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Wyeast · Mar 9, 2008, 06:43 PM

Hello,
I am installing a 100 amp subpanel in my workshop 125' away from my house. I have two ground rods in the ground by the shop, and was wondering if it was necessary to run 4 wires from the house to the shop (two hots, one neutral, one ground), or could I just run the three wires from the house (two hots, one neutral), and use the equipment ground from the ground rods by the shop.
Thanks, Brian

stanfortyman · Mar 10, 2008, 04:23 AM

OK, real quick because I have to get going.

A ground rod does not provide a ground. There are reasons for them but you do NOT get your "ground" from ground rods. Just trust me on this one.

You DO need a 4-wire feeder to your shop. The neutrals and ground stay isolated at the remote panel.

The ground rod(s) are required in any case where there is a feeder run to a remote/detached structure.

Wyeast · Mar 17, 2008, 12:31 PM

Thanks for the answer, stanfortyman.
So, I will run all 4 wires from the house to the shop (two hots, one neutral, and one bare ground). Would I connect the wire from the ground rods by the shop to the bare ground wire that comes from the house, and connect them both to the chassis of the panel at the shop?? The Neutral will be isolated from the box at the shop.

I plan on using #2 copper wire for the two hots, and the neutral. What size wire should I use for the bare ground wire coming from the house, and also from the ground rods by the shop??

If the "ground" doesn't come from the "ground rods", then why do they call them that, and what do they do?
Thanks, Brian

donf · Mar 17, 2008, 12:50 PM

No,

According to the NEC, a "B" or secondary panel that is outside of the residence cannot be grounded by the main panel in the house.

If you connect the ground from the residence and the ground from the sub panel, you may end up with a "Difference of Potential" to the two ground points. The "Difference" can cause a "Floating ground" because of the ambient current in between the two earth ground points.

If your "B" panel or Subpanel are within the same building, then you can use the ground on the SEP to the "B" panel.

stanfortyman · Mar 17, 2008, 01:11 PM

Don, you need to explain this a little better, because the way it is written it is completely wrong.

A non-service panel (sub-panel) should/can be grounded using an equipment grounding conductor. This is actually a change for the 2008 NEC making it mandatory that an EGC is run with the feeders to a remote sub-panel.

So to say a "main" panel cannot ground a "sub-panel" is in direct contrast to the NEC.

Wyeast, in lieu of typing a long dissertation about ground rods here is what the NEC has to say:

"Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation."

Wyeast · Mar 19, 2008, 05:41 PM

Holy smokes, this is complicated. In layman's terms, what should I do? I have the ground rods in the ground by the shop. I will soon run some wires from the house to the shop. The advise from Stan is to run two hots, one neutral, and a bare ground wire from the house.

So, should I do anything with the ground wires from the ground rods by the shop?

Also, I plan on using #2 copper wire for the two hots, and the neutral. What size wire should I use for the bare ground wire coming from the house, and also from the ground rods by the shop??

I appreciate all your help on these questions.
Take care, Brian

mr500 · Mar 19, 2008, 06:31 PM

Originally Posted by Wyeast

Holy smokes, this is complicated. In layman's terms, what should I do? I have the ground rods in the ground by the shop. I will soon run some wires from the house to the shop. The advise from Stan is to run two hots, one neutral, and a bare ground wire from the house.

So, should I do anything with the ground wires from the ground rods by the shop?

Also, I plan on using #2 copper wire for the two hots, and the neutral. What size wire should I use for the bare ground wire coming from the house, and also from the ground rods by the shop???

I appreciate all your help on these questions.
Take care, Brian

Yeah it get confusing FAST on this forum. I have done what u are doing now and from help in this forum it went off great. IMO, and I'm not electrician, if u pull #2 for hots and neutral, then a #6 would go out to your shed from the main panel for ground( don't have to be bare ). ANd then Out of your panel to the grounding rod would be #8.

ALSO since this is not an attached building (to your house), the grounds and neutral in the shed panel box must be kept separate. They can however be landed on same buss bar in the main panel at the house.

mr500 · Mar 19, 2008, 06:37 PM

Originally Posted by Wyeast

Thanks for the answer, stanfortyman.
Would I connect the wire from the ground rods by the shop to the bare ground wire that comes from the house, and connect them both to the chassis of the panel at the shop???.

Thanks, Brian

Yes.. the ground rod(s) will be terminated on the EQUIPMENT GROUND buss in your sub panel. WHICH is also where the ground coming from your house will land as well. . If you don't have an equipment ground buss in your sub panel you can buy them and use machine screws (I used self tap) to attach it to the metal panel. Then land (connect) all your grounds onto this buss using ONE screw for each wire. Use a good acorn clamp that is rated for burial when you tie in the gnd wire to the gnd rod...

Mike

KISS · Mar 19, 2008, 07:55 PM

Some babbleing:

A single service needs a single ground or reference point. So, there is the necessity of running the ground from the sub to the main panel.

But, that ground is useless when it comes to a lightning strike. Hence the need to a local ground.

This is like the normal teir and most everyone stops here.

Sensitive equipment locations such as a plant floor may need an AC ground and a DC ground and isolation transformers with one end of the secondary attached to a ground rod.
In some locations there are two grounds which stay separate. They are green and green with yellow. One is clean and used for a point of reference and the other is protective ground. It's safe to ignore what I just said, but grounding is not a simple concept.

In residential, the Cable TV, the telephone NID and the house power SHOULD share a single ground that is not to far away.

mr500 · Mar 19, 2008, 08:47 PM

Wow. You can learn from reading and doing. Whaooo!!