Multiple A/C Units_Share Service 1

[wilsonv]

Hi everybody, I'm working on the calculations for a strip mall consisting of 10 tenant spaces. 2-10kVA Heating units for two spaces and 8-8kVA Heating units for the remaider spaces. Cooling load is less than Heating, so only the heating load is considered. Is there any rules that allow derrating the 10 units in order to size the service or do I have to use 84kVA? Since chances that all units run at the same time are very, very low. Each tenant space will have a panelboard which will feed the Heating units. The supply voltage is 120/208Y 3-phase 4-wire and the Heating units are rated for 208V 1-phase.

Thanks.

 

[BJC]

The chances that they will all be on at any given time is low. The chance that they will all be on sometime is certain. Some day in winter during a cold spell they will all be on.

 

[davidbeach]

Some day in winter during a cold spell they will all be on.

And, if the power has been off for a while, you can be certain that they will all be on at the same time for as long as it takes for them to catch up once the power comes back on.

 

[corona]

Do you have other loads, indoor/outdoor lighting, receptacles, motors, etc., etc.
How are you metering?
Individual spaces, one meter?

Corona

 

[wilsonv]

Thanks BJC and davidbeach for yours posts. Yours post make sense. The simple things are the ones that bite you in...It is summer time thats why...

 

[steelerfan28655]

Are you calculating the load to determine the main breaker size for each unit, or the main breaker size for the whole complex?

Or are you trying to determine the proper size of transformer to supply all of them with?

On the load side of the meter, I don't believe the NEC will allow you to diversify the load.

On the line side of the meter, take the total load (kVA) of all units and multiply by 70% to size the transformer.

Happiness is a way of travel, not a destination.

 

[wilsonv]

Corona, each tenant space will have a meter and yes there are other loads but not motor loads.

steelerfan28655, I'm trying to size the service main breaker for the main disconnect. The utility company will size the transformer. I think you are right, the NEC don't let you diversify this type of load. Is just that with all the lighting, show window lighting, sign lighting receptacles & misc. the calculations result in a 800A service 782A to be exact(Non-Continuous + Continuous). The problem is that if the owner has to add more load right after it is built because some tenants will require it, based on the calculations the service may not be enough to accomodate the new load. Until all the spaces are fully occupied we wont have any data from the Utility company or can't perform a study to determine the actual demand from the building to prove that the actual demand is lower than the calculated per NEC. Is a 11,500 sq-ft building, to me 800A service seem to be a lot for this building.

 

[davidbeach]

At 208Y/120V, 782A is 281.5kVA. That over 11,500 sq-ft is 24.5VA/sq-ft. Unless you are in an area with extreme temperatures (cold only matters with electric heat) that should be enough. On the other hand, it the code calculations say 782A, I would never use an 800A service, but would increase to 1000A or even 1200A (most of the cost of the increase is likely to be in the 800A to 1000A step and very little more from 1000A to 1200A).

 

[wilsonv]

Thanks davidbeach, I will explain our client and suggest to go with at least 1000A service.

 

[alehman]

25VA per sf seems very high. You may wish to evaluate how the heating equipment was sized.

 

[lengould]

It might save you money to put in a control system which can shut down or cycle the heaters in sequence to ensure that only eg. 60% are running at any one time. Provided some addurance, the code will allow you to then derate. Might cost more than just uprating the service though, and on the coldest day might leave you with come chilly tenants.


Pechez les vaches.

 

[mc5w]

A heftier service will run cooler and live longer. Also, when most electricians build heavy circuits they only have 1/2 to 3/4 of the wire strands conducting when the service is new. All other things being equal it will be cheaper to install two 800 amp services than a single 1200 amp service.

This large of a building is also allowed to have a dedicated sevice lateral from the utility transformer to each tenant, what is known as the large area building rule. Also, each tenant will have a fire rated wall between the other tenant which would qualify this structure as 10 buildings for electrical service purposes. Therefore, you can have up to 6 service switches for each tenant. For instance, on some utilities commercial electric heat has to be on its own meter because of discount rates such as Ohio Edison's rate 22 seasonal rate.

You also did not tell us how many square feet for each tenant. The other loads can add up really fast and could also necessitate a second air conditioner. You also did not tell us what kind of climate that you have or what kind of electrical rates that you have. If you live in an area where power is cheap such as Bonneville Power Administration tenants will tend to plug in and turn on all kinds of power hungry toys.

For what an air conditioner costs you could upgrade these units to air source heat pumps which will save your tenants a lot of money curing mild weather. Here in Cleveland, Ohio about 75% of the heating season can be handled by an air source heat pump. Just remember than a heat pump needs to be on an elevated base so that a foot of snow will not block it. I have also installed York's relay box for a heat pump that is added onto a fossil fuel furnace.

 

[wilsonv]

mc5w, thanks for the info on the large area building rule. I'll evaluate this alternative. The tenant spaces are 8ea-1,000sq-ft, 1ea-1,500sq-ft and 1ea-2,400sq-ft. I'm waiting on a confirmation of the size for the A/C units for each tenant space, I should have it in the following days.

But also the load for the Show Window Lighting as calculated per NEC is a lot. All the tenant spaces have open window from side to side. The calculated load is (238 l-ft*200 VA/l-ft) 47.6 kVA. Do you know of any rule that allow me to derrate this type of load. I have seen some similar buildings with a 800A service and with tenants using a lot of lighting, so I'm just wondering how they can do it.