Estimating electrical load for building addition

[Eng]

Hello,

I am working with a client who would like to put a 21,000 sq ft addition on to their school. They only want a study and cost estimate done for now. I will be able to determine the new mechanical load, but what VA/sq ft number should I be using for the general use spaces? This is a school and we will be well below the 3VA/sq ft for lighting load (I will still use this number), but what about receptacle load?

Thank you

 

[petersonra]

Hello,

I am working with a client who would like to put a 21,000 sq ft addition on to their school. They only want a study and cost estimate done for now. I will be able to determine the new mechanical load, but what VA/sq ft number should I be using for the general use spaces? This is a school and we will be well below the 3VA/sq ft for lighting load (I will still use this number), but what about receptacle load?

Thank you

one would think that somewhere an architect would have a book that has more or less standard specs for schools. It might well vary a lot depending on what kind of school it is.

personally, my guess is that these days receptacle loads will be higher than one might think due to the prevalence of computers and other things that need to be plugged in.

 

[JDBrown]

For specific receptacle loads, use the actual load of the equipment. Otherwise, for incidental loads (teacher's cell phone charger, radio, etc.), you can probably just count it as part of the 3W/sq.ft. But make sure you talk to your client and find out the specific use of each room, especially when it comes to computers. Will there be a dedicated computer lab? Will each classroom have a few computers? Is there one per student in every classroom? How about projectors?

Is there going to be a wood/metal/auto shop in the addition? A Home Ec classroom, maybe (although I don't think many schools even offer Home Ec anymore)? You'll need to include load for any equipment in these specialty rooms.

They likely won't be able to nail down firm numbers for you at this stage, but you'll need to get a reasonable estimate if you're to have any hope of giving them useful information at the end of your study.

Good luck!

 

[Eng]

For specific receptacle loads, use the actual load of the equipment. Otherwise, for incidental loads (teacher's cell phone charger, radio, etc.), you can probably just count it as part of the 3W/sq.ft. But make sure you talk to your client and find out the specific use of each room, especially when it comes to computers. Will there be a dedicated computer lab? Will each classroom have a few computers? Is there one per student in every classroom? How about projectors?

Is there going to be a wood/metal/auto shop in the addition? A Home Ec classroom, maybe (although I don't think many schools even offer Home Ec anymore)? You'll need to include load for any equipment in these specialty rooms.

They likely won't be able to nail down firm numbers for you at this stage, but you'll need to get a reasonable estimate if you're to have any hope of giving them useful information at the end of your study.

Good luck!

They are looking at a dedicated computer lab and expanding their kitchen. Those two areas I am comfortable providing estimates for. It's the unknown areas of general use that I am concerened with. Obviously the 3VA/Sq ft lighting load is much more than the energy codes allow and thus it is possible that the general use receptacles could be considered included in the 3VA/sq ft.

 

[JDBrown]

They are looking at a dedicated computer lab and expanding their kitchen. Those two areas I am comfortable providing estimates for. It's the unknown areas of general use that I am concerened with. Obviously the 3VA/Sq ft lighting load is much more than the energy codes allow and thus it is possible that the general use receptacles could be considered included in the 3VA/sq ft.

I would see if you can find out if they use computers in the classrooms (or if they have plans to do so). At the school where my brother used to teach, the students used laptops in every class (and they were always plugged in). As you might imagine, the number of receptacles required goes through the roof, and the power consumption is quite a bit more than when they're using a pencil and paper.

If you really can't find out anything from the client about what they plan to do in the classrooms, then they will deserve what they get (an estimate that's not based in reality). At that point, it will be time for some educated WAGging on your part.

 

[Smart $]

They are looking at a dedicated computer lab and expanding their kitchen. Those two areas I am comfortable providing estimates for. It's the unknown areas of general use that I am concerened with. Obviously the 3VA/Sq ft lighting load is much more than the energy codes allow and thus it is possible that the general use receptacles could be considered included in the 3VA/sq ft.

You can use the optional method of 220.86 for schools. The calculation is based entirely on area...

Get the demand data for 1yr period to determine existing loads [220.87].

 

[petersonra]

I would be more worried about having adequate numbers of outlets in appropriate locations than actual power consumption. A typical laptop does not take a whole lot of power but if you have 30 people in a room and they all want to plug in simultaneously, that is a lot of outlets.

 

[Smart $]

I would be more worried about having adequate numbers of outlets in appropriate locations than actual power consumption. A typical laptop does not take a whole lot of power but if you have 30 people in a room and they all want to plug in simultaneously, that is a lot of outlets.

While that's one of the reasons I suggested using the optional method, I've never actually used 220.86. In looking at it, I'm left wondering how to implement...???

 

[Eng]

At this point we are only pulling together a study and cost estimate so that the school can go to referrendum. Based on the past years peak demand, it appears we have adequate capacity to servce the new addition. I do still need to show the calculations however.

When I look at 220.86 (NEC 2011, Handbook Calculation Example), it states to calculate the overall deman of a 100,000 sq ft building based on T220.86. I do not see how they are coming up with 1.15MVA/100,000 sq ft = 11.5VA/sq ft by using T220.86. How is the 1.15MVA determined.

Thanks.

 

[Smart $]

At this point we are only pulling together a study and cost estimate so that the school can go to referrendum. Based on the past years peak demand, it appears we have adequate capacity to servce the new addition. I do still need to show the calculations however.

When I look at 220.86 (NEC 2011, Handbook Calculation Example), it states to calculate the overall deman of a 100,000 sq ft building based on T220.86. I do not see how they are coming up with 1.15MVA/100,000 sq ft = 11.5VA/sq ft by using T220.86. How is the 1.15MVA determined.

Thanks.

Best guess is branch circuit loads are determined per Part II, summed up, then divided by area. That gives you the VA/ft?...

Let's see using some arbitrary numbers...

100kVA total branch circuit load ? 2,000ft? = 50VA/ft?

2,000ft? ? 3VA/ft? ? 100% = 6,000VA ............ (50 > 3)
2,000ft? ? 17VA/ft? ? 75% = 25,500VA ............ (50 > 3 and 50 > 20, so 20-3=17)
2,000ft? ? 30VA/ft? ? 25% = 15,000VA ............ (50-20=30)
Total service or feeder demand = 46.5kVA

 

[shamsdebout]

Hello,

I am working with a client who would like to put a 21,000 sq ft addition on to their school. They only want a study and cost estimate done for now. I will be able to determine the new mechanical load, but what VA/sq ft number should I be using for the general use spaces? This is a school and we will be well below the 3VA/sq ft for lighting load (I will still use this number), but what about receptacle load?

Thank you

I have used 3 VA/ft2 - 6 VA/ft2 for a similar type of space and it has worked out well. Basically taken from historical data.