Sub panel grounding

sluggermike · 11-09-2007, 06:04 AM

I plan on putting a sub panel in to accommodate the power requirements for an endless pool. When I put in a new main panel a couple years ago, I the neutral and the ground wires were all attached to one bus bar. Do all sub panels need to have a separate bus bar for the ground and one the neutral, and why?
Thanks

duke2043 · 11-09-2007, 08:20 AM

When you put in subpanel make sure that the bond screw is out you will need to seperate neutrals and grounds,when you run wire from main panel make sure you run 3-wire with ground

Piedmont · 11-09-2007, 10:25 AM

I think subpanels are best left for electricians. Grounds & neutrals can only be connected together at the main panel (if it has a combined neutral/ground buss), your subpanel MUST have a seperate ground & neutral buss. Unlike main panels the neutral buss on subpanels are floating (no contact) the ground buss connects to the metal.

When you get a panel it can be a main or sub, there's often a green metal screw on the neutral buss. If it's a main panel you screw it in which makes that buss have contact with the metal frame and makes that buss a combined neutral/ground which is what you're familiar with. If it's a subpanel you DO NOT screw it in, which keeps the neutral floating (no contact with metal frame) and have a seperate buss for the ground.

It's just what I read in some wiring books and I'm just a DIYer so take what I say with a grain of salt. I'd never do a subpanel myself, you really should get a qualified electrician.

Stubbie · 11-09-2007, 12:50 PM

In 2008 the NEC will require all sub-panels to have separate neutral bars and ground bars. There is one exception that exists at present that is allowing a 3 wire (H-H-N) feed to a detached building.

So you asked the why?

It's called objectionable current. We bond the neutral and ground at the service equipment because the only wanted return for fault current and system current to the source (transformer) is the service neutral. This simply means virtually all current that returns to the main disconnect enclosure will use the service neutral due to its low impedance to the source. It will not follow the grounding wires in any appreciable value to the dwellings grounding electodes which are also connected to the system neutral bar. This route has too high of an impedance so kirchoffs current law rules here.... the current will almost entirely follow the service neutral back to the source. However don't confuse what I'm saying to imply current will always take the lowest path of resistance. It will take all paths but most will take the path of lowest resistance. If we open the service neutral somewhere between the transformer and the main disconnect then the only path back to the source is along the grounding electode conductors bonded at the neutral bar of the service equipment and connected to the water pipe and grounding rods or whatever your using for an electrode. Then current will use this path of higher impedance because it becomes the only path. It will then flow through the earth to the ground rods of the serving transformers. This is absolutely not wanted as you can guess. See below graphic for illustration. Take note that you lose your ground fault protection in a service neutral "open" condition and a breaker will not trip as shown. This is because the impedance is so high on the return through earth that current flow is too restricted and there isn't enough current coming through the breaker on a ground fault to trip it.

SUB-PANELS

When we go further downstream (load side) of the main disconnect as to a sub-panel the same occurs if we maintain a 3 wire feed or a 4 wire feed bonded ground and neutral or not in the sub-panel. This is because all current on the load side... objectionable or not... ends up back at the main disconnect where it will "choose" the service neutral to get back to the source. So if we run a 3 wire feed to a detached building sub-panel we want the neutral and ground bonded because the only path back to the main disconnect panel for current whether it is fault current or system current is the feeder neutral between the sub and the main disconnect. There is no equipment ground with the feeder so only one path exists. Note: All sub-panels that are located in the same structure with the main disconnect must have 4 wire feeders.
Ok so now lets look at a 4 wire feed to a sub-panel. Only difference is we run an equipment ground wire or metal conduit along with the neutral and hot wires (H-H-N-G). If we bond the neutral and grounds at the sub-panel neutral bar we have now created two paths back to the service equipment. Both paths are low impedance (copper wire or aluminum wire or metal conduit) So return current from the branch circuits served by the sub will pretty much split evenly.....half on the equipment ground and half on the neutral as it returns to the source. So if we would have an open neutral of the feeder to the sub all current will now follow the equipment ground because we bonded it to the neutral return at the sub-panel. Again Kirchoffs current rule wins. We do not want any current on the equipment ground except fault currents and these will exist very briefly because they will be stopped when the circuit breaker opens. Equipment ground is for human safety so we dang sure don't want current hiding on the equipment ground unknown to us. Especially if we are using metal conduit as our equipment ground. So we do not bond safety ground to the neutral bar in the sub-panel when we use 4-wires utilizing a wire for equipment ground or 3-wires with metal conduit as our equipment ground. As the below graphics show.

