# No Idea how to do a load calculation.

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#### lahabra

##### New Member
Can anyone explain how to do a load calculation on a house?

My Plan
-----100A main
-----100A Sub Panel
------------------------
20A 240v AC 1,800 BTU
20A 240v AC
------------------------
20A 240v AC 1,800 BTU
20A 240v AC
------------------------
15A AFCI air conditioner 700 BTU
20A microwave
20A kitchen counter L.
20A kitchen counter R.
20A Disposal
20A Dish wash
20A Bath rm.
20A GFCI Whirlpool tub
20A Washer
20A Dryer
15A AFCI smoke alarms
15A AFCI bed rm. Outlets and lights
15A lights
15A Frige/stove
15A general outlets 1.
15A general outlets 2.
15A general outlets 3.
15A outside lights
15A GFCI outside outlets

This is a small 600sqâ€™ house.

#### Bill Arden

##### Computer Programmer
-----100A main
-----100A Sub Panel
20A 240v * 2, AC 1,800 BTU
This is a small 600sqâ€™ house.

The "diversity" concept of doing load calculation is based on likley uses and not the amperage of the circkuts. For example I got a pile of 20A outlets and ran a lot of 20A circuits in my garage, but I don't actually expect to use that much current at the same time.

So you need to know
1. Number of bathrooms. (probably one)
2. Number of bedrooms. (probably two given the size)
3. Non human loads.
Hot Water heater, Stove, AC, Dish wash, Whirlpool tub, Washer, Dryer

#### 480sparky

##### In the Trades
You don't have the needed information to do a calculation. They are not done by # of circuits and their ampacities.

Go to Annex D of the NEC and look at the examples. You can get an idea on how it's performed by looking at them. Then go to Article 220.

#### lahabra

##### New Member
Go to Annex D of the NEC,......Then go to Article 220.

Thanks for the pointer,...I tried to find the NEC Article you mentioned but I lack the computer searching skills.

#### Alectrician

##### DIY Senior Member
Duh.............google "residential load calculations".

#### Bob NH

##### In the Trades
This is by way of example based on understanding of and estimates from your load definitions. You will have to adjust for some things but it is clear that 100 Amps is enough. I have probably included more than you need.

You seem to have a lot of circuits for the size of house but that doesn't increase the load except where you have a separate circuit for washer and gas dryer.

Can anyone explain how to do a load calculation on a house?

My Plan
-----100A main
-----100A Sub Panel
------------------------
1. 20A 240v AC 1,800 BTU Seems very small BTUs for a 20 Amp circuit, but use Amps x 240.
20A 240v AC
------------------------
2. 20A 240v AC 1,800 BTU Seems very small BTUs for a 20 Amp circuit, but use Amps x 240.
20A 240v AC
------------------------
3. 15A AFCI air conditioner 700 BTU Seems very small BTUs for a 20 Amp circuit, but use Amps x 120.
4. 20A microwave Microwave Watts rating, say 1500 Watts
5. 20A kitchen counter L. 1500 Watts
6. 20A kitchen counter R. 1500 Watts
7. 20A Disposal Use Amps x 120 = Watts. or 1200 Watts per HP
8. 20A Dish wash Use Amps x 120 = Watts, estimate 1500 Watts
9. 20A Bath rm. Included in lighting
10. 20A GFCI Whirlpool tub Use motor amps x 1.2 x 120 + heater watts = Watts; say 1500 Watts.
11. 20A Washer 1500 Watts
12. 20A Dryer Laundry circuit 1500 Watts
13. 15A AFCI smoke alarms Included in lighting
14. 15A AFCI bed rm. Outlets and lights Included in lighting
15. 15A lights Included in lighting
16. 15A Frige/stove Assume gas stove. Use 1500 Watts; probably more than needed.
17. 15A general outlets 1. Included in lighting
18. 15A general outlets 2. Included in lighting
19. 15A general outlets 3. Included in lighting
20. 15A outside lights Included in lighting but add installed light watts
21. 15A GFCI outside outlets Included in lighting

22. This is a small 600sqâ€™ house. 3 Watts per square ft x 600 = 1800 Watts. Use gross area including garage and basement.

Loads that can be added together with demand factor applied.

Add all of the loads except A/C:
1500+1500+1500+1200+1500+1500+1500+1500+1500+1800= 15,000 watts.

