Calculating the size of a panelboard.

[toro45]

Hello;

I thought I'd ask if anyone has any rule of thumb or knows calculations for sizing an electric panel. I have done this before but I have always taken a precaution by oversizing the panels. Here is an example:

1) I have a 35,000 square foot area and I am sizing a panel for the area.
2) I am going with 2VA per square foot because I am taking misc. power as well as receptacles into consideration.
3) The total power for the area will be 70,000VA.

Here is how I have typically solved this problem when I am trying to recommed a 120/208v panel for this area:
70000/(208*1.73)=194A

I would use a 250 main breaker panel for this application.
(Assume that nothing is known about how much the circuits are going to be used)

Is this wrong? How can I get a more precise size for this panelboard without oversizing the main breaker. The NEC codebook doesn't help much.

Thanks

 

[wareagle]

It appears to me that all you have accounted for is general lts and rec. What about all the other equipment that will be installed? What type of facility is this?

 

[mpparent]

Listen to wareagle my friend....


A rule of thumb table that I have has, for instance, an office building lighting w/ft^2 as:

@ 10-25k ft^2 = 1.72w/ft^2

General purpose receptacle loads at a high VA/ft^2:

1.5VA/ft^2

AC requirements:

6VA/ft^2

Electric Water Heating:

5kW/10k ft^2

Bear in mind, these are quick and dirty #'s. You need to do a thorough design to really size your service.

Mike

 

[Senselessticker]

IEEE 241 - "Grey Book" is a great resource if you are dealing with a commercial type of area. I agree with wareagle and mmparent...what about the other loads for the building? 2VA / sq ft is a VERY low estimate. For example, normal fluorescent lighting itself can consume about 2 VA / sq. ft. Power distrubution in just about *any* type of building is much more art than science. There's not a right/wrong way to do things in most cases assuming your are designing in accordance with applicable codes. Again...the Grey Book is a great resource for commercial areas. If you dealing with an industrial area, try the Red Book.

 

[jbelectrical]

I agree with mpparent. It's best to design the electrical plans before sizing the service.

The only time we use square footage calculations is when we're dealing with a "shell" building in which exterior walls are put up during the first phase of construction and tenant spaces are completed in the second phase. We do this in order to adequately size service entrance equipment (Usually meter centers) and the conduits being routed to each tenant space.

Anyways, the square footage calculations that mpparent gave appear to be more detailed than the ones I've used (Or have been *told* to use). For an office-type occupancy, I figure somewhere between 17 and 20 VA per square foot.

 

[EEJaime]

All the gentlemen are correct, you may also want to find out what the space has the possibility of being used for. If it will be a Medical Office Building the loads increase, if it will house outpatient radiology clinics, the loads will increase. If it will have a large retail facility with merchandise lighting, the lighting loads will double. If each tenant will have their own Air Condioning unit vs a central HVAC system the loads will differ. Any processing, manufacturing space will have significantly higher loads.

Is this an "electrical panel" for the entire building or just lights and receptacles for this 35ksf? Are building HVAC loads, elevators, etc..., served by seperate "House Panels"?

As one of the fellows in this forum often states, the answer to your question is..."It depends".

 

[pitsof]

Hi toro45

Everything all guy mensioned is 100% correct i MUST SAY! WE resently had a problem by just doing few calculations and making conclusions there after. For example you can have two breakers with the same capacity but different fault current.What I want to say is take everything into consideration before taking a move.