Grounding a detached garage: "sub-panel" vs "new service"

[FunfDreisig]

I've been reading several threads with differing advice on whether electrical service for a detached garage requires 3 or 4 wires. IOTW whether it requires a ground wire in addition to the two hots and common.

From what I've read so far, the need for a ground wire back to the meter/main panel depends on...
1 - Whether you are complying w/ 2008 NEC code vs earlier versions.
and/or
2 - Whether you are wiring it as a "sub-panel" vs "new service".

Could someone please explain what exactly a ground wire does that is not accomplished with ground rods at the detached garage?

Thanks in Advance - Funf Dreisig

 

[walrus]

I've been reading several threads with differing advice on whether electrical service for a detached garage requires 3 or 4 wires. IOTW whether it requires a ground wire in addition to the two hots and common.

From what I've read so far, the need for a ground wire back to the meter/main panel depends on...
1 - Whether you are complying w/ 2008 NEC code vs earlier versions.
and/or
2 - Whether you are wiring it as a "sub-panel" vs "new service".

Could someone please explain what exactly a ground wire does that is not accomplished with ground rods at the detached garage?

Thanks in Advance - Funf Dreisig

A fault current is trying to get back to the source, a transformer on the pole in most cases. Most people think, electricity travels to ground, yes it can but its trying to get back to the source, Xo on the transformer. The ground doesn't do that very well, while a properly sized wire will. The grounding electrodes at a detached building are there for lightning protection. There are thousands of detached buildings in the US with 3 wire sub panels, wired correctly don't seem to be an issue.

 

[sberry]

In this world with so many utilities, phone and gas lines, etc run 4 wires. There are some cases where 3 is still legal but it is not a good idea for most homeowner type shops.

 

[FunfDreisig]

A fault current is trying to get back to the source, a transformer on the pole in most cases. Most people think, electricity travels to ground, yes it can but its trying to get back to the source, Xo on the transformer. The ground doesn't do that very well, while a properly sized wire will. The grounding electrodes at a detached building are there for lightning protection. There are thousands of detached buildings in the US with 3 wire sub panels, wired correctly don't seem to be an issue.

Thanks for the clear explanation. I hadn't considered the difference between "grounding" for lightning vs electrical fault.

Funf Dreisig

 

[FunfDreisig]

In this world with so many utilities, phone and gas lines, etc run 4 wires. There are some cases where 3 is still legal but it is not a good idea for most homeowner type shops.

FWIW I'm not trying to avoid running 4 wires. I'm just trying to understand why it is (sometimes) required now, when it wasn't before

For example, does anyone know of a case where a "3 wire sub-panel" in a detached garage caused a problem?

Funf Dreisig

 

[walrus]

FWIW I'm not trying to avoid running 4 wires. I'm just trying to understand why it is (sometimes) required now, when it wasn't before

Its always required now at least in 08 code. in 05 code one could do 3 wire as long as there were no metallic paths between the buildings. I'm assuming there were instances where metallic paths showed up after the install. Phone, ethernet, cable are all metallic paths so...

 

[Aceman]

For example, does anyone know of a case where a "3 wire sub-panel" in a detached garage caused a problem?

If that neutral were to ever come loose or develop a poor connection in a 3-wire system current will continue to try and seek a path back on anything metallic. Water lines, coax, gas pipe, etc. It has the potential to become a very dangerous situation.

 

[thdewey]

If that neutral were to ever come loose or develop a poor connection in a 3-wire system current will continue to try and seek a path back on anything metallic. Water lines, coax, gas pipe, etc. It has the potential to become a very dangerous situation.

X2 = back-up ground.

 

[sberry]

It would try to seek a path on anything metalic loose connection or not.

 

[Aceman]

It would try to seek a path on anything metalic loose connection or not.

You're absolutely right.

That's why you had to be absolutely sure there wasn't and never would be any other metallic paths between buildings fed with a 3-wire.

Or just run a 4 wire and never worry about it again.

 

[383astro]

4 wire is red/black/white and Green right? red/black is the power, white ground and green neutral? I am asking because I thought about adding a new box on my garage.

 

[Elroy]

4 wire is red/black/white and Green right? red/black is the power, white ground and green neutral? I am asking because I thought about adding a new box on my garage.

A Green neutral ??? Nope, Sorry

How about White for neutral.

Green is for ground.

 

[nadogail]

Green or Sometimes Green with a yellow stripe is Ground the World Around.

Neutrals are white or sometimes a natural gray.

When I taught Electrical Wiring some years ago. I had my students learn "Green Is Ground, The World Around" and "Black To Brass Will Save Your ***" Green and White are sacred colors, to be used only for their intended purpose.

Black, Red, Yellow, Blue, Brown, Orange & Violet are not reserved for any special use. Sometimes custom dictates that a color be used for a purpose, but custom is not law.

 

[FunfDreisig]

OK we've covered the 3 wire vs 4 wire setup for a breaker box SUB-PANEL on a detached garage and why 4 wires are better in this case. But we haven't really covered the second half of my OP....

If the detached garage was getting a "new service" (i.e. a second meter) attached to the SAME transformer there would only be 3 wires coming into the top of the breaker box -- two hot and a neutral. And there would be a bare cooper wire coming into (usually bottom of ) the breaker box that would be attached to a honkin' cooper ground rod stuck into the real ground.

So if I'm reading the comments in this thread correctly. The issue with not using a ground wire for a SUB-PANEL in a detached garage, is the risk of electricity in the detached garage seeking a "path to ground"* via some metallic path via the breaker panel to which the sub-panel is attached. IOW electricity bridging between two "circuits" (e.g. a circuit on the main panel and a circuit on the sub panel). Is that right?

