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Old 04-24-2011, 01:59 PM   #1
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Question about sub panel ground


10 years ago I had a workshop built off the back of my attached garage. At that time I installed a 50 amp sub panel in there.

This is a 1975 home and the main panel in the basement is a 3 wire so I followed suit and ran a 3 wire to the sub panel. My home is being sold and the home inspector told the buyer that the sub panel needs to be 4 wire.

The need for separate neutral/ground is understandable but how is this possible since the main panel is only 3 wire? I understand that a separate ground can be run but the 3rd wire already serves as ground so isn't that redundant?

Short of rewiring the entire house, how is this supposed to be handled?

Thanks for the help.

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Old 04-24-2011, 02:45 PM   #2
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Question about sub panel ground


99% of homes are a 3 wire service. The power company does not give you a ground. You create the ground when you install the service, this is done by takeing a wire to the street side of the water meter or to ground rods installed outside.


I beleive in the US you used to be able to run a 3 wire to an outbuilding and create your ground with a ground rod at the subpanel. If this is an old install and there is a ground rod at the subpanel, then it probably is a legal install and they have no ground to fight you on it.

Someone with more knoweledge of the NEC will let you know more details on this rule.

In Canada I beleive you would need a 4 wire service.

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Old 04-24-2011, 03:13 PM   #3
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Question about sub panel ground


unless a 4 wire feed to the outbuilding was required when it was installed, tell the home inspector (who is likely not a licensed electrician or even a truly qualified electrical inspector) to take a hike. If it was installed when 3 wire was legal, it is grandfathered.

unless something has since been changed that would make the install illegal.

the one situation I can think of is when 3 wire was legal, there could be no other electrically conductive pathways between the house and the outbuilding. If there is such a path, a 3 wire would not be legal even when 3 wire feeds were legal.


Quote:
I understand that a separate ground can be run but the 3rd wire already serves as ground so isn't that redundant?
No, it isn't. A ground and a neutral are different and provide different benefits.
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Old 04-24-2011, 03:42 PM   #4
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Question about sub panel ground


While your existing 3 wire sub-panel may well be code compliant under the NEC when it was installed, if the buyer wants to play hardball he can make you bring it up to current code or negotiate a price to do so. Your choice may be to comply or lose the sale.

I would tell your real estate agent that the home inspector is quashing the sale and that the installation is code compliant based on the year it was installed. Agents can place pressure on the home inspector to "recognize the error of his ways". I am not suggesting that the inspector overlook a legitimate issue but he should research the code in affect at the time of the install.

BUT it is a tough sellers market, it might be cheaper to comply than to fight.
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Old 04-24-2011, 04:04 PM   #5
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Question about sub panel ground


The OP states the workshop is off the back of the attached garage. This may not have ever been legit.
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Old 04-24-2011, 04:29 PM   #6
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The OP states the workshop is off the back of the attached garage. This may not have ever been legit.
please explain why you believe so.
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Old 04-24-2011, 07:24 PM   #7
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Question about sub panel ground


I am envisioning a house with an attached garage and the workshop is attached to the garage, ie this is not a detached structure and would not be eliglble for the 3 wire feeder exception.
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Old 04-24-2011, 08:35 PM   #8
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Quote:
Originally Posted by Jim Port View Post
I am envisioning a house with an attached garage and the workshop is attached to the garage, ie this is not a detached structure and would not be eliglble for the 3 wire feeder exception.
You might be right. As I re-read the original post, I can see where you can have that interpretation. I read it that it was not connected but I could.....


I could be.......


I just can't say it Jim. I may not be right sometimes but I just cannot admit to being wrong.

so, we need some clarification. Is this workshop physically connected to the garage which is physically connected to the house?

If the shop and the garage are attached, Jim would be correct and it would require a 4 wire feeder.
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Old 04-24-2011, 08:50 PM   #9
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Question about sub panel ground


That's right, the garage is attached to the house and the shop is attached to the garage.

Sounds like I will need to look into a grounding rod.

Thanks for your help.
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Old 04-24-2011, 09:02 PM   #10
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Question about sub panel ground


Quote:
Originally Posted by Richo View Post
That's right, the garage is attached to the house and the shop is attached to the garage.

Sounds like I will need to look into a grounding rod.

Thanks for your help.
A ground rod is for high voltage events like lightning. It has nothing to do with properly grounding a subpanel in an attached structure.

You need a 4 wire feeder to the subpanel.
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Old 04-24-2011, 10:26 PM   #11
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Question about sub panel ground


So how do you get a 4 wire feed to the panel when the main panel in the house has a 3 wire feed?
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Old 04-24-2011, 10:50 PM   #12
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Question about sub panel ground


At the service is the only place were it is allowed to bond the neutral to ground. Bonding other locations can lead to objectionable current where it should not be. You do not want current flowing on parts that should not carry voltage.
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Old 04-24-2011, 11:18 PM   #13
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Question about sub panel ground


driving a rod will not solve this problem. It was improperly installed to start with.
Inside a dwelling all feeders must be 4 wire. your service is 3 wire (all of them are) the fourth wire is actually "created" inside the main panel and must be shared to all sub panels(in attached buildings).


You therefore must add a conductor to your feeds. this conductor must be either in the same cable or conduit. it can not be run along side.
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Old 04-24-2011, 11:41 PM   #14
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Question about sub panel ground


Quote:
Originally Posted by Richo View Post
So how do you get a 4 wire feed to the panel when the main panel in the house has a 3 wire feed?
you just take 4 wires to the subpanel from the main panel. In the main panel, the ground and the neutral will be connected to the same effective point. In the subpanel, the ground and neutral will be separated. The ground will be bonded to all metal within the system while the neutral will be bonded to only the neutral bar in the panel.

there is one saviour here if it was installed in metal conduit. Was it installed in metal conduit?
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Old 04-25-2011, 07:30 AM   #15
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