Main Bonding Jumper

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jbellino

jbellino

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Location
Central Florida
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Journeyman Electrician
Hello,

I'm dying to get to the bottom of something that has been puzzling me for a long time, in regards to bonding neutral to ground at the first means of disconnect.
Specifically, my question is about bonding the neutral to the grounding terminal strictly at the first means of disconnect rather than also bonding the two at the sub panel. I understand that the former works and what goes wrong when the latter is done (neutral return path flowing through both the neutral wire, the ground path, and everything bonded to ground). But I'm having some trouble understanding why this is.

We know that the power from the utility transformer travels in one big loop going through the meter, the service, the sub panel, the branch circuit, and all the way back in reverse order. So if the service (or first disconnecting means) is part of this entire loop going to and from the utility line, and all of the metal components in the branch circuits are connected to the service ground hub, which is bonded to the neutral, why wouldn't we also run into this problem of neutral current flowing through the grounding path/bonded metal parts here? I understand that current wants to return to its source. But both the subpanel and the first means of disconnect are upstream from the branch circuits, closer to the source. So this is confusing to me.

Hopefully the way I've presented this question makes sense.
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You have presented your question in a way that makes sense and asking it is proof that you are above average in understanding grounding and bonding. The basic rule is that the NEC starts at the service disconnect. Neutral current does flow on the metal parts at the service but we want to keep it off all the other metal parts we as electricians have control of inside the building.
 
Welcome to the forum.

Simply put, up to the service main, where the premises equipment grounding system begins, where the local electrodes land, and where the premises neutral zero-voltage reference is, the neutral and any metallic enclosures and raceways are considered to be one conductor.

Thus, there is no parallel pathway like a sub-panel feeder would have if its neutral's load end was bonded to the EGC.


Added:
To quote myself from another thread:

The basics: On the line side of whichever enclosure houses the main disconnect (and ignoring the meter), there is no EGC, there is only the neutral.

The main disconnect is where the premises grounding system begins as separate conductors, and its connection to the service neutral is the main bonding conductor. The grounding electrodes (rod(s), water pipe, building steel, etc.) must also connect to the service neutral at (or before) this point.

Any circuits leaving this enclosure, whether branch circuits or feeders, must include an EGC separate from the neutral (if there is one), and said neutral should never be bonded to ground again. Except for switching, fusing, etc., we treat the neutral as we would any ungrounded (hot) conductor.
 
LarryFine said:
Welcome to the forum.

Simply put, up to the service main, where the premises equipment grounding system begins, where the local electrodes land, and where the premises neutral zero-voltage reference is, the neutral and any metallic enclosures and raceways are considered to be one conductor.

Thus, there is no parallel pathway like a sub-panel feeder would have if its neutral's load end was bonded to the EGC.


Added:
To quote myself from another thread:

The basics: On the line side of whichever enclosure houses the main disconnect (and ignoring the meter), there is no EGC, there is only the neutral.

The main disconnect is where the premises grounding system begins as separate conductors, and its connection to the service neutral is the main bonding conductor. The grounding electrodes (rod(s), water pipe, building steel, etc.) must also connect to the service neutral at (or before) this point.

Any circuits leaving this enclosure, whether branch circuits or feeders, must include an EGC separate from the neutral (if there is one), and said neutral should never be bonded to ground again. Except for switching, fusing, etc., we treat the neutral as we would any ungrounded (hot) conductor.
Click to expand...
Exceptions including grounding the X2 on a control transformer in an MCC cabinet?
 
So even though technically it's not its first means, it's where it begins the new SDS, therefore ok to do. Thanks for the planted seed.
 
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