Receptacle Branch Circuit Design Calculations – Part Six


In article 
" Receptacle Branch Circuit Design Calculations – Part Three ", I stated that a Receptacle in dwelling units may serve one of the following loads:
  1. General-use Receptacle Loads, 
  2. Small appliance Loads, 
  3. Laundry Load, 
  4. Cloth dryer Load, 
  5. Fastened-in-place Appliance loads, 
  6. Household cooking appliances load, 
  7. Heating and air conditioning loads, 
  8. Motor loads. 


I explained the first two types in the following articles:



In the following paragraphs, I will explain Where and how to distribute each load outlets in a dwelling building as per NEC code.

You can review the following articles for more information:





3- Laundry Load Branch circuits





Definition:
As per NEC section 550.2 Laundry Area is An area containing or designed to contain a laundry tray, clothes washer, or a clothes dryer.


3.1 Applied NEC Rules for Laundry branch circuit



There are many NEC rules that control the location and load of Laundry branch circuit including:

  • 210.11 (C)(2) Required Laundry Branch Circuits in Dwelling Units 
  • 210.50 (C) Appliance Receptacle Outlets 
  • 210.52 (F) Laundry Areas 
  • 220.52 (B) Small-Appliance and Laundry Loads in Dwelling Unit 
  • 210.23 (A) Permissible Loads for 15- and 20-Ampere Branch Circuits 


3.2 Rules controlling Laundry Branch Circuit



Rule#1: Number of Laundry branch circuits

In each dwelling unit, At least one 20-ampere branch circuit shall be provided to supply the laundry receptacle outlet(s).



Rule#2: Dedicated usage of Laundry branch circuits

The laundry branch circuits required in a dwelling unit(s) by shall supply only the receptacle outlets specified for Laundry use and this circuit shall have no other outlets supplying other loads.(see below image)






Rule#3: Multifamily dwellings, Laundry in public area

As per NEC section 210.25(A), Branch circuits in each dwelling unit shall supply only loads within that dwelling unit or loads associated only with that dwelling unit.
So, In multifamily dwellings building, if Laundry facilities are provided on the premises and are available to all building tenants (as a common usage). the laundry branch circuit will not be added to each individual dwelling unit.
This Laundry branch circuits for public or common areas are required to be supplied from a separate “house load” panelboard. This requirement permits access to the branch-circuit disconnecting means without the need to enter the space of any tenants. The requirement also prevents a tenant from turning off important circuits that may affect other tenants.



Rule#4: special case, no laundry facilities permitted

As per Rule#3, In other than one-family dwellings where laundry facilities are not to be installed or permitted, a laundry receptacle shall not be required.



Rule#5: Location of receptacle outlet used for Laundry

Appliance receptacle outlets installed in a dwelling unit for specific appliances, such as laundry equipment, shall be installed within 1.8 m (6 ft) of the intended location of the appliance.


3.3 Calculation of Laundry load


A- For feeder and service calculation purposes:



Rule#6: Laundry branch circuit load

In each dwelling unit, a load of not less than 1500 volt-amperes shall be included for each 2-wire laundry branch circuit installed.
Where additional laundry branch circuits are provided, they also are calculated at 1500 volt-amperes per circuit.


Important!!!
If Rule#3 above is applicable, then no Laundry load will be added for feeder calculation of each dwelling unit and the Laundry branch circuits load will be added to the “house load” panelboard.



B- For branch circuit requirements (conductor ampacity & size and over-current protection) calculation


This will be explained in coming articles, however you can review the calculation for maximum number of allowable receptacles on a branch circuit which was discussed before in previous article 
" Receptacle Branch Circuit Design Calculations – Part Three ".



3.4 How to specify the required Type of receptacles for Laundry branch circuits?

A- GFCI Type

The type of 15- or 20-ampere, 125-volt receptacle that is required to be supplied from the laundry branch circuit will depend on the washing machine location as follows:
a- If the washing machine is located in the bathroom: 

As per NEC section 210.8 A (1), There are no exceptions to the bathroom GFCI requirement. So, the 15- or 20-ampere, 125-volt receptacle must be GFCI protected.
b- If the washing machine is located in the Laundry Area or in a kitchen: 

The 15- or 20-ampere, 125-volt receptacle will not be GFCI protected.



