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-   -   Sub panel sizing and distance question (http://www.diychatroom.com/f18/sub-panel-sizing-distance-question-22115/)

fburke 06-11-2008 08:25 AM

Sub panel sizing and distance question (pic added)
 
I want to run a sub panel to my shed. With a measuring wheel it comes out to about 143 feet from my 200A service in my house so Iíll call it 150 just to be safe.

Should I go with a 60 amp and run 4 # 8ís ? What size conduit should I run and how deep does it need to be.

The shed isnít that big roughly 10 x16 I donít weld so the most I would probably run at this point is an air compressor lights ect. but if I have to do the work I might as well do it right and be done with it.

Right now it is fed off of my porch circuit with a direct bury 12/2 I just bought the house last year so I inherited this issue.

http://i56.photobucket.com/albums/g178/fburke/shed.jpg
I have noticed the voltage loss is too great for this wire I have not been able to run my air compressor more than about 95 lbs of pressure without blowing the 15 amp breaker even then at start up I always knock out my florescent lights. so I have not used it much in fear of running the unit.

I know itís not the compressor because when I have it in the house it works fine, however it really should be on a 20 amp circuit even though the specks only call for a 15A it does draw a little more on start up even the reviews I read on this compressor state that.

J. V. 06-11-2008 09:14 AM

If your wire to the shed is 12/2 direct burial, you can put a 20 amp breaker in the slot. The distance is an issue also. The load (power consumption) at the shed is the only way to determin voltage drop. In your case you don't know what your load will be, right? If it were my shed I would bump the wire up one size and keep the sub panel at 60 amp. Sub panels are not wired the same as service panels. Someone here will be able to give you the exact wire size for the distance.

BigJimmy 06-11-2008 09:23 AM

Quote:

Originally Posted by J. V. (Post 129538)
Someone here will be able to give you the exact wire size for the distance.

See http://www.mikeholt.com/documents/fr...Calculator.xls

fburke 06-11-2008 10:01 AM

Quote:

Originally Posted by J. V. (Post 129538)
load will be, right? If it were my shed I would bump the wire up one size and keep the sub panel at 60 amp.


at this point thewre is no sub panel in the shed it just feeds into an outlet then feeds the recepticals and lights....the sub panel is what i am adding so i have some room to play with and it will be safe.

BigJimmy 06-11-2008 10:47 AM

Quote:

Originally Posted by fburke (Post 129549)
at this point thewre is no sub panel in the shed it just feeds into an outlet then feeds the recepticals and lights....the sub panel is what i am adding so i have some room to play with and it will be safe.

I recently install a sub in my garage for this same reason. I went with a 60A Square D QO, 6 position panel for this which for my purposes is in itself overkill. I did install a single 20 GFCI recept (on its own ckt) just for the compressor.

Not concerning VD, the feeder for my panel is protected by a 40A 2-p breaker. I installed 4-#6 because this was what I had on hand. 8 awg. THWN would be fine for your application as long as the OCPD is rated 50A or less.

Back to the voltage drop, if you use the calculator, depending on the breaker size that you protect the circuit with, you may need to up the conductor size. BUT, keep in mind the following:

1. If you use the current value that coincides with the breaker size feeding the panel, this will be overly conservative. The compressor will be the only really significant load and the greatest draw will be on start (based on the LRA or FLA data from the nameplate).

2. Keep in mind that the NEC does not require conductors to be sized to take into account VD. This is covered in FPN's which are recommendations but still is a good idea. But as JV mentioned, you should be concerned with what the anticipated load will be, not just calculate VD on a panel that is running at rated capacity.

3. Assuming that you will be running RNC (PVC) underground, 1" is OK per the tables for either 8 or 6 awg. But, if it was me, I'd go with 1-1/4" esp. if you choose 6 awg.

Jimmy

J. V. 06-11-2008 11:11 AM

Quote:

Originally Posted by fburke (Post 129549)
at this point thewre is no sub panel in the shed it just feeds into an outlet then feeds the recepticals and lights....the sub panel is what i am adding so i have some room to play with and it will be safe.

I meant stick with the 60 amp panel if thats what you want. In the space you mention 60 amp is a bit overkill. But you can put any size you want in the shed. But you must protect the feeder with the correct size breaker. You can put a 100 amp breaker panel in there if you want. Ps...don't forget the ground rods.

BigJimmy 06-11-2008 12:46 PM

Quote:

Originally Posted by J. V. (Post 129562)
Ps...don't forget the ground rods.

And don't forget to not bond the neutral bus. And don't forget the 6-handle rule.

J. V. 06-11-2008 03:57 PM

Dittos

chris75 06-11-2008 04:14 PM

Your only allowed 1 feeder to a building, so that existing line going to the shed will have to be removed if it in fact enters the shed.

chris75 06-11-2008 04:16 PM

Quote:

Originally Posted by BigJimmy (Post 129593)
And don't forget the 6-handle rule.

THis is generally a moot point since most panels in order to be rated as Service Equipment must be supplied by a main breaker.

pcampbell 06-11-2008 04:30 PM

What is the 6 handle rule?

chris75 06-11-2008 04:42 PM

Quote:

Originally Posted by pcampbell (Post 129651)
What is the 6 handle rule?

Article 225.33 Maximum Number of Disconnects
(A) General. The disconnecting means for each supply permitted by 225.30 shall consist of not more than six switches or six circuit breakers mounted in a single enclosures, or in or on a switch board.


But like I said, just about every panel is listed only as service equipment when only fed from a main breaker.

BigJimmy 06-11-2008 06:08 PM

Quote:

Originally Posted by chris75 (Post 129645)
THis is generally a moot point since most panels in order to be rated as Service Equipment must be supplied by a main breaker.

Chris-

Help me here. I was under the impression, based on my interpretation of the definition of Service Equipment in art. 100 that a panelboard does not qualify as service equipment when installed as a sub.

Jimmy

chris75 06-11-2008 06:33 PM

Quote:

Originally Posted by BigJimmy (Post 129681)
Chris-

Help me here. I was under the impression, based on my interpretation of the definition of Service Equipment in art. 100 that a panelboard does not qualify as service equipment when installed as a sub.

Jimmy


if its a panel installed in a detached structure than its not a sub-panel... with that said, if you take a look at the definition,

Service Equipment. The necessary equipment, usually consisting of a circuit breaker(s) or switches(s) and fuse(s) and their accessories, connected to the load end of the service conductors to a building or other structure, or an otherwise designated area, and intended to constitute the main control and cutoff of the supply.


Article 225.31 requires a disconnect on a seperate building or structure,

THis next section requires that the disconnect required in 225.31 meet the following.

225.36 Suitable for Service Equipment.
The disconnecting means specified in 225.31 shall be suitable for use as service equipment.


Now, after all that... (whew!) when you open up a panel it will have a sticker that says only suitable for service equipment when supplied by a single main.



This is probably way over the average DIY head. but there it is in a nutshell.


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