Confused on Ground / Neutral Bonding

[kmacht]

I started the wiring of my garage this weekend. I have everything run except for the service feed wire from the house to the sub panel. I am confused on one thing though. The sub panel is in a detached garage. I thought that you had to have the neutral and the ground seperate from each other. The panel I have has the neutral bar on the left side, a ground bar on the right side and a large plate that goes across the top that connects them to each other. The center of that plate has a hole in it that will allow you to bond it to the sub panel box. I thought that I could just remove this plate to seperate the ground and neutral but it appears to be holding the ground and neutral bar in place. So what am I missing? Did I understand this wrong and the bar is ok, just without the bonding screw? Do I have to cut the bar? Help.


Keith

 

[kmacht]

I have a secondary/followup question. In trying to figure out my grounding question by searching the net I also found a few posts that stated if a detached structure is closer than 6' to the main structure then it is considered attached. The wall of my garage closest to the house is 5' away. So do I still wire the building as detached (i.e. driving new ground rods 8' deep seperated by 6') or do I wire it as attached and just run a ground wire from the house panel?

Keith

 

[mrb]

you run a ground wire from the house to the garage, this is never optional. As far as 6ft and ground rods or not, i am not aware of this rule. Can someone cite a code section?

as far as the neutral / ground bar -what do the instructions for your panel say? Can you post a picture? It could be the jumper is made to be removed, or that could just be your neutral bar and you need to install a ground bar.

 

[kmacht]

The instructions were pretty non existant. The only thing it had in there was how to mount it to the wall (where the screws go) and that there was a green bonding screw that could be installed if required by code. I think you are right in that I will need a seperate grounding terminal. I will see if I can get a picture tonight.

Keith

 

[trbomax]

The separate ground buss must be bought seperate.You can get them at big box,they fit int little lugs on the back of the panel and attach with 1 or 2 self tapping screws. There is usually a green screw in the panel that connects the nuetral to the panel . That should be removed if you are useing it as a sub.

edit) the 4th wire (ground) goes to the separate ground buss.

 

[Charles (in GA)]

The panel I have has the neutral bar on the left side, a ground bar on the right side and a large plate that goes across the top that connects them to each other. The center of that plate has a hole in it that will allow you to bond it to the sub panel box. I thought that I could just remove this plate to seperate the ground and neutral but it appears to be holding the ground and neutral bar in place. So what am I missing? Did I understand this wrong and the bar is ok, just without the bonding screw? Do I have to cut the bar? Help.


Keith

Is both of the bars insulated from the box, probably with plastic underneath them?

This sounds like one of the newer, "fancy dancy" panels that has a neutral bar on either side for ease of wiring installation. If they both are insulated from the box/cabinet, and the only way of grounding either of them to the cabinet is via the green screw, then its apparent that they are both neutral bars, and you will need to add a ground bar screwed directly to the cabinet.

What brand and model is it?

Charles

 

[BillK]

Keith,
Your electrical inspector should be the one to ask, he will have to sign off on the permit. You did get a permit ?

 

[kmacht]

The panel is a GE sold at Home Depot. I don't know the model number off the top of my head but it is sold as a contracters pack with 5 breakers and the 100 amp main breaker included. I did some more investigating last night after looking through your posts. It does appear that both sides will be neutral. There was a ground symbol next to some screw holes to the right of the neutral bar. I bought a ground bar kit last night and installed it there. I also removed the green screw bonding the neutral bar to the metal frame. I think I am good to go.

The only open question I have is on the requirement for a seperate set of ground rods installed 6' apart and wired to the sub panel. Can someone explain the logic behind this? If I already have a ground wire and a neutral wire going back to the main panel then why do I need a second ground?

Keith

 

[avc8130]

Keith,
Easiest thing is to call the town inspector and ask him if ground rods are required. Your "detached but close" condition is where the confusion comes in. Call, ask, do, inspect, pass, enjoy.
ac

 

[dipper]

I've never heard of a 6' rule, detached is detached. Best bet is to call the inspector and do what he wants. It seems there is a different way to hook up a subpanel depending on who you talk to and the ground bar/rod requirements. I found the best way is to do it exactly the way the inspector wants. that will always make the inspection go easier.

 

[jhelrey]

When you hook up the wires from the panel in the house to the garage, this is the order.

Hook up black to the breaker, white where the others are hooked, and ground on the bar. White and ground might be put in the same place.

When you do the garage, hook up black to the breaker, neutral to one side, ground to the other. Break the connection between the two. Even then, you don't technically have to. I have never gotten an explanation why but it's how I have always wired up panels.

