Load calcs for an apartment

[anonymousparky]

When doing load calcs for the service of an apartment building, do you calculate each unit first then add? I have a confusing question that I am unsure of proper procedure. Normally, you would account for all ranges for all the units, then deduct accordingly. This problem shows 2) 8kw ranges per unit and 13 units. Would I go off of the 26-30 count of ranges, which would be 27kw or would I go off 2 ranges, which would be 11kw times 13 units?

 

[Dennis Alwon]

When doing load calcs for the service of an apartment building, do you calculate each unit first then add? I have a confusing question that I am unsure of proper procedure. Normally, you would account for all ranges for all the units, then deduct accordingly. This problem shows 2) 8kw ranges per unit and 13 units. Would I go off of the 26-30 count of ranges, which would be 27kw or would I go off 2 ranges, which would be 11kw times 13 units?

Are you going to use 220.84?

If you are doing the ranges then I would go off the 26 ranges and get 24kw using col. B Where are you getting 27kw

 

[anonymousparky]

Are you going to use 220.84?

If you are doing the ranges then I would go off the 26 ranges and get 24kw using col. B Where are you getting 27kw

I was using table 220.55. 26-30 ranges. I guess it is 24 kw. I looked in the column to the right of it for not over 12kw. My bad.

I am new to load calcs, and trying to learn. Single family is easy enough but the multifamily is tricky at times. Is there a better thread for learning them?

 

[JoesBagelandDeli]

Are you going to use 220.84?

If you are doing the ranges then I would go off the 26 ranges and get 24kw using col. B Where are you getting 27kw

26/3 = 8.66->9

9*2 = 18
Demand factor from Table 220.55 : 28%

18 * .28 * 8kW = 40.3 kW

40.3 / 2 = 20.16kW

20.16kW * 3 = 60.5 kW
...
I thought this was the process for applying 220.55.

 

[augie47]

would the key not rely on the service being 3 phase or single phase ?

 

[Dennis Alwon]

would the key not rely on the service being 3 phase or single phase ?

Yes I was not thinking 3 phase but it probably is set up as that.

 

[david luchini]

I was using table 220.55. 26-30 ranges. I guess it is 24 kw. I looked in the column to the right of it for not over 12kw. My bad.

I am new to load calcs, and trying to learn. Single family is easy enough but the multifamily is tricky at times. Is there a better thread for learning them?

Table 220.55, Column C for 26 ranges would be 41kW.

Column B for (26) 8kW ranges would be 49.9kW.

You could use the lower value from Column C.

 

[JoesBagelandDeli]

This is why I stick with the Optional Method.

The current draw @ 60.5kW 3ph will be less than 41kW 1ph, but i'm still surprised that the demand kW is so much higher for 3ph.

It's also strange to me that the same code section can be applied so differently depending on whether the design is for 1ph or 3ph.

 

[david luchini]

This is why I stick with the Optional Method.

The current draw @ 60.5kW 3ph will be less than 41kW 1ph, but i'm still surprised that the demand kW is so much higher for 3ph.

It's also strange to me that the same code section can be applied so differently depending on whether the design is for 1ph or 3ph.

Use Col. C. The demand for the ranges will be 49.5kW instead of 60.5kW

 

[JoesBagelandDeli]

Use Col. C. The demand for the ranges will be 49.5kW instead of 60.5kW

For a 3-phase service? How are you getting to 49.5kW?

 

[david luchini]

For a 3-phase service? How are you getting to 49.5kW?

9*2=18. The demand for 18 ranges is 33kW.

33kW * 1.5 = 49.5kW.

 

[wwhitney]

9*2=18. The demand for 18 ranges is 33kW.

33kW * 1.5 = 49.5kW.

But in the typical case of a 13 unit building supplied by 3 phase power, each apartment would only get 120/208V "single" phase. So with 2 ranges per apartment, the 26 ranges are arranged as 10, 8, and 8 across the 3 phases, not 9, 9, 8.

