# Sub-panel size



## dmanola7 (May 2, 2009)

I have a job where there is a brand new 225a square d homeline outdoor panel that is full to the brim and (3) twin breakers have already been installed. I would like to free some space in this panel by adding a sub-panel on side of it. I plan to put (2) 40a a/c circuits in it and some lighting and general receptacle circuits also. Would my best bet be a 100a or 125a sub-panel?


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## TxElectrician (May 21, 2008)

your best bet would be to figure the load on the existing panel and see what it can accommodate.


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## Speedy Petey (Sep 30, 2003)

100 or 125 amp panel is the same thing basically. I forget why some are rated 125a, but there is some reason for it.

It is not worth feeding it with a true 125a anyway. A 100A sub-feed should be more than enough for any residential sub-panel. Especially if you are just moving a few circuits over to make room.


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## Tiger (Nov 21, 2007)

dmanola7 said:


> I have a job where there is a brand new 225a square d homeline outdoor panel that is full to the brim and (3) twin breakers have already been installed. I would like to free some space in this panel by adding a sub-panel on side of it. I plan to put (2) 40a a/c circuits in it and some lighting and general receptacle circuits also. Would my best bet be a 100a or 125a sub-panel?


I install QO, not Homeline and I'm not familiar with exterior panel sizes but my SOP is as follows:

Install the largest main panel available. In QO interior that's 40 circuit.
Install all the 240 volt circuits in the main panel.

If it needs a subpanel, install the largest 100A panel available. In QO interior that's a 32 circuit.

Of course as TxElectrician states, it all starts with a load calculation to determine the correct size main distribution panel.


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## Magnettica (Dec 19, 2006)

If I were you I would put those new A/C circuits in the main panel and relocate a few of the lighting circuits to the sub panel.


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## hbelectric (Oct 8, 2007)

Like already said, you can use a 100/125/225 amp panel, doesn't matter. What matters is your wire size and breaker feed size.
100amp, 8/10/12 space is fine. Don't put in a 32 space. I would'nt transfer the A/C to the main, if the sub is right next to the main.
Your just adding larger wires to a already packed panel.

#3 wire, 100amp breaker, 100/125 amp load center


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## woodchuck2 (Feb 27, 2008)

I am going to agree with magnettica here, i have always put the larger loads on top of the buss closest to the breaker and work downward with smaller loads. I would also put the sub-panel feed up top too.


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## ampman (Apr 1, 2009)

dmanola7 said:


> I have a job where there is a brand new 225a square d homeline outdoor panel that is full to the brim and (3) twin breakers have already been installed. I would like to free some space in this panel by adding a sub-panel on side of it. I plan to put (2) 40a a/c circuits in it and some lighting and general receptacle circuits also. Would my best bet be a 100a or 125a sub-panel?


 this sounds like a diy question ---just saying


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## 480sparky (Feb 1, 2009)

How many spaces are in the panel? 20? 30? 40?

Now, how many circuits are allowed in the panel? 20? 30? 40?

If you have a panel that can take 40 circuits, but there's only 20 or 30 spaces, you have plenty of 'room' left for more tandem breakers. A sub-panel would be a total waste.





If, however, you have a _40-space panel with 43 circuits_, you have an illegal installation. If the house only needs a 200a service (a load calc can determine this), then 43 circuits is just silly. Some breaker sharing can be done to get the total number of circuits back down to 40 or less.

If, however, the load calc shows 200a is not enough to carry the load, then a new service is in order.


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## dmanola7 (May 2, 2009)

480sparky said:


> How many spaces are in the panel? 20? 30? 40?
> 
> Now, how many circuits are allowed in the panel? 20? 30? 40?
> 
> ...


 
The main panel has 40 circuits and 3 tandem breakers were installed. The cover to the panel wont fit on snug and it looks like a rats nest inside. I dont know why the original electrician didnt add a sub-panel inside from the get-go. A sub-panel really needs to be added for any extra circuits in the future.


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## dmanola7 (May 2, 2009)

ampman said:


> this sounds like a diy question ---just saying


what the hell is a "diy"?:bangin:


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## 480sparky (Feb 1, 2009)

dmanola7 said:


> The main panel has 40 circuits and 3 tandem breakers were installed. The cover to the panel wont fit on snug and it looks like a rats nest inside. I dont know why the original electrician didnt add a sub-panel inside from the get-go. A sub-panel really needs to be added for any extra circuits in the future.