B- Tamper-Resistant type


Tamper-Resistant receptacles are intended to increase safety for children.

A single receptacle or a duplex receptacle located within dedicated space for appliance that, in normal use, is not easily moved from one place to another ( like dishwashers, refrigerators, washing machines, and the like ) shall not be required to be tamper-resistant.



4- Clothes Dryer Load

4.1 Applied NEC Rules for Clothes Dryer Load


There are many NEC rules that control the location and load of Clothes Dryer including: 

  • 210.50 (C) Appliance Receptacle Outlets 
  • 220.54 Electric Clothes Dryers in Dwelling Unit(s) 
  • 220.53 Appliance Load — Dwelling Unit(s) 


4.2 Rules controlling Clothes Dryer Load



Rule#1: Location of receptacle outlet used for Clothes Dryer

Appliance receptacle outlets installed in a dwelling unit for specific appliances, such as laundry equipment, shall be installed within 1.8 m (6 ft) of the intended location of the appliance.



Rule#2: Circuit ratings for Clothes Dryer

High-wattage cord-and-plug-connected loads, such as electric ranges, clothes dryers, and some window air conditioners, may be connected to a 208-volt or 240-volt 3 phase, 4 wire circuit.
Same can be done by using single phase multi-wire circuits that can supply both line-to-line and line-to-neutral connected loads as in below image.





Rule#3: Not Fastened-in-Place Appliances

As per NEC section 220.53, electric ranges, clothes dryers, space-heating equipment or air conditioning equipment must not be included with the number of appliances that are fastened in place.
Also, All portable small Appliances for kitchen and others are not Fastened-in-Place Appliances.



4.3 Calculation of Clothes dryer load


A- For feeder and service calculation purposes:




Important!!!
If a dwelling unit doesn’t include a clothes dryer, no Load for clothes dryer will be added for feeder calculation of this dwelling unit.



Important!!!
A combination of clothes washer and clothes dryer (see below image) will be handled in calculations as it is a clothes dryer. But the branch circuit and 125V receptacle outlet for laundry is still required i.e. minimum Laundry requirements is mandatory while clothes dryer requirements is not.





Important!!!
Kilovolt-amperes (kVA) shall be considered equivalent to kilowatts (kW) for clothes dryer loads.


First: As per NEC Standard calculation method:




Rule#4: clothes dryers load in NEC Standard calculation method

As per NEC section 220.54, the load for household electric clothes dryers in a dwelling unit(s) shall be either 5000 watts (volt-amperes) or the nameplate rating, whichever is larger, for each dryer served.



Important!!!
The NEC does not prohibit applying the full load of all dryers to a service and/or feeder calculation especially when all the clothes dryers are in a common Laundry area in a multi-family dwelling because It is possible that all dryers will be operating at one time in a laundry facility. But when each tenant has separate clothes dryer, it is unlikely that all dryers will be in operation simultaneously.  So, Table 220.54 Demand Factors for Household Electric Clothes Dryers will be used.





In multifamily dwelling, where two or more single-phase dryers are supplied by a 3-phase, 4-wire feeder or service, follow the below steps:

  • Step#1: Assign the maximum number of dryers connected between any two phases. 
  • Step#2: The total number of clothes dryers between two phases = 2 x the maximum number of clothes dryers connected between any two phases ---- (Result#1) 
  • Step#3: 2-phase Total “connected load” for clothes = totals of all the clothes dryers loads x (Result#1) ----- (Result#2) 
  • Step#4: Based on The total number of clothes dryers between two phases (Result#1), assign the demand factor from Table 220.54. 
  • Step#5: 2-phase Total “demand load” for clothes dryers = (Result#2) X Demand factor ----- (Result#3) 
  • Step#6: service 3-phase Total “demand load” = (Result#3) X 3/2 


Example#1:


What is the minimum service load for a 4.7 KW combination clothes washer and dryer?


Solution:
A combination of clothes washer and clothes dryer will be handled in calculations as it is a clothes dryer.

