How to size main circuit breaker?

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jaehlee99

Member
Location
Canada
Good Morning Forum

I am working on design apartment, I have two questions.

Each dwelling unit has around 25kW connected load(100% demand factor) wth 208V, single phase. This main circuit breaker will protect four dwelling units.

Can you advise me how to size main breaker in specific ?

Do I need to separate circuit breaker for metered switchboard ?

Please see the attached file.

Thanks in advance

James
 

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  • QUESTION.pdf
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Smart $

Smart $

Esteemed Member
Location
Ohio
If you really want to know how to size it, at a minimum, you have to do an Article 220 load calculation first.

Just looking at the diagram, and with your no-demand load is 25kW/apt comment, you could throw in a 400AF/AT breaker and do the load calc further upstream.
 
kwired

kwired

Electron manager
Location
NE Nebraska
Does NEC apply or Canada code?

If Canada code - not so many here know it's requirements all that well.

Not clear what the source is but if NEC applies it may have too many service or main disconnecting means.
 
J

jaehlee99

Member
Location
Canada
Smart $ said:
If you really want to know how to size it, at a minimum, you have to do an Article 220 load calculation first.

Just looking at the diagram, and with your no-demand load is 25kW/apt comment, you could throw in a 400AF/AT breaker and do the load calc further upstream.
Click to expand...



Thanks for your reply.

What kind of things should I count load calculation further for upstream ?

Please see the load calculation of unit.

Thanks

James
 

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  • CALCULATION SHEET-unit.pdf
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J

jaehlee99

Member
Location
Canada
kwired said:
Does NEC apply or Canada code?

If Canada code - not so many here know it's requirements all that well.

Not clear what the source is but if NEC applies it may have too many service or main disconnecting means.
Click to expand...


Thank for your reply.

Even this is Canadian Code, but it would be very similar to NEC code.

What does the source mean in your reply?
 
kwired

kwired

Electron manager
Location
NE Nebraska
iwire said:
I don't think kwired looked at the one line. There is no issue with the disconnecting means.
Click to expand...
I see at least 9 service/main disconnects unless there is more ahead of this that is not shown. If the breaker being asked about in question 1 in red gets installed you eliminate 8 mains.

Or else I am missing something.
 
kwired

kwired

Electron manager
Location
NE Nebraska
jaehlee99 said:
Thank for your reply.

To the left side incoming power utility, if it was main bus, does system have problem?
Click to expand...
That is what I was initially trying to get at. If there is no service disconnect to the left of what is shown then you have more then the maximum of six service disconnecting means that the NEC allows.

If you put one where you are asking if one is needed, then you will be under six service disconnecting means.
 
J

jaehlee99

Member
Location
Canada
kwired said:
I see at least 9 service/main disconnects unless there is more ahead of this that is not shown. If the breaker being asked about in question 1 in red gets installed you eliminate 8 mains.

Or else I am missing something.
Click to expand...

Hi Kwired,

Thank for your comment.
If I need to install new breaker between metered switch and main bus, how to size it for this system.

Thanks in advance
 
kwired

kwired

Electron manager
Location
NE Nebraska
jaehlee99 said:
Hi Kwired,

Thank for your comment.
If I need to install new breaker between metered switch and main bus, how to size it for this system.

Thanks in advance
Click to expand...
You need load details and use art 220 which may allow certain demand factors to come up with the calculated load - this just gives you minimum sizes needed for what you have - you can always design for future additions.
 
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