And the reason that the a new service is different is that the best "path to ground" is the ground rod at the detached garage breaker panel. Not some other metallic path that connects the detached garage to another nearby structure that happens to get it's electricity from the same transformer. Is that correct?

Thanks again - Funf Dreisig

* in this case I'm using the term "path to ground" to include a path back to the power company transformer.

 

[2level]

FunfDreisig,
I don't have your answers, but I hope someone here does, and I have a related question or two. I'm
interested in the wiring configuration on service equipment (not wired as a subpanel).
When you have two 200amp panels wired directly from the meter lugs, and there is an additional metallic path between the two buildings and each panel is wired as seperate service equipment, does 2008 NEC require that 4th wire? If it's not required, should it be?

 

[ddawg16]

As I understand the code......you can have mutiple grounds.

Now it's important to understand that when the neutral comes in from the pole....you are not sure that it is at ground....that is why you have a big ground rod near the water pipe and the ground wire connected to that rod and the water pipe and it runs back to your panel where the ground and neutral are connected together. This sets up the neutral as being at ground reference.

Important point....if you run a sub panel off your main panel, the neutral in the sub does NOT get tied to the earth ground in the same panel. They want only one reference for neutral....but it's ok to have another ground rod at the sub...you just don't connect the neutrals and grounds together at that panel.

However, if you have 'new' service from the pole...you bring up a good point. I'm going to assume that since you now do NOT have a connection between that new service and your existing panel at the house, it would be treated as if it was your neighbors house...hence, your own ground and the netrual and ground would be tied together.

Need one of the experts to chime in and confirm or correct.

 

[walrus]

As I understand the code......you can have mutiple grounds.
.

Multiple grounds all bonded together

 

[jtillery]

I have a related but slightly different question. There are 2 buildings on my property (house and a commercial building), both have seperate electric services. I am upgrading the panel in the commercial building, and saw a couple things that didn't make sense to me.

In the commercial building, I have a 200 amp disconnect that is fed from the meter (2 hot wires and a bare ground). The panel is within 1 foot of the disconnect and the neutral and ground are not bonded. It appears that the bare ground goes to power company's transformer, but I am not 100% sure. The old panel had a bare ground that went to the main water line where it came into the building.

The house has a 100 amp main breaker panel that is fed from a seperate meter, it is also grounded to the water line. I could not find any indications of a ground rod for this service either.

Is it acceptable to have the grounds from the 2 services bonded? The water service is shared between the house and commercial building so it would be bonded there. I thought I saw an aritcle in the NEC about this situation, but I spent an hour or two looking again and couldn't find it. Also, I am trying to figure out if I should have a ground rod for each building.

 

[walrus]

They may have been code when installed without ground rods. If you water line is copper and travels any distance its probably a better ground than a rod would be.

 

[jtillery]

They may have been code when installed without ground rods. If you water line is copper and travels any distance its probably a better ground than a rod would be.

You brought up a good point, I think the last time it was inspected was back in the 50s when the house and building were built. The water line comes up in the center of the building, so it is probably 100-125' from the house. It is an 1 1/2'" line.

It sounds like it would be an acceptable ground for a signle service, but is it safe to have the house and the building grounded to it, since they are seperate electric services?

Thanks

 

[kbs2244]

You will have 3 wires coming from the pole.
The inside the building ground/netural wires are joined at the breaker box and run on the third wire out to the pole.
At the pole they run down the side of the pole to the base of the pole hole.
But there is also a ground rod or plumbing connection as a back up.
The code is depending on the power co to have a good connection from the breaker box and the bottom of the pole.

 

[Stargeezer]

FYI- I'm getting very close to having my garage electrical finsihed. This weekend should see it done! It was a 75 foot underground run to a detached shop. 100 Amp panel in garage from a 200 main panel out on the pole that feeds 100 amps to the house too.

Everything was grounded. 3 wires plus #4 green ground wire in the buried conduit, ground wire from shop panel to twin 20 foot rebar grounds under the garage slab, and ground wire connected to the metal building frame. My electricain grounded it three ways; plus using metal conduit throughout the shop as well.

 

[oldgoat]

If I remember right when they wired my garage they moved service to the garage and split 100 amp to the garage and another 100 to the house and replaced the box there also. None of it was ran through metal conduit, just the gray plastic. I know when it was inspected the inspector had them add another ground rod and told them that they didn't need to run 4 wires. After he left the owner said that it always seems to depend on the inspector which way is right though. He didn't agree with it, but he did change it to what the inspector wanted.

 

[SportFury59]

Going to start on electrical soon on my new detached garage. When doing concrete floor had to have rebar come up through floor near panel box so ground is to rebar. Inspector said rods in ground are no longer accepted on new construction. My concrete guy was unaware of this code and "resisted" doing this but I told him the inspector was adamant on this. The inspector told me most concrete guys he comes in contact with are resisting this rule. He said rule came into law Sep '08.

 

[mrb]

thats called a UFER ground, basically if you have steel in the slab you have to stub a piece of the rebar up out of the footing and bond it.

 

[walrus]

Going to start on electrical soon on my new detached garage. When doing concrete floor had to have rebar come up through floor near panel box so ground is to rebar. Inspector said rods in ground are no longer accepted on new construction. My concrete guy was unaware of this code and "resisted" doing this but I told him the inspector was adamant on this. The inspector told me most concrete guys he comes in contact with are resisting this rule. He said rule came into law Sep '08.

What kind of slab? Is it insulated? Footers are where the UFER is supposed to come from

 

[bimmer1980]

I've always thought that the grounding capabilities of concrete are less than desirable. I can understand tying the rebar into the grounding system, but to only use the rebar as the ground seems like an oxy *****.......

FYI, "fine home building" has a great book on wiring stuff that I have found extremely helpful. the author covers may of the electrical service and sub panel questions......