Advice on 200a main service and 100a sub


Old 08-16-14, 09:52 PM
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Join Date: Aug 2014
Posts: 4
Smile Advice on 200a main service and 100a sub

First off, hello and thank you in advance for any help and advice.
Long story short, I needed to upgrade a 100amp to 200amp service to install a hot tub. I've worked in construction for a long time so I know how to install, but I don't know all the current codes for electric. I just need some advice for peace of mind.
What I have: For main - 200 amp Homeline 30/60; 25'' 4/0-4/0-4/0 ser cable; 2" pvc conduit, connectors and weatherhead, new 200a meter socket (poco approved); two 5/8 copper clad ground rods and clamps; 4ga ground wire; 100amp breaker for sub and all needed breakers.
For subpanel - 100amp Square D QO load center (original main); 35' 2/2/2/4 service cable; re-using current breakers; 50a breaker for spa.
For spa - 50a spa panel with gfci built in; 6/6/6/8 feed wire(sub to spa panel); 6/6/8 wire (panel to spa); appropriate conduit and connectors.
I know that the sub and spa panel need to have isolated neutrals. Just need to figure out how to do the QO sub panel, the spa panel comes setup. Any input or advice is very welcome and appreciated!
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Old 08-17-14, 04:00 AM
Join Date: Jun 2008
Location: VA.
Posts: 813
I guess this is what you're asking but the you will connect the 2 hots to the main breaker.
If the neutral bar is insulated from the can, remove any bonding screw/strap and hook all the neutrals to that bar. I'm pretty sure QO panels are insulated.

You'll need to purchase a ground bar and move all grounds there.

Keep in mind that, if you have a 3 wire range and/or dryer that uses SE cable, the bare wire in that cable is NOT a ground, it's a neutral.

Also, you only need a #6 for ground rods.
Old 08-17-14, 05:09 PM
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Join Date: Aug 2014
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Thanks for the quick reply wirenut1110. Yeah, I knew the basic hookup of the sub panel but the ground/neutral bar had me a bit confused. I'd never encountered installing a sub in the same building during my construction work. You answered that with the separate ground bar and I'll look to see if the neutral has an isolation screw. As for the ground wire, I read on another forum(poco's) that my power company wants #4 for the grounding to the rods and #6 to the water main inlet. At any rate if its a bit over kill no biggie because I had the wire. Also, I knew about the 220v connections already, but thanks for being thorough.

A question on the breaker arrangement: does it matter where I put the 100a breaker in the main panel? Should it be at the top closest to the main breaker? I will not have that many spaces used because my house is relatively small (under 1300sqft.). Electric stove, living room, dining/1st FL bed, kitchen, microwave, fridge, bath, 2 bedrms upstairs, 100a for sub, furnace. Subpanel will have dryer, spa, basement/workshop, back rm, second fridge. Does this look like an appropriate setup and does this fill up my amp requirements? Or do I have room for expansion if needed?
Old 08-17-14, 05:47 PM
Join Date: Jun 2008
Location: VA.
Posts: 813
I try to put 100 amp breakers on a buss with lower amperage breakers as to not overheat the buss.

As far as your capacity, a load calculation would determine that.
You can go hereMike Holt Electrical Formulas
scroll down to residential load calculations and download the spreadsheet.

You enter values and it calculates the load for you.
Old 08-17-14, 07:00 PM
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You will need to check the bus stab rating to see if breakers can be installed across from the 100 amp breaker.

I would move the large loads to the new panel and feed the old panel with a smaller feeder..
Old 08-18-14, 08:27 AM
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Join Date: Jan 2010
Location: USA
Posts: 9,405
I read on another forum(poco's) that my power company wants #4 for the grounding to the rods and #6 to the water main inlet.
Are you absolutely sure about that? That would be just the opposite of the NEC. I believe I'd contact the power company and question that requirement because it is wrong.
Old 08-18-14, 10:20 PM
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Join Date: Aug 2014
Posts: 4
Thank you guys for the responses!
I think I'll just leave the 100a without any across from it.
Pcboss, I need the setup I described because of location in the house, costs, and loads needed in specific location. I didn't want to have to go back and forth through the house with wire. The dryer is on the back end of the house where the work shop and spa are (closest to the sub), and the stove is considerably closer to the main. I could leave the dryer in the main but then I would need new wire as the house was wired horribly. 50 feet of it to be exact. Is there anything wrong with the way I described how I'm doing it?
Thanks CasualJoe, I double checked and I remembered the info backwards. Good thing it was only the ground rods wire I was changing so it wouldn't have been too small. The water supply ground is already there but now I'll check to see if it is #4 wire.
Old 08-25-14, 09:41 AM
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Join Date: Aug 2014
Posts: 4
New question on residential load calculations if you don't mind. When I'm calculating electrical load for the main do I use 100a * 240 for the sub load, or do I calculate the load in the sub panel (as a service calculation) and then add that to the main panel's calculations?

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