4 -WIRE or 3 WIRE with metal conduit as equipment grd to detached building only method allowed as of 2008 NEC

47_47 · 11-09-2007, 01:02 PM

Great explaination Stubbie, I knew where they should or shouldn't be bonded, but your explaination has made it finally clear as to why and what could happen if not done correctly.

robertmee · 11-09-2007, 02:02 PM

Stubbie, I wanna know where you get all these great diagrams

Stubbie · 11-09-2007, 02:27 PM

Sure, right click on the diagram then choose properties, the location will be listed. Most of this type come from mike holt links. I show diagrams because I think most DIY people are just as smart as anybody given the information with which to learn. Diagrams shorten that learning curve.
Also I like the people and professionals here at DIY chatroom and I want posts that will give DIY the needed information to do their electrical work safely. Diagrams are good teaching aids... simple as that.
It also provides an excellent data base and reference for those searching the forums. So in my replies I make sure that keywords are present to aid people in getting the information they are seeking.
Also if you are doing a project electrical or otherwise and are interested in a diagram then go to google images and put in grounding and bonding and all the images I posted will show up. For instance put in multiwire circuit and diagrams of multiwires will be listed...often times a lot of other related images will show also.

Stubbie

sluggermike · 11-09-2007, 02:41 PM

Thanks for all your help. I will need to study the information and diagram before I put in the sub panel. I sometimes feel like I am attending a class on becoming an electrician when I go on this forum. I really appreciate all it.
Thanks again
Slugger

frenchelectrican · 11-09-2007, 02:47 PM

Stubbie:
seems you are becoming a teacher in here but anyway you really can make the point very clear with the photos show here even other people used diffrent langange but the answer will be the same.

offhand note :

I am working on the 08 code cycle info and get it ready after the first of the year.

FOR DIY"S keep in your mind that most electricians here will mention the code related items most will useally mention 05 code [ some will use 02 code cycle ]
unless some type of local admendments show up then will use that part for that area only

Merci, Marc

Stubbie · 11-09-2007, 02:58 PM

Hi Marc

First let me say you are one sharp cookie yourself...you have paid your dues so to speak... and not much gets by you when someone asks a question here. Collectively both the DIY and professionals here make this a fine place to hang out. BTW.. I don't know it all.... not even close....anybody thinks the electrical trade is a cake walk is not hitting on all cylinders. There is another professional who is administator on another electrical forum who alway says "You don't know what you don't know"...how true that is in our trade.

Take care Marc

Andy in ATL · 11-09-2007, 03:40 PM

Well done Stubbie! MIKE HOLT RULES!!!

Stubbie · 11-09-2007, 04:59 PM

I want to make one observation in the case of an open service neutral or sub-panel non-service neutral (especially if 3 wire and bonded). Your going to have all kinds of crazy things happening to your homes electrical and thats one thing that makes this really dangerous for the homeowner cause they go to the breaker or fuse panel and grab the metal cabinet door to check their breakers wondering what is going on. Little do they know its loaded with current. Throw in a ground fault and your going to have that current at 120 volts.
Few times I have found where the sub-panel had separate neutral and ground but they did not carry the ground back to the service main panel disconnect. Instead they just thought it was ok to ground the panel to the ground rods. Any ground fault occurs in this situation and all metal will come to 120 volts and no breaker will trip. I found this out the hard way on one occasion.

I also made one oversite and that is that until you go on the 2008 NEC a 3 wire feed to an outbuilding will be allowed. But 3 wires (H-H-N) are not allowed to detached buildings if any other metallic paths exist between the buildings...like a water line for example. In this case you must run an equipment ground with the feeder to the detached building regardless of what code cycle your area is working under.

Stubbie

Piedmont · 11-09-2007, 05:28 PM

Dumb question, what's bonded mean as in "especially if 3 wire and bonded"?

My guess, bonded means the ground is not a physical wire but rather some type of conduit? Like, a 3 wire bonded means there's 3 wires in a metal tube!?

Is armored cable then, considered a 2 wire bonded if it's 14/2?

Andy in ATL · 11-09-2007, 05:58 PM

Piedmont, Let's see if I can type fast and beat Stubbie!

From the 2005 NEC: Bonding (Bonded). The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed.

From the 2008 NEC: Bonded (Bonding). connected to establish electrical continuity and conductivity.

The NEC recognizes that a surprisingly large percentage of electricians, from Masters to apprentices, struggle with the very question that you, a DIYer posed. I want to make it VERY CLEAR that all the "amateurs" following this thread and grasping it should be very proud of yourselves. Bonded, in plain english (feel free to jump in, smarter guys) means we are connecting everything that is 1.metallic, and 2. might come into contact with electricity, together and bringing it (with the very least resistance possible) back to the electrical supply source.

It should be noted that we also connect all these parts to the ground (planet Earth). THIS IS NOT NEARLY AS IMPORTANT as making sure everything can get back to the source.

Why do we do this? In electricity, when the resistance goes down, the amperage goes up. OK you might ask....why do I want all the metallic parts in my house at a very high amperage??? Ain't that dangerous??

The opposite, in fact!! The quicker you get the amperage up, the quicker your breaker trips. Now I'll press "submit Reply" and see if I beat Stubby or Marc!

Stubbie · 11-09-2007, 07:00 PM

Examples of bonding

Bonding jumper from neutral bar to case of load center could also be a green screw

Water pipe bond

This one is interesting anybody see a problem with this one?? .