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Discussion Starter · #1 ·
Hello,
I'm currently going to a vocational school and am stuck on a subject. It's regarding grounding the neutral conductor.
I know the neutral conductor that's coming from the transformer to the panel after the meter should be grounded and that's why both grounds and neutrals are bonded at this point. It is when a sub panel is added that the ground and neutral should be separated. My issue is this, when two separate conductors are pulled (one for the ground and one for the neutral) along with the two other hots (as in an SER cable) these two conductors the bare ground and the jacketed neutral are actually bonded at the main panel because you can't physically separate at the main panel. For this reason, the two conductors are physically coupled to begin with. When they arrive at the sub panel they then should be separated. BUT, my issue with this is that they are still bonded at the main panel. If I put an ohm meter at the sub panel I can verify that they are not open. So why even bother separating them when they are coupled to begin with? Also what's the reason for not using the bonding screw at the sub? The two conductors (ground and neutral) from the main will bond the ground bar and the neutral bar anyway because as I said they are bonded at the main to begin with... A sub with the bonding screw removed will have the neutral bar and the ground bar isolated from each other BUT as soon as the ground and neutral arrive at the sub the isolation ends regardless of the green bonding screw. I've physically tested it. Can someone explain this to me please. Thank you.
 

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All neutrals and grounds are bonded together at one end (at the service). They're supposed to be. They need to be separate at the other end because one is intended to be a normal circuit conductor, and the other is not. So you want to keep the normal current flow on the neutral where it belongs. The EGC should not see current flow except in the event of a fault, at which point it directs it back to the service, where it ties back to the neutral and ultimately back to the source.

For your sub panel keep neutral and ground isolated but you sure as hell better be bonding the enclosure to the ground bus.
 

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Discussion Starter · #3 ·
Sorry but you just repeated what I said. I'm aware of these facts but when you bring two bonded conductors to a sub and try to separate them that's where I get confused. It's like bringing one conductor from the main and then splitting it in half and labeling it as separate it. They are not going to be separated. Also these conductors will bond the isolated ground bar and neutral bar at the sub because they are touching at the main where they left off...

All neutrals and grounds are bonded together at one end (at the service). They're supposed to be. They need to be separate at the other end because one is intended to be a normal circuit conductor, and the other is not. So you want to keep the normal current flow on the neutral where it belongs. The EGC should not see current flow except in the event of a fault, at which point it directs it back to the service, where it ties back to the neutral and ultimately back to the source.

For your sub panel keep neutral and ground isolated but you sure as hell better be bonding the enclosure to the ground bus.
 

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Sorry but you just repeated what I said. I'm aware of these facts but when you bring two bonded conductors to a sub and try to separate them that's where I get confused. It's like bringing one conductor from the main and then splitting it in half and labeling it as separate it. They are not going to be separated. Also these conductors will bond the isolated ground bar and neutral bar at the sub because they are touching at the main where they left off...
You didn't say anything about normal circuit current. I did. That's the key to keeping neutrals and grounds separate after the service.

Line to neutral current supplies loads through a hot conductor and a neutral conductor. You run a ground wire with it to bond non-current carrying metallic parts, like conduit and enclosures, in the event of an insulation failure on a hot conductor. If the hot contacts the bonded metal, it is a ground fault and generally has low impedance, resulting in a brief (but large) current spike on your grounding system. Ultimately this works its way back to your subpanel ground bar, then the feeder ground wire, then to the service ground bar, over the main bonding jumper, and then through the neutral back to the source. A ground fault is a short circuit.

Just cause two wires are connected at one end doesn't mean anything. The hot legs of a 240v single phase circuit are technically the same piece of wire. But you don't wanna reconnect them later.
 

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You're not just bringing one equivalent conductor and "splitting it in half." Youre bringing one insulated conductor and testing it just like a hot: keeping it isolated from ground, using insulated splices like wire nuts, etc. the other conductor is your bare and you're connecting it to any and every piece of metal in your electrical system that's not supposed to be part of a circuit.

This way you keep circuit current on the nice isolated neutral at least all the way back to the service. At that point they become one again but unless you lose your neutral connection, current should mostly stay on the appropriate path you provided back to the source.
 

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You don't want a current carrying conductor (neutral) to have a parallel path (ground)back to the source. Ground is your safety wire only as Eric pointed out.
 

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Discussion Starter · #7 ·
Respectfully I did mention the normal circuit current; I said "along with the two other hots (as in an SER cable)". Please bare with me, I do understand that one leg of the hot with the neutral supply 120v loads and I do understand that the ground bonding to metallic boxes and appliances serves to trip the circuit in case of a fault. Let me ask you, if you put an ohm meter at the ground bar and the neutral bar in a sub panel don't you get evidence of a closed circuit? I tested it yesterday at the lab. I tested an empty sub without the bonding screw it was OL at the voltmeter. Then I put the screw in I got 000.1 resistance. I removed the screw and got OL. Then when we added the ground to the ground bus bar and the neutral to the neutral bus bar I tested 000.1 between the supposedly separated bars...
Also I know the hots come from the same transformer but isn't separating them is for the purpose of splitting to voltage as in 120 versus 240?
You didn't say anything about normal circuit current. I did. That's the key to keeping neutrals and grounds separate after the service.

