Why We Don’t 3wire Sub Panels but Bond At FMOD

[Small Electrician]

Example…
200amp upgrade with new meter and panel ourside
Original fuse box inside became sub fed
3 wire sub panel with seu
In a dwelling
Grounds and neutrals tied together

If the bare neutral conductor was lost, what would happen? Wouldn’t the load try to balance between the 2 phases? And then the rest of the current would try to flow onto any metal that’s bonded and energize it and become a shock hazard?

Wouldn’t the same shock hazard be if the service neutral was lost? (Lost service neutral is much more likely than lost sub panel bare neutral conductor). At the first MOD ground rods should also be bonded (but it’s a very high resistance parallel path) and possibly water line or gas line bonded.

Trying to understand why it’s so important we don’t 3wire sub panels (with bonded grounds and neutrals).

 

[electrofelon]

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Trying to understand why it’s so important we don’t 3wire sub panels (with bonded grounds and neutrals).

The main reason is because then you would have neutral current flowing on every metal thing between the main and the sub. Current flowing means difference of potential so you would have all this exposed metal stuff with current flowing on it with different potentials. Could be a shock and fire hazard. Yes the same issue exists with a service and everything upstream from it, but just ignore that .

 

[Small Electrician]

The main reason is because then you would have neutral current flowing on every metal thing between the main and the sub. Current flowing means difference of potential so you would have all this exposed metal stuff with current flowing on it with different potentials. Could be a shock and fire hazard. Yes the same issue exists with a service and everything upstream from it, but just ignore that .

Now when a sub panel is 3 wired (especially when an original main becomes a sub after a service upgrade), if a ground fault were to occur, the breaker would still trip (even though it’s a 3wire sub instead of 4) as long as grounds and neutrals are bonded together, correct?

 

[Dennis Alwon]

Now when a sub panel is 3 wired (especially when an original main becomes a sub after a service upgrade), if a ground fault were to occur, the breaker would still trip (even though it’s a 3wire sub instead of 4) as long as grounds and neutrals are bonded together, correct?

Yes, but remember current will flowing in every pathway. So a 120v circuit will have current on the neutral and the ground wire. We don't want current on the ground as its purpose is to bond all metal pathways together so if there is a ground fault then the circuit will trip rather than carry current as when it is bonded together with the neutral.

 

[Small Electrician]

Yes, but remember current will flowing in every pathway. So a 120v circuit will have current on the neutral and the ground wire.

So isn’t the case with any service since we bond all grounds and neutrals at FMOD? Because current takes all paths

 

[Dennis Alwon]

So isn’t the case with any service since we bond all grounds and neutrals at FMOD? Because current takes all paths

At the service we run 3 wires so we don't have current flowing back thru the ground wire. When there is a metal pipe between the service disconnect and the meter there can be objectional current however, for some reason, the code allows this.

 

[electrofelon]

I can give you a good example of the safety issues with a three wire feeder: I was working in a mill building that had multiple units that were temporarily fed with three wire feeders, neutral and ground bonded. We were replacing the feeders with four wire MC. The way it worked out, most of the MC runs were landed on the live temp fed tenant panel and we were working them back. As we dragged the MC's over EMT runs, ducting, and all sorts of other metal stuff, there were sparks and arcing to the MC sheath. Of course these were pretty small sparks, but would get worse with higher loads, and especially during a fault. Imagine two things close together, sparking, and then someone throws some blankets over it.....

 

[CoolWill]

In the case of the main neutral becoming open, the shock hazard is low because all the grounds in the house will rise to the same potential as the open neutral due to being bonded to it. If everything is at the same voltage, no current can flow. But if the 3 wire sub panel neutral is open, potential exists between that panel's EGC and other EGCs from the main panel.