Bonding neutral and ground bars ?

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Old 10-20-16, 08:02 PM
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Bonding neutral and ground bars ?

Reading quite a bit of material. Understand that in the main panel, one must bond neutral and ground bars together so that if a short occurs, the ground wire in the circuit has direct neutral access to the incoming circuit, as opposed to actully having to flow through a ground rod, to the electrical pole and finally to transformer. I get this.

On the other hand, everything I read says that neutral and ground bars should NOT be bonded in a sub-panel. Theory is that stray currents in the neutral from an appliance won't be present in the ground circuit, thereby reducing possibility of shock.

Here's what I don't get. When you install a sub-panel, you run neutral to the main panel neutral, and sub-panel ground to main panel ground. Neutral and ground may not be bonded in the sub-panel but are fer sure grounded in the main panel.

So... what's the point... if the ground and neutral wires are bonded anywhere, they are bonded everywhere. Can someone explain?

TIA
 
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Old 10-20-16, 08:12 PM
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The point is.....

The neutral carries only the neutral current flow and the ground only carries the ground fault.

If you had a combined neutral/ground wire between your main panel and your sub panel.....and you lost it. What would happen ? You'd have voltage on the ground. You could have live appliance cabinets. You'll have a major imbalance between your two power legs sending high voltage into 120v devices.

You don't ever want voltage to appear on the ground.
 
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Old 10-20-16, 08:22 PM
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I apologize for being dumb. If I run a ground wire from the sub-panel ground bar to the main panel ground bar, AND, I run a neutral wire from the sub-panel neutral bar to the main panel neutral bar, AND, I bond the neutral and ground bars in the main panel, how are the neutral and ground bars not bonded in the sub-panel, by virtue of the fact that neutral/ground wires are bonded in the main panel?
 
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Old 10-20-16, 08:36 PM
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It's a fairly esoteric point. If you bond the neutral and ground anywhere but at the service panel, there will be current flowing in the ground wire, because two wires in parallel will divide the current flow in proportion to their relative resistance. So if the neutral and ground are the same gauge, half the current will flow in the neutral wire and half in the ground wire.

Now because wire is not a perfect conductor, meaning zero resistance, any current flowing in a wire will cause a voltage to be developed across the wire. That means that the ground at your subpanel will not be at zero volts relative to earth ground, which is what you want it to be for safety reasons, but will be some (fairly small) voltage higher than zero. So that's one problem. Your subpanel ground, and all the grounds connected to it are not really at ground potential.

Now suppose your neutral wire developed a bad connection at some point (happens). Now more or even all of the circuit current is flowing through the ground wire and you have no way of knowing this. If the ground wire then developed (or had from the beginning) a bad connection, or was disconnected by an unsuspecting homeowner or electrician working on the main panel, all of sudden all the metal parts of all the equipment connected to the subpanel ground would be energized to full line voltage, an extremely dangerous condition.

Granted, this would be an unusual situation, but the whole point of the grounding system is to ensure safety under fault conditions. It is designed so one can count on the ground being at zero volts under all but extremely unlikely conditions.
 
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Old 10-21-16, 06:36 AM
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If you insist on saying it that way, it is an altruism that if A is bonded to B and B is bonded to C then A is bonded to C. So you will get near zero resistance if you measure between the neutral prong hole and the ground prong hole at any receptacle up in the house where all the wiring is up to date and correct, because neutral and grounded are bonded at the main panel. (Always turn off power before doing any resistance or continuity measurements.)

But you do not want any alternate routes for normal neutral current from outlets back to the main panel (here the panel or box with the first master switch for the home) in the form of the ground wires (equipment grounding conductors running between those locations. So you do not bond (create interconnections between) neutral and ground downstream of the main panel.

The ground wire is not an alternate route for neutral current up in the house even if the neutral path is broken due to damage or whatever. In this case the current should stop and the circuit remain broken until it is repaired,
 

Last edited by AllanJ; 10-21-16 at 07:03 AM.
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Old 10-21-16, 09:27 AM
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Thanks to CarbideTipped... Duh... light bulb on... yes, of course, bonding in both the subpanel and main panel creates parallel circuits for the neutral wire, and current flows in both wires in accordance with Ohm's law and parallel circuits.

And, thanks to AlanJ who anticipated my A bonded to B bonded to C question.

Cheers.

PS: Ordered all brand new outdoor GFCI outlets, new boxes and new in use weatherproof covers for the campground electrical repair/upgrade project that started this whole thing.
 
