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Old 02-11-2008, 11:19 PM   #16
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Sub-panel grounding question.


gesoneguy

One of us can explain the reason in detail... but in short... Ground rods are primarily for protection of equipment from huge voltage events like lighting. The equipment grounding conductor ran with the feeder is to allow fault current to get back to the center tap of the serving transformer (source) so that a breaker will trip in the sub-panel. that is why the ground and neutral are bonded in the main panel so fault current and neutral current can utilize the service neutral to get back to the source since there is not a grounding wire ran with the lateral or overhead drop to your meter. If you bond the ground and neutral in the sub-panel then you have a parallel path back to the main panel for neutral and ground. In a non-fault condition basically half your neutral current is using the ground wire and the other half is using the neutral in the feeder to the detached structure. You do not want current on your grounding wire unless a fault exists. So you don't bond them at the sub. The other issue is if the neutral opens for whatever reason and you have the ground bonded to the neutral....now the ground starts carrying all the neutral current and you also will have neutral current on the metal of your sub-panel. Make sense?

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Old 02-11-2008, 11:20 PM   #17
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Sub-panel grounding question.


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gesoneguy

One of us can explain the reason in detail... but in short... Ground rods are primarily for protection of equipment from huge voltage events like lighting. The equipment grounding conductor ran with the feeder is to allow fault current to get back to the center tap of the serving transformer (source) so that a breaker will trip in the sub-panel. that is why the ground and neutral are bonded in the main panel so fault current and neutral current can utilize the service neutral to get back to the source since there is not a grounding wire ran with the lateral or overhead drop to your meter. If you bond the ground and neutral in the sub-panel then you have a parallel path back to the main panel for neutral and ground. In a non-fault condition basically half your neutral current is using the ground wire and the other half is using the neutral in the feeder to the detached structure. You do not want current on your grounding wire unless a fault exists. So you don't bond them at the sub. The other issue is if the neutral opens for whatever reason and you have the ground bonded to the neutral....now the ground starts carrying all the neutral current and you also will have neutral current on the metal of your sub-panel. Make sense?

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Old 02-11-2008, 11:43 PM   #18
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Sub-panel grounding question.


That applies if the L&AandBC panelboard is in the same building as the service equipment. And a service rated disconnect is located and grouped at the service equipment. NEC 225.36 requires the disconnect at a detached building to be service rated. In an mlo panel it will state on the spec sheet that it is suitable as service equipment if 6 breakers or less and it is not used as a L&Aand BCP. So the minute you make it a L&AandBCP it is no longer rated as service equipment and you must have a single throw main disconnect which is essentially the same requirement for the dwelling.

Look here this is where I had my mind changed a few years ago when I believed the six handle rule was acceptable.

http://forums.mikeholt.com/showpost....3&postcount=12
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Old 02-12-2008, 02:27 AM   #19
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Sub-panel grounding question.


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That applies if the L&AandBC panelboard is in the same building as the service equipment. And a service rated disconnect is located and grouped at the service equipment. NEC 225.36 requires the disconnect at a detached building to be service rated. In an mlo panel it will state on the spec sheet that it is suitable as service equipment if 6 breakers or less and it is not used as a L&Aand BCP. So the minute you make it a L&AandBCP it is no longer rated as service equipment and you must have a single throw main disconnect which is essentially the same requirement for the dwelling.

Look here this is where I had my mind changed a few years ago when I believed the six handle rule was acceptable.

http://forums.mikeholt.com/showpost....3&postcount=12
Yes, yes, good show, old chap. Not that it matters much, but I'm in complete agreement with you concerning the L&ABCP and the 6 disconnect rule. Besides that, a main is always a good idea, especially on a detached structure, even if you could get away with an MLO.

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Old 02-12-2008, 08:49 AM   #20
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Sub-panel grounding question.


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I've run #2 from the Meter Box to the sub-panel, and grounded it at the sub-panel with two grounding rods 6ft apart. !

What's the thing about 2 ground rods. I've seen it mentioned on this forum many times. Are a bunch of you guys in a part of the country where you can't get the "less than 25 ohms" (NEC 250.56) met? I live in clay and swamp country and have never had to use 2 ground rods. Just wondering.
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Old 02-12-2008, 09:22 AM   #21
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Sub-panel grounding question.