Demand factor for optional calculation: 100% of first 10,000 Watts + 40% of balance = 10,000 + 0.4 x 5000 = 12,000 See NEC 220.80

Add the AC; I'm allocating 6000 Watts but you need to get the nameplate ratings.

Total = 18,000 Watts

18,000/240 Volts = 75 Amps so 100 Amps is OK.

#### lahabra

##### New Member
Thanks for reviewing my plan. I feel better having your input.

20A Bath rm. Included in lighting

I was wandering why you didnâ€™t count the bathroom circuit separately? Was that a typo?

#### lahabra

##### New Member
Add all of the loads except A/C:

Do you always ad the AC in last or did you do it because we donâ€™t have its name plate rating?

Thanks

#### Bill Arden

##### Computer Programmer
You might also want to add in the load for an electric hot water heater and an electric stove for future use.

Edit:
>"5. 20A kitchen counter L. 1500 Watts"
>"6. 20A kitchen counter R. 1500 Watts"

Why would this be separate for the total house load?
You are unlikely to plug in two microwaves at the same time.

Edit:
Don't forget to check local zoning laws and the power utility rules. My local utility does not allow 100A main panels anymore for houses even though they tend to use 15Kw pole transformers.

Last edited:

#### Bob NH

##### In the Trades
You might also want to add in the load for an electric hot water heater and an electric stove for future use.

Edit:
>"5. 20A kitchen counter L. 1500 Watts"
>"6. 20A kitchen counter R. 1500 Watts"

Why would this be separate for the total house load?
You are unlikely to plug in two microwaves at the same time.

You would only provide for electric water heater and range if that will be the source of energy for those processes. You would not if you have gas for water heating and cooking.

Load of two kitchen counter circuits is included because 220.82(B)(2) explicitly requires it, along with 1500 for EACH laundry branch circuit.

It is fairly common to have multiple loads operating in the kitchen. I have tripped the breaker when an electric teakettle and a countertop microwave were plugged into the same 20 Amp circuit. An electric griddle combined with either will also trip the circuit. I have marked my circuits A and B and given instruction that not more than one high-power device be operated on a circuit.

Relative to some of the other inquiries, some loads are explicitly included in the lighting load (and no additional load allowance required) per code instruction of 210.14(J), as follows:
(1) Bathroom branch circuits of 210.11(C)(3)
(2) Outdoor outlets per 210.52(E) and Basements and Garages per 210.52(G)

The A/C is added at the end because 220.82 separates (B) general loads subject to application of demand factors and (C) Heating and air conditioning loads subject to application of different demand factors. (C)(1) requires 100 percent of the nameplate ratings of the A/C. It requires the largest of the heating or cooling load but not both.

Last edited:

#### JWelectric

##### Electrical Contractor/Instructor
Bob

Above you stated that; â€œRelative to some of the other inquiries, some loads are explicitly included in the lighting load (and no additional load allowance required) per code instruction of 210.14(J), as follows:
(1) Bathroom branch circuits of 210.11(C)(3)
(2) Outdoor outlets per 210.52(E) and Basements and Garages per 210.52(G)â€ although you used 220.82 for the calculation.

I disagree with this statement in so much as if you are using the Optional feeder and service load calculation then only 220.82 will apply.
In other words you answer should have been;

(B) General Loads. The general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of the following loads:
(1) 33 volt-amperes/m2 or 3 volt-amperes/ft2 for general lighting and general-use receptacles. The floor area for each floor shall be calculated from the outside dimensions of the dwelling unit. The calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.
(2) 1500 volt-amperes for each 2-wire, 20-ampere small-appliance branch circuit and each laundry branch circuit specified in 220.52.
(3) The nameplate rating of all appliances that are fastened in place, permanently connected, or located to be on a specific circuit, ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and water heaters.
(4) The nameplate ampere or kVA rating of all motors and of all low-power-factor loads.

To refer back to 220.14 will mean that the standard calculation is what is being used. The two do not intermix.

Also it needs to be pointed out that the optional calculation can only be used for a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater.
If the calculated load using the optional calculation comes up to less than 100 amps then the standard calculation must be used,

In this case with the limited information given it wouldnâ€™t much matter as the standard calculation still comes up to less than 100 amps and 230.79(C) comes into play;
(C) One-Family Dwelling. For a one-family dwelling, the service disconnecting means shall have a rating of not less than 100 amperes, 3-wire.