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Old 10-21-16, 04:25 PM
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To follow up on AlanJ's comments:

The whole point of the ground wire then is to provide a path to ground if the hot wire should somehow come into contact with any part of the appliance/receptable that could also come into contact with the human using it. An example would be a hot wire rubbed raw in a metal toaster.

Without the ground wire, an unsuspecting human has the unintended ability to complete the short circuit when she touches the toaster while standing on a damp floor.

With the ground wire, besides presumably providing a much lower resistance path to ground, the circuit breaker would most likely be immediately tripped due to an overcurrent situation the instant the hot wire made contact with the toaster case.

What a ground wire cannot protect against is situation where the neutral current is traveling through a human instead of the neutral wire, as in a hair dryer in the bathtub; hence the need for the additional protection of a GFCI outlet or breaker.

Do I have that right?
 
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Old 10-21-16, 05:52 PM
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Yes, that is the basic explanation.

You should note also that there are two "grounds". There is the EGC (Equipment Grounding Conductor) which we have been talking about. There is also the GEC (Ground Electrode Conductor) which is basically bonding to metal pipes and one or more ground rods. That can not be used as an alternate path for the EGC because dirt can be rather high resistance. It is strictly for atmospheric charges and bonding metallic pathways such as steel or copper pipes.

If you were to say connect the ground on a receptacle to a ground rod instead of the EGC and there was a short the fault might not trip the breaker as quickly because of the higher resistance. Worse perhaps because of the possible higher resistance of the GEC there could be a potential voltage between the GEC and EGC so if you touched to metal chassis one grounded via EGC and one connected to the EGC you could get shocked.
 
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Old 10-21-16, 06:01 PM
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Another thing to remember is that the electricity is trying to get back to its source. It is not like water filling up a piping system and flowing everywhere until the pipes are filled.
 
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Old 10-23-16, 12:44 PM
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There is the EGC (Equipment Grounding Conductor) which we have been talking about. There is also the GEC (Ground Electrode Conductor) which is basically bonding to metal pipes and one or more ground rods.
I first learned about this dogs years ago when I installed an HP minicomputer in my business. The equipment required an isolated ground receptacle in order to control noise. The installing electrician explained the need for a orange outlet which separated the ground circuit from the EGC. He drove an 8 foot rod into the ground for the isolated ground, then connected the box to the standard EGC.

A few years later, I was having issues with a desktop PC at my house abruptly rebooting or shutting down. My house is old enough that it is all ungrounded two wire outlets, so I had used a 3 to 2 prong adapter in the outlet to power the PC. I pounded a rod into the ground, and fastened a wire from the adapter to the rod. My desktop PC never shut down or rebooted after that. I conclude that the power supply couldn't handle noise ungrounded.

I realize that this setup did not provide EGC grounding protection but it did keep my PC running, and it's all long gone now... it was a Gateway... gives you some idea of how long ago that was.

Oddly enough, if I plug a surge protector into the outlet/adapter, it lights up saying it has enough ground to function as a surge protector but I wonder if this is really the case, given the higherl resistance of the ground rod.
 
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Old 10-23-16, 01:05 PM
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The ground rod has nothing to do with the ground pin on the receptacle.
 
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Old 06-11-18, 07:48 PM
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Great discussion keep it up. Possible reference a text book. I get the code rule but not the purpose. I recently had some wiring cleaned up and a tester revealed a neutral-ground reversed. I have been here over 40 years and added lots of stuff. At one time I had 3 sub-boxed for my shop. Illness change my days in the shop. So recently I got a residential electrician to clean it all up. He added a duplex receptacle where I had and old porcelain fixture with an incandescent bulb. The light worked ok for 15 years but removing it revealed at Gnd/neutral reversal. I had to have it replaced as the code will not allow an bulb exposed it must have a lens. Now how do I find the reversal.
 

Last edited by billwhit1; 06-11-18 at 07:51 PM. Reason: spelling
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Old 06-12-18, 07:18 AM
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A tester that has a readout "ground neutral reversal" is probably a gadget you want to toss into the trash can.

You want to own a simple analog voltmeter (the kind with a moving needle). You should measure about 120 volts hot to neutral, about 120 volts hot to ground, and about zero volts neutral to ground.

A broken or incorrectly installed ground wire (equipment grounding conductor) or incomplete grounding can cause a tester of the aforementioned kind to give off the aforementioned readout.
 
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