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Load calculation + subpanels

6684 Views 11 Replies 5 Participants Last post by  AllanJ
I'm working on a <1000 sq ft home w/detached garage and 100 amp service, I know just enough about load calculations to get confused when it comes to how subpanel loads fit in. Here's a list of questions:

Square footage: as I understand it the first step in any load calc is running a square footage formula to establish the general lighting/receptacle load. This accounts only for the area one wants to calculate the load for, right? So, in the case of a home with a main panel and two subs one would do the square footage calculation for 3 separate areas. I can elaborate on what's connected where if need be.

Safety factor: a few sources indicate that NEC calls for the gross electrical load for a subpanel to be multiplied by 1.25, can anybody confirm or even cite the NEC section that covers this?

Outdoor lighting/receptacles: some methods call for an accounting for all lighting/receptacle loads NOT in the main living area (outdoor/detached garage areas) in a separate calculation; this usually involves multiplying each receptacle by 180 (watts) and adding this to the sum total wattage of all fixtures. So in the case of the detached garage, does this calculation take the place of the square footage-based general load portion?

Watts to amps: most sources say that once I have my total wattage simply divide this by 240 (volts) to get my amperage load. At least two sources say to divide by 230 (???). Being that I'm in the US I think 240 makes more sense, where in the world is 230 coming from?

Maximum load vs service: finally, how close can my total load amperage get to my service amperage? With 100 amp service am I good with any number short of 100 or am I limited to 80% of that?

At this point I've looked up so many online sources that I'm dizzy. Seems crazy but almost every load calc method I find varies slightly from the previous. Any help I can get on figuring this out would be GREATLY appreciated.
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Great, that helps answer my question on how to handle the garage. In my case it looks like it'll get treated as outdoor space. No permanent loads in the garage aside from perhaps the garage door opener and an occasionally used window a/c. Here's a run-down:

light fixtures = 710 watts
receptacles = 1,980 watts
g. door opener = 720 watts
window a/c = 1,350 watts

gross sum total = 4,760 watts

(sum total X 1.25*)/240 = 24.8 amps

*any word on whether this portion is actually needed?

So with or without the 1.25 adjustment along with a "worst case" scenario of running a/c and constantly opening/closing the door looks like I'm good with the 30 amp subpanel currently in place.

Now, back to how the square footage math is supposed to fit for the remainder. Due to how and where it's installed the main panel only houses 2 subpanel feeds and the general lighting and house receptacle circuits (2). I think it might make sense to split the single house receptacle circuit at some point which would bring me to 3 circuits aside from the sub feeds. The house subpanel located in the dining room accomodates the 2 required appliance circuits, 1 laundry circuit, the 1 bathroom circuit as well as assorted dedicated circuits for a total of 12 circuits. When figuring the square footage math do I omit the areas served by the house subpanel and just include the living and bedroom areas? Finally, when doing the calculation for the house subpanel do I leave out the square footage math and treat it like the garage subpanel?

Just in case my rambling isn't making sense this is where I'm coming from: garage sub gets calculated as shown above, house sub gets calculated the same way; neither subpanel area is included in the square footage "general load" math. All remaining living areas (living and bedrooms) get covered in the square footage calculation which might give me something like the following:

garage subpanel = 24.8 amps

house subpanel = 55.6 amps (est, no math done yet)

main = 10.5 amps (837 sq. ft. X 3 watts)/240

giving me a total sum load of 90.9 amps

Does this seem right or is there some seriously flawed thinking on my part? If so please, please tell me where I go off the rails.
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Thanks for the link jbfan, that calculator is pretty similar to what's available on the Mike Holt forum. Everything there and how it's done is pretty familiar but notice there's nothing about outdoor fixtures/receptacles, so aside from the door opener and a/c unit in my detached garage does nothing else in there get accounted for? At least one other method indicates that outdoor fixtures/receptacles need to be figured in with an additional calculation.

