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Old 03-16-2014, 12:38 AM   #1
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100 Amp Subpanel ?'s


Have 250 amp going into the house. However my main panel is small and I believe is only 200 amp? Will have to check. Either way it's full and I would like to add a 100 amp sub panel.

I want to put it in the basement which is about 40-50 ft away from main panel in the garage. I am looking for an actual diagram and size of wiring to do this. I've done plenty of electrical work but after reading I have become more confused with panels needing breakers some not and the grounding bars.

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Old 03-16-2014, 01:02 AM   #2
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100 Amp Subpanel ?'s


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Originally Posted by jbean8510 View Post
Have 250 amp going into the house. However my main panel is small and I believe is only 200 amp? Will have to check. Either way it's full and I would like to add a 100 amp sub panel.

I want to put it in the basement which is about 40-50 ft away from main panel in the garage. I am looking for an actual diagram and size of wiring to do this. I've done plenty of electrical work but after reading I have become more confused with panels needing breakers some not and the grounding bars.
You don't have 250 amps going into a house.

Look at the panel and see what the largest breaker is.

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Old 03-16-2014, 08:01 AM   #3
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100 Amp Subpanel ?'s


I think you mean 240 volts, not 250 amps.

Have you done a load calculation? I doubt you need 10ⁿ0 amps for the basement.
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Old 03-16-2014, 08:14 AM   #4
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100 Amp Subpanel ?'s


This topic has been discussed many times on this forum, do a search for "add subpanel" or similar you will find dozens of threads. The actual process is pretty simple if you have electrical experience. You add a 100A 2 pole breaker to your existing panel, and run 3 wire with ground cable to the new panel. Two hots, typically red and black, one neutral (white), and a ground wire (typically green). At the new panel, you keep the neutral and the ground separate, which may require you to remove the bonding screw from the panel. Find the threads, this process is discussed at length in some of them, my post is the short version.

You need to discuss cable size with your local wiring inspector. In my jurisdiction, #2 copper with a #6 ground wire is necessary. However, you will find numerous on line discussions that indicate that #4 copper is acceptable, or sometimes #3 is indicated. It seems to be a question of how the local authority interprets the NEC, which is why I suggest discussing this issue first with your wiring inspector. You can also use aluminum wire, however it has to be larger than the copper wire, and you will need to use an aluminum wire rated panel and breaker.

As to whether you need a main breaker in the subpanel, that should also be discussed with your wiring inspector. If the panels are in sight of each other, I believe the inspector may allow for main lugs only on the subpanel, but in my town you always need a main breaker. Sizing the main breaker is a little tricky.

In principle, the upstream feeder wires are protected by the 100A breaker in the main panel. In my town, the subpanel breaker is sized consistently with the main panel breaker, or smaller if the rating of the subpanel is less than 100A in your case. The curious question arises if you purchase a 150A rated subpanel, can you put a 150A rated main breaker in it? Perhaps so, because all the downstream wires are protected by their own breakers, and the upstream wires are protected by the 100A main breaker, so the only thing being protected by the subpanel main breaker is the panel itself. But this is a call for the wiring inspector. Good luck with the project.
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Old 03-16-2014, 08:56 AM   #5
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100 Amp Subpanel ?'s


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As to whether you need a main breaker in the subpanel, that should also be discussed with your wiring inspector. If the panels are in sight of each other, I believe the inspector may allow for main lugs only on the subpanel, but in my town you always need a main breaker. Sizing the main breaker is a little tricky.

This is an unusual requirement and is not part of the NEC. The NEC does not require subpanel to have a main breaker as long as the sub is in the same structure.






In principle, the upstream feeder wires are protected by the 100A breaker in the main panel. In my town, the subpanel breaker is sized consistently with the main panel breaker, or smaller if the rating of the subpanel is less than 100A in your case. The curious question arises if you purchase a 150A rated subpanel, can you put a 150A rated main breaker in it? Perhaps so, because all the downstream wires are protected by their own breakers, and the upstream wires are protected by the 100A main breaker, so the only thing being protected by the subpanel main breaker is the panel itself. But this is a call for the wiring inspector. Good luck with the project.

Again a unusual requirement not called for in the NEC. a main breaker in a sub panel is merely a disconnect and can be any size; typically the same or larger than the breaker feeding it from the main panel.

