Demand load calculation

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Jimmy4645

Member
I have a question whether a 100 amp is sufficient or do I need a 200 amp (panel/service). Voltage is 120/240 single phase.

Lighting Load: 2,640VA
Receptacke load: 15,480VA
Motor/Fan: 390VA
HVAC Load: 6,280VA

I took the lighting load @ 125% and derated the receptacle load ( 1st 10KVA @ 100% and the remainder @ 50%). I came up with a total load of 22,050VA (91.9 Amps) @ 120/240V, single phase.

This panel will have a main and is fed directly from a meter. This load is for a Salon located in a retail strip mall.

Since the demand amps are 91.9 amps do I need to go with a 200 amps and not a 100 amps because 91.1 amps exceeds 80% of the 100 amps rule??? (i.e. you cannot load up a panel more than 80% of its ratings. 80 amps for a 100 amp panel and 160 amps for a 200 amp panel?

Thanks guys
Appreciate any help!
 

bfletcher

Member
Location
New York
Don't you hate it when your load is right in that gray area?
I would provide a 200A service but only provide a 125A or 150A main breaker for the panel. I don't think the utility would provide anything less than the 200A meter.
Is there any chance of expansion for this salon? If so, this may be something to take into consideration when sizing the service.
 

Jimmy4645

Member
There is no room for expansion, which makes me believe a 100 amp is sufficient. The issues are that the space has an existing 100 amp panel already and the owner does not want to upgrade if it is not necessary per code. And I?m wondering if my 91.9 demand Amps is ok on a 100 amp panel per code.

My bad. Wasn?t clear. The 80% is the maximum rating of the main breaker, which pretty much means the breaker cannot exceed 100% non-continuous load and 125% continuous load..
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
More to the point, we are allowed to load the breaker to a total of (125% of the continuous" PLUS (100% of the non-continuous). But we already know what that total is: it is 91.1 amps. That is less that the rating of a 100 amp breaker, and therefore we can load the breaker to that value, plus a bit more.

All that said, if it were my decision to make, I would use a 150 panel, and ask the utility for a meter capable of handling that value. This being a salon, it is possible that the receptacle load will be more than the load calculation takes into account. If they get busy, they may add another chair or two, and turn on all the hair dryers and other equipment at the same time. The NEC gives us minimum for safety. But as designers and installers we should also think about keeping the client satisfied. They would not be satisfied if their hair dryers keep tripping the breakers, or if the total load keeps tripping the main breaker. A few extra bucks now would be worth their investment, and I would put it to them in that manner.
 

Npstewart

Senior Member
I was always told also that you should never load a breaker over 80% of its rated capacity. I dont think its a requirement in the NEC however it seems widely accepted. Anyone have any thoughts on this?

For example, lets you have a 10KW non continuous load and the voltage is 208-3, that comes out to about 27.8 amps. I would use a 35A or 40A breaker. I suppose technically you could use a 30A breaker? Any opinions?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
For example, lets you have a 10KW non continuous load and the voltage is 208-3, that comes out to about 27.8 amps. I would use a 35A or 40A breaker. I suppose technically you could use a 30A breaker? Any opinions?
For a non-continuous load, I'd use a 28a breaker if there was one, so a 30a is adequate, especially if it makes the difference between #10 wire and #8 wire.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
A 28Amp breaker??? Where the hell would you get a 28amp breaker from???
He did say "if there was one." He was making the point that you could use one, that the load could be that close to the breaker rating, if the load was not continuous.

 

Npstewart

Senior Member
Ok lol, I didnt see that until now :)

So I suppose my 80% rule of thumb goes with all the other rules of thumb, good for not much....Theres a lot of myths that get started and its hard to seperate truth from fiction sometimes
 

kwired

Electron manager
Location
NE Nebraska
I don't work with mnay 120/240 systems, where exactly did you derive that from?

Voltage has nothing to do with what amount of amperage will trip the breaker the same rules apply to what ever voltage/number of phases you are accustomed to.

I was always told also that you should never load a breaker over 80% of its rated capacity. I dont think its a requirement in the NEC however it seems widely accepted. Anyone have any thoughts on this?

For example, lets you have a 10KW non continuous load and the voltage is 208-3, that comes out to about 27.8 amps. I would use a 35A or 40A breaker. I suppose technically you could use a 30A breaker? Any opinions?

If your calculations included 125% factors for continuous loading your calculated load has already accounted for the 80% you mentioned. If you limit the calculated load to 80% in this case you have derated twice.
 

dbuckley

Senior Member
I'd just advise a sanity check on what equipment the salon is actually going to use, and make sure that when its all switched on (as it will be one day) you are able to operate without busting a breaker.
 
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