Square D QO Sub Panel

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schwarz633

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In order to accommodate a portable generator circuit and mechanical manual interlock I'm installing a 200A main lug only QO 40 space panel next to my existing 200A main breaker 40 space panel, as I need additional breaker spaces. I will be powering the sub panel from a 2-pole 100A breaker in the main panel. As best I can determine, the largest conductor the neutral/ground bar in the main panel will accept is #4. The smallest conductor the 100A 2-pole breaker will accept is #4. The smallest conductor the sub panel main lugs will accept is #6. I have a 1-1/2" rigid nipple between the panels. So I guess I will be using (3) #4 THHN and (1) #8 ground, does that sound right? Am I correct in my understanding that the sub panel load side conductors may not pass through the main panel?
 
schwarz633 said:
In order to accommodate a portable generator circuit and mechanical manual interlock I'm installing a 200A main lug only QO 40 space panel next to my existing 200A main breaker 40 space panel, as I need additional breaker spaces. I will be powering the sub panel from a 2-pole 100A breaker in the main panel. As best I can determine, the largest conductor the neutral/ground bar in the main panel will accept is #4. The smallest conductor the 100A 2-pole breaker will accept is #4. The smallest conductor the sub panel main lugs will accept is #6. I have a 1-1/2" rigid nipple between the panels. So I guess I will be using (3) #4 THHN and (1) #8 ground, does that sound right?
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You size based on ampacity and load.
So if you need a 100A breaker then on a 200A service you'll need #2 to the sub. #8 ground is okay for a 100A breaker per 250.122.

schwarz633 said:
Am I correct in my understanding that the sub panel load side conductors may not pass through the main panel?
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No. It's allowed. There are fill limitations, and per code you're supposed to put signage on the main telling where the disconnecting means are in the sub, (although I never see it done and it seems like overkill when the panels are right next to each other).
 
schwarz633 said:
In order to accommodate a portable generator circuit and mechanical manual interlock I'm installing a 200A main lug only QO 40 space panel next to my existing 200A main breaker 40 space panel, as I need additional breaker spaces. I will be powering the sub panel from a 2-pole 100A breaker in the main panel. As best I can determine, the largest conductor the neutral/ground bar in the main panel will accept is #4. The smallest conductor the 100A 2-pole breaker will accept is #4. The smallest conductor the sub panel main lugs will accept is #6. I have a 1-1/2" rigid nipple between the panels. So I guess I will be using (3) #4 THHN and (1) #8 ground, does that sound right? Am I correct in my understanding that the sub panel load side conductors may not pass through the main panel?
Click to expand...

You can't use a 100A breaker for 4 AWG in this application, it does not meet the criteria allowing undersizing the conductors, either downsize the breaker to 90A, or upsize to 3 AWG.
 
I just became aware of the LK100AN neutral lug kit which will allow me to terminate the larger wire. Didn't know those were a thing. I'll stick with the 100A CB and use #3 copper wire.
 
schwarz633 said:
In order to accommodate a portable generator circuit and mechanical manual interlock I'm installing a 200A main lug only QO 40 space panel next to my existing 200A main breaker 40 space panel, as I need additional breaker spaces. I will be powering the sub panel from a 2-pole 100A breaker in the main panel. As best I can determine, the largest conductor the neutral/ground bar in the main panel will accept is #4. The smallest conductor the 100A 2-pole breaker will accept is #4. The smallest conductor the sub panel main lugs will accept is #6. I have a 1-1/2" rigid nipple between the panels. So I guess I will be using (3) #4 THHN and (1) #8 ground, does that sound right? Am I correct in my understanding that the sub panel load side conductors may not pass through the main panel?
Click to expand...
Those QO panels usually have a limited number of ports (maybe 3 or 4) on the neutral bus that accept up to maybe 1/0 AWG I believe.
 
If the left panel is the service panel and has the bond, then the neutrals and grounds are OK together; it's not clear to me from the photo. OTOH, the top end of the far-right neutral bar doesn't look like it's properly secured, so that wants attention.
 
Depending on actual load I likely would have run either #4 or #6 neutral unless maybe this is 208/120 and/or if there is no two pole breakers to be supplied. Chances are your neutral never sees more than ~70% either ungrounded conductor sees so even those sizes are sort of overkill.
 
acrwc10 said:
I would also address the moisture issue in the 1st panel and separate the grounds and neutrals that are under 1 terminal screw.
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It appears to me that placing the ground and neutral in the same lug may have been approved in 1995 when the house was built. I guess I could add a couple of ground bars to bring it into compliance. Not sure it's worth the trouble, but worth considering.

DoubleTap.JPG
 
schwarz633 said:
It appears to me that placing the ground and neutral in the same lug may have been approved in 1995 when the house was built.
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That's not what the text you quoted says. The only change that occurred in 2002 was that the practice went from being an implicit violation of 110.3(B) per the requirements of UL67 to it also becoming an explicit violation of (2008) 408.41.

Cheers, Wayne
 
wwhitney said:
That's not what the text you quoted says. The only change that occurred in 2002 was that the practice went from being an implicit violation of 110.3(B) per the requirements of UL67 to it also becoming an explicit violation of (2008) 408.41.

Cheers, Wayne
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Ok, I'll go along with that, but apparently in 1995 the local AHJ didn't have a problem with it. In an attempt to make things right I'm contemplating adding a ground bar to each side above the existing neutral/ground bars and terminating all of the grounds on those. Do I just drill/tap those into the enclosure with no additional wire back to the existing neutral/ground bar? I'm also starting to 2nd guess the #8 ground wire I ran from the main panel neutral/ground bar to the sub panel LH ground bar. Doesn't that in effect constitute a 2nd "bond"?
 
schwarz633 said:
Do I just drill/tap those into the enclosure with no additional wire back to the existing neutral/ground bar?
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Not unless a bonding screw or jumper is added. I don't see one in the old panel.

To open up more neutral terminals, like to add a neutral lug, you could theoretically connect all of the #14 EGCs to one #14 pigtail, all of the #12s to one #12 pigtail, etc., or even all EGCs to one of the largest size.

1673385678170.png 1673385732200.png
 
...and the bonding screw in the above photo was green to begin with. Looks like a lot of oxidation in the top of that panel.
 
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