article 210.23 Branch circuits may be installed for specific loads. An individual
branch circuit can supply any load that is within the rating of the circuit.
5. Article 210.52 Dwelling unit receptacle outlets location & spacing.
6. Article 220.12, Table 220.12 Unit lighting load for dwelling occupancies shall not
be less than 3 VA per sq. ft. In determining load on the “VA per sq. ft.” basis,
outside dimensions of the building shall be used, not including open porches,
garages, nor unfinished and unused spaces unless adaptable for future use.
7. Article 210.11(C)(l) For small appliance load, including refrigeration equipment,
in kitchen, pantry, family room, and breakfast room, two or more 20-amp
appliance circuits shall be provided.
8. Article 210.11(B) When the “VA per sq. ft.” calculation is used the load shall be
apportioned evenly among branch circuits, according to their capacities.
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NEC BRANCH CIRCUIT CALCULATIONS
9. Article 210.11(C)(2) At least one 20 amp circuit must be provided for laundry
receptacle outlet(s). This circuit shall have no other outlets.
10. Article 210.11(C)(3) At least one 20 amp circuit required for bathroom receptacle
outlet(s). This circuit shall have no other outlets.
11. Article 220.82(B)(2) Feeder load at 1500 VA for each small appliance and
laundry circuit as required by NEC 210.11(C)(1) and (C)(2).
12. Article 230.79(C) For a one family dwelling, the service disconnect shall not be
less than 100 amps, 3-wire.
13. Article 422.11(E) Rating or overcurrent device on circuit for single nonmotoroperated appliance shall:
1. Not exceed that marked on the appliance.
2. If the overcurrent protection device is not marked and the appliance is
rated l3.3A or less, not exceed 20A.
3. If the overcurrent protection device is not marked and the appliance is
rated over 13.3A, not exceed 150% of the rating of the appliance.
14. Article 220.53 It is possible to apply a 75% demand factor to the nameplate rating
of 4 or more appliances fastened in place other than electric ranges, clothes
dryers, space-heating equipment, or ac equipment served by the same feeder or
service in one, two, or multi family dwellings.
15. Article 220.54 The load for household electric clothes dryers shall be either 5,000
VA or name plate rating which ever is larger.
Branch Circuits, Article 220.10
210.11(A) Number of Branch Circuits - The number of branch circuits shall be
determined from the total computed load and the size and rating of the circuits used. In
all installations, the number of circuits shall be sufficient to supply the load served. In no
case shall the load on any circuit exceed the maximum specified by section 220.18.
The number of branch circuits required are equal to the (total connected load) divided by
(the circuit breaker used times the voltage of the circuit).
Number of branch circuits (#BC) =
LOAD____________
CIRCUIT BREAKER X VOLTAGE
Examples:
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NEC BRANCH CIRCUIT CALCULATIONS
1. A store building is supplied by a 240/120 3 phase 4 wire delta system. Determine
the minimum number of two wire, 20 amp circuits required for 150 general
purpose receptacles.
Solution: Load = 150 rec. * 180 VA = 27,000 VA
Circuit Breaker size = 20 A, Voltage = 120 V
#BC = _____Load __= ___27,000 ___= __27,000__
CB * Voltage
20 * 120
2,400
#BC = 11.25 or 12 circuit breakers
2. An office building measures 150 feet by 125 feet and is supplied by a 240/120
Volt 3 phase 4-wire Delta system. Determine the minimum number of 20A, twowire branch circuits required for the general purpose receptacles.
Solution: Receptacle load = 1 VA/square foot (Article 220.14(K)(2))
Load = 150 ft. x 125 ft. * 1 volt-ampere/sq ft = 18,750 VA
Circuit Breaker size = 20 A, Voltage = 120 V
#B.C. = ____Load_____ = _18,750 = 7.8l or 8 circuit breakers
C.B. X Voltage
2,400
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