Sub panel bonding question

[Mac25]

Yes there will have to be continuity between the neutral and the ground at the pony panel, they are bonded up at the utility service. Most folk are still missing the point of keeping neutral current in the neutral and no neutral current in the ground path.

I must be one of the confused folks....having been a mechanic for 20+ years, I have a pretty good understanding o how electricity flows, but I am by no means a master home electrician. I understand the purpose of the neutral and ground and I know that electricity takes the path of least resistance. I also know that if two objects have continuity and voltage is induced, it will flow between the two objects. So if my neutral and ground have continuity and a small amount of current is flowing on the neutral, it will also flow to the ground, so I guess I dont really understand the bonding screw? I have continuity at my sub panel between the neutral and ground bars with or without the bonding screw tightened againstthe panel or removed. Am I looking into this way to much to comprehend? Thanks for all the replies!!

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[arlen4720]

I must be one of the confused folks....having been a mechanic for 20+ years, I have a pretty good understanding o how electricity flows, but I am by no means a master home electrician. I understand the purpose of the neutral and ground and I know that electricity takes the path of least resistance. I also know that if two objects have continuity and voltage is induced, it will flow between the two objects. So if my neutral and ground have continuity and a small amount of current is flowing on the neutral, it will also flow to the ground, so I guess I dont really understand the bonding screw? I have continuity at my sub panel between the neutral and ground bars with or without the bonding screw tightened againstthe panel or removed. Am I looking into this way to much to comprehend? Thanks for all the replies!!

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Draw it out on paper, and you will understand. Put in grounding electrodes and all. I would like to ask that you don't keep asking the same question until you do this.

 

[Mac25]

Draw it out on paper, and you will understand. Put in grounding electrodes and all. I would like to ask that you don't keep asking the same question until you do this.

I would like to ask you not to reply to the thread then. I am just trying to understand the principle.

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[arlen4720]

I would like to ask you not to reply to the thread then. I am just trying to understand the principle.

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I didn't mean to offend you, I'm just trying to help you understand. The only to visualize it is on paper, just try it, and you will understand. That's all I'm trying to say.

 

[CobyRupert]

I must be one of the confused folks....having been a mechanic for 20+ years, I have a pretty good understanding o how electricity flows, but I am by no means a master home electrician. I understand the purpose of the neutral and ground and I know that electricity takes the path of least resistance. I also know that if two objects have continuity and voltage is induced, it will flow between the two objects. So if my neutral and ground have continuity and a small amount of current is flowing on the neutral, it will also flow to the ground, so I guess I dont really understand the bonding screw? I have continuity at my sub panel between the neutral and ground bars with or without the bonding screw tightened againstthe panel or removed. Am I looking into this way to much to comprehend? Thanks for all the replies!!

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You are measuring continuity at the sub panel between the neut. and gnd. because they are connected together at the main panel (and hopefully not at the sub panel).
The bonding screw is just that. To bond the neutral and gnd. bus (at one location if it's being used as a main panel(service entrance) location.
"So if my neutral and ground have continuity and a small amount of current is flowing on the neutral, it will also flow to the ground" - Uhh, if you mean the small current produced by the battery in your Ohm (continuity) meter, then yes, you've created a circuit from the positive probe to the main panel and back to the negative probe on the ground and neutral wires. But during normal operation only neutral current should flow on the neutral conductor. That's why you only bond gnd & neut. in 1 place. If bonded at the sub panel, neutral current would have a parallel path on the ground wire back to the neutral in the main panel.
BTW: Neutral current doesn't flow "to the ground", it seeks to return to the transformer it originated from. We however like to make this side of the transformer winding pegged to 0 volts, aka "neutral" or "grounded".

 

[buickanddeere]

If the meter was switched to AC volts and a couple of heavy 120V loads were putting 25amps onto the neutral at the pony panel. Then something to the order of
1 to 3 volts would be measured between the neutral and ground bars in the pony panel.

 

[jefwyn]

I thought one of the problems with bonding the neutral and the ground at the sub panel, was that current can flow on both the neutral and ground and breakers may not trip at the sub panel. Is this correct?

Although neutral current does flow back to the pole transformer, the transformer neutral goes to ground via ground rods at the pole or back to the sub station transformer via the ground grid. Am I off base on this statement?

 

[CobyRupert]

I thought one of the problems with bonding the neutral and the ground at the sub panel, was that current can flow on both the neutral and ground and breakers may not trip at the sub panel. Is this correct? Although neutral current does flow back to the pole transformer, the transformer neutral goes to ground via ground rods at the pole or back to the sub station transformer via the ground grid. Am I off base on this statement?

Breakers will still trip, the problem with neutral current flowing on the ground is there's resistance in that wire, and current flowing through a resister will develop a voltage across that resistance. (Remember Ohm's Law? voltage=current x resistance) Then everything "grounded" by that ground wire has a raised voltage, including panel cases, switch boxes, etc... Animals and electronics can be sensitive to this "stray voltage".
Real ground, as in earth/dirt is a poor conductor for current to use to get back to the transformer, that's why we run a ground conductor and bond it to the neutral in one spot so that the fault current to this "ground system" is large (enough to trip breaker).
Short answer: The transformer is "grounded" to peg the neutral to a zero reference, but it doesn't have to be. There are some "ungrounded system" that are not. Especially in industrial settings where you want to keep equipment running even when you have a ground fault; - the fault current isn't high enough to trip the breaker.

 

[MNBobcat]

I suspect that if the sub-panel had the ground and neutrals bonded together, that any GFCI wired off that sub-panel would immediately trip. Anyone know for sure?

 

[quicksandfarmer]

I suspect that if the sub-panel had the ground and neutrals bonded together, that any GFCI wired off that sub-panel would immediately trip. Anyone know for sure?

It won't. The GFCI trips when the current through the neutral isn't exactly the same as the current through the hot wire. Regardless of where the neutral is bonded to the ground, as long as it is upstream of the GFCI device all of the neutral current goes through the neutral on the GFCI. If the neutral touches ground downstream of the GFCI it provides a parallel path for some of the neutral current and will trip the GFCI immediately.