Thread: sub panel grounding

The house has two buildings, a main house and a garage/ guest house that are attached. The service panel for the main house feeds (3 wire - no ground) a 60 amp sub panel in the garage. The guest quarters (attached to the garage) has a separate power drop, so the main and garage are on one drop and the guest quarters is on a separate drop. The Sub panel for the garage is grounded with the ground bar bonded to the neutral bar (I believe this part is correct as a neutral fault would need sufficient current traveling through the ground to trip the breaker. (In a 4 wire feeder system, the ground and neutral are isolated in the sub panel). Here is the rub. The equipment ground for the sub panel for the garage is attached to the ground rods for the service panel for the guest quarters. So, the main house panel is grounded to a UFR and 100 feet away, the sub panel for the garage (fed by the main panel) is grounded by the same two ground rods that ground a separate service panel for the guest quarters. There are no metalic lines or pipes connecting the main house and the garage/guest house.
So, Specifically what are the dangers. I do not have enough knowledge about grounding systems to know if this is a real problem or not. My concern was that, since the ground and neutral were bonded in the sub panel, and those, in turn were bonded to the same ground as the other service panel, current might flow through the ground of the sub panel and energize the ground wire in the circuits for the guest house. But, I wondered if the ground rods would prevent this by directing any extra current into the ground and not into the ground wire of the guest quarters circuit. If there is a problem, what are my options to make the system safer?
I know this is long and confusing, but any help would be greatly appreciated.
Thanks,
Sam

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I'm no expert but 2 things I can tell you. Both ground wires have 0 voltage potential, both are at ground potential. There can be no shock hazard.

Nevertheless, you should always comply with the code standards when it comes to electrical safety, so a look at the code book is called for.

#2 It is not correct to ground the neutral in the remote location. Someone else can explain why. I believe the neutral bus should be isolated from the panel and ground. It is the same configuration as a 4-wire system, with only the location of the ground contacts separated. JMO

I'm obviously no expert, either. I thought the ground and neutral should be separated at the panel (like in a four wire system), But, from what I've read, then a neutral fault would go to ground only, and not through the neutral to the transformer (as in a 4 wire system). Since the ground has a fairly high resistance, it might prevent the breaker in the sub panel from tripping. This could lead to an over load on the wires. That's why the ground and neutral are bonded at the sub panel (still could be wrong on this, though). I guess a lightning strike would automatically go to ground because that is where they originate. But, since the resistance in the ground is greater than the resistance in the wires, I am confused as to whether an open neutral in the sub panel circuit could, in fact, energize the ground wire in the guest quarters (since both the sub panel and separate guest quarters panel are connected to the same ground rods). Thanks John for taking the time. You were the only one to respond.
Sam

Originally Posted by John Kogel

I'm no expert but 2 things I can tell you. Both ground wires have 0 voltage potential, both are at ground potential. There can be no shock hazard.

You'd be surprised. Do a little research on "ground loops." If the two ground sources are separated by a significant enough distance you can find yourself with a not insignificant voltage potential.

Originally Posted by sam sanders

I'm obviously no expert, either. I thought the ground and neutral should be separated at the panel (like in a four wire system), But, from what I've read, then a neutral fault would go to ground only, and not through the neutral to the transformer (as in a 4 wire system). Since the ground has a fairly high resistance, it might prevent the breaker in the sub panel from tripping. This could lead to an over load on the wires.

Circuit breakers open when the current flow exceeds the operating design. It doesn't matter if that current is to neutral, to ground, or to another hot leg (as in a 240V device). If you want a circuit to open on a path fault not involving an over-current situtation you'll need a GFCI or AFCI breaker.

That's why the ground and neutral are bonded at the sub panel (still could be wrong on this, though).

Yes, you are.

I guess a lightning strike would automatically go to ground because that is where they originate. But, since the resistance in the ground is greater than the resistance in the wires, I am confused as to whether an open neutral in the sub panel circuit could, in fact, energize the ground wire in the guest quarters (since both the sub panel and separate guest quarters panel are connected to the same ground rods). Thanks John for taking the time. You were the only one to respond.
Sam

Lightning will take ALL possible paths to resolve the voltage potential. In the situation you describe, that would include back voltage on the neutral conductor to the garage/guest house where it would be resolved to the additional earth ground located there.