Line to neutral current supplies loads through a hot conductor and a neutral conductor. You run a ground wire with it to bond non-current carrying metallic parts, like conduit and enclosures, in the event of an insulation failure on a hot conductor. If the hot contacts the bonded metal, it is a ground fault and generally has low impedance, resulting in a brief (but large) current spike on your grounding system. Ultimately this works its way back to your subpanel ground bar, then the feeder ground wire, then to the service ground bar, over the main bonding jumper, and then through the neutral back to the source. A ground fault is a short circuit.

Just cause two wires are connected at one end doesn't mean anything. The hot legs of a 240v single phase circuit are technically the same piece of wire. But you don't wanna reconnect them later.
 

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Draw 2 horizontal lines on a piece of paper and connect them at one end. That is your ground and neutral connected together at the main panel. If you connected them together at other points (sub panel) then you would have neutral current also riding on the ground wire. Tying them together at one point only, keeps the neutral current on the neutral only.

Connect the lines together at other points if you need to see the parallel current path on paper.
 

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Discussion Starter · #9 ·
Oh my God, you're awesome. I just realized that the meter is what's causing the loop to close. Now it's clear to me.
Thank you 8V71 I could hug you right now...
Draw 2 horizontal lines on a piece of paper and connect them at one end. That is your ground and neutral connected together at the main panel. If you connected them together at other points (sub panel) then you would have neutral current also riding on the ground wire. Tying them together at one point only, keeps the neutral current on the neutral only.

Connect the lines together at other points if you need to see the parallel current path on paper.
 

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Ok combined neutral and ground. Sounds ok. Will it work, yes? Now what happens if the neutral becomes compromised going to the sub panel? You end up with 120 volts on the grounding system. A serious hazard.
 

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Ok I read your comment. 8V71 is right. When everything is hooked up a meter will show close to zero ohms on a sub because of the main bond.
 

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Thank you 8V71 I could hug you right now...

Hey hey hey. Easy there fella.

Now, to easily understand why this is done, connect the lines at the sub, erase the neutral (or worse, both) lines (to simulate a fault), and see where the neutral current will go.

With a 120V load on the panel, every grounded receptacle will have an energized ground and everything plugged in with a grounded plug will have energized metal parts.
 

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Really good question! A good understanding of these fundamental concepts now will greatly help down the road. Un-balanced neutral currents can drive you nuts and your scenario is probably the most common one that you will run across.
 

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Meter? loop? Your one point your equipment bonding system connects to your grounded conductor is your main bonding jumper. If you look in a service panel it is a green screw or a piece of 6 wire (for 200 amps) The circuit is complete when a hot touches any bonded metal. There will be a low impedance path, that causes current to flow, and trip the overcurrent protective decice.

The meter is allowed to be bonded to the grounded conductor (neutral) via 250.80. The grounding electrode conductor is a diffrent system and is allowed to connect to 3 points (sometimes less depends on poco), the grounded conductor at the drip loop, the meterbase, or the main disconnect.

The the first step to understand grounding, is to study the terms associated with these systems and understand what they all mean.

Ex. Grounded conductor
Grounding electrode
Grounding electrode conductor
Main bonding jumper
Equipment grounding conductor
Equipment bonding jumper
 

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Open up the book and begin at article 250. Read all of it. Then checkout the definitions section in article 110. Objectionable current flow is a very, very bad thing and IMO you're not an electrician until you understand it, clearly understand it. i'm just saying it's important to know, that's all.
 

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Open up the book and begin at article 250. Read all of it. Then checkout the definitions section in article 110. Objectionable current flow is a very, very bad thing and IMO you're not an electrician until you understand it, clearly understand it. i'm just saying it's important to know, that's all.
Well said, I see it the same way.:thumbup: The irony is that objectionable current is the most common type of code violation. Its safe to say that very few understood it, and many still don't. I see droves of apartments from the 60s, 70s and 80s with subpanels fed with 3 wires, bonding jumpers in place, SEU run out of them for ranges one and on. Commercial techs running grounds to the neutral bar and vice versa. Separately derived sources being screwed up are another biggy.

Grounding and bonding are essentials to the trade but poorly understood. Apprentices need to be taught these concepts in depth, not just connecting wires and bending conduit.
 

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Discussion Starter · #17 ·
Thank you, thank you, thank you!
You have no idea how frustrating that was to me and how many people I’ve asked before I asked here.
That was so illusive! I will never look at my multi meter the same way again.
It’s simple yet so illusive. Then again, it only becomes simple when it gets figured out.
Let me scream it again one more time and I’ll leave you alone.
THANK YOU!

Draw 2 horizontal lines on a piece of paper and connect them at one end. That is your ground and neutral connected together at the main panel. If you connected them together at other points (sub panel) then you would have neutral current also riding on the ground wire. Tying them together at one point only, keeps the neutral current on the neutral only.

Connect the lines together at other points if you need to see the parallel current path on paper.
 

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You are welcome but I don't understand how your meter fits onto this. I probably shouldn't ask. :laughing:
 

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Discussion Starter · #19 ·
Oh man! Sorry 8V71 your grade has been lowered from an A+ to a B+; it seems you indirectly solved my issue. :laughing:
I’ll elaborate when I come back, my girlfriend is nagging me I need to give her a ride...
My thank you(s) are still valid though:thumbup:

You are welcome but I don't understand how your meter fits onto this. I probably shouldn't ask. :laughing:
 

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Keep this in mind David: Electricity doesn't just seek the easiest path, it takes all paths available to it relative to resistance.
 
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