Thank you so much Stubbie! Let me see if I'm thinking right. We're basically adding layers of safety, and creating a fault path back to the main service panel, so that IT can deal with a problem, should it arise?

I understand that there is not a grounding wire with the overhead drop to the meter, but I'm having trouble understanding this line. Could you break that down a bit more. Again, thanks!


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that is why the ground and neutral are bonded in the main panel so fault current and neutral current can utilize the service neutral to get back to the source since there is not a grounding wire ran with the lateral or overhead drop to your meter.
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Old 02-12-2008, 10:03 AM   #22
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Sub-panel grounding question.


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Originally Posted by gesoneguy View Post
Thank you so much Stubbie! Let me see if I'm thinking right. We're basically adding layers of safety, and creating a fault path back to the main service panel, so that IT can deal with a problem, should it arise?

I understand that there is not a grounding wire with the overhead drop to the meter, but I'm having trouble understanding this line. Could you break that down a bit more. Again, thanks!
An electrical fault needs a complete circuit in order to trip the breaker. The complete circuit would be from the utility transformer, through your meter equipment, down your circuit conductors, through the fault, back on the ground wire, back to the neutral at the meter, and finally arriving back at the transformer.

In other words, when you hear about something "grounding out", just know that the electricity isn't really going into the earth. It is looking for the path back to the neutral at the transformer. Tying the ground wire and neutral together provides this path.

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Old 02-12-2008, 11:38 AM   #23
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Sub-panel grounding question.


I'm not sure I can improve on Inphases reply but you are forcing me to make the long answer ....I've posted a diagram to show the path of fault current if a short to ground occurred on a metal receptacle box load side of the service equipment. The red arrows show the path fault current will take back to the center tap of the transformer.

The service neutral is the only path neutral current and fault current (if there is a fault) can travel to the transformer once you leave the main panel disconnect. This is where you bond the load side branch circuit grounded legs (white wires or neutrals). Notice the service neutral connects to the neutral/ground bar at the main service equipment. From there through the meter termination to the transformer it is the only path back to the center tap of the transformer. All neutral current and fault currents (if they occur) must be able to flow to the center tap. So by bonding the equipment ground and branch circuit neutrals to the neutral bar/ground bar (they are one and the same in the main disconnect enclosure for the service equipment) this allows these currents access to the service neutral for the electrical trip (complete circuit) to the transformer center tap. This is called the 'low impedance path' that must exist for neutral current (at all times) and fault current when faults occur.
In the case of a fault you must have large, if not massive, current flow through the circuit breaker in order for it to trip instantaneously on detecting the fault. In other words you want it to trip real quick to clear the fault current and deenergize the branch circuit where the fault occurs. In order for this to happen it must have the lowest impedance ( resistance if you will) to the center tap of the transformer. You do not rely on the earth grounding such as ground rods for a low impedance path back to the source. The impedance to earth for typical house voltages is much to high and will not allow enough current to flow thru your breaker to the fault point back to the neutral/ground bar via the equipment ground conductor then out to your ground rods and water pipe which are also bonded to the service equipment neutral /ground bar. Remember in this example we are visualizing the service neutral has opened for whatever reason causing current whether neutral or fault to find another way to the source. Since you also have a big copper wire running out to your grounding electrodes from the neutral bar this becomes the path of choice or 'the new lower impedance path'. But a ground rod in the earth lets say has 15 ohms of resistance to earth (actually impedances but bear with me) In a 120 volt circuit that is 120/15 = 8 amps of current....not anywhere near enough to trip a 15 amp circuit breaker so your metal box where that fault is occurring becomes energized at 120 volts and is an electrocution hazard. However having a near 0 resistance path to the center tap when the service neutral is used allows massive amps to flow and the breaker will trip....example 120/.25 = 480 amps. Rather a crude example of comparison but you get the idea.
The idea of the bonding requirements of the sub-panel I think you understand. Equipment grounding conductors (for safety) are never to have current other than fault current on them and then only breifly when the breaker clears that fault current when it trips. Any current flowing on the equipment grounding conductors in feeders or branch circuits is considered objectionable current and is not wanted. This is why no connection is allowed between neutral and ground load side of the service disconnecting means. Hope I haven't confused the hell out of you. Here is the diagram. In our example of the open service neutral look at the diagram and picture the neutral or fault current traveling the grounding electrode conductors out to the rods and water pipe and then trying to go through the dirt over the the ground rod of the transformer and then up to the center tap of the transformer.
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Sub-panel grounding question.-bonding-diagram.jpg  

Last edited by Stubbie; 02-12-2008 at 12:29 PM.
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Old 02-13-2008, 11:29 PM   #24
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Sub-panel grounding question.