Originally Posted by Corn Walker

You'd be surprised. Do a little research on "ground loops." If the two ground sources are separated by a significant enough distance you can find yourself with a not insignificant voltage potential.

I know what a ground loop is. He is discussing two ground leads terminated at one point, no loop there.

Good point on the breaker stuff. I think the poster might be confusing normal breaker operation with GFCI or AFCI operation.

Originally Posted by John Kogel

I know what a ground loop is. He is discussing two ground leads terminated at one point, no loop there.

From his description it appears there is one grounding point at the main house, a 3 wire feeder to a remote panel where the neutral from the first panel (which I presume was bonded to ground) is bonded together with the ground which is fed from yet another panel located in the guest house. That creates a potential for a ground loop, with the earth serving as one loop leg and the neutral conductor the other. However I'm not clear on the distances involved, particularly between the garage panel and the guest house panel, and between the guest house panel and the grounding system.

It's probably nothing to worry about, but I had a situation with two building separated by a few hundred feet where the shielding from a phone feeder cable was bonded to ground in both buildings and created a ground loop with enough current for you to feel it.

"Re: sub panel grounding"

What a bummer ... grounding a sub is rough on its hull, yeah, that certainly can ruin some hull panels.

[quote=Corn Walker;133204]From his description it appears there is one grounding point at the main house, a 3 wire feeder to a remote panel where the neutral from the first panel (which I presume was bonded to ground) is bonded together with the ground which is fed from yet another panel located in the guest house. That creates a potential for a ground loop, with the earth serving as one loop leg and the neutral conductor the other. However I'm not clear on the distances involved, particularly between the garage panel and the guest house panel, and between the guest house panel and the grounding system.

That's right. Except the ground wires from both the garage sub panel and the guest house service panel are both connected by acorns to the same two copper ground stakes (the ground wire for the sub panel doesn't run through the Guest house panel prior to entering the ground. The ground wires from the two panels are only connected in the ground via the stakes). The distance between the Main house panel and the Guest house service panel is about 80 feet. the sub panel is another 15 feet beyond the Guest house panel. The ground stakes for the sub and guest panels are between these two panels (guest house and sub). There are no metallic connections that connect the guest house/garage and main house (no phone lines, pipes, etc).

I'm learning while you are discussing this. Thanks

sidebar: Jerry, why demean your vast knowledge of the NEC with your petty "sub" nomenclature shenanigans? everybody says it- get over it!

To avoid ground loops only ground the neutral at one place, preferably the main entrance panel. A lot depends on the resistivity of the earth around that specific area. Also, the distance between the panels which would then determine whether each place should have its own independent system. Since it sounds like all these buildings are all grounded at the same ground rod system then i would keep all neutrals together and only ground the neutral at one place, the entrance panel. I hope this makes sense.

Originally Posted by Mike Locurcio

sidebar: Jerry, why demean your vast knowledge of the NEC with your petty "sub" nomenclature shenanigans? everybody says it- get over it!

Because SO MANY QUESTIONS posted here are so simply answered when one simply discards the "sub" and simply thinks "service equipment" or "other-than service equipment".

That ONE SIMPLE change in thinking clears up a lot of misconceptions and thinking problems - so - get over it! And stop using the "sub" term, many questions will have answers which become self-evident when that happens.

So, as you stated ... Get over it!

It also helps when there are paragraphs, but I did not go there - but paragraphs DO make it easier to read and understand what is being written and asked.

To Mike:

Paragraphs, like this:

Originally Posted by sam sanders

The house has two buildings, a main house and a garage/ guest house that are attached.

The service panel for the main house feeds (3 wire - no ground) a 60 amp sub panel in the garage.

The guest quarters (attached to the garage) has a separate power drop, so the main and garage are on one drop and the guest quarters is on a separate drop.

