COMMUNITY FORUM

MWSBIKE

01:03PM | 08/14/08
Member Since: 08/13/08
8 lifetime posts
Bvelectrical
I have added a 100A 240V sub-panel to my detached garage. I ran 1.5" conduit, 42 ft worth and buried it 18". Here is my mistake...everything works but I think this is a code problem: I pulled 3 #2 THHNs, 2 hots (120V) and 1 for the neutral. At the panel, I have separated the neutral and ground buses, bonded the panel box and drove a ground rod. Electrical geeks in my office said I should have pulled a ground along with the 3-#2s. Ok, like I said probably a code issue but everything works fine.

So the questions to the group are:

1. What's the REAL risk?

2. If the risk is real or I might get hosed on inspection when I go to sell the house, what is the best fix? A direct buried ground? Seems pointless if I have a ground rod.

Thanks in advance.

househelper

07:05AM | 08/15/08
Member Since: 03/31/05
265 lifetime posts
The ground rod has nothing to do with the grounding (bonding) of the two panels. It only provides protection from high voltage spikes such as lightning and contact with high voltage power lines.

With the neutral and ground separated and no ground wire back to the main panel, your ground (bare) wires have no connection with the main and are essentially floating in the subpanel.

If you have no other metallic paths between the garage and main house, and you are still under the 2005 NEC, you may bond the neutral and ground bars (and the enclosure) and make this a compliant installation.

MWSBIKE

09:02AM | 08/15/08
Member Since: 08/13/08
8 lifetime posts
I have two grounds (well three - 1 for the meter base, 1 from main panel in house to cold water line, and 1 for the sub-panel). So how would my grounds and ground bus be "floating" if the ground bus in the sub panel has a ground wire/rod combination. The sub-panel neutral is tied to the main panel neutral bus via the #2 conductor AND the sub-panel box is connected to the ground bus,wire, rod combo.

The only place a ground is not existant is inside the PVC conduit b/w the main and sub panels.

Everything has a path where it is supposed to go.

Anyway, to be compliant would I have to eliminate the sub-panel ground rod and instead directly bury a ground wire follwing the path of the PCV conduit? Seems worthless since it would have direct contact just like the sub panel ground rod.

What am I missing?

househelper

09:33AM | 08/15/08
Member Since: 03/31/05
265 lifetime posts
A detached building with a subpanel needs its own ground rod, regardless of if there are three wires or four wires feeding it.

The earth is a very poor conductor. If your resistance to ground at the grounding rod is 25 ohms (typical) and you have a 120V fault to ground, that is only about 5A flowing back to the main panel, hardly enough to trip a breaker.

Billhart

08:35PM | 08/15/08
Member Since: 04/25/05
1916 lifetime posts
There is the equipment grounding conductor. That is what connects to the ground bus in the panel and runs to the equipment for permant installations or to the ground pin on a receptacle.

The equipment grounding conductor serves 2 purposes.

One to make sure that any metal in that building is at the same potential. The only way to guarntee that is that no current be flowing through the "ground" under normal conditions.

The other purpose is as a return path for fault current. If the hot shorts out in an appliance or tool the current has to flow back through the equipment grounding conductor to the transformer ground. The "other ground" the earth is not good enough conductor to trip the breaker.

The the equipment grounding conductor is either bonded to the neutral at the sub-panel or a separate equipment ground conductor is run from the sub-panel back to the main panel where it is bonded to the neutral.

The other "ground" is the ground electrode system. It consists of one or electrodes (ground rods, under ground metal water pipes, electrodes embedded in the footers and others).

The purpose of the electrodes is to provide a path to handle any surges from nearby lightning strikes.

Each separate building requires their own ground electrode system, whether a 3 wire (2 hots, neutral and bonded to the ground) or 4 wire (2 hots, neutral, and equipment grounding conductor back to the main panel and the neutral bus is isolated from the grounds).

If a 3 wire system properly installed then all of the metal in the 2nd building will be at the same voltage.

But since current is flowing through the neutral back to the main panel there is a voltage drop and thus there will be a volage difference in the "ground" at the house and the "ground" at the outbuilding.

This is no problem unless there is a metalic path that connects the 2 buildings such as a water pipe or tv cable.

What you have is neither fish nor fowl and is VERY DANGEROUS.

TimBonham

07:28PM | 08/16/08
Member Since: 01/09/07
197 lifetime posts
"Anyway, to be compliant would I have to eliminate the sub-panel ground rod and instead directly bury a ground wire follwing the path of the PCV conduit?"

Why do that? Just pull an additional wire through the PVC conduit for a ground wire. There should be plenty of room in a 1.5" conduit. Just make sure to lube it up well when pulling it (and, obviously, cut off all the power in that line while doing this!)