So, the minimum service load for a 4.7 KW combination clothes washer and dryer = 5000 watts = 5 KW



Example#2:


In a multi-family dwelling with 10 dwelling units, assuming the load of 10 single-phase dryers is connected as evenly as possible to the 3-phase system (3 dryers connected between phases A and B, 3 dryers connected between phases B and C, and 4 dryers connected between phases A and C), as in below image.




Solution:

3-phase Dryer load without applying the 220.54 demand factor = 5500 VA x 10 = 55,000 VA 

Step#1: The maximum number of dryers connected between any two phases is 4

Step#2: Twice the maximum number of dryers connected between any two phases is used as the basis for calculating the demand load = 4 dryers x 2 = 8 dryers

Step#3: 2-phase Total “connected load”= 8 x 5500 VA = 44,000 VA
Step#4: from Table 220.54, for 8 dryers, demand factor = 60% 

Step#5: 2-phase Total “demand load”= 44,000 VA x 0.6 = 26,400 VA (connected between two phases of the 3-phase system)

Step#6: service 3-phase Total “demand load” = 26,400 VA x 3/2 = 39,600 VA



Second: As per NEC Optional calculation method:




Rule#5: Application of NEC Optional calculation method

NEC Optional calculation method will be used if the following condition is verified:
  1. The service-entrance or feeder conductors have an ampacity of at least 100 amperes.




Important!!!
If the service-entrance ampacity calculated by the optional method is less than 100A, re-calculate with using the standard method.



Important!!!
In NEC Optional calculation method, for a multifamily dwelling, Table 220.84 “Optional Calculations — Demand Factors for Three or More Multifamily Dwelling Units” will be used if the following conditions are verified:
  1. If No dwelling unit is supplied by more than one feeder
  2. Each dwelling unit is equipped with electric cooking equipment.
  3. Each dwelling unit is equipped with either electric space heating or air conditioning, or both.






Important!!!
The optional calculation can be used, provided all of the conditions for using table 220.84 listed above are met. Otherwise, the calculation for the multifamily dwelling is performed by using standard calculation method.



Important!!!
In Optional calculation method, note the following:
 The dryer load is not subject to an individual demand factor as required by table 220.54 for NEC Standard calculation method.
The nameplate of the clothes dryer will be used, even if it is less than 5000 watts.



In a multi-family dwelling, for Optional calculation method, follow the below steps: 


  • Step#1: Total “connected load” for clothes dryers = sum of all nameplate rating of clothes dryers loads in each dwelling unit---- (Result#1). 
  • Step#2: Based on the total number of dwelling units in this building, assign the demand factor from Table 220.84. 
  • Step#3: 3-phase service Total “demand load” for clothes dryers = (Result#1) X Demand factor. 


Example#3:


What is the one-family dwelling load contribution for a 4.5KW clothed dryer?


Solution:
The minimum load using the NEC standard calculation method for a clothes dryer = 5000 watts 

The load using the NEC optional calculation method for a clothes dryer = nameplate rating = 4500 watts



Example#4:

Find the solution for the same multi-family dwelling building in example#1 by using the NEC optional calculation method.


Solution:
Step#1: Total “connected load” for clothes dryers =5500 VA X 10 = 55,000 VA
Step#2: from Table 220.84, For 10 dwelling units, the demand factor = 43 %. 

Step#3: 3-phase service Total “demand load” for clothes dryers= 55,000 VA x 0.43 = 23,650 VA
Service ampacity = 23,650VA /(208 V X1.732) = 66 A < 100 A, So the calculation must be repeated using NEC standard calculation method. 



B- For branch circuit requirements (conductor ampacity & size and over-current protection) calculation


This will be explained in coming articles, however you can review the calculation for maximum number of allowable receptacles on a branch circuit which was discussed before in previous article 
" Receptacle Branch Circuit Design Calculations – Part Three ".



4.4 How to specify the required Type of receptacles for Clothes dryer load?


Same as that in paragraph 3.5 for Laundry receptacles.



In the next article, I will explain Receptacle branch circuits that serve other types of loads. Please, keep following.



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1 comment:

  1. Hello Eng. Ali,

    This is only for information. In example-2, it should be 4 dryers connected b/w phases A & B, since there are 4 dryers are shown in pictorial representation.

    ReplyDelete

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