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Hope I haven't confused the hell out of you.
No way! That totally makes sense. Thanks SO MUCH for the explanation! I didn't realize there was actually a line back to the center tap, but when I went out and looked at the line coming into my house.. of course there is.

I'm sorry for all the rookie questions, but I'm really excited to finally start grasping this stuff. The only other thing I don't get is when to bond and not to bond. The panel box inside my house IS bonded. What determines that? Or was my box wired wrong?

Oh yeah, and is there an advanced book I can get that has good diagrams like that? All the ones at the home centers are baby stuff like how to strip a wire.

Thanks again!
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Old 02-14-2008, 12:54 AM   #25
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Sub-panel grounding question.


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No way! That totally makes sense. Thanks SO MUCH for the explanation! I didn't realize there was actually a line back to the center tap, but when I went out and looked at the line coming into my house.. of course there is.

I'm sorry for all the rookie questions, but I'm really excited to finally start grasping this stuff. The only other thing I don't get is when to bond and not to bond. The panel box inside my house IS bonded. What determines that? Or was my box wired wrong?

Oh yeah, and is there an advanced book I can get that has good diagrams like that? All the ones at the home centers are baby stuff like how to strip a wire.

Thanks again!

When to bond... that is a frequent question, asked by 25 year electricians! It is not something to be ashamed of, especially as a laymen, because sooooo many electricians don't even have a clue!

Basically, bonding the neutral only happens at one point in the electrical system: the first means of disconnect. After that point, there are two separate wires, the ground and the neutral. Never to meet again. This is to avoid parallel paths for neutral current and to keep an accidentally opened neutral from energizing the exposed metal cases of other devices.

The Code allows for the bond to occur at 3 places actually: the neutral in the meter can, the neutral of the overhead service drop, or the first disconnecting means, but never more than one at a time! The vast majority is done in the disconnect. By disconnect, I mean either a separate switch that turns your panel off, or a main breaker panel.

Think of the ground wire as a dedicated path back to the neutral to complete the fault circuit, and not having anything to do with the actual Earth (even though that does have it's own function).

You can find great books on the subject by either Mike Holt or Tom Henry, and others.

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Old 02-14-2008, 01:14 AM   #26
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Sub-panel grounding question.


EDIT: I see I am going to be a little redundant with Inphase but I will leave the post anyway.

Quote:
I'm sorry for all the rookie questions, but I'm really excited to finally start grasping this stuff. The only other thing I don't get is when to bond and not to bond. The panel box inside my house IS bonded. What determines that? Or was my box wired wrong?
Your house panel is where the main service disconnect is located. This is the first overcurrent protection device the service entrance conductors land on after they come out of the meter. This disconnect in your case is a main breaker installed in a panel basically attached to your meter enclosure. This makes this the service equipment because this is where that main disconnect for all power to your dwelling is located. It is here that you would bond your neutral and ground in the majority of cases. So that both fault current and neutral current can utilize the service neutral to get to the source. The neutral current needs to get there for house circuits to work. And fault current needs to get there for breakers to trip. So both ground wires for safety and neutral wires for circuit operation will land on the same bars at the service equipment. The neutral bar(s) acts as the bonding mechanism to the service neutral for the ground wires and neutral wires. Bonding simply put is the joining of metal parts to provide for a low impedance path back to the source for fault currents. Now don't forget that the metal of your main panel must be bonded to the neutral bar so that any fault imposed on that metal will cause a breaker to trip. So if you look at your sub-panel picture you will see that green screw. If you run that green screw in it threads into the metal of the breaker panel and bonds that metal to the neutral bar. Now any fault to the metal has a path thru that green screw to the neutral bar to the service neutral and out to the center tap of the transformer.

