Receptacle Branch Circuit Design Calculations – Part Three


In the previous article 
Receptacle Branch Circuit Design Calculations – Part One ",  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: 

  • Dwelling, One-Family: A building that consists solely of one dwelling unit. 
  • Dwelling, Two-Family: A building that consists solely of two dwelling units. 
  • Dwelling, Multifamily: A building that contains three or more dwelling units. 



2- Receptacle Branch circuit ratings and permissible loads


In no case shall the load exceed the branch-circuit ampere rating. The following are the permissible Receptacle Branch circuit ratings in dwelling buildings:


A) 15- and 20-Ampere Branch Circuits

15- and 20-Ampere Branch Circuits shall be permitted to supply: 

1- Only lighting units: this case explained before in previous article 

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:

  • If it is not fastened-in-place, it can have a rating of up to 80 % of the branch circuit rating as in TABLE 210.21(B)(2). 
  • If it is fastened-in-place, other than luminaires, it shall not exceed 50 % of the branch-circuit ampere rating.


Important!!!
Using fastened-in-place equipment is not permitted for the small-appliance branch circuits, laundry branch circuits, and bathroom branch circuits required in a dwelling unit and these branch circuits shall supply only their receptacle outlets required by the code.



B) 30-Ampere Branch Circuits


A 30-ampere branch circuit shall be permitted to supply utilization equipment in any occupancy. A rating of any one cord-and-plug-connected utilization equipment shall not exceed 80 percent of the branch-circuit ampere rating. 


Important!!!
A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.



C) 40- and 50-Ampere Branch Circuits


A 40- or 50-ampere branch circuit shall be permitted to supply cooking appliances that are fastened in place in any occupancy.



3- Selecting Receptacle rating for a branch circuit


A- Receptacle rating, general

Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall conform to the values listed in Table 210.21(B) (3), or, where rated higher than 50 amperes, the receptacle rating shall not be less than the branch-circuit rating.



B- Single Receptacle on an Individual Branch Circuit (see below image)

A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit. For example, a single receptacle on a 20-ampere individual branch circuit must be rated at 20 amperes.





C- Receptacle supplying Total Cord-and-Plug-Connected Load


a Receptacle shall not supply a total cord-and plug- connected load in excess of the maximum specified in Table 210.21(B)(2).





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


For Receptacle Branch circuit that supplies Cord-and-plug-connected loads 1440 volt-amperes, nominal, or less or less than 1⁄4 hp.


B) 208-volt or 240-volt circuit, nominal, between conductors


For Receptacle Branch circuit that supplies High-wattage cord-and-plug-connected loads, such as electric ranges, clothes dryers, and some window air conditioners.



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:

  • 180 volt-amperes for each single receptacle,
  • 180 volt-amperes for each multiple receptacle (duplex or triplex) on one yoke,
  • 90 volt- amperes per receptacle for multiple receptacles (four or more). 





Important!!!
If a receptacle is dedicated for a specific device, then the actual load is used and If this dedicated load is continuous, then the 125% overrate is appropriate.


To calculate the Maximum allowable number of receptacles on a branch circuit, make the following steps: 

  • Step#1: Determine the maximum circuit power by Multiply the branch circuit voltage and amperage.
  • Step#2: Then divide by 180 volt-amperes. 

The result from 
Step#2 = the Max. Allowable single, duplex or triplex receptacles or a combination of them on a branch circuit. 


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:

Then divide the power by the load per receptacle
For 20-amp circuit: 

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:


  • Step#1: Calculate the total receptacles load for the whole building as explained above. 
  • Step#2: Calculate the total receptacle circuit load in VA by multiplying its voltage by its amperage as follows: 
  1. For 15-A Branch circuits = 120 V × 15 A = 1800 VA 
  2. For 20-A Branch circuits = 120 V × 20 A = 2400 VA 
  • Step#3: Divide the total receptacle load for the whole building by the maximum load per circuit to determine the minimum number of circuits. 



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: 

Step#1:
The number of receptacles is unknown, so a receptacle load of 1 VA/ft2 can be calculated:
Area = 80 ft × 120 ft = 9600 ft2 

Total 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 

Step#3:
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. 


Important!!!
As per NEC section 220.14(J), limit using the unit value (1 VA/ft2) for banks and office buildings but it can be used also (as approximate) for other types of buildings including dwelling ones.



7- Receptacle branch circuits Calculations in dwelling buildings

In the broad sense, Receptacle in dwelling units may serve one of the following loads:

  1. General-use Receptacle Loads,
  2. Small appliance Loads,
  3. Laundry Load,
  4. Fastened-in-place Appliance loads,
  5. Cloth dryer Load,
  6. Household cooking appliances load,
  7. Heating and air conditioning loads,
  8. Motor 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


Rule#1: Main rule (see below image)

Receptacles shall be installed such that no point measured horizontally along the floor line of any wall space is more than 1.8 m (6 ft) from a receptacle outlet.





The wall space is a wall unbroken along the floor line by doorways, fireplaces, archways, and similar openings and may include two or more walls of a room (around corners), as illustrated in Exhibit 210.27. The Minimum length for a wall space is 2 ft. 


Important!!!
The wall space behind the swing of a door is included in the measurement. This does not mean that the receptacle outlet has to be located in that space, only that the space is included in the wall-line measurement.



Rule#2: electric baseboard heaters built-in receptacle (see below image)

If there is a permanently installed electric baseboard heaters (longer than 12 ft.) equipped with factory-installed receptacle outlets or outlets provided as a separate assembly by the manufacturer shall be permitted as the required outlet or outlets for the wall space utilized by such permanently installed heaters (see image below) . Such receptacle outlets shall not be connected to the heater circuits.






Rule#3: Receptacle designed for intended use

Receptacle designed for intended utilization equipment or practical room use may be placed in corners, may be grouped, or may be placed in a convenient location. For example, receptacles in a living room and family room that are intended to serve home entertainment equipment or home office equipment.



Rule#4: floor Receptacles (see below image)

Receptacle outlets in floors shall not be counted as part of the required number of receptacle outlets unless located within 450 mm (18 in.) of the wall.
  





Rule#5: kitchen and dining areas counters receptacles other than that used for small appliances (see above image)

Receptacle outlets installed to serve kitchen or dining area counters (for small appliances) cannot also be used as general-use receptacles for an adjacent wall space and in this case general-use receptacle branch circuits must be added to serve such locations.



Rule#6: receptacles not under main rule

The following receptacles can't be used as general-use receptacles ( rule#1 will not apply for it):
A receptacle that is Part of a luminaire or appliance, or
A receptacle that is controlled by a wall switch, or
A receptacle that is Located within cabinets or cupboards, or
A receptacle that is located more than 1.7 m (5.5 ft) above the floor.



Important!!!
A receptacle controlled by a switch may result in the occupant using an extension cord, run from an outlet or device that is not controlled by a switch, to supply appliances or equipment that require continuous power, such as an electric clock. So, this receptacle not considered as general-use receptacle.




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.




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