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Old 03-16-2014, 10:20 AM   #6
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100 Amp Subpanel ?'s


Quote:
Originally Posted by Daniel Holzman View Post

You need to discuss cable size with your local wiring inspector. In my jurisdiction, #2 copper with a #6 ground wire is necessary. However, you will find numerous on line discussions that indicate that #4 copper is acceptable, or sometimes #3 is indicated. It seems to be a question of how the local authority interprets the NEC, which is why I suggest discussing this issue first with your wiring inspector. You can also use aluminum wire, however it has to be larger than the copper wire, and you will need to use an aluminum wire rated panel and breaker.
They are interpreting the code wrongly, that is the problem. #4 cu. should only be used on the service conductors at 100 amps. #3 cu. must be used after the main breaker for 100 amps... People use the wrong tables, that's the only issue.


Quote:
Originally Posted by Daniel Holzman View Post
As to whether you need a main breaker in the subpanel, that should also be discussed with your wiring inspector. If the panels are in sight of each other, I believe the inspector may allow for main lugs only on the subpanel, but in my town you always need a main breaker. Sizing the main breaker is a little tricky.
main breakers are only required in sub-panles in detached structures. Another case of the, this is how we're doing it in my town mentality....
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Old 03-16-2014, 01:29 PM   #7
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100 Amp Subpanel ?'s


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They are interpreting the code wrongly, that is the problem. #4 cu. should only be used on the service conductors at 100 amps. #3 cu. must be used after the main breaker for 100 amps... People use the wrong tables, that's the only issue.




main breakers are only required in sub-panles in detached structures. Another case of the, this is how we're doing it in my town mentality....

The rule allowing # 4 copper to a 100 amp sub panel is correct , at least in the 2011 code book . Did not have a 2014 code book handy .

It says for residential services and feeders . This would be a feeder .

But as has been stated , it all comes down to what the AHJ says .

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Old 03-16-2014, 01:31 PM   #8
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100 Amp Subpanel ?'s


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Originally Posted by WyrTwister View Post
The rule allowing # 4 copper to a 100 amp sub panel is correct , at least in the 2011 code book . Did not have a 2014 code book handy .

It says for residential services and feeders . This would be a feeder .

But as has been stated , it all comes down to what the AHJ says .

God bless
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Sorry, but you are reading it wrong... It has nothing to do with a AHJ, it has to do with being able to read the NEC. To sum it up for you, you can only use #4 if it is going to supply the ENTIRE load, a sub-panel obvious does not meet this requirement.

Last edited by stickboy1375; 03-16-2014 at 01:33 PM.
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Old 03-16-2014, 03:03 PM   #9
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100 Amp Subpanel ?'s


Agreed....
310.15(B)(7).

service to panel 100 a main breaker cu #4 (seu al 2).............does not apply to sub panels from that panel.

the way I see "feeder" in that article.

service to Main breaker enclosure cu #4 (seu al 2). Main breaker enclosure to remote main panel cu #4 (ser al 2).
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Old 03-16-2014, 03:13 PM   #10
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100 Amp Subpanel ?'s


If the service is 100A, you could use #4 to feed a 100A sub panel.

If the service is larger than 100A you have to use #3.
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Old 03-16-2014, 03:18 PM   #11
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If the service is 100A, you could use #4 to feed a 100A sub panel.

If the service is larger than 100A you have to use #3.
sounds good.

100 amp service. #4 to sub panel. someone upgrades 100 amp service to 200 amp. now what?
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Old 03-16-2014, 03:25 PM   #12
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sounds good.

100 amp service. #4 to sub panel. someone upgrades 100 amp service to 200 amp. now what?
You'd have to feed the sub with a 90A breaker.
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Old 03-16-2014, 03:29 PM   #13
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You'd have to feed the sub with a 90A breaker.
thank you
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Old 03-16-2014, 03:33 PM   #14
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If the service is 100A, you could use #4 to feed a 100A sub panel.

If the service is larger than 100A you have to use #3.

I should clarify...

You could use #4 in the first scenario, if the service used #4. If, for some reason it used #3, you couldn't use the #4.

This is based on the 2008/2011, the 2014 has a new, F-ed way to calculate what needs to be used.
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Old 03-16-2014, 04:16 PM   #15
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100 Amp Subpanel ?'s


What's going on in this basement that the OP thinks it needs a 100a subpanel?

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