Pull the cover to the panel in your house and you will see what I mean.

 

[kmacht]

I have the wiring down as to what goes where, what I am looking for is an explanation of why a second grounding point (2 ground rods 6' apart) are needed. That is what code calls for and what the inspector will want, I am just curious as to why.

Keith

 

[avc8130]

Oh, so you accept the fact that you have to install 2 ground rods for the garage...but you want to UNDERSTAND why?

Screw it, just do it. Get it inspected and enjoy your new power. Leave the "why" to the smart folk that write the codes. LOL

ac

 

[Stuart in MN]

Here's the short answer to the question: a single rod is permissible, but only if it has a resistance to the earth of 25 ohms or less. Getting that low a resistance can be difficult, plus it's not a simple matter to be able to measure it accurately. Article 250.56 of the NEC says that if a single rod doesn't meet that requirement, a second rod shall be installed. Nine times out of ten, it's easier to just go ahead and install two rods to begin with.

 

[kmacht]

Yeah, I accept that I have to put two rods in because I don't have the equipment to check to make sure 1 is enough. The problem is that here in connecticut, half of the state is sitting on top of granite. I have had a heck of a time digging anything deeper than the topsoil goes. I am really not sure how I am going to get two rods to go down 8'. I still don't understand why it is needed though. Why isn't the ground going back to the house panel enough? I was hoping that someone who understands the reasoning behind the NEC could shed some light so at least I don't feel like I am wasting my time just to meet a regulation that does nothing for functionality or safety.

Keith

 

[mrb]

not finding fault with anyone in particular, but it seems like everyone here is in a hurry to build their garage, so they pour concrete, put up a building, and then try to figure out the electrical. If the electrical were considered when beginning construction, you would stub up a piece of rebar (UFER ground) from your footing, bond that, and not even have to mess with ground rods. Actually if a ufer ground exists (you have a footing with rebar in it) you HAVE to bond it -not optional, but it seems like most inspectors on garage projects never make mention of it.
/rant

 

[Stuart in MN]

There are other options for places with rocky soil. The rods can be driven in at a 45 degree angle, or if even that isn't enough they can be buried in a 30 inch deep trench. You can also use a plate electrode, a grounding ring, or as mrb pointed out, for new construction an Ufer ground embedded in the footing is standard practice these days.

The thing is, like most electrical questions there isn't necessarily only one correct answer. The grounding section of the NEC is pretty complex, plus the advice you get on the Internet is worth what you pay for it. It's worth a call to the local inspector to see what will be acceptable in your particular area.

 

[kmacht]

That is exactly what I did when I poured the concrete pad and wired our hot tub. I added a piece of rebar that stuck through the top and was able to use that. The garage was different. I had an existing attached garage that I tore down, poured an extension to the slab and put up a much larger detached garage. I did put rebar in the extension to the slab but there was no way for me to find where the existing rebar was in the old slab and therefore no way to tie the extension and rebar into it and have a stub sticking up. I hope that makes sense. The old slab was poured over 50 years ago and I have no way to even tell if there is rebar in it or not.

What is really confusing me is why I need a second ground. Why isn't the ground wire running back to the house box sufficient? I can see the reasoning for needing if back when a 3 wire feed from the house to a sub panel was legal, if the neutral broke, you needed a ground path or else you become the ground path. With a neutral and a ground run to the box, it would take both wires being broken to cause a short.

Keith

 

[kmacht]

I already talked to the inspector. He wants the two grounding rods. He couldn't tell me why other than that is what the code requires. He did mention the 40 degree angle but the chances of me not hitting a rock at any angle for 8' is pretty slim.

Keith

 

[fefarms]

I already talked to the inspector. He wants the two grounding rods. He couldn't tell me why other than that is what the code requires. He did mention the 40 degree angle but the chances of me not hitting a rock at any angle for 8' is pretty slim.

Keith

You are trying to satisfy two different provisions of the NEC.

1) Every feeder or branch circuit requires an equipment grounding conductor. This can be satisfied by metal conduit or a separate dedicated wire. In your case this provision is satisfied by the fourth wire that you are running back to the service entrance panel. The intention here is to provide a fault current path to avoid energizing metallic objects in contact with live wires, and ensure tripping of circuit breakers proteting the feeder or branch circuit.

2) Every BUILDING (provided with more than a single 20 amp branch circuit) requires its own grounding electrode means. This is what is causing the requirement for ground rods. Other exposed metal in the building (such as metallic plumbing, conduit, and electrical panels) is required to be bonded to these ground rods. The equipment grounding conductor is required to be bonded as well. The intention here is to provide a low impedance path for sudden electrical surges, such as might be induced by nearby lightning strikes.

