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-   -   Yet another sub panel question (http://www.diychatroom.com/f18/yet-another-sub-panel-question-108172/)

jboggz 06-19-2011 08:35 PM

Yet another sub panel question
 
Sorry for opening this.

I currently have a detached building that was wired before we moved in. The building has a couple of florescent lights on a 20A breaker and a couple of receptacles on another 20A breaker. The building sub panel is feed from the main panel with a 50A double pole breaker. Wiring configuration for this sub panel is 2 hots and 1 neutral. No ground bar exists in the sub panel and the grounds are terminated on the neutral bar.

The buried cable has gone bad to this sub panel and needs to be replaced.

My plan is to change the double pole breaker from a 50A to a 40A. My cable run is about 110ft. I will be using PVC conduit for the new cable.

Question: Will I need to install a ground bar in the sub panel? If so where will this ground bar terminate (back to the main panel or install a grounding rod at the building)?

Question: Using THWN will #8 cu be large enough wire for a 40A breaker with a run of 110ft? Or should I switch to a 30A double pole breaker? (I want to stick with #8 as I can get it second had cheaply. New, never before used second hand cable).

I have a camper parked beside the detached building. It would be nice to use this as an office from time to time but not really a necessity. The camper takes a 30A feed. If used as an office, only the AC and computer would be used (sorry, no beer in the fridge in the office :wink:)

Question: Would a 30A double pole be enough for the camper and the 2 other circuits? Really the only thing that runs on the other circuits are low voltage garden lights, set of 10.

I appreciate in advance any replies. As far as code, Iím told my state has adapted the 2008 NEC. So any code that can be referenced would be great.

jbfan 06-19-2011 08:46 PM

A ground bar, and 2 grounds rods need to be installed at the subpanel.
#8 and a 40 amp breaker will work fine, although it is a code violation to remark the wire another color.
2 hots, a neutral and a ground are required from the main to the sub.
A 30 amp breaker for the camper should work fine.

jboggz 06-19-2011 09:01 PM

Thanks for the fast reply.

The grounding is what confuses me I guess. So I should tie the sub panel ground bar to the 2 ground rods and also back to the main panel ground? Can the grounds also be #8 or will I need to downsize to #10?

Any specs on the ground rods would be appreciated (distance apart if required and if any distance from the sub panel needs to be maintained).

Thanks again

jbfan 06-19-2011 09:12 PM

You can use #8, but #10 is all that is required.
Grounds rods 6' apart, and I try to put the first one as close to the panel and shed as I can.

jboggz 06-19-2011 09:27 PM

Thanks! :thumbup:


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