Need help with installing 400 amp (320) service panel


Old 11-22-13, 11:00 AM
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Join Date: Nov 2013
Location: USA
Posts: 1
Need help with installing 400 amp (320) service panel

Hello to all,

New member here. Retired carpenter as my trade. Have wired houses that I have built for myself with 200 amp service.

now to my situation. After talking to local power company and what they require, it appears best option for providing new service to a workshop I am building next to house will be to replace existing 200 amp service panel with a CSED combination box. I have been looking at Square D boxes for this and per my power company's requirements, it appears that Square D box QU816D400SL

Name:  QU12L400SL dwg _3313 with notes.jpg
Views: 22074
Size:  51.6 KB will be acceptable. I have also verified my underground service to home is 320 amp underground service wire from an pedestal box transformer on ground.

Is this the most economical way to go to do this? Or is there any other suggested better ways, ie. cost? My purpose is to provide power to my workshop which should be no further than 50 ft. away. It will require the use of a rotary phase converter, as I have 3 phase woodworking machines. This phase converter will use a 30amp breaker and any additional power needs will be general lighting, single phase power outlets, and a possible mini split heat pump. Of course other open breakers will always be needed eventually.

Do I have to replace my existing 200 amp panel box and use the one above to service the home, or does the existing 200 amp box stay where is and fed by the above box? (after looking at diagram, this now appears to mount on outside of house where existing meter box is installed). If that is correct, not sure I like that idea, as too much service interruption. Does this box feed a separate 200 amp cutoff box also? I am confused on this wiring setup and need help in analyzing this and what I need to do if this is the suggested majority opinion.

My initial plan was to just install a 100 amp shutoff and feed to 100 amp service to this building. I do not have to install mini split system (25 amp) and 100 amp should handle rest. However after thinking, may not be wisest move but I have to weigh this against cost, as it is always, and will always be the overriding factor as far as I am concerned. Feed to workshop will have to be underground, and I assume a new meter socket will be installed by power company to handle this new combination service, assuming this is how I go.

At this point, I am leaning to above diagram install, but not sure if this box is most economical way to go. My experience in this is 320 amp service is none. However, I do not want to hire this job out. I enjoy learning how to do this, and what better way than on my own home. By the way, no electrical inspections in my county. However, all houses I have ever built were always 3rd party inspected for obvious reasons. I doubt I will get thiis inspected, but will have it carefully looked at by an industrial electrician to make sure all looks good.

I am not sure where to start, but asking questions is always best place to begin. I can use existing 200 amp service in garage if it needs to be taken out in place of new service above. Appreciate all comments but one. Please do not advise me to have a qualified electrician do this. I plan on doing all this according to national code.

30 amp for phase converter, 25 amp for mini split, have 45 amps left for lights, plugs, etc. Now maybe I am just plain foolish to consider installing the combination service entrance panel. Not getting needing extra amps for all extra money and time spent to do it.

thanks in advance.


Last edited by Greg Brohey; 11-22-13 at 11:46 AM. Reason: see bold print
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Old 11-22-13, 04:56 PM
PJmax's Avatar
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Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 46,221
Welcome to the forums.

I have read thru this several times and have not grasped the entire concept.
I understand you want a 100A service to the garage.

You said you have an underground 320A feeder feeding........ ? Your 200A service ?
Old 11-23-13, 09:19 AM
Tolyn Ironhand's Avatar
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Join Date: Nov 2007
Location: Twin Cities, MN
Posts: 12,285
Welcome to the forums!

I can not read your picture, but it doesn't matter because my advice will still be the same. I highly doubt you would need a residential 400 amp service. A 200 amp would serve you just fine. 200 amps is a TON of power. There are good ways to get this, but as PJ posted, I am not following your post 100%.

Is this a new place or do you have an electrical service on the property now?
If there is, where is it? How many amps?

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