In article " Receptacle Branch Circuit Design Calculations – Part Three ", I stated that a Receptacle in dwelling units may serve one of the following loads:
 Generaluse Receptacle Loads,
 Small appliance Loads,
 Laundry Load,
 Cloth dryer Load,
 Household cooking appliances load,
 Fastenedinplace Appliance loads,
 Heating and air conditioning loads,
 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:
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 ".
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:
 Applied NEC Rules for Heating and air conditioning loads
 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:

Important!!!
For condition#1 in Rule#1
above, If the maximum demand data for a 1year period is not available, the
calculated load shall be permitted to be based on the maximum demand (measure
of average power demand over a 15minute period) continuously recorded over a
minimum 30day 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 1year period from 220.87(1) x 125%
New Load = Continuous loads x
125 % + Noncontinuous 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 peakdemand meter has been connected to the service for more than a year. The maximum demand data from the peakdemand 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 1year 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 Codecompliant
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 singlephase service. The service can be fed from a
120/240volt (V) or 208Y/120V system but must be a 3wire 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:
 Where Additional AirConditioning Equipment or Electric SpaceHeating Equipment Is Not to Be Installed.
 Where Additional AirConditioning Equipment or Electric SpaceHeating Equipment Is to Be Installed.
A Where Additional AirConditioning Equipment or Electric SpaceHeating 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 onefamily dwelling; the existing kitchen will also be renovated. The area is 1,500 square feet. The existing dwelling has the following loads:
 Two smallappliance 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 smallappliance branch circuits will be installed, thus making a total of four smallappliance branch circuits.
No additional air conditioning equipment or electric spaceheating 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 generaluse receptacle load = 2,000 ft2 x 3 VA /ft2= 6,000 VA
Second: calculate the smallappliance branch circuit and laundry branch circuit load as specified in [220.83(A)(1)]
The smallappliance and laundry branch circuits load = (four smallappliance branch circuits + one Laundry branch circuit) = 1,500 VA x 5 = 7,500 VA.
Third: calculate The Fastenedinplace appliance load, as specified in 220.83(A)(3)(a)
The Fastenedinplace 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 voltamperes 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 AirConditioning Equipment or Electric SpaceHeating 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 airconditioning or spaceheating, so the following
will be applied for these loads:

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 spaceheating 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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