Bonded Neutral…..

[Seven-Delta-FortyOne]

I’m having a serious mental block about this right now, and I even drew out a diagram amd I still can’t answer my question.

If you have a generator with a bonded neutral, and you use it for standby power, I know it’s a Code violation. I’m not wondering about that.

But I’m having trouble seeing where the multiple return paths would be. Suppose a pretty standard setup with a power inlet box right next to the panel, usually PVC nipple, and a flexible genny cord.

Even in a ground fault condition, I don’t see why fault current would go anywhere but directly back to the source, which is the generator.

What am I missing here?

 

[ActionDave]

You have it right.

 

[LarryFine]

But I’m having trouble seeing where the multiple return paths would be. Suppose a pretty standard setup with a power inlet box right next to the panel, usually PVC nipple, and a flexible genny cord.

The EGC of the generator cord would be bonded to the neutral at both ends.

 

[Seven-Delta-FortyOne]

The EGC of the generator cord would be bonded to the neutral at both ends.

Correct.

But as far as being able to come in contact with it, it would be only in the generator power cord.

 

[dereckbc]

I think what you are missing is understanding a Separately Derived System and a non-SDS. You can have the neutral bonded to the generator frame and be code compliant configured as a Separately Derived System. All SDS means is the Transfer Switch switches the neutral. Give NEC 250.30 a read.

One more confusion factor might be comparing a permanent stationary standby generator and a portable genny on wheels. The same rules would apply. If the portable neutral is bonded to the frame, the switch interface would be required to be SDS switching the neutral circuit disconnecting neutral from the utility.

 

[LarryFine]

I think what you are missing is understanding a Separately Derived System and a non-SDS. You can have the neutral bonded to the generator frame and be code compliant configured as a Separately Derived System. All SDS means is the Transfer Switch switches the neutral.

To try to help with this explanation, let me see if I can make it simple-ish:

With an SDS, each source is bonded, the load is not bonded, and the neutral is switched.

With a non-SDS, each source is not bonded, the load is bonded, and the neutral is not switched.

So, either the sources are bonded, or the load is bonded, but not both.

One of each type of source is a problem.


What I'm wondering is whether a bonded-neutral generator can be used without a separate EGC to avoid a parallel pathway, and resembles a utility supply?

 

[Seven-Delta-FortyOne]

No. I understand SDS and non-SDS systems.

Ive installed both, but rarely SDS for home standby generators.

I was wondering about the actual physics of a generator system with a bonded neutral, installed as a non-SDS, but no bonded metallic paths. Such as you would have with a flexible generator cord, run to a power inlet box, directly attached to a panel with a breaker interlock.

I believe that the chance of getting a shock from parallel return paths, in this situation, is zero.

 

[electrofelon]

No. I understand SDS and non-SDS systems.

Ive installed both, but rarely SDS for home standby generators.

I was wondering about the actual physics of a generator system with a bonded neutral, installed as a non-SDS, but no bonded metallic paths. Such as you would have with a flexible generator cord, run to a power inlet box, directly attached to a panel with a breaker interlock.

I believe that the chance of getting a shock from parallel return paths, in this situation, is zero.

If the inlet box is fed with say EMT or MC, neutral current would flow on the EMT or MC. Same as a sub panel with a bonded neutral and a metallic wiring method.

 

[ActionDave]

If the inlet box is fed with say EMT or MC, neutral current would flow on the EMT or MC. Same as a sub panel with a bonded neutral and a metallic wiring method.

No different than a meter can and a service disconnect.

 

[electrofelon]

No different than a meter can and a service disconnect.

I concur. However Neutral currents flowing on stuff at the service is essentially unavoidable (unless the NEC mandated an appropriate combination of non metallic raceways or enclosures.

 

[dereckbc]

I don’t see why fault current would go anywhere but directly back to the source, which is the generator.

Do not forget Conservation of Energy Laws. Laws of physics cannot be violated like man-made laws. Concerning electrical energy, every last drop of current returns to its source. It cannot go anywhere else.

You asked if anyone could be shocked if you wired an SDS genny to a non-SDS circuit. The answer is yes. Imagine placing a heavy load on either L! or L2, resulting in a significant neutral current. The current would incur a voltage drop between the frame of the genny and earth. So it is possible.

