Branch Circuit Design Calculations – Part Eleven


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. Household cooking appliances load, 
  6. Fastened-in-place Appliance loads, 
  7. Heating and air conditioning loads, 
  8. Motor loads. 

I explained the first six 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:



7- Heating and air conditioning loads – Part Three

In the previous Article 
 " Heating and air conditioning loads – Part One ", I explained the following points:

  1. Applied NEC Rules for Heating and air conditioning loads 
  2. Feeder and service calculation of Heating and air conditioning loads by two methods: 
  • First: As per NEC Standard calculation method,
  • Second: As per NEC Optional calculation method.

The rules applied in second method differ from a dwelling unit type to another. I explained the rules for first dwelling type “A single dwelling unit” in Article 
 " Heating and air conditioning loads – Part One "

Also, I explained the rules for dwelling unit type “multifamily dwelling and two dwelling units” in Article 
" Heating and air conditioning loads – Part Two "

Today, I will continue explaining the rules for feeder and service calculation of Heating and air conditioning loads as per NEC Optional calculation method for the forth case of dwelling unit which is existing dwelling unit. 


Forth: Existing Dwelling Unit



Important!!!
The purpose of doing service and feeder calculations for an existing dwelling unit is to determine if the existing service or feeder is of sufficient capacity to serve a required additional loads or not.




Rule#1: Conditions Of adding new loads to An Existing Dwelling Unit

As per NEC section 220.87, Additional loads may be connected to existing services and feeders under the following conditions:

  1. The maximum demand kVA data for a minimum 1-year period (or the 30-day alternative method from the exception) is available.
  2. The maximum demand at 125 percent plus the new load does not exceed the ampacity of the feeder or rating of the service.
  3. The feeder has overcurrent protection in accordance with 240.4, and the service has overload protection in accordance with 230.90.





Important!!!
For condition#1 in Rule#1 above, If the maximum demand data for a 1-year period is not available, the calculated load shall be permitted to be based on the maximum demand (measure of average power demand over a 15-minute period) continuously recorded over a minimum 30-day period using a recording ammeter or power meter connected to the highest loaded phase of the feeder or service, based on the initial loading at the start of the recording. The recording shall reflect the maximum demand of the feeder or service by being taken when the building or space is occupied and shall include by measurement or calculation the larger of the heating or cooling equipment load, and other loads that may be periodic in nature due to seasonal or similar conditions.




Important!!!
For condition#2 in Rule#1 above, apply the NEC standard Calculation method to get the total load as follows:

Total Load = Existing Load Value + New Load

Where:
Existing Load Value = Max demand Value for a 1-year period from 220.87(1) x 125%
New Load = Continuous loads x 125 % +  Non-continuous loads x 100 %





Important!!!
If condition#2 in Rule#1 above is not verified, you need to increase ampacity of the feeder and/or rating of the service to be able to add new loads to an existing dwelling unit.



Example#1:

A business owner wants to add equipment to an existing building. The new equipment will not replace any of the existing equipment. The existing service has a rating of 1,200 amperes (A). A peak-demand meter has been connected to the service for more than a year. The maximum demand data from the peak-demand meter shows a demand of 743A.

The new equipment that has a calculated load (in accordance with standard load calculation procedures) of 251A. Does this existing service have an ampere rating high enough for the existing loads and the new loads?



Solution:

Existing Load Value = Max demand Value for a 1-year period from 220.87(1) x 125% = 743 x 125% = 929 A

New Load = 251 A

New Service Load = Existing Load Value + New Load = 929 + 251 = 1,180 A

Since the existing service is rated 1,200A, this installation will be Code-compliant





Rule#2: Conditions Of Application Of NEC Optional Calculation Method For An Existing Dwelling Unit

As per NEC section 220.83, The NEC optional calculation method for an existing dwelling can only be used if the dwelling is supplied by a single-phase service. The service can be fed from a 120/240-volt (V) or 208Y/120V system but must be a 3-wire system.



For existing dwelling unit, it shall be permissible to calculate the total load by using the NEC optional calculation method in the following two cases:

  1. Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is Not to Be Installed. 
  2. Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is to Be Installed. 



A- Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is Not to Be Installed



 
Rule#3: Calculation of electrical load as per NEC Optional Calculation Method For An Existing Dwelling Unit – Case (A)

In this case, the calculation is almost identical to the calculation method in 220.82(B) for single family dwelling. The only difference is the amount of load that is rated at 100 percent, in 220.82(B) for single family dwelling it was 10 KVA but in 220.83 for existing dwelling it will be 8 KVA.



The following percentages shall be used for existing and additional new loads:


Load (kVA)
Percent of Load
First 8 kVA of load at
100
Remainder of load at
40



Important!!!
The NEC optional calculation method for an existing dwelling shall Include all of the existing loads and the new loads.


