Show Window Calculations

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starbright28

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Here's another design/calculation that I'm working on.

We have a tenant building that can be split into 3 different tenants. It's like a strip mall. Right now we are designing for bare minimim.

First - I showed 3 show window receptacles for each window it has. Basically just one receptacle above each window.

My question is this: how do you go about calculating this for VA? Do you take the 220 times your voltage to get your VA? or what is the best way to explain this to me in computing it for a panelboard schedule.

Thanks in advance.
 
I can?t guess where you came up with 220 times voltage. Show window lights are never going to need 220 amps, and it is amps that you multiply by voltage to get VA.

220.14(G) gives you two choices for calculating VA for show windows. One is to use the VA per outlet, as described in other sections of 220. That would mean 180 VA per duplex outlet, above each window. The other method is to count 200 VA for every foot of the window?s width.
 
you know Charlie, I was trying to remember from the code book and guessed wrong!!! I was thinking of what you quoted - 200VA.

So I tried using the method of 200VA per ft. My calcs seemed high. We have one window 12' in length, and others around 9'.

I didn't dare ask the engineer on how he calculated this, since sometimes they are done by "guessing".

So was I on the right track of actually figuring this out?
 
Point 1:

If you have one window at 12 feet and two others at 9 feet each, you have a total of 30 feet, and need to allow for 6000 VA. That does not mean the owner will want to install 6000 watts of lighting. It?s just that we, the designers and installers, have to include that much power in the service calculation.

Point 2:

You should never be afraid to ask something of an engineer. We don?t know it all, and what we do know we should be happy to share. We should also not be afraid to admit when we don?t know something. And if we are guessing (sometimes that is appropriate, especially early in a project) then we had better make it clear that this is a guess and not the results of a calculation. If that is not the way the engineers in your office are willing to communicate with you, then I would call that a sad situation.

Point 3:

You should never rely on your memory of what the code says. I suggest you look up ?Charlie?s Rule? either on Google or within this Forum.
 
Point 1
So if I know the distance of the windows, that is how many VA I apply for that receptacle. We show receptacles for show windows.

Point 3
I know sometimes that trying to remember something from the code book is not always the best thign to do, but I do know people who can rattle off code articles with there code section number. The MN test for Journeyman and Masters used to be a memory section that you had to rely on remember stuff from the code books and law & regs book. That sucked. They now did away with that.

Point 2
I have found that some things that I have questioned don't sit well with the engineer in house. I asked him on many things on how he figures things out. With one of my posts that I asked about the Lighting Calcs, and how to figure them out, well he took the information that I found out hard. I found out that what I found how to do the calculation was the right way, and the way he did it was the very conserative way but not exactly the right way either. He won't help me with that anymore, and puts up this front about it. The lighting salesmen asked me if he gave me information that he had on how to do these things. I said - "what information". I never recieved anything. I told that salesmen, you know the story, I don't get hardly anythign from him. I have to pry it from him. So I just dropped it around him, and if I do my own calcs (which some projects I am) then I do it the right way.

So I may have steered clear of the subject a bit with the replies to the points. Main thing is - was I doing the calculation correct in the first place?
 
starbright28 said:
Main thing is - was I doing the calculation correct in the first place?
Click to expand...
Not sure I understand this question. I will say that nothing you mentioned in the first post of this thread was a correct way to do this calculation. So if you had a way of doing it, and did not describe your way in the first post, then I do not know if you had been doing it correctly. You are going to have to help us help you on this.
 
This is how I calcualted for show window.

the window is 10'-0" horizontal, and I have one receptacle above it.

I took 10'(200) = VA
VA = 2000

Some of the windoes are 12' in length. Each window only have 1 receptacle above it.

I guess since I read in the code book that this was one way to do it, i went and did it.

Does my question seem to make sense now?
 
Your basic approach is OK, but I do see one concern. The 12 foot window is calculated at 2400 VA. With a 120 volt power source, that gives you a current of 20 amps. The 10 foot window is calculated at 2000 VA, which gives you a current of 16.7 amps.

You are not permitted to load a 20 amp circuit to more than 80% (i.e., 16 amps), if the load is considered ?continuous.? That word means ?turned on for more than 3 hours at a time.? Lighting in general, and certainly show window lighting, would be treated as a continuous load. So you need more than one 20 amp circuit to supply power to the show window lighting. Therefore, you need more than one receptacle.