I'm afraid my questions are getting lost so I'll try a different approach. I think one of my mistakes is working out the load calcs for the subpanels and attempting to work backwards from there. For the sake of discussion let's forget about the subpanels for a bit and start with my 100 amp service and the list of circuits I'd like to supply. Let's just say my main panel can easily accomodate all these circuits without the need for subpanels and my total load amperage comes out to 90.9 amps. Since this would be a theoretical max load and highly unlikely it would every get to this point continuously this should be fine on 100 amp service, right?

Now, in the real world where my main panel will not accomodate anything close to 12 circuits I'll have to distribute things among two subpanels;one for the detached garage and the other for the majority of house circuits. Without me inserting any pre-conceived ideas about how this should be done, how exactly should it be done?
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jbfan, regarding this:

"If you only have 12 circuits in the main panel, you need a bigger panel."

There already so much going on that I didn't really want to get into main panel history but here it is. The current 100 amp service/meter/panel replaced the original 60 amp service around 15-20 years back. The original panel was tiny and housed something like 4 circuits for the entire house and garage. When the new panel was installed (prior to my ownership) there seems to have been little thought given to it's location/installation method and how it might affect any plans to accommodate much more than the original 4 circuits; let's just say that after the 5th things would get ugly. So I can deal with hassle/expense of updating/relocating the meter/panel or install a centrally located subpanel to accommodate the new house circuits I'd like to add; does the subpanel approach not make more sense?
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brric, regarding this:

"Perhaps one thing you may not understand is that the 3 watts per square foot includes the lighting and general use receptacles. You do not add up all the potential recep and lighting loads individually when doing load calcs"

I do get this and better understood it once the second half of AllanJ's response sunk in. Among the things I wasn't picking up on was that the purpose of a load calculation is to establish where your load demands put you in relation to existing service amperage, nothing more. The big unanswered question is how do I determine which circuits I can house in each of my subpanels. Also, I'm including some pics of the worksheets out of a book I was referencing, this might better illustrate where my confusion is coming from...

This is the sheet for load calculations:

Here are my problems with this sheet:

1. In highlighted section 4 you see where it mentions dealing with fixture/receptacle loads in outdoor/detached areas. I think this kind of jibes with how AllanJ suggested I handle the garage but I rarely see something like this is any other calculators.

2. In highlighted section 9 you see watts are being converted by dividing by 230. This book isn't more than 10 years old and I'm fairly certain the most current printing will be exactly the same so I'm just writing this off to lazy fact-checking on the publisher's part.

3. Most importantly, if you look closely, the HVAC loads are not being excluded from the "-10k/40%" adjustment. As I understand it, the greater of ones HVAC loads MUST be factored in at 100% without exception.

This is for dealing with subpanels:

My problems with this sheet are:

1. Section 2 talks about an NEC required adjustment for calculating subpanel loads, this is the only place I've seen mention of this adjustment.

2. Section 3, there's 230 again.

3. Lastly, no idea how I'm supposed to deal with detached garage here. Not being "living space" it would make sense to skip the square footage basic lighting calculation; that would imply I'm supposed treat it as "outdoor space" and itemize each fixture/receptacle per sheet 1.

4. Furthermore, what if I'm attempting to calculate a subpanel load for a panel that doesn't house any general lighting or electrical circuits at all (small appliance/laundry/dedicated circuits only)? Doesn't make sense to me to include a square footage based number where no general lighting/electrical loads are involved.

So as you can see, not knowing any better I'm finding it difficult to distinguish what's legit and what isn't within these two sheets. Based on the number of discrepancies I should probably just write them both off entirely?

Provided that a total load calculated at 90.9 amps is ok on 100 amp service (nobody confirmed) then I would only need to determine which circuits I can house in each of my subpanels. Rather, confirm that the existing garage panel configuration is fine and that my proposed circuit additions to the house subpanel would be ok. Would be extremely helpful if somebody could explain the proper method for this.
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Fix for broken links in previous post:

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