Example#2:


An addition will be built onto an existing one-family dwelling; the existing kitchen will also be renovated. The area is 1,500 square feet. The existing dwelling has the following loads:
  • Two small-appliance branch circuits; 
  • One laundry branch circuit; 
  • Two attic roof ventilators rated 506 VA each; 
  • One motor rated 1,176 VA; 
  • A range rated 8,000 VA; 
  • A clothes dryer rated 5,000 VA; 
  • And a water heater rated 4,500 VA. 

The service supplying this existing dwelling is 120/240V, single phase, and is rated 100 A.
The addition built onto this dwelling will add 500 square feet of floor area. Therefore, the total calculated floor area will be 2,000 square feet.
The existing range will be replaced by a new 12,000 VA range.
The kitchen renovation will also include two additional appliance loads: a dishwasher rated 1,380 VA and a kitchen waste disposer rated 864 VA. Two more small-appliance branch circuits will be installed, thus making a total of four small-appliance branch circuits.
No additional air conditioning equipment or electric space-heating equipment will be installed

Is this 100A service of sufficient capacity to serve these additional loads? If not, what size service is required?



Solution:

When calculating an existing dwelling unit, include all the existing loads and all the additional new loads.

First: calculate the general lighting and general as specified in [220.83(A)(1)]

The general lighting and general-use receptacle load = 2,000 ft2 x 3 VA /ft2= 6,000 VA

Second: calculate the small-appliance branch circuit and laundry branch circuit load as specified in [220.83(A)(1)]

The small-appliance and laundry branch circuits load = (four small-appliance branch circuits + one Laundry branch circuit) = 1,500 VA x 5 = 7,500 VA. 

Third: calculate The Fastened-in-place appliance load, as specified in 220.83(A)(3)(a) 
The Fastened-in-place appliance load= Loads of (Two attic roof ventilators + motor + a dishwasher + kitchen waste disposer) = 506 + 506 + 1,176 + 1,380 + 864 = 4,432 VA 

Forth: calculate other loads as specified in 220.83(A)(3)(b), (c), (d) 

Other loads = new range load + clothes dryer load + water heater load = 12,000 VA + 5,000 VA + 4,500 VA = 21,500 VA 

Fifth: Before applying the demand factor, the total for the existing loads and the new loads = 6,000 + 7,500 + 4,432 + 21,500 = 39,432 VA 

Sixth: In accordance with 220.83(A), calculate the first 8 kVA (8,000 VA) of load at 100 percent and the remainder of load at 40 percent. 

Demand of existing loads and the new loads = 8,000 VA x 100 % + (39,432 – 8,000) x 40% = 20,573 VA 

Seventh: Find the minimum ampere rating by dividing volt-amperes by voltage. 

The minimum ampere rating required for this service = 20,573 VA ÷ 240 V = 85.7 A = 86 A.

Since the service supplying this dwelling is rated 100A, it is of sufficient capacity to serve these additional loads.



B- Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is to Be Installed



Rule#4: Calculation of electrical load as per NEC Optional Calculation Method For An Existing Dwelling Unit – Case (B)

In this case, the calculation is identical to that of case (A), But In this case, there are additional loads for of air-conditioning or space-heating, so the following will be applied for these loads:

  1. The larger connected load of air-conditioning or space-heating, but not both, shall be used.
  2. The following percentages shall be used for existing and additional new loads:

  • Air-conditioning equipment at  100%
  • Central electric space heating at %100
  • Less than four separately controlled space heating units at 100%




The percentages in below image shall be used for existing and additional new loads.






Important!!!
The NEC optional calculation method for an existing dwelling shall Include all of the existing loads and the new loads.



Example#3:


In example#2, before the job starts, the homeowners change their mind. They decide to install a central air conditioning system that is rated 7,200 VA.

Is this 100A service of sufficient capacity to serve these additional loads plus the central air conditioning unit? If not, what size service is required?

Solution: 

Use rules as specified in 220.83(B), for existing and additional new loads where additional air conditioning equipment or electric space-heating equipment is to be installed.

From example#2, the Demand of existing loads and the new loads = 20,573 VA
As per Rule#4 above, the air conditioning system load must be added to the calculation at 100 percent of nameplate rating 
So, the Demand of existing loads and the new loads = 20,573 + 7,200 = 27,773 VA 
The minimum ampere rating required for this service = 27,773 VA ÷ 240 V = 115.7 A = 116 A , which exceeds the 100 A old service rating 
In accordance with 240.6(A), the next standard ampere rating above 116 is 125 amperes. Therefore, the minimum size service for this dwelling is 125 amperes. 





In the next Article, I will explain the Motor loads values for branch circuit calculation in dwelling units. Please, keep following.




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