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View Full Version : Installing sub-panel at new shop....



Kevin Slankard
03-24-2008, 2:28 AM
I just wanted to run this buy you guys before I head to Lowes tomorrow.

I have an external panel at my house that is fed directly from my meter can. The neutral and grounds all run to the Neutral Bus bar. This seems to be normal for a main panel, as I've read up on tonight. This panel is grounded with a grounding rod.

I'm installing a 40amp double pole breaker in this box and gonna run power to my shop. I'm plan on running 2 hots, 1 neutral, 1 ground. The hots will be 8awg stranded wire and the neutral and ground will be 10awg stranded wire.

I apparently bought a main box instead of a sub panel box because there is no ground bus bar in the box, only a neutral bus bar. Reading on the net tonight, seems like I'm going to have to install a ground bus bar since this is a sub panel.

Correct me if I'm wrong, but I feel confident to this point. Now, here is where I think I'm right, but could be wrong:

Since the main box only has a neutral/ground common bus bar, I shouldn't run a ground from that bar to the ground to my new sub panel. I should only run a neutral from the neutal/ground bus bar and then install a ground rod and connect it to my new sub panel grounding bar. Clear as mud right? My new shop is not attached to my house. Its about 30ft away and as of now, ungrounded in any way.

Please feel free to correct me if I'm wrong. I didn't realize there were so many codes and what not. When I started this I was going to just run a ground and neutral from my main box to my new box and connect both wires to the neutral bus bar as I saw on my main panel. But, this doesn't seem to Kosher now. If anyone can find a pretty good write up on this I'd love to read it. I'm not quiet following why this is good practice for the main box and not a sub panel.

Thanks for listening. I'll have some pics up of the new shop soon. I have a few circuits ran inside for some outlets and lights and a few pieces of OSB up to see how the finished product is going to look. I'm now double and triple checking my main connecting before proceding.

Kevin

Kevin Slankard
03-24-2008, 4:34 AM
More reading has been done. This is where I think I stand:

I'm going to have to install a ground bus bar to my new sub panel. Currently it only has the neutral bus bar. These must be seperate.

Other than my two hot wires, I'll run a #10 neutral from my main panel Neutral/Ground bus bar to my subpanel Neutral bus bar.

I'll also run a #10 ground wire from my main panel neutral/ground bus bar to my new sub panel ground bus bar(that I have to install).

I'll also drive a grounding rod and connect it to my new sub panel grounding bus bar.

Does this sound correct?

If I may ask, do I really need the ground wire coming from the neutral/ground bus bar on my main panel to come to my new sub panel grounding bar when I'm going to drive a grounding rod for the panel? Other than that, I think I'm good.

Thanks again

Kevin

Bernie May
03-24-2008, 8:01 AM
Most of what you suggest follows general code. You do need a ground wire to the separated buss in your subpanel. Grounding bar is optional, usually excluded in an attached garage. The neutral is usually the same size as the two hots. I would consider running NM 6 cable that includes a #10 ground and upgrading to a 50 AMP breaker.

David Parker
03-24-2008, 10:01 AM
I'm also contemplating putting in a sub-panel myself. I found the following site helpful.

http://ths.gardenweb.com/faq/lists/wiring/2005015726007600.html

Dave

Thom Sturgill
03-24-2008, 11:32 AM
As I understand it, you NEVER install a second ground rod. The 'earth potential' is NOT the same at two points even just 30' away and you will thus have current running on the ground wire. I do know that for this reason, you should never ground both ends of a 'shield' or 'drain' wire in communications circuitry.
Perhaps someone will chime in with the specific code requirement, its been many years since I read a code manual.

Rob Russell
03-24-2008, 3:38 PM
Kevin,

The neutral must be the same size as the 2 hots - all 3 of those are current-carrying conductors.
It sounds like you're pulling individual conductors - THHN/THWN. If that's the case, #8 copper THHN/THWN is good for 50A. If you decide to pull #6 copper THHN/THWN, you'd be able to use a 60A breaker. For up to a 60A subpanel, a #10 copper Equipment Grounding Conductor is acceptable.
In terms of driving additional ground rods, it depends on whether your shop is in an attached or detached space. If it's attached, you don't need additional ground rods. If it's a detached building, ask your local AJH (Authority Having Jurisdiction, aka the inspectors) what they want. I checked with my local AHJ on running power out to a shed and expected to need to drive ground rods, but they specifically did not want me to have the second set of rods.Rob
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