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Old 07-07-2017, 09:22 AM   #1
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Sub-panel Continuity Question


Hello All,
I've removed the green grounding screw from my subpanel, and checked continuity between the neutral bar and the ground bar, with no wires hooked up on either of the two bars. I have no continuity, the two bars are isolated from each other.
When I install the neutral conductor and ground to the correct bus bars, I now show a very slight continuity between the neutral and ground bar, with a reading of .02 on my multimeter, (I presume because the neutral and ground are connected at the main).

Is this correct?

Also, when I look at voltage from an outlet with a circuit from the subpanel, I show household voltage between the ground and the hot lead, as if it were an outlet powered from the main.

So, what is the point of disconnecting the neutral and ground bar at the subpanel, if I still have continuity residing from the connected neutral and ground at the main?

Thanks for the help
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Old 07-07-2017, 09:39 AM   #2
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Re: Sub-panel Continuity Question


Basically, you don't want (the NEC does not want) a current carrying bare wire (EGC wire). ...except of course when it is acting in a fault to trip a breaker.

You have a neutral to do that that is an insulated conductor.

If you don't float the neutral at a subpanel, your bare ground wire (EGC) would be a current carring conductor from the sub to the main.

(Yes... In a sense they are the same wire at ground reference and joined at the main, but their functions are different. An insulated neutral that normally carries current, and think of it as an emergency bare EGC that functions only in an amergency to clear a fault.)
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Old 07-07-2017, 10:27 AM   #3
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Re: Sub-panel Continuity Question


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Originally Posted by MTN REMODEL LLC View Post
You have a neutral to do that that is an insulated conductor.

If you don't float the neutral at a subpanel, your bare ground wire (EGC) would be a current carrying conductor from the sub to the main.
I'm pretty sure I understand this concept, which is why I have such a negligible reading on the multimeter between neutral and ground.
It looks as if the wiring at the subpanel is correct, and the current is following the neutral return.

So a little more, and what concerns me, is the subpanel is mounted to a large steel RV port, and the RV port is grounded with multiple steel anchoring rods, sunk deeply into the ground. Basically, the entire RV port is the Earth Ground.
What concerns me, is in the event of a direct short to ground, at the circuit on the subpanel mounted on the steel RV Port, what is keeping me from getting electrocuted if I were to touch the steel RV port, before the subpanel circuit breaker were to trip? Or, would this be an impossibility because if I were to touch the RV port, at the same exact time the short to ground were to happen, the current would seek the Earth Ground through steel, instead of myself, because steel would be a much better conductor, instead of an ungrouded me?
Or, would the short to ground follow the ground / neutral conductor back to the main where it is grounded there?
Thanks for the help

Last edited by South Taco; 07-07-2017 at 10:29 AM.
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Old 07-07-2017, 11:11 AM   #4
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Re: Sub-panel Continuity Question


Well... (Realizing I'm only a GC and not an electrician or electrical engineer).... electricity/current/amps will follow ALL paths to ground...but in ACCORDANCE with it's paths resistivity according to OHMs law..

The earth (which we do refer to as ground) is basically high resistivity and little
current would flow through "earth ground" (if any). And generally speaking (unless dripping wet and depending on your body chemistry) you have a high resistivity.

And understand, that your subpanel is grounded back to the main which is tied to your neutral right back to the transformer which has very low resistivity.

I think this is a correct statement........ but if a ground fault occured, and in the moments before a breaker functioned, and you were touching your car port and you were well grounded to earth, theoretically, yes you would be energized and a conductor..... but that is theoretically as you and the earth have a very high resistivity and very little current would be involved (if any).

But that is the reason that you don't want a EGC bare ground always energized as a conductor.... because your car port would be energized and if you were standing in water or grabbed a good ground and you had a bad neutral, you could/would possibly get shocked.
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Old 07-07-2017, 11:24 AM   #5
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Re: Sub-panel Continuity Question


Also electrical safety design is for "when things happen". And a couple of things which commonly happen is a neutral can become loose/disconnected. And wires can be cut through during construction/remodeling - just the neutral to the subpanel could be cut by a drill or a saw (possible).

So if the neutral is bonded to the ground at the subpanel and the only wire carrying that to the main panel is cut/disconnected.... Then everything with a grounded metal case connected to the subpanel would suddenly become hot! Zap!