Ironically in your situation, using a portable genny wired as SDS and appropriate transfer switch, the EGC serves double duty; it is both your EGC and GEC for the genny. If references the genny frame to earth, and carries fault current.

 

[wwhitney]

No different than a meter can and a service disconnect.

Neutral current can flow on the EGC between the two neutral-EGC bonds (I'm going to call the metallic service conduit an EGC for this discussion, even if it's not quite the right term). For a meter can and service disconnect, those are consecutive and usually close to each other--a limited amount of EGC carrying neutral current.

If your transfer switch is the service disconnect, then the same is true. But if the transfer switch is downstream, for just a subpanel, then the EGC between the transfer switch and the service panel (with the other N-G bond) would be carrying neutral current.

Cheers, Wayne

 

[Seven-Delta-FortyOne]

But if the transfer switch is downstream, for just a subpanel, then the EGC between the transfer switch and the service panel (with the other N-G bond) would be carrying neutral current.

Only during a fault condition though, correct?

Because unlike an improper N/G bond at a sub-panel, the generator is a different source, so there is no reason for current to go back through the main panel, then back to the generator.

 

[wwhitney]

Only during a fault condition though, correct?

No. With a generator with bonded neutral and an EGC connected to a transfer switch which doesn't switch the neutral, there's a loop: (grid-wise) upstream neutral to service neutral-EGC bond to upstream EGC to generator EGC to generator neutral-EGC bond to generator neutral back to upstream neutral.

So on generator all load neutral current will divide between the intended path, through the generator neutral connection, and the other way around the loop. Also, if the generator is connected but the transfer switch is set to on grid, the loop is still present and grid neutral current will likewise flow through the generator neutral-EGC bond.

Cheers, Wayne

 

[Seven-Delta-FortyOne]

Just had to draw it out again.

I see where the current would split at the sub-panel neutral bar, some going directly back to the generator, and some through the neutral back to the main panel, and returning on the EGC.

 

[LarryFine]

Neutral current can flow on the EGC between the two neutral-EGC bonds (I'm going to call the metallic service conduit an EGC for this discussion, even if it's not quite the right term). For a meter can and service disconnect, those are consecutive and usually close to each other--a limited amount of EGC carrying neutral current.

Remember, there is no EGC ahead of the service main; everything is the neutral, even conduit and enclosures.

If your transfer switch is the service disconnect, then the same is true. But if the transfer switch is downstream, for just a subpanel, then the EGC between the transfer switch and the service panel (with the other N-G bond) would be carrying neutral current.

Not if it's wired correctly; the bond in the transfer switch should be removed.

 

[wwhitney]

Remember, there is no EGC ahead of the service main; everything is the neutral, even conduit and enclosures.

Yes, as I commented, it's not an EGC. If you like, we can call it the "unintentional neutral conductors" vs the "intentional neutral conductor". But they are components that would be EGCs if they weren't upstream of the service disconnect.

Not if it's wired correctly; the bond in the transfer switch should be removed.

Certainly, but that wouldn't avoid the neutral-EGC loop in the scenario under discussion. The two bonds are at the generator and at the service.

Cheers, Wayne

 

[wwhitney]

What I'm wondering is whether a bonded-neutral generator can be used without a separate EGC to avoid a parallel pathway, and resembles a utility supply?

Certainly it can electrically, so I assume your question is whether the NEC allows it? Excellent question, someone needs to pour through Article 250 to answer it. : - )

Cheers, Wayne

 

[shortcircuit2]

Certainly it can electrically, so I assume your question is whether the NEC allows it? Excellent question, someone needs to pour through Article 250 to answer it. : - )

Cheers, Wayne

250.32(B) disallows this... but the Exception No. 2 may permit no EGC

 

[electrofelon]

Certainly it can electrically, so I assume your question is whether the NEC allows it? Excellent question, someone needs to pour through Article 250 to answer it. : - )

Cheers, Wayne

As far as I know, there are 4 provisions in the code for using the neutral as a bonding conductor, with two being only for existing installations (other than a service).

1. Existing ranges and dryers
2. Existing feeders to detached structures
3. Outdoor SDS (is it 250.30(A)(1) exception?)
4. Medium voltage MGN systems.