dwelling unit calculation question

[electrofelon]

I am studying for my Washington administrator's test and have a few questions about load calcs I was hoping to get some help with:

1. If i am calculating the service size for, say, a 20 unit apartment building, regardless if I use the regular or alternate calc, do I calculate the load for a single unit and multiply by 20 or calculate it in one step as if the whole complex was one dwelling unit (and get to use the demand factors for multiple ranges, dryers, and appliances - if using the regular calc)?

2. I am going thru the examples in Annex D of the handbook which brought up another question. Regarding 430.24 (conductors that supply motors sized at 125%). When do I add in 25% of the largest motor load to a calc? For anything that has a motor? What about a dishwasher, thats got a motor and do I only do it once for the largest thing with a motor?

3. Finally, when doing a calculation for an industrial facility that has both continuous and non continuous loads, the final calculated load of course includes 125% of the continuous loads. What I am confused on is what is this value used for (the one with the 125%), sizing the OCPD or sizing the conductors, or both? It appears in this example I am working through they use the figure with the 125% to size the OCPD but then size the conductors without it. Is this correct, seems odd? Code reference?

Thanks I really appreciate the help.

 

[david luchini]

I am studying for my Washington administrator's test and have a few questions about load calcs I was hoping to get some help with:

1. If i am calculating the service size for, say, a 20 unit apartment building, regardless if I use the regular or alternate calc, do I calculate the load for a single unit and multiply by 20 or calculate it in one step as if the whole complex was one dwelling unit (and get to use the demand factors for multiple ranges, dryers, and appliances - if using the regular calc)?

Calculate the load as if it was one unit and apply the demand factors. You would have 20 dryers with a 38% demand factor, and 20 ranges with a max demand of 35kW, etc.

2. I am going thru the examples in Annex D of the handbook which brought up another question. Regarding 430.24 (conductors that supply motors sized at 125%). When do I add in 25% of the largest motor load to a calc? For anything that has a motor? What about a dishwasher, thats got a motor and do I only do it once for the largest thing with a motor?

Add 25% of the largest motor on a feeder per 430.24.

3. Finally, when doing a calculation for an industrial facility that has both continuous and non continuous loads, the final calculated load of course includes 125% of the continuous loads. What I am confused on is what is this value used for (the one with the 125%), sizing the OCPD or sizing the conductors, or both? It appears in this example I am working through they use the figure with the 125% to size the OCPD but then size the conductors without it. Is this correct, seems odd? Code reference?

Thanks I really appreciate the help.

The 125% of the continuous applies to both the OCPD and the feeder conductors (215.3 and 215.2(A)(1)). In most instances, if your conductors are sized for your breaker you would be covered. However, there could be instances where this doesn't work. If you had 80A continuous and 20A non-continuous you would need a c/b of at least 120A, so a 125A c/b would be the standard size. Although #2 Awg with an ampacity of 115 is allowed on a 125A c/b per 240.4(B), you would have to use #1 Awg because the feeder ampacity has to be at least 120.

 

[erickench]

In my opinion I would use the standard method to size the subfeeders to each apartment panel and the optional method to size the service conductors.

 

[electrofelon]

Thanks David.

All what you say makes sense, however I still am not following the example in the code book. If you or anyone else as a 2008 handbook its example D3(a) in Annex D, the "undgrounded feeder conductor part". In this example the conductors need a .7 adjustment for number of conductors and a .96 adjustment for temp. They take 125% of the continuous load and 100% of the noncontinuous load and get 136 amps, I follow up to here. Next they apply adjustment factors on the actual load, without the 25%?? I am not sure what they are doing that for. Then in the end they say you can use 2/0 copper, 195 amps because 195*.7*.96= 131, but dont we need to get 136? Confused. Thanks

P.S. Eric, that was another question I forgot to ask. When do you use which calc? Could you elaborate on why you prefer the way you said?

 

[david luchini]

Thanks David.

All what you say makes sense, however I still am not following the example in the code book. If you or anyone else as a 2008 handbook its example D3(a) in Annex D, the "undgrounded feeder conductor part". In this example the conductors need a .7 adjustment for number of conductors and a .96 adjustment for temp. They take 125% of the continuous load and 100% of the noncontinuous load and get 136 amps, I follow up to here. Next they apply adjustment factors on the actual load, without the 25%?? I am not sure what they are doing that for. Then in the end they say you can use 2/0 copper, 195 amps because 195*.7*.96= 131, but dont we need to get 136? Confused. Thanks

It is a bit confusing. You can see in the example that the actual load on the circuit is 99,000VA (56.6kVA cont + 42.4kVA non-cont) or 119A at 480V, 3ph.

The addition of the 25% for the continuous load comes from 215.2. Reading 215.2(A)(1) it says that "the MINIMUM feeder-circuit CONDUCTOR SIZE, BEFORE the application of any adjustment or correction factors..."

From this, the example arrives at 136A. So the minimum feeder size BEFORE any adjustment factors must be 1/0AWG, though the circuit would see only 119A max.

After applying the adjustment factors to the 2/0AWG, they have an ampacity of 131 which is large enough to carry the load of 119A, and is larger than the minimum allowable size required by 215.2(A)(1).

Applying the adjustment factors to a 1/0AWG would give an ampacity of 114, which is too small to carry the load of 119A. Hope this makes sense.

 

[electrofelon]

Ok had to read over it a few times but I got it now. The word "before" plays a key role in there, Thanks, appreciate it.