MAIN SERVICE PANEL CAPACITY?

Question

My son lives in Columbus, GA, and has built a workshop in the back yard, about 50 ft from the service panel in the house. The structure is 360 sq ft and will be used for computer repair, hobby work (RC aircraft), and craft work. We estimate the max total load in the shop, including lighting and HVAC, won't be more than 15-20A at any one time. (No welding or monter air compressors.) Trying to save some money, he wants to wire the building himself, having it inspected when through. Actual connection to a service panel would be done by a licensed electrician.

I have advised him to contact the city electrical inspector's office to find out what their standards are, and also to get documents pertaining to wiring practices, standards, etc.

A few questions have come up in our conversations:

How does the service rating (he has 150A service) relate to the total amps obtained by adding up all the breakers? He is wondering if he may have to add a sub panel just to feed the workshop, or could he add another breaker (there is room for a few more breakers).
Having built a few workshops in my time, I have always used wire larger than that required for anticipated loads, not just for safety reasons, but to minimize voltage drop (I'm a ham and hate sagging feeds). I recommended he feed the shop with larger than necessary conductors for the same reason. Then I took another look at this from an inspector's viewpoint: why such heavy service? What are you really going to do out there? Could this open up an unneccesary bag of worms? I am familiar with the trap of feeding 15A receptacles with 12-ga.

TIA for any advice!

Cheers-- Terry--WB4FXD Edenton, NC

Bud-- · Accepted Answer

The minimum size of the service (150A) is determined by a calculation that includes the square footage, specific loads like air conditioning and other factors. The total of the breakers can be much larger than that. Technically a calculation would have to be made to see if a larger service is needed, but for the load you are adding it would be very unlikely to matter. Adding a subpannel fed from a breaker is no different from adding a breaker in the panel - just add the breaker.

Larger wire for future as well as voltage drop - both good ideas. Around here I have heard of hassles from inspectors based on the possibility of running a business in the garage - a zoning issue. IIRC it involved feeders large enough for a welder and possibility of car repair. I would guess you will have no problem with the inspector.

15A receptacles can be fed with 12 ga and connected to a 20A circuit (if there is more than one receptacle - duplex outlet is 2). A duplex 15A receptacle is rated for 20A total combined from both halves.

bud--

gfretwell · Answer

You could probably get by with a single multiwire circuit, fed with #12/3 wire @ 20a (2 pole breaker) and have two 120v 20a circuits and a

240v circuit. That is your biggest bang for the buck. You get more flexibility if you run a feeder and install a sub panel in the outbuilding. You will need a ground rod for that plan. The 2/4 circuit panel should be all you need and you could probably do fine with 30a breaker, 10/3 wire. Greater flexibility would be to pull 8/3 and a 40a breaker but that is more money. You get what you pay for.

You have to be careful with "advice" since folks who are not paying the bill tend to advise overbuilding these things.

Terry · Answer

=>The minimum size of the service (150A) is determined by a calculation that includes the square footage, specific loads like air conditioning.....

Thanks, Bud. That makes sense as I see it. I have passed along your comments and all I can do is keep insisting he work with the city so he doesn't have do any serious tear-outs at the end!!

Cheers-- Terry--WB4FXD Edenton, NC

Terry · Answer

=>You could probably get by with a single multiwire circuit, fed with =>#12/3 wire @ 20a (2 pole breaker) and have two 120v 20a circuits...

Thanks for the advice, and I have passed it along to my son. I'm sure he will take all I have sent him and discussed with him, then digest it all, and finally do whatever he wants!! I just hope he talks with the city before rather than after so as to avoid a costly tear-out if he did it wrong!!

Thanks again--

Terry--WB4FXD Edenton, NC

gfretwell · Answer

The thing that becomes "costly" is digging up the wire. Be sure it is deep enough. Everything else is fairly easy to fix if they catch you cheating. Safest is to go through the permit/inspection process and that usually gets you "free" advice from the building department at plan review time. They will walk you through things like burial depth, wiring methods, GFCI requirements and such. That will depend on the ultimate finish of the building interior. A tip, GFCIs are cheaper than property taxer so you should probably stay primative, requiring GFCI protection, until you clear the permit.. Then you can make the inside pretty. I would keep the GFCI.

MAIN SERVICE PANEL CAPACITY?

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