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#189354 - 10/03/09 02:13 PM 220 calculations  
watersparkfalls  Offline
Joined: Mar 2005
Posts: 220
Washington...Not DC
I wired a 300 foot addition(4 lights and 10 plugs, I added it onto an existing circuit which feeds the living room plugs(no lights in there)and kitchen light and dining light. The inspector says I have to count that square footage(kit,dining and addition)to calculate my branch circuit which is feeding these. When did we start calculating for single branch circuits? Have I been missing something? This house has the number of circuits required per square footage, including the addition. So why is he making me calculate individual circuits to the existing and addition? Can anyone shed some light on this? I could pull a new circuit...but that's not what I bid, so for future bidding, if he is correct on this branch circuit/SQ FT I won't get burned again.
Thanks guys,

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#189355 - 10/03/09 03:21 PM Re: 220 calculations [Re: watersparkfalls]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,118
They get that from 210.11(B) and a healthy bit of "interpretation".


(B) Load Evenly Proportioned Among Branch Circuits. Where the load is calculated on the basis of volt-amperes per square meter or per square foot, the wiring system up to and including the branch-circuit panelboard(s) shall be provided to serve not less than the calculated load. This load shall be evenly proportioned among multioutlet branch circuits within the panelboard(s). Branch-circuit overcurrent devices and circuits shall be required to be installed only to serve the connected load.

Basically if those loads were evenly proportioned before and you add load to one circuit they are not evenly proportioned anymore.
You would have to provide a calculation that showed you did not overload that circuit.

Greg Fretwell

#189356 - 10/03/09 04:14 PM Re: 220 calculations [Re: gfretwell]  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
I think I can see the inspector's issue.

When you build a house, you have to do a load calculation. What he's saying is that you also need to do this when you add to the house. In this calculation, there would be two questions to answer:

First, is the service to the house large enough to support the addition? Does adding 300 sq. ft. overload your service?

The second is whether you have enough power for that addition. That is, using the 3 watts/sq. ft. rule and accounting for fixed equipment (like an air conditioner), is the circuit large enough to feed it all?

Let's look at an imaginary 15 amp 120v circuit:
3 watts / sq. ft.
15 amps x 120 volts = 1800 watts
1800 / 3 = 600 sq. ft.
That circuit can serve a maximum of 600 sq. ft.

#189357 - 10/03/09 06:13 PM Re: 220 calculations [Re: renosteinke]  
watersparkfalls  Offline
Joined: Mar 2005
Posts: 220
Washington...Not DC
Thanks Guys, hey Reno do we have to take the 80% into our calculation?
1440/3=480 Sq ft

#189358 - 10/03/09 06:25 PM Re: 220 calculations [Re: watersparkfalls]  
HotLine1  Offline

Joined: Apr 2002
Posts: 6,864
Brick, NJ USA
80% is for continuous loads (more than 3 hrs 'on') commercial. It does not apply to resi.

That's a good practice to 'limit' your resi loads (ie: 80%)


#189399 - 10/05/09 07:17 PM Re: 220 calculations [Re: HotLine1]  
mikesh  Offline
Joined: Jun 2006
Posts: 613
Victoria, BC, Canada
Dang that looks complicated. Canada just says 1 outlet = 1 amp for general purpose so a 15 amp circuit can have a max of 12 outlets including general lighting outlets but obviously not switches. Special loads at the connected load and we calculate a house up to about 1000 sq ft is 5000 watts for basic plugs and lights and add another 1000 watts for each added 1000 square feet. Heat, 6000 w for the range, and appliances over 1500 watts are added separately at %25 after the range has been accounted for.
If a house calculates under 100 amps they still have to wire for at least 100 amp service.
the philosophy winds up almost the same service size but we never get more than 12 plugs and or lights on a circuit.

#189400 - 10/05/09 07:34 PM Re: 220 calculations [Re: mikesh]  
renosteinke  Offline
Cat Servant
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Mikesh, the procedures I used apply only to residential 'general purpose' circuits.

We do have a similar method for receptacles in places that are not residences, and it's fairly similar to yours. For residences, there is no limit to the number of receptacles on any one circuit. (Note that I'm talking about code rules - not good design!)

I'm actually impressed that the inspector was 'aware' enough to question the loading of the circuit, and the effect the addition may have had on the original house's load calculations. Applying those calculations to an individual circuit is something that can lead to some lively debate! IMO, the inspector is on shaky ground, making this ruling by inference ... but then, that's why we call him the AHJ.

Still, a 300 sq. ft. addition is pretty large - perhaps as many as three decent bedrooms can fit in that space. The last place I lived - a duplex - each unit was less than 400 sq. ft. I think that some consideration should be made for the demand of this space.

#189401 - 10/05/09 07:55 PM Re: 220 calculations [Re: renosteinke]  
gfretwell  Offline

Joined: Jul 2004
Posts: 9,118
Mike the US load calcs are based on square footage because they assume that is the best measure of what the load will be for the general usage receptacles. The actual number of receptacles is more related to the layout of the room so counting them is not as important as the size if the area they supply. People will figure out how to plug in everything they want, no matter how many receptacles you have.

Greg Fretwell

#189433 - 10/08/09 12:00 AM Re: 220 calculations [Re: gfretwell]  
jdot77  Offline
Joined: Mar 2007
Posts: 16
Riverview, Fl
Guys, I have to disagree with the load calcs here. If you have ever done a load calc for a residental home, we all know that the calculation always comes out way low. The issue here is a code issue. No where in the code does it state a limit on how many recepticles you can put on a circuit for residental. If you are in a Commericial building that is another story 180va per recepticle. Also Balancing the load should not be the issue in this ciruit either. I dont think the inspector is right on this one watersparks. but he is the autority having jurisdiction. Just my thoughs wink


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