I explained the essential definitions for receptacles branch circuit and the different types of receptacles. Also, in the previous article " Receptacle Branch Circuit Design Calculations – Part Two ", I explained basic principles, GFCI protection and AFCI protection for receptacle branch circuits. Today, I will explain the Receptacle Branch circuit calculations for both Dwelling and Non-dwelling buildings as follows.First: Receptacle Branch circuit calculations in dwelling buildings 1- Essential definitions: As I mentioned before in previous article " Branch Circuit Design Calculations – Part Two " that the Dwelling Building is A single unit, providing complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, cooking, and sanitation.Dwelling buildings include the following types:
2- Receptacle Branch circuit ratings and permissible loads
A) 15- and 20-Ampere Branch Circuits 15- and 20-Ampere Branch Circuits shall be permitted to supply:
2- Only utilization equipment: with condition that the combined load for all utilization equipment must not exceed the branch circuit rating. 3- Combination of both: in the case the permissible rating of the utilization equipment will depend on its type as follows:
B) 30-Ampere Branch Circuits
C) 40- and 50-Ampere Branch Circuits
3- Selecting Receptacle rating for a branch circuit A- Receptacle rating, general
B- Single Receptacle on an Individual Branch Circuit (see below image)
C- Receptacle supplying Total Cord-and-Plug-Connected Load
4- Voltage ratings for Receptacle Branch circuit In dwelling units the voltage rating for Receptacle Branch circuit shall not exceed the following:A) 120 volts, nominal, between conductors
B) 208-volt or 240-volt circuit, nominal, between conductors
5- The Maximum allowable number of receptacles on a branch circuit As per NEC section 220.14(I), Receptacle outlets load (see below image) shall be calculated at not less than:
To calculate the Maximum allowable number of receptacles on a branch circuit, make the following steps:
Example#1: How many receptacles can be placed on a 120-volt, 20-amp circuit? How many can be placed on a 120-volt, 15-amp circuit?Solution:
Step#1:Determine the maximum circuit power (for 20-amp circuit) = 120 V × 20 A = 2400 VA Determine the maximum circuit power (for 15-amp circuit) 120 V × 15 A = 1800 VA Step#2:
Maximum allowable number of receptacles = 2400 VA / 180 VA = 13.3 For 15-amp circuit: Maximum allowable number of receptacles = 1800 VA / 180 VA = 10 So, A 120-volt, 20-amp circuit can supply 13 receptacles. A 120-volt, 15-amp circuit can supply 10 receptacles.
6- The Minimum number of receptacle branch circuits for bank or office buildings As per NEC section 220.14(J), if the number of receptacles is unknown so for bank and office buildings, we can calculate the receptacles load by multiplying the area in ft2 by the unit value (1 VA/ft2). To get the required number of receptacle branch circuits for bank or office buildings make the following steps:
Example#2: Determine the total receptacle load for an 80 ft × 120 ft office building? And determine the number of 15-amp circuits needed to supply the load. Noting that the number of receptacles is unknown.Solution:
The number of receptacles is unknown, so a receptacle load of 1 VA/ft2 can be calculated: Area = 80 ft × 120 ft = 9600 ft2Total Receptacle load = 1 VA/ft2 × 9600 ft2 = 9600 VA Step#2:To determine the number of circuits required, first calculate the allowable load for a single circuit: The allowable load for a single circuit = 120 V × 15 A = 1800 VA
Divide the total receptacle load by the maximum load per circuit to determine the minimum number of circuits: Minimum number of 15-A receptacle Circuits = 9600 VA / 1800 VA = 5.33 This is the minimum number, so round up to six circuits.
7- Receptacle branch circuits Calculations in dwelling buildings In the broad sense, Receptacle in dwelling units may serve one of the following loads:
In the following paragraphs, I will explain Where and how to distribute each load outlets in a dwelling building as per NEC code. 1- General-use Receptacle Loads 1.1 locations of General-use Receptacle Loads as per NEC section 210.52
In the next article, I will continue explaining Where and how to distribute each receptacle load outlets in a dwelling building as per NEC code. Please, keep following. |