Also, does 220.55 really require us to use the same load for 10, 8, and 8 as for 10, 10, and 10? Seems like the first case should be a lower load.

Cheers, Wayne

 

[anonymousparky]

I'm glad more people have had imput. It was single phase in the problem. I originally used column C by mistake. It should have been B. My confusion lay more in whether I would take 2 ranges for the 1 unit, then take that number by 13 units, or if I would go off 26 ranges for 13 units. I can't remember if it was optional or standard.

 

[david luchini]

I originally used column C by mistake. It should have been B.

Use Column C instead of Column B. It gives you a lower demand.

or if I would go off 26 ranges for 13 units. I can't remember if it was optional or standard.

You would use 26 ranges for 13 units. If it was the optional calculation, you wouldn't be using 220.55.

 

[JoesBagelandDeli]

But in the typical case of a 13 unit building supplied by 3 phase power, each apartment would only get 120/208V "single" phase. So with 2 ranges per apartment, the 26 ranges are arranged as 10, 8, and 8 across the 3 phases, not 9, 9, 8.

Also, does 220.55 really require us to use the same load for 10, 8, and 8 as for 10, 10, and 10? Seems like the first case should be a lower load.

Cheers, Wayne

How are you getting 10, 8, and 8? How would any pair get two ahead? I was so flustered i had to draw this out, and I'm seeing 9,9,8. I must be missing something. Please explain.

 

[wwhitney]

How are you getting 10, 8, and 8? How would any pair get two ahead? I was so flustered i had to draw this out, and I'm seeing 9,9,8. I must be missing something. Please explain.

I'm assuming each apartment only has a 120/208V "single phase" supply. So with 13 apartments, 5 will be supplied A-B, say, and 4 B-C, 4 C-A. With 2 ranges per apartment, that means 10 ranges will be A-B and 8 B-C, 8 C-A.

Cheers, Wayne

 

[Dennis Alwon]

I'm assuming each apartment only has a 120/208V "single phase" supply. So with 13 apartments, 5 will be supplied A-B, say, and 4 B-C, 4 C-A. With 2 ranges per apartment, that means 10 ranges will be A-B and 8 B-C, 8 C-A.

Cheers, Wayne

That's an interesting way to look at it. I assume the question normally would understand 3 phase to each apt but in reality that usually is not the case

 

[wwhitney]

but in reality that usually is not the case

Assuming a test question, if it specifies 13 apartment, 2 ranges per apartment, I infer it's not just to get you to multiply 2 * 13 = 26, there's no particular reason to test people on that one extra multiplication step. A reasonable test writer would just write it for 26 ranges if that's the idea.

But by specifying 13 apartments, 2 rangers per apartment, coupled with the knowledge that apartments generally get only 2 ungrounded conductors, now you have to think about the range arrangement across ungrounded conductors. So instead of being a pointless extra multiplication step, it becomes an interesting feature worth incorporating into a test question.

Cheers, Wayne

P.S. It's probably contrary to the the current text in 220.55, but it seems to me a better and very realistic way to deal with asymmetric range loading on the ungrounded conductors is simply to calculate each conductor separately, using the actual number of ranges connected to that conductor, and using half of the corresponding table values.

 

[JoeStillman]

Assuming a test question, if it specifies 13 apartment, 2 ranges per apartment, I infer it's not just to get you to multiply 2 * 13 = 26, there's no particular reason to test people on that one extra multiplication step. A reasonable test writer would just write it for 26 ranges if that's the idea.

But by specifying 13 apartments, 2 rangers per apartment, coupled with the knowledge that apartments generally get only 2 ungrounded conductors, now you have to think about the range arrangement across ungrounded conductors. So instead of being a pointless extra multiplication step, it becomes an interesting feature worth incorporating into a test question.

Cheers, Wayne

P.S. It's probably contrary to the the current text in 220.55, but it seems to me a better and very realistic way to deal with asymmetric range loading on the ungrounded conductors is simply to calculate each conductor separately, using the actual number of ranges connected to that conductor, and using half of the corresponding table values.

Sounds like you've written Test Questions before.