Bonding takes place throughout the dwelling premise wiring system, this includes your equipment ground wires and metal conduits and jumpers from the metal boxes to the ground wires and jumpers to the green screws threaded into the metal yokes of your switches and receptacles. You are bonding (joining) all metal that is likely to become energized in a fault condition. So that no matter where a fault occurs the fault current has a low impedance path back to the source to allow a breaker to trip. Any break in that bonding path and the metal from where that break exists and the origin of the fault will come to line voltage and a breaker will not trip. Touch any metal in that area and you will get zapped. Maintain that low impedance pathway of bonded metal back to the source and your breakers will trip. Once tripped and the breaker clears the fault any attempt to reset it will fail forcing you to locate the fault and correct it in order to reset the breaker.

Now as for where do you bond neutral and ground. This generally only occurs at the service equipment where the main disconnect is located. You do not bond neutral and ground load side of the service disconnect (there are a few exceptions). Or house side if that makes more sense to you. Remember the neutrals carry current when something is operating on a branch circuit so those white wires will shock you just as well as the hot ones. So if I were to connect them to ground anywhere else (lets say up in your bedroom at a receptacle) I would energize my equipment ground wire on that branch circuit and in turn energize all metal connected to it including the metal housings of any 3 prong grounded appliances plugged into the receptacles on that branch circuit.

You do not bond equipment and ground at sub-panels when you have other metallic paths between the service equipment dwelling and the detached building or if the sub is located in the same dwelling with the service equipment. In these cases you run 4 wires... 2 hots a neutral and an equipment ground for the reasons I stated earlier.

As for books ..google mike holt he probably has the best books out there from basic to advanced with the best graphics you will see. His books explain code and bonding/ grounding, electrical theory but do not necessarily show you how to wire your house.
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Sub-panel grounding question.-main-panel-bond-1.jpg  

Last edited by Stubbie; 02-18-2008 at 04:49 PM.
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Old 02-14-2008, 05:11 AM   #27
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Sub-panel grounding question.


More diagrams.... I probably should add that the diagrams I have posted are ones that I have drawn and are intended as guides and teaching aids. They are not however substitutes for electrical common sense and the knowledge necessary for a complete installation that is safe and code compliant. If you have any doubts you should verify with a qualified electrician.


The first diagram shows a 3 wire feed to a detached building no metal paths other than the feeder exist. Ground and neutral are bonded in this case in the sub-panel. Not a good idea but is code legal until 2008 NEC is adopted by your local authority which prohibits this installation. Some local jurisdictions have not allowed 3 wire feeds for some years.

The first diagram shows a fault at a receptacle metal box in the detached building with a 3 wire feeder. The feeder neutral has broken or opened at the sub-panel neutral bar breaking the fault path back to the main panel and service neutral. There is now no way for a breaker to trip and all metal on the fault path to the sub-panel including the metal of the sub-panel comes to line voltage. It is important to understand the danger with this. With the neutral opened all your 120 volts loads are going to stop working. First thing your going to do is go over and check the breaker panel. Your going to grab the metal door of the breaker panel to look at the breakers unknowing that breaker panel metal is energized. I think you get the picture.
Second diagram shows the added safety of a 4 wire feeder . Click on images to enlarge.
Attached Thumbnails
Sub-panel grounding question.-open-neutral.jpg   Sub-panel grounding question.-open-neutral-4-wire-feeder.jpg  

Last edited by Stubbie; 02-16-2008 at 01:02 AM.
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Old 02-14-2008, 05:02 PM   #28
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Sub-panel grounding question.


good stuff
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Old 02-15-2008, 08:00 PM   #29
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Sub-panel grounding question.


Awesome! Thanks so much InPhase & Stubbie! Your advice really helped a lot. You guys rock!
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Old 02-15-2008, 09:05 PM   #30
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Sub-panel grounding question.


Your welcome

I hope you learned something that will be useful someday. Just remember that service neutral (the grounded conductor) coming to the house from the transformer is the aorta of the electrical system. Lose it and fault and neutral currents cannot get back to the transformer center tap. That creates all kinds of issues with the electrical system but that is another long story...

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