The Sub panel for the garage is grounded with the ground bar bonded to the neutral bar (I believe this part is correct as a neutral fault would need sufficient current traveling through the ground to trip the breaker. (In a 4 wire feeder system, the ground and neutral are isolated in the sub panel).

Here is the rub. The equipment ground for the sub panel for the garage is attached to the ground rods for the service panel for the guest quarters.

So, the main house panel is grounded to a UFR and 100 feet away, the sub panel for the garage (fed by the main panel) is grounded by the same two ground rods that ground a separate service panel for the guest quarters. There are no metalic lines or pipes connecting the main house and the garage/guest house.

So, Specifically what are the dangers.

I do not have enough knowledge about grounding systems to know if this is a real problem or not. My concern was that, since the ground and neutral were bonded in the sub panel, and those, in turn were bonded to the same ground as the other service panel, current might flow through the ground of the sub panel and energize the ground wire in the circuits for the guest house.

But, I wondered if the ground rods would prevent this by directing any extra current into the ground and not into the ground wire of the guest quarters circuit. If there is a problem, what are my options to make the system safer?

I know this is long and confusing, but any help would be greatly appreciated.
Thanks,
Sam

Now, going through the above brings up the following:

Originally Posted by sam sanders

The house has two buildings, a main house and a garage/ guest house that are attached.

The service panel for the main house feeds (3 wire - no ground) a 60 amp sub panel in the garage.

The guest quarters (attached to the garage) has a separate power drop, so the main and garage are on one drop and the guest quarters is on a separate drop.

Okay, what we have in the above is: a) there are two buildings; b) 1 building is supplied by TWO SERVICES - which is not allowed.

The Sub panel for the garage is grounded with the ground bar bonded to the neutral bar (I believe this part is correct as a neutral fault would need sufficient current traveling through the ground to trip the breaker. (In a 4 wire feeder system, the ground and neutral are isolated in the sub panel).

Here is the rub. The equipment ground for the sub panel for the garage is attached to the ground rods for the service panel for the guest quarters.

Which is already a problem as that one structure (that one building) should only have ONE service to it.

So, the main house panel is grounded to a UFR and 100 feet away, the sub panel for the garage (fed by the main panel) is grounded by the same two ground rods that ground a separate service panel for the guest quarters. There are no metalic lines or pipes connecting the main house and the garage/guest house.

The first question which pops up is how does Sam know the house is grounded to a concrete encased electrode? Is that just an assumption?

Not that it matters that much, the second structure (the building containing the garage and the guest house) is served by two services, which is not allowed - which means the entire installation needs to be gone over by a licensed and competent contractor (which obviously excludes the contractor who wired it - IF a contractor wired it that way).

The first question which pops up is how does Sam know the house is grounded to a concrete encased electrode? Is that just an assumption?

Not that it matters that much, the second structure (the building containing the garage and the guest house) is served by two services, which is not allowed - which means the entire installation needs to be gone over by a licensed and competent contractor (which obviously excludes the contractor who wired it - IF a contractor wired it that way).

[/QUOTE]


I bought the house and built the addition on the main house, upgraded the service on the main house and grounded it to the rebar in the footings in the addition. The three wire feeders to the garage were already there, just hooked them up to the new main panel. This set up allowed the garage to be on the same power as the house so the renter in the guest house would not be paying for electricity to the garage (to which they do not have access. Since I now have a shop in there, it is a significant amount of electricity.

I had a licensed electrician do the work, but the feeders/panel to the garage were existing so the city inspector didn't look at the configuration too closely. Electrician says it's fine (but such statements often have nothing to do with code).

I thought it didn't look right. Thought I'd pick some other brains. I was hoping there was a safe way to keep the existing service set up.

Disclosure: Sometimes I read the NEC and I get a little confused translating the rules into certain field situations. (e.g., "You can't have two services to the same building." I interpreted that as two main services and just didn't recognize that the sub panel set up fell under this statement - can't explain why). Sometimes the most obvious elements of a situation escape me when focused on a problem - stupid brain.

So, does anyone have any suggestions (besides get a new electrician) to get the main house and garage on the same meter and the guest quarters on a separate meter while maintaining code compliance?

