Modular Building Demand Factor Reductions

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bduda

Member
Location
Pennsylvania
We are working on a project where sixteen modular units will be joined together to form a temporary bus terminal. Each of the sixteen units will have it's own subpanel (most are 125 AMP)that feeds HVAC electric heating/cooling package units along with the normal lighting and receptacle circuits. There will be a main distribution panel set up to feed these subpanels. Service is 120/240 single phase.

These units are being supplied design/build and I am reviewing the calculations to begin coordination with the local power company. The loads for the HVAC units (10,560 watts per phase) are making up about 40% of the total load resulting in a large service size.

The modular manufacturer indicates that they do not endorse or suggest the use of any demand factor reductions and that they would be at the direction of the contractor. They further go on to admit that properly factored, they could substantially reduce the building service requirements. It seems highly unlikely that all sixteen HVAC units would ever run at once. What, if any, demand factor reduction factors could be applied to these HVAC units? They'll do a receptacle demand reduction per 220-13 but thats nothing compared to the HVAC demand.

Any insight or recommendations from someone who has dealt with this type of installation would be helpful.

Bradley G. Duda, P.E.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: Modular Building Demand Factor Reductions

I believe HVAC units are often designed to run continuously on the hottest or coldest day of the year. Thus, I think it is possible that all would run at the same time, depending on the climate. In addition, since a unit could run for more than 3 hours, its feeder should have an additional 25% capactiy per the NEC.

So I get 89 amps per unit x 1.25 = 110 amps per unit. That doesn't leave much power for lights and receptacles.

So for 16 units at 125 amps per, that is 2000 amps at 240 single phase. If the units were 480V, 3 phase, you would probably have only a 800A service.

Steve
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: Modular Building Demand Factor Reductions

Steve:
I am not sure I would agree with your figures. The equipment may be designed to allow it to run continuously, but that does not mean that it must be treated as a ?continuous load,? for the purposes of a service calculation. The cooling portion of the HVAC system does not need to be multiplied by a factor of 125%. That factor only applies to the largest single motor. I believe that the heating portion of the HVAC system also does not need the 125% factor. That is because I believe that no single heating unit is likely to run for 3 hours without a break.

Bradley:
The bad news is that the NEC does not have a demand factor for HVAC equipment. The hopeful news is that you may be able to take advantage of 220.21. You can make your own call on the percentage of the HVAC loads that are likely to be in use simultaneously. It would probably require a sealed calculation, and even that might not be acceptable to the local authority.

By the way, what was the source of your information regarding the HVAC load? Did it come from the vendor, or did you use the NEC?s motor tables from article 430?
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: Modular Building Demand Factor Reductions

These sound just like what we wire for job trailers.

Depending on the typical weather conditions in your area I would go easy on demand factors.

You have to keep in mind these units have next to nothing for insulation, the windows (single pane) and doors leak a lot.

In the winter the heat runs a lot and in the summer the AC runs a lot.

Also it sound like there will be a lot of traffic in and out putting higher demands on the heat and cooling systems.

Steve's figure of 89 amps makes sense as the panels are usually 125s as you said and the breaker for the HVAC will be one 2 pole 90 or sometimes one 2 pole 40 and a 2 pole 50. One for fan and compressor the other for resistive heat.

I can tell you we ran 7 or 8 of these units out of two 200 amp 3 phase panels. I know that is not much info, but I never took any readings as they worked. ;)
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Re: Modular Building Demand Factor Reductions

OK, I'll agree with Charlie B that you don't have to multiply the HVAC load by 1.25. I was thinking of branch circuit conductors (424.3(B) for example).

I thought 220.21 only applied where you can be pretty sure things aren't running at the same time: like heating and air conditioning loads. I'm not sure you can apply it to multiple units of the same type.

Steve
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Re: Modular Building Demand Factor Reductions

Originally posted by steve66: I thought 220.21 only applied where you can be pretty sure things aren't running at the same time: like heating and air conditioning loads. I'm not sure you can apply it to multiple units of the same type.
That is why I used such weasel words (sorry, I meant to say ?terms of precision ambiguity? :D ) as ?hopeful news,? and ?may be able to take advantage,? and ?you can make your own call,? and ?might not be acceptable to the local authority.? Bradley will have to do some thinking, before putting his PE Seal on a calculation. I?m not at all sure I?d be willing to use mine.
 
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