 

[RUSSDON11817]

I know this is an old thread, but the answer to the question "Why is it necessary to provide (2) separate ground rods if the ground is already in the subfeed from the main panel?" is as follows:

1. The NEC and the local Authority Having Jurisdiction require that all remote buildings be grounded through both a grounding conductor included in the sub-feed AND a separate grounding electrode system consisting of 2 or more of the following: metal cold water pipe, UFER concrete encased electrode, or ground rod. A second, supplemental ground rod may be installed if there are not any other electrodes present to constitute a "system".

2. Equipotential plane (equal electrical potentiality between multiple objects or surfaces). Enough resistance may exist in the grounding conductor included with the sub-feed that a difference in electrical potential may exist between the outbuilding and the main building's electrical grounding systems. This could present a shock hazard if a person is able to touch both building's metallic frames or electrical parts at the same time--either directly or indirectly through a conductive material that may span to both structures (i.e. an extension cord, concrete slab, water, etc). By installing a separate grounding electrode system, the equipotential plane is established, ensuring any difference in potential to ground is eliminated or reduced.

That's the whole truth on the matter.

 

[DekeT]

It's simple. You need 2 ground rods because the codes are written by the electrical supply industry and not by the electricians.

 

[wyliesdiesels]

.....What is really confusing to me is why I need a second ground. Why isn't the ground wire running back to the house box sufficient? I can see the reasoning for needing if back when a 3 wire feed from the house to a sub panel was legal, if the neutral broke, you needed a ground path or else you become the ground path. With a neutral and a ground run to the box, it would take both wires being broken to cause a short.

Keith

You are confusing 2 different things that have 2 different functions. A ground rod @ a detached building is for things like lightning strikes. The egc/ground wire running back to the service main is there so that objectionable currents are kept from other grounding paths/metal that runs between the buildings! A 3 wire feed is still allowed as long as there is no other path! (NEC 250. )

A neutral is not part of the GES and neutrals should NOT be used for grounding! So if a neutral broke and its a 3 wire feed with no alternate grounding paths, there would be no way for u to become the ground path! This is also the reason why neutral and ground should NEVER be bonded in a sub panel because if the neutral did break and the egc was bonded to the neutral then there could be current flowing over the egc and possibly on metal fixtures such as faucets, water pipes, etc.

I'm not sure what u mean by "it would take both wires being broken to cause a short." That doesn't make sense! A short to what? When a wire is broken, its called open not short! A short would be like when a neutral contacts a hot!

 

[ddawg16]

Yea....old thread....good subject....

For anyone now building a garage....chances are the AHJ will accept the Ufer ground....

And for a detached garage...seperate neut and earth...

 

[MrMark]

I know this is an old thread, but the answer to the question "Why is it necessary to provide (2) separate ground rods if the ground is already in the subfeed from the main panel?" is as follows:

1. The NEC and the local Authority Having Jurisdiction require that all remote buildings be grounded through both a grounding conductor included in the sub-feed AND a separate grounding electrode system consisting of 2 or more of the following: metal cold water pipe, UFER concrete encased electrode, or ground rod. A second, supplemental ground rod may be installed if there are not any other electrodes present to constitute a "system".

2. Equipotential plane (equal electrical potentiality between multiple objects or surfaces). Enough resistance may exist in the grounding conductor included with the sub-feed that a difference in electrical potential may exist between the outbuilding and the main building's electrical grounding systems. This could present a shock hazard if a person is able to touch both building's metallic frames or electrical parts at the same time--either directly or indirectly through a conductive material that may span to both structures (i.e. an extension cord, concrete slab, water, etc). By installing a separate grounding electrode system, the equipotential plane is established, ensuring any difference in potential to ground is eliminated or reduced.

That's the whole truth on the matter.

Never have heard about this NO. 2 at all. The grounding electrode system is for lightning, to reference the neutral to earth, to dissipate static and something else that I can't remember. I think you are thinking of something else.

Do you have a cite?

 

[Gooch]

Never have heard about this NO. 2 at all. The grounding electrode system is for lightning, to reference the neutral to earth, to dissipate static and something else that I can't remember. I think you are thinking of something else.

Do you have a cite?

He is correct. It is for both.

 

[MFortie]

Well, the two ground rods have to do with grounding potential. My inspector explained that the dampness of the soil plays a big part in providing the grounding. Here in the dry Ca 'desert', it's common practice (and required) to install two grounding points (rods, cold water, UFER).

