Electrical Engineering - MAIN SERVICE PANEL CAPACITY?

Author

MAIN SERVICE PANEL CAPACITY?

Terry

2006-04-27, 10:21 am

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:

1. 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).

2. 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--

2006-04-27, 12:21 pm

Terry wrote:
>
> 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:
>
> 1. 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).
>

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.

> 2. 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.
>

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@aol.com

2006-04-27, 3:21 pm

On Thu, 27 Apr 2006 13:16:46 GMT, katamasouth@hotmail.com (Terry)
wrote:

>
>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,
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

2006-04-27, 4:21 pm

On Thu, 27 Apr 2006 10:21:01 -0500, Bud-- <remove.BudNews@isp.com>
wrote:

=>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

2006-04-27, 4:21 pm

On Thu, 27 Apr 2006 14:04:42 -0400, gfretwell@aol.com wrote:

=>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@aol.com

2006-04-27, 5:21 pm

On Thu, 27 Apr 2006 18:35:37 GMT, katamasouth@hotmail.com (Terry)
wrote:

>On Thu, 27 Apr 2006 14:04:42 -0400, gfretwell@aol.com wrote:
>
>
>=>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

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.

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