Demand Calculation

[bigpapa400lb]

A single family dwelling is supplied by a 120/240 volt, single phase service. The dwelling has 3200 square feet and the following electrical equipment:

15,000 VA fixed space heater at 240V
14,000 VA Range at 240V
6,000 VA clothes dryer at 240V
1,000 VA disposal motor at 120 V
1,500 VA fan motor at 240 V
Five other non-motor, 120V, fastened in place appliances, with total combined nameplate rating of 10,500 VA

Using the standard calculation method method, calculate the total load for the dwelling's service.

3,200 x 3va(general lighting) 9600
1500 x 2(small appliance) 3000
1500 x 1(Laundry) 1500

Total 14100

First 3000 @ 100% 3000
Remaining 11100 @ 35% 3885

Total General Lighting 6885

15000 Fixed space heater @ 100% 15000
14000 Range 240V col C 1 Range 8KW 8800
6000 VA Dryer 6000
1000 VA Disposal Motor(largest motor 125%) 1250
1500 VA fan motor @ 240v 1500/2 750
10500 Appliance load @ 75% 7875

Total Demand is 46560

Answer key says 46685 VA
Somehow I am missing 125 VA?

 

[charlie b]

The disposal is not the largest motor. The fan is. Adding 25% of the 500 VA difference between the fan and the disposal gives you the 125 VA you are missing.

 

[charlie b]

Now let me ask two questions:

1,000 VA disposal motor at 120 V

Why isn't this included with the appliance loads, so that the 75% factor of 220.53 can be applied to it?

1500 VA fan motor @ 240v 1500/2 750

I don't see why you get to divide the 1500 VA by 2, and only count it as 750VA. Can you give me the reference?

 

[bigpapa400lb]

Now let me ask two questions:Why isn't this included with the appliance loads, so that the 75% factor of 220.53 can be applied to it? I don't see why you get to divide the 1500 VA by 2, and only count it as 750VA. Can you give me the reference?

Because motors would need a neutral that what I was thinking. But there are phase to phase motors.

 

[charlie b]

Because motors would need a neutral that what I was thinking. But there are phase to phase motors.

Is that a response to the question about the disposal or the question about the fan?

Here are my thoughts about the 1500 VA fan. You are not adding the load on each phase separately, like you would do if you were trying to balance the loads. You are looking for the total load. The total load of that one item is 1500 VA. It would divide out at 750 VA on each of the two phases to which it would be connected. But it is still a total of 1500.

But if you count it as 1500, your answer will be 750VA higher than that of the "answer key." If you were to include the disposal in the 75% demand factor, that would reduce your total by 250 VA. So now you would be 500 VA higher than the answer key's total. I don't know how to reconcile the differences.

 

[gndrod]

Is that a response to the question about the disposal or the question about the fan?

Here are my thoughts about the 1500 VA fan. You are not adding the load on each phase separately, like you would do if you were trying to balance the loads. You are looking for the total load. The total load of that one item is 1500 VA. It would divide out at 750 VA on each of the two phases to which it would be connected. But it is still a total of 1500.

But if you count it as 1500, your answer will be 750VA higher than that of the "answer key." If you were to include the disposal in the 75% demand factor, that would reduce your total by 250 VA. So now you would be 500 VA higher than the answer key's total. I don't know how to reconcile the differences.

I agree. That 1500va fan motor in the standard method [220.53] FIP can be 100% if under four in-place loads. It is only factored 75% DF in four or more. rbj

 

[charlie b]

It is only factored 75% DF in four or more. rbj

Take a close look at the question (I missed this the first time through). It says that there are five other fixed-in place appliances, so the 75% DF can be applied to them. I think the disposal should be included with the "five others," so that it gets the 75% DF also.

 

[gndrod]

Take a close look at the question (I missed this the first time through). It says that there are five other fixed-in place appliances, so the 75% DF can be applied to them. I think the disposal should be included with the "five others," so that it gets the 75% DF also.

Thanks Charlie, I missed the five. I agree the FIP includes the disposer. The largest motor can be applied to the disposer [220.50] [430.24] if its the largest...so va x 25% is additional I believe. rbj

 

[S'mise]

I agree with 430.24 (highest rated motor) at 125%

1500va fan is rated more than the 1000va disposal
so; 1500x1.25=1875va for fan

I agree the disposal should be calc at 75% also.

I get 47,185va Still, does not match the key.

 

[charlie b]

Still, does not match the key.

Perhaps the key is wrong.

 

[S'mise]

yes, that drives me bonkers

Its best when they list all their numbers so one can tell where they went wrong. (or the author)

 

[gndrod]

yes, that drives me bonkers

Its best when they list all their numbers so one can tell where they went wrong. (or the author)

The way the FIP statement lined up is where I see confusion. I recalculated the FIP (please feel free to comment) as such:

FIP disposer....1000 + Fan motor....1500 + 5 other NPR's 10500 total= 13000 x 75%= 9750 va

Accordingly: 6885 + 9750 + 6000 + 8800 + 15000 + 375 (largest motor) = 46810 va

BigPapa, Maybe I missed something, could you look this over and confirm? Tx rbj