I would have placed one receptacle each at the 1/3 point and the 2/3 point, and I would power them from separate circuits. That would be at 3? 4? and 6? 8? for the 10 foot window, and 4? and 8? for the 12 foot window.
 
Charlie,
I don't see anything in the code that forces me to install show window receptacles to equal the VA calculated @ 200 VA per foot. The code only requires a minimum of one receptacle per each 12 feet of show window or major fraction thereof. The 200 VA per foot is a service calculation not a branch circuit calculation. If only one receptacle is needed, only one circuit is needed. It is acceptable to determine the circuit ampacity by calculating the load per receptacle at 180 VA or 200 VA per linear foot, whichever is appropriate for the way the owner will be using their show window. There is a great difference in a show window for a restaurant which may have a "beer sign" in the show window, and a jewelry store that has many lights to make the product look good. If a future tennant needs additional receptacles the service will have adequate capacity for them to be added. If the electricin knows that whatever will plugged into the show window receptacle(s) exceeds 180 VA they should include it in the calculation.
 
You are right, haskindm. I had thought of that as well. I should have made it clear that the "I would do" statements were design considerations, and not code requirements. So the thing that needs to be done is to find out what the owner plans to install, in terms of lighting fixtures and bulbs, and to make sure the installed receptacle(s) can handle that load.
 
Charlie and haskindm:

Since the show window receptacles we wer showing on our plans are for bare minumum with no certain in mind who the tenants would be.

I explained by email to the Architect that we have to show at least 1 show window receptacle by the window because it was code. Especially when the project is a shall building for 3 tenants.

So as far as calculation of a show window (for sizing for a branch circuit to show on a panelboard) do you just use the 180VA or what should be the calculation??
 
I'm not a fan of this engineer which you say guesses calculations...

You calculate VA by multiplying volts by amps.

One duplex receptacle is 180VA each per code. If all three receptacles are on the same circuit, then the total VA for that circuit would be (3*180VA) 540VA.

One 20A circuit can have up to (20A*120V*80%) 1920VA on it, per code. Although I never go over 1800VA.
So in reality, you have have up to 10 duplex receptacles on one 20A circuit.
 
i could rant and rave, but i'll save that for a personal post.

I know how VA comes about, but when it pertains to show windows there seems to be a couple of ways you could come up with the VA, according to code (or my interpertation of it)
 
Your Engineering Process is Broken!

Your Engineering Process is Broken!

starbright28 said:
you know Charlie, I was trying to remember from the code book and guessed wrong!!! I was thinking of what you quoted - 200VA.

So I tried using the method of 200VA per ft. My calcs seemed high. We have one window 12' in length, and others around 9'.

I didn't dare ask the engineer on how he calculated this, since sometimes they are done by "guessing".

So was I on the right track of actually figuring this out?
Click to expand...

When I was a young engineer (about 50 years ago) I was trained in a practice of doing design calculations and checking drawings. You might consider adopting something like it. The practice was:

1. Someone (draftsperson, or engineer, or whatever) makes a drawing or analysis or calculation. If it's an analysis or calculation, the work is shown on the paper. Like your 5th grade teacher always said; "Show your work".

2. Some responsible person checks that drawing or calculation or analysis.
a. If it was a drawing, the check-print was shaded in yellow for those parts confirmed correct, and corrections shown in red. The finished check-print was a sea of yellow on the "blueprint" with islands of red. The check-print was filed (kept) after the corrections were made. The final drawing was signed by the draftsperson, checker, and responsible engineer.

b. If it was a calculation or analysis, there was a verification shown for each significant aspect of the analysis. The requirements, assumptions, equations, and arithmetic were all checked.

If the person whom you describe as an "engineer" does his calculations by guessing, and can't show you the work and citations of applicable code, then he doesn't deserve the title and he is doing a disservice to the company. He is leaving you to get into trouble by multiplying when your should divide because he isn't teaching you.

Your work and what you learn there are important parts of the education you need to pass your next examination.
 
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Bob:

I not only have heard this practice of check over your work, but I used to have to do that everyday. Now my work here I am the one putting it on paper and when I get redlines, rarely, I will still tend to highlight. I guess it was something drilled into me.

So to get back on the topic of show windows, how would you calculate the following according to code? The topic I am still not understanding even though a few posts have made reference on how to do these calculations.

You have a window in front of a tenant space that is 8ft wide. This window is on both sides of the door coming into the tenant space. Since a tenant space will have a business that will want something to "show" in the windows, how do you go about designing this to code? And what (show) the calcualations would be for this?

Thanks.
 
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