Originally Posted by sam sanders

So, does anyone have any suggestions (besides get a new electrician) to get the main house and garage on the same meter and the guest quarters on a separate meter while maintaining code compliance?

Single service drop, dual meters. Assuming this is a legal ADU you shouldn't have a problem getting them to set up a second meter point for the garage. You would then abandon the feeder between the main house and the garage.

Alternatively, get an average of the rental unit's electricity usage for a year and include it in the monthly rent. Then abandon the feeder from the main house to the garage/shop and instead feed it from the ADU service equipment.

You could also install a private meter and bill the tenant for their usage.

Google E-mon for an idea.

A fire separation wall effectively makes this situation 'two buildings', electrically speaking. This is sometimes the simplest way to rectify a mess. Just a fyi note.
Bob Smit, County EI

Originally Posted by bob smit

A fire separation wall effectively makes this situation 'two buildings', electrically speaking. This is sometimes the simplest way to rectify a mess. Just a fyi note.
Bob Smit, County EI

Bob,

Do do that properly, after the fact, is not always easy. (bold and underlining is mine)

Building. A structure that stands alone or that is cut off from adjoining structures by fire walls with all openings therein protected by approved fire doors.

Note that it specifically states "fire walls", which does not include any of the lesser variations such as "fire partitions", neither of which are likely to be found, or even constructed, in a dwelling unit or it garage.

FIRE PARTITION.

A vertical assembly of materials designed to restrict the spread of fire in which openings are protected.

FIRE WALL.

A fire-resistance-rated wall having protected openings, which restricts the spread of fire and extends continuously from the foundation to or through the roof, with sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall.

Jerry; That is why I stated, "sometimes" and "FYI".
Just informing our viewers of the option. I have suggested this, albeit in commercial applications, and the option was utilized at a cost/labor savings by the Builder and/or Electrical Contractor.

One should note that the separation usually runs 'deck to deck': as in lowest floor to roof.
Bob Smit, County EI

Originally Posted by sam sanders

The Sub panel for the garage is grounded with the ground bar bonded to the neutral bar (I believe this part is correct as a neutral fault would need sufficient current traveling through the ground to trip the breaker.)

No. The sub panel HAS to have ground and neutral separate - don't connect them. (But separate them, in your case.) Make sure the ground bus is mechanically connected to the sub panel enclosure and that neutral bus is not mechanically connected to enclosure

Originally Posted by sam sanders

The equipment ground for the sub panel for the garage is attached to the ground rods for the service panel for the guest quarters.

That is OK as long as the ground bus and neutral bus in the sub panels are not connected together.

The neutrals and grounds should only be connected together (bonded) in ONE LOCATION, this location is the main disconnect, usually your main panel (not always). They should never be bonded anywhere on the load side of the main service.

Originally Posted by sam sanders

So, Specifically what are the dangers.

The way I understand the wiring in the sub panel as you have it, neutral & ground connected inside the sub panel, if the feed neutral coming from the house were to disconnect you'll have neutral return current flowing on ground in the garage. Right now the flow is hot to neutral. If the neutral is cut you'll have hot to ground, which you don't want. Oh it could cause fire, electrocution, death. So, def separate the ground & neutral in the subs.

Originally Posted by bob smit

Jerry; That is why I stated, "sometimes" and "FYI".

I got that, but the ease of doing that is not as one would think.

Just informing our viewers of the option. I have suggested this, albeit in commercial applications, and the option was utilized at a cost/labor savings by the Builder and/or Electrical Contractor.

One should note that the separation usually runs 'deck to deck': as in lowest floor to roof.

When done correctly, it takes a lot more than just that described above, it takes re-engineering the entire structure so that ...

Originally Posted by Jerry Peck

FIRE WALL.

... with sufficient structural stability under fire conditions to allow collapse of construction on either side without collapse of the wall.

... the "structure" on one side of the fire wall (the fire side) can collapse without collapsing the fire wall or the structure on the other side of the fire wall (the non-fire side of the wall).

Just wanted to point that out so everyone would consider the total costs, engineering, and work involved in doing that (yes, it can be done, but at great cost).