100A Sub Panel Wire Size ?

[Schwarz633]

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 QO 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? I had planned on using something larger than the #4, but I don't see how it can be terminated in the main panel. Am I correct in my understanding that the sub panel load side conductors may not pass through the main panel?

 

[wwhitney]

As best I can determine, the largest conductor the neutral/ground bar in the main panel will accept is #4.

That is surely not correct. It may be that your current neutral bars can only accept #4 and smaller. But there is a lug kit that mounts to the neutral bar, taking up two holes, and allowing a larger conductor to be terminated to it. Be sure to use a torque screwdriver/wrench for all your terminations.

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?

No. A #4 Cu conductor has a 75C ampacity of 85A, so it would need a 90A breaker, along with a load calculation showing that the connected load is no more than 85A. You can use #3 ungrounded conductors which have a 75C ampacity of 100A. The neutral could stay #4 if the calculated load on the neutral is 85A or less, which would be likely if you have any double pole breaker in the subpanel.

I had planned on using something larger than the #4, but I don't see how it can be terminated in the main panel. Am I correct in my understanding that the sub panel load side conductors may not pass through the main panel?

No. The only restriction is that your conductors on the load side of the service disconnect can not pass through raceways with service conductors. In enclosures there's no restriction.

Cheers, Wayne

 

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[Schwarz633]

I have ordered the LK100AN lug kit and have installed #3 copper and a 100A breaker.

 

[Afjes]

Ah, one small thing maybe to consider.

You may want to while you can move the new panel over to the right a bit more than you have it now.
Use a longer nipple between panels. I don't think that since the panels are so close to each other that the panel covers will go on properly, they may overlap each other.

 

[Schwarz633]

Ah, one small thing maybe to consider.

You may want to while you can move the new panel over to the right a bit more than you have it now.
Use a longer nipple between panels. I don't think that since the panels are so close to each other that the panel covers will go on properly, they may overlap each other.

I'm using surface mount covers, they are the same height and width as the load center enclosures. I've also added (2) 15 lug ground bars in the spaces above the neutral/ground bars in the left panel. I will be moving all of the grounds to the new ground bars as they are all now terminated with a ground and neutral in the same lug. I guess this was a gray area in the code that was clarified some time after this original installation was approved by the AHJ in 1995. I don't want some future home inspector waving the red flag on that situation.