With the separate ground/neutral and not bonded at the subpanel. If you loose either or both wires going to the subpanel, no problem so far as someone being electrocuted.
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Old 07-07-2017, 12:29 PM   #6
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Re: Sub-panel Continuity Question


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Originally Posted by Justin_99 View Post
So if the neutral is bonded to the ground at the subpanel and the only wire carrying that to the main panel is cut/disconnected.... Then everything with a grounded metal case connected to the subpanel would suddenly become hot! Zap!
OK...I believe I understand the concept. Let's say I neglected to isolate the ground and neutral bars at the subpanel. If the neutral conductor, carrying the return current, were disconnected/broken, then the ground would now be the current carrying conductor. Because the ground is bonded to the RV Port, then anyone touching the RV port would be electrocuted. Correct?

By disconnecting the ground and neutral at the subpanel, then if the neutral becomes broken, then the ground would take over as neutral/return due to the fact that they are connected at the main...Thus, the RV Port is not the current carrying conductor.

If this is correct, then how would you ever know if the neutral is broken? Unless, you take readings at an outlet, and find there is no current between the Hot and Neutral at the outlet...I think I just answered my own question.

Last edited by South Taco; 07-07-2017 at 12:33 PM.
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Old 07-07-2017, 12:45 PM   #7
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Re: Sub-panel Continuity Question


Not quite. If the neutral is broken between the panel and the subpanel, then everything on the subpanel stops working. The EGC can't conduct the power back to the panel, because there is no bonding anywhere after the breakage. And that's also how you'll know that the neutral is broken.

Also, if there is an improper connection between neutral and ground, and the neutral gets disconnected upstream from the connection, you are right: the ground takes over as return path. However, if you touch you RV port, most likely you won't even feel it, because your resistance is normally much higher than the EGC. This is no different than a bootlegged ground, which does work in most cases (even though it's illegal and dangerous). However, at that point the game gets quite dangerous. If the EGC gets disconnected, you are completely screwed. Or if somehow your resistance gets low, and the EGC resistance gets higher.
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Old 07-07-2017, 01:03 PM   #8
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Re: Sub-panel Continuity Question


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Originally Posted by csab_ View Post
Not quite. If the neutral is broken between the panel and the subpanel, then everything on the subpanel stops working. The EGC can't conduct the power back to the panel, because there is no bonding anywhere after the breakage. And that's also how you'll know that the neutral is broken.
Ahh...makes sense, the circuit between neutral and ground is broken, therefore the subpanel does not work.

However, from the subpanel, there is a 6ga 4-wire connecting an RV 14-50 plug, which the RV connects to.
So, we have an Main Panel > Subpanel > RV connector.

So in the above scenario, if the neutral is broken, because the 14-50 RV plug has a neutral connected to the subpanel neutral bar, and the 14-50 RV Plug also has a ground connected to the subpanel ground bar, is it possible the ground still remains the current carrying conductor back to the main, if the neutral were to become disconnected between the main and subpanel.

I suppose I should not have left out the part about the RV plug circuit off the subpanel

So here's what I have:
* A 200 amp main, which powers a 100 amp subpanel. The main and subpanel is connected with a 3 conductor 1/0 + Ground (Notre Dame)
* The 100 amp subpanel has two circuits: A double pole 50 amp breaker powering the 14-50 RV plug, which the RV plugs into, and a 20 amp breaker powering a 12'x14' Utility Shed with two outlet's.
* The subpanel neutral bar and ground bar are not bonded, they are isolated from one another.

That's all my surprises
Thanks for the help

Last edited by South Taco; 07-07-2017 at 01:15 PM.
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Old 07-07-2017, 01:15 PM   #9
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Re: Sub-panel Continuity Question


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Originally Posted by South Taco View Post
Ahh...makes sense, the circuit between neutral and ground is broken, therefore the subpanel does not work.

However, from the subpanel, there is a 6ga 4-wire connecting an RV 14-50 plug, which the RV connects to.
So, we have an Main Panel > Subpanel >RV connector.

So in the above scenario, if the neutral is broken, because the 14-50 RV plug has a neutral connected to the subpanel neutral bar, and the 14-50 RV Plug also has a ground connected to the subpanel ground bar, is it possible the ground still remains the current carrying conductor back to the main, if the neutral were to become disconnected between the main and subpanel.
No, because the neutral and the ground are not connected at the plug.

So if neutral is broken between panels, then the current can go Main Panel -> Subpanel -> Plug -> RV -> Plug -> Subpanel and then it can't flow back to the main panel, so you'll notice that there is no power at the RV. There is n o point where the power can jump from the neutral to the ground.
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Old 07-08-2017, 03:48 PM   #10
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Re: Sub-panel Continuity Question


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Originally Posted by csab_ View Post
No, because the neutral and the ground are not connected at the plug.
There is n o point where the power can jump from the neutral to the ground.
Oh...Didn't think about that.
At any rate, I believe my original question of a very low reading on continuity from the ground to neutral bars at the subpanel is correct, and I wired everything correctly.

Thanks for everyone's input
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