#### Bob NH

##### In the Trades
See comments embedded in the post.

Bob

Above you stated that; â€œRelative to some of the other inquiries, some loads are explicitly included in the lighting load (and no additional load allowance required) per code instruction of 210.14(J), as follows:
(1) Bathroom branch circuits of 210.11(C)(3)
(2) Outdoor outlets per 210.52(E) and Basements and Garages per 210.52(G)â€ although you used 220.82 for the calculation.

I disagree with this statement in so much as if you are using the Optional feeder and service load calculation then only 220.82 will apply.
That position is not consistent with the examples in Appendix D. Can you cite examples or other references? See remark below regarding use of 220.14 for both standard and optional calculation. Bob_NH

In other words you answer should have been;

(B) General Loads. The general calculated load shall be not less than 100 percent of the first 10 kVA plus 40 percent of the remainder of the following loads:
(1) 33 volt-amperes/m2 or 3 volt-amperes/ft2 for general lighting and general-use receptacles. The floor area for each floor shall be calculated from the outside dimensions of the dwelling unit. The calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.
(2) 1500 volt-amperes for each 2-wire, 20-ampere small-appliance branch circuit and each laundry branch circuit specified in 220.52.
(3) The nameplate rating of all appliances that are fastened in place, permanently connected, or located to be on a specific circuit, ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and water heaters.
(4) The nameplate ampere or kVA rating of all motors and of all low-power-factor loads.

To refer back to 220.14 will mean that the standard calculation is what is being used. The two do not intermix. The standard feeder and load service calculation is in Part III, and the optional calculation in Part IV. Neither of those include section 220.14, which is not specific to either method of load calculation. Therefore, I believe it applies generally. That is consistent with the examples in Appendix D where neither the standard nor optional calculations include the bathroom circuit as a separate element of the load. Bob_NH.

Also it needs to be pointed out that the optional calculation can only be used for a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater.
If the calculated load using the optional calculation comes up to less than 100 amps then the standard calculation must be used. Not correct. The ability to use the optional calculation depends on the ampacity of the feeder or service circuit being at least 100 Amps; not on the calculated load being at least 100 Amps.

In this case with the limited information given it wouldnâ€™t much matter as the standard calculation still comes up to less than 100 amps and 230.79(C) comes into play;
(C) One-Family Dwelling. For a one-family dwelling, the service disconnecting means shall have a rating of not less than 100 amperes, 3-wire.

#### JWelectric

##### Electrical Contractor/Instructor
That position is not consistent with the examples in Appendix D. Can you cite examples or other references? See remark below regarding use of 220.14 for both standard and optional calculation. Bob_NH

Yes just as you have done in you next statement. The standard is contained in Part III and the Optional is contained in Part IV

The standard feeder and load service calculation is in Part III, and the optional calculation in Part IV. Neither of those include section 220.14, which is not specific to either method of load calculation. Therefore, I believe it applies generally. That is consistent with the examples in Appendix D where neither the standard nor optional calculations include the bathroom circuit as a separate element of the load. Bob_NH.

I agree that there is no additional load for the bath circuit but it must be on a 20 amp circuit as outlined in 210.11(C)(3).
What I am trying to say is that all that is used in the Optional calculation is the three parts of 220.82. There is no reason to look elsewhere for what is included or not included in the calculation.

If the calculated load using the optional calculation comes up to less than 100 amps then the standard calculation must be used. Not correct. The ability to use the optional calculation depends on the ampacity of the feeder or service circuit being at least 100 Amps; not on the calculated load being at least 100 Amps.

Then these words will need to be edited;
(A) Feeder and Service Load. This section applies to a dwelling unit having the total connected load served by a single 120/240-volt or 208Y/120-volt set of 3-wire service or feeder conductors with an ampacity of 100 or greater.

To make this a little easier to understand take out all the numbers and read it as;
(A) Feeder and Service Load. This section applies to a dwelling unit having the total connected load served by a single set of 3-wire service or feeder conductors with an ampacity of 100 or greater.

Here it clearly says that the dwelling unit having a total connected load of a 100 amps or more that is supplied by feeders or service conductors.
In this case it wouldnâ€™t matter because either calculation comes up to less than 100 amps and 230.70 kicks in.

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