 
I would do a load calc to see if the existing 200a has the capacity for more circuits. I realize some people go nuts when it comes to circuits and fill up their panels with 20 amp breakers that have 2 amp loads on them. Other times, a 200a panel is right on the edge, and adding more would mean an increase in the service size.

If 200 amps is more than plenty, I would first see if some circuits could be combined to free up some spaces. You'll need to do this anyway since the panel is in violation anyway.




dmanola7 said:


> what the hell is a "diy"?:bangin:


Um..........do it yourself.


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## Tiger (Nov 21, 2007)

480sparky said:


> If, however, you have a _40-space panel with 43 circuits_, you have an illegal installation. If the house only needs a 200a service (a load calc can determine this), then 43 circuits is just silly. Some breaker sharing can be done to get the total number of circuits back down to 40 or less.
> 
> If, however, the load calc shows 200a is not enough to carry the load, then a new service is in order.


I agree with most of 480sparky's usual good advice, but IMHO it's better to add a subpanel (a good design allows for future expansion).


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## dmanola7 (May 2, 2009)

480sparky said:


> I would do a load calc to see if the existing 200a has the capacity for more circuits. I realize some people go nuts when it comes to circuits and fill up their panels with 20 amp breakers that have 2 amp loads on them. Other times, a 200a panel is right on the edge, and adding more would mean an increase in the service size.
> 
> If 200 amps is more than plenty, I would first see if some circuits could be combined to free up some spaces. You'll need to do this anyway since the panel is in violation anyway.
> 
> ...


Thanks Sparky, it is actually a 225A service so I know for sure it has room for extra circuits. The majority if the load is lighting and general use receptacles. There are (2) 40A A/C ckts, 30A dryer & a 50A Oven.


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## 480sparky (Feb 1, 2009)

dmanola7 said:


> Thanks Sparky, it is actually a 225A service so I know for sure it has room for extra circuits. The majority if the load is lighting and general use receptacles. There are (2) 40A A/C ckts, 30A dryer & a 50A Oven.


Is it a 225a service, or a 225a max. rated panel being used at 200a?

I've never seen a 225 amp service. Next size up from 200 for me is 400.

Again, there may be _room_ for more circuits, but can a 200a/225a service handle the increased load? Just because you have checks in the checkbook does not mean there must be money in the account. Only a proper load calculation will bear out the truth.


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## Magnettica (Dec 19, 2006)

dmanola7 said:


> Thanks Sparky, it is actually a 225A service so I know for sure it has room for extra circuits. The majority if the load is lighting and general use receptacles. There are (2) 40A A/C ckts, 30A dryer & a 50A Oven.


For the A/C we need the MCA (minimum circuit ampacity). The 40 amp breakers are for short-circuit protection of the conductors, not the A/C. 
The dryer load is calculated at 5,000 VA
The oven load is calculated at 8,000 VA

If the A/C's minimum ampacity was say 35 amps (times 2), that's 16,800 VA

16,800
5,000
8,000
---------

29,000 VA/ 240 = 124 amps


It's also amazing how quickly I forgot all this load calc stuff. I had to look at the book to remind how to do one. What kind of heating system is in this house?


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## Magnettica (Dec 19, 2006)

woodchuck2 said:


> I am going to agree with magnettica here, i have always put the larger loads on top of the buss closest to the breaker and work downward with smaller loads. I would also put the sub-panel feed up top too.


Case in point, I know it is highly unlikely that both condensing units will turn on at the same time, but if they do there is a chance of that 100 amp circuit breaker overloading and shutting off. There is less of a chance of that happening with the 200 amp breaker.


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## dmanola7 (May 2, 2009)

Magnettica said:


> For the A/C we need the MCA (minimum circuit ampacity). The 40 amp breakers are for short-circuit protection of the conductors, not the A/C.
> The dryer load is calculated at 5,000 VA
> The oven load is calculated at 8,000 VA
> 
> ...


I believe I saw a 70A in there for the heat


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## dmanola7 (May 2, 2009)

dmanola7 said:


> I believe I saw a 70A in there for the heat


None of the circuits in the panel are labeled, which is something else I have to do when I go back.


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## dmanola7 (May 2, 2009)

480sparky said:


> Is it a 225a service, or a 225a max. rated panel being used at 200a?
> 
> I've never seen a 225 amp service. Next size up from 200 for me is 400.
> 
> Again, there may be _room_ for more circuits, but can a 200a/225a service handle the increased load? Just because you have checks in the checkbook does not mean there must be money in the account. Only a proper load calculation will bear out the truth.


Yeah its definitely a 225A main breaker square D Homeline


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