 

[Norcal]

Well, the two ground rods have to do with grounding potential. My inspector explained that the dampness of the soil plays a big part in providing the grounding. Here in the dry Ca 'desert', it's common practice (and required) to install two grounding points (rods, cold water, UFER).

If metallic water line of sufficient length exists it must be used as a grounding electrode, this is not optional.

Edit: If it is there it has to be used, to put it short & simple.

 

[wyliesdiesels]

Never have heard about this NO. 2 at all. The grounding electrode system is for lightning, to reference the neutral to earth, to dissipate static and something else that I can't remember. I think you are thinking of something else.

Do you have a cite?

No 2 is correct! I've been shocked before because of this exact scenario!

Wow! I didn't realize this was such an old thread! Would be interesting to find out how the OP ended up doing his wiring since he had so many misnomers about electricity which were corrected by so many above! Too bad we didn't see it back then!

 

[MrMark]

He is correct. It is for both.

I seriously doubt it.

Do YOU have a cite?

He is confused on the requirement of tying ALL grounding electrodes together under separate systems. This has nothing to do with the requirement of ground rods or two ground rods; just that if you have multiple ground electrodes you must tie them all together to avoid differences in potential which create current flow if something conductive is interposed. That is absolutely not their purpose - to create some equiplane or whatever was posted.

 

[MrMark]

http://ecmweb.com/content/bringing-grounding-down-earth

Here's a cite.

 

[MFortie]

If metallic water line of sufficient length exists it must be used as a grounding electrode, this is not optional.

Edit: If it is there it has to be used, to put it short & simple.

Yup, agreed. Most our stuff down here seems to be PVC though. I should say, most of MY stuff that I've run across...

 

[Speedy Petey]

No.2 is absolutely NOT correct. Or at least I should say it is not accurate.

Grounding electrodes have NOTHING to do with touch potential.

The site MrMark linked is great. The "Grounding vs Bonding" articles are a wealth of information.

 

[kossuth]

That is exactly what I did when I poured the concrete pad and wired our hot tub. I added a piece of rebar that stuck through the top and was able to use that. The garage was different. I had an existing attached garage that I tore down, poured an extension to the slab and put up a much larger detached garage. I did put rebar in the extension to the slab but there was no way for me to find where the existing rebar was in the old slab and therefore no way to tie the extension and rebar into it and have a stub sticking up. I hope that makes sense. The old slab was poured over 50 years ago and I have no way to even tell if there is rebar in it or not.

What is really confusing me is why I need a second ground. Why isn't the ground wire running back to the house box sufficient? I can see the reasoning for needing if back when a 3 wire feed from the house to a sub panel was legal, if the neutral broke, you needed a ground path or else you become the ground path. With a neutral and a ground run to the box, it would take both wires being broken to cause a short.

Keith

Well, the reason you want the separate ground for the detached garage is many fold. Here are few.

1. Your building has lightning potential. You want that potential to reach earth through your earth ground vs coming back into the house and if that arch gets into your main panel well..... Game on sparky .

2. You and your family's safety. A circuit breaker works off the concept that if a current exceeds a safe level it trips, well it trips. When the breaker trips is based off the amperage it's rated for, but many things dictate what size of wire is used for what. IE a 20 amp circuit generally uses 12 AWG wire, but in longer run situations you would want to step the wire up to 10 AWG to reduce resistance and heat. Every wire has resistance, and generally the larger the wire the less the resistance and the shorter the wire the less resistance. Being breakers work off the idea of detecting a short to ground (a lack of resistance between the hot and ground conductor) could in theory trick a breaker into not tripping when in reality the load is excessive and is burning your wiring up (hopefully not your garage with it).

There's alot more to it, but those are two major reasons right there.

 

[Speedy Petey]

2. You and your family's safety. A circuit breaker works off the concept that if a current exceeds a safe level it trips, well it trips. When the breaker trips is based off the amperage it's rated for, but many things dictate what size of wire is used for what. IE a 20 amp circuit generally uses 12 AWG wire, but in longer run situations you would want to step the wire up to 10 AWG to reduce resistance and heat. Every wire has resistance, and generally the larger the wire the less the resistance and the shorter the wire the less resistance. Being breakers work off the idea of detecting a short to ground (a lack of resistance between the hot and ground conductor) could in theory trick a breaker into not tripping when in reality the load is excessive and is burning your wiring up (hopefully not your garage with it).

OK, but the earth, and earth grounding has NOTHING to do with this.
Breakers don't detect a short to "ground". They detect a short to the bonded grounding conductor, or bonded metal parts. The voltage is seeking it's source, NOT the earth or ground.