MWSBIKE

09:17AM | 08/19/08
Member Since: 08/13/08
8 lifetime posts
Seriously, thanks for an intelligent explanation. Beats being told, "you dont do it that way".

Ok, so your explanation is motivation to do one of two things: 1. Attempt to pull a ground through the 1.5" conduit, or 2. Run a direct buried ground.

So a few more questions:

1. Would I use fish tape to pull the ground? I have three sweeping 90's between my two LBs and I remember having trouble getting the three conductors gettng through. Cant imagine a stiff bare ground wire going through.

2. If I go the direct buried ground route would I run this separate from the ground rod? From your explanation it sounds like I would want to but I cant help but think that unless I ran a ground that was either in conduit or insulated for direct burial that it would itself act as just another ground "rod". yes/no?

3. Not a question but a statement. I cannot do the proper three wire installation any longer because I cut the bar that connected the neutral and ground buses.

4. In the event of an short in a saw/drill/welder, etc. you mentioned that the current must have a path back to the transformer ground? How does this happen (whether it is installed properly or how I have it now)?

Thanks in advance.

MWSBIKE

09:32AM | 08/19/08
Member Since: 08/13/08
8 lifetime posts
2. If I go the direct buried ground route would I run this separate from the ground rod (FOR THE SUB PANEL)? From your explanation it sounds like I would want to but I cant help but think that unless I ran a ground that was either in conduit or insulated for direct burial that it would itself act as just another ground "rod". yes/no? IN OTHER WORDS IT WOULDN'T BE ABLE TO ACT LIKE AN EQUIPMENT GROUND, RIGHT?

3. Not a question but a statement. I cannot do the proper three wire installation any longer because I cut the bar that connected the neutral and ground buses (IN THE SUB-PANEL..NOT THE MAIN).

4. In the event of an short in a saw/drill/welder, etc. you mentioned that the current must have a path back to the transformer ground? How does this happen (whether it is installed properly or how I have it now)?

SEEING IF I AM RIGHT: IN MY CASE, IT WOULDN'T HAVE A PATH EXCEPT FOR THE GROUND ROD WHICH AS WAS POINTED OUT EARLIER WOULD HAVE TOO MUCH RESISTANCE TO TRIP A BREAKER IN THE MAIN OR SUB-PANEL.

IN A CORRECT 4-WIRE INSTALLATION, IT WOULD RUN BACK THROUGH THE BARE EQUIPMENT GROUND, ACROSS THE BAR IN THE MAIN PANEL THAT CONNECTS THE GROUND BUS AND NEUTRAL BUS, AND ALONG THE SERVICE NEUTRAL BACK TO THE TRANSFORMER...RIGHT?

Billhart

10:26AM | 08/19/08
Member Since: 04/25/05
1916 lifetime posts
BTW, if it works for you can go with the 3 wire install by adding a jumper between the ground and neutral bus.

You have the right ideas.

I would not try to run the equipment grounding conductor back to the ground rods at the main panel. Connect them in the panel like the other wires. Mainly because that is what everyone expects and reduces confusion later.

I have not tried to add that large of wire to conduit, but I know that the wires can twist and make it very hard to get in another.

You might be better off pull the 3 and then repulling all 4.

Be sure and use wire lub. And have some one pushing as you pull.

Might need to rig up a wood tripole with something like a boat winch to help pull the wires.

You can direct bury copper wire for an electrode called a Ground Ring. But I am not sure of the spec on to use that as an electrode. But based on that I would assume that it would be OK to run a bare COPPER wire for the equipment grounding conductor.

But that is not standard and would require cross checking many different parts of the code and I don't have time for that.

MWSBIKE

10:50AM | 08/19/08
Member Since: 08/13/08
8 lifetime posts
"I would not try to run the equipment grounding conductor back to the ground rods at the main panel. Connect them in the panel like the other wires. Mainly because that is what everyone expects and reduces confusion later."

I knew I would confuse people with what I said earlier. I was referring the the sub-panel ground rod. In other words, running a bare ground from the sub-panel ground bus out of the garage and passing by the ground rod within inches prompted me to ask. Nevermind though, I know what you mean about running a ground from one equipment ground bus in the sub-panel to the ground bus in the main panel.

Boat winch? Forget it...its getting buried. Shoot and I was looking forward to asking my wife to go buy fish tape (and explaining that it's for keeping fish together).
Click_to_reply_button Inspiration_banner

INSPIRATION GALLERY



Post a reply as Anonymous

Photo must be in JPG, GIF or PNG format and less than 5MB.

Reply_choose_button

captcha
type the code from the image

Anonymous

Post_new_button or Login_button
Register

Follow_banner_a
Newsletter_icon Google_plus Facebook Twitter Pinterest Youtube Rss_icon
 
webapp1