Can I use a demand factor?

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thinfool

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Kentucky
I usually have jobs where an engineer does all the figuring, but not in this case.

I have to quote a service and feeders for a portable classroom building. It consists of 13 - 13'wide mobile sections.

11 of the sections each have a 125 amp 1ph load center with lights, recepts and a wall pack hvac unit with 10kw of electric heat.

2 of the sections have a 150 amp 1ph load center with lights, recepts and a wall pack hvac unit with 15kw of electric heat.

I want to furnish a 208/120Y service with the appropriate feeders to each section.

How do I size the main service disconnect and distribution panel? Is there a demand factor that applies here? Otherwise I can build a 1000 amp service, but that seems like overkill.

Thanks in advance...!

JP
 
Re: Can I use a demand factor?

How about treating it like a school?

220.34 for optional method or 220.10 for standard method.

You dont give all the details such as square footages, but you may find that something less than 600A 208Y/120V. 3-PH 4-W could serve the entire thing.
 
Re: Can I use a demand factor?

I'm not sure you have enough information. You should be able to get manufacturer's panel schedules, which would show the calculated loads. My experience is that each section typically has a calculated load of over 100A, otherwise they would use 100A panels instead of 125A. There's no diversity on the HVAC, although you may be able to take the smaller of the actual heating and actual cooling loads. This has probably already been factored into the manufacturer's load calculations. The only real area for diversity would be receptacles (220.44), but it's unlikely that you'll have 10kva of receptacles.

With 12 modular units you'll have 8 units connected to each phase. (7 X 125A) + (1 X 150A) = 1025A worst case. Unless you have documentation showing that the panels are loaded at less than their rating I'd recommend using a 1200A service. That's guaranteed to calculate OK. If I was reviewing the plans I would not accept a 1000A service without complete panel schedules.

As a side note, many of the panel schedules I've seen from modular manufacturers are not, in my opinion, calculated properly. Continuous loads are not always calculated at 125%. Also, it's common for them to sum the total KVA load and divide by 240V instead of calculating L1 and L2 separately. Since it's my seal on the service drawings I always recalculate the panels, and I generally find that my calculation is higher than the manufacturer's.
 
Re: Can I use a demand factor?

As to treating this as a school, please be aware of the note to 220.86 in the 2005 NEC Handbook:

Many schools add small, portable classroom buildings. The air-conditioning load in these portable classrooms must comply with Article 440, and the lighting load must be considered continuous. The demand factors in Table 220.86 do not apply to portable classrooms, because those demand factors would decrease the feeder or service size to below that required for the connected continuous load.
 
Re: Can I use a demand factor?

Thinfool,
We are finishing a job in Pittsburgh where we put in 16 modulars to serve as a temporary bus terminal. We ended up with a 1600A 120/208 Y service, although we had a commercial kitchen too w/ a 200 A subpanel.

I can tell by your specs that you are probably dealing with Williams Scotsman? I went through the same issues in trying to apply a demand factor, but the electric heat is a killer. Unless you have some type of energy management system that would prevent all of the units from kicking on at once (say on Monday morning when they turn the thermostats up from 60 deg to 68 deg, or if you have a power outage for an hour and they all kick back on automatically, then you have to design the service to handle the loads from all HVAC units on at once. Unfortunately, 98% of the time, they will cycle randomly, but there are times when you will have them all on. All other diversity factors are negligible in comparison to the heating load.

The modular manufacturer would not endorse or even talk about diversity factors. So they leave it in the service designer's hands. You can calculate and cite code references, but always must consider the realities of the situation.
 
Re: Can I use a demand factor?

When you say "wall pack AC units," does that mean that they use 120 VAC? How about the heating system: it is also 120V? If they are rated for 240 volts, then you may need to look closer, before using a 120/208V service.
 
Re: Can I use a demand factor?

I do not believe article 220.34 applies since these are portable classrooms. Below table 220.34 there is a statement "This section shall not apply to portable classroom buildings."
 
Re: Can I use a demand factor?

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...probably dealing with Williams Scotsman?
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Yep, you hit the nail on the head. Thanks for your input, it's a big help!

JP
 
Re: Can I use a demand factor?

Charlie,
They HVAC units are package type with electric heat strips. In the case of Williams Scotsman, they are "Bard" brand 208-230 rated. It gets kind of messy having a 120/208V 3 phase main distribution panel and all modular unit subpanels are single phase 120/240V; makes balancing the phases a little interesting. We only had to put a buck boost on a sewage pump that was only available 240 V single phase.

What was interesting about our site was the modulars were 800' from the right of way. At our request, power co. allowed us to run UG primary to within 20' of the bldg and set Xfmr. Set a high voltage switch at right of way. Cost about $28K just for PoCo work. Otherwise, our secondaries would have been large parallel conductors at that distance.
 
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