transfer switch and shared neutrals

[iwire]

Phil Forget about the neutral, just put it out of your mind.

Now to make sure we are talking about the same thing.

We are talking about an existing MWBC in an existing panel.

Now we come along to install the type of transfer switch shown above....like the Genrac unit.

We find we only want to power one side of the multiwire branch circuit on generator so we only 're-route' one side of the circuit through the transfer switch factory supplied harness.

Is that correct?

If so that is a direct violation of 210.4(A) and it has nothing to do with the neutral.

It has to do with the circuit being capable of being supplied from two separate sources and that will happen each time the utility power comes back on while the generator is running.

Let me as you this.

With one side on utility power and the other side on generator power what would the voltage been between these two legs?

 

[goldstar]

Phil Forget about the neutral, just put it out of your mind.

I can't because it has everything to do with what were discussing (if, in fact, we are discussing the same thing)

We are talking about an existing MWBC in an existing panel.

This is correct but not necessarily a 240 volt circuit. I'm talking about two (2) 120 volt circuits on opposing phases with a shared neutral (I believe that's what the OP was asking). If we're talking about a 240 volt circuit then you are correct

We find we only want to power one side of the multiwire branch circuit on generator so we only 're-route' one side of the circuit through the transfer switch factory supplied harness.

Is that correct?

Yes, but I didn't re-route the shared neutral. That remains firmly landed on the neutral buss in the MBP and all the MWBC circuit wires originate in the MBP irrespective of whether they are transferred over to the generator. Remember, I'm not supplying utility power from the manual xfer switch panel. I'm only supplying power from the generator in a power fail condition on a per circuit basis.

If so that is a direct violation of 210.4(A) and it has nothing to do with the neutral.

If your statement is correct then any circuit transferred by this particular xfer switch panel would be in violation of 210.4(A) making this entire panel an NEC violation and that's a discussion for another day (and BTW, I can buy into that line of thinking).

It has to do with the circuit being capable of being supplied from two separate sources and that will happen each time the utility power comes back on while the generator is running.

Here's where I think we're having a problem. Each transfer switch in the xfer switch panel has a "utility-off-generator" position. If we're only transferring one of the 120 volt circuits it is not possible to supply the circuit simultaneously from both sources nor is it possible to back-feed into the other circuit or back to the utility.

Let me ask you this.

With one side on utility power and the other side on generator power what would the voltage be between these two legs?

Zero volts. There would be no measurable reference between any phase leg of the generator to the utility phase legs unless you bring the neutral back into play. That's the only common tie and why it has to be part of this discussion.

 

[rlMutch]

Sorry to butt in, but maybe this will help. (Or maybe it won't) A multi-wire branch circuit with two hots connected to opposite phases of the same system, and supplying equal (balanced) loads, will have no current flowing on the neutral. The two hots are 180 degrees out of phase, or is it 90?, anyway, they cancel each other out. If one of those loads is supplied by the utility, and the other is supplied by the generator, it's possible that the neutral could carry the sum of both loads, depending on the utility/generator phase relationship, which would be a violation of 210.23. Also, since one part of the circuit would originate from the utility supply, and the other would originate from the generator supply, it's clearly a violation of 210.4(A).

 

[iwire]

I can't because it has everything to do with what were discussing (if, in fact, we are discussing the same thing)

It does not have 'everything to do with it' the neutral has a small part in it.

This is correct but not necessarily a 240 volt circuit. I'm talking about two (2) 120 volt circuits on opposing phases with a shared neutral (I believe that's what the OP was asking). If we're talking about a 240 volt circuit then you are correct

Splitting a straight 240 circuit would not violate 210.4 as a straight 240 circuit is not a MWBC and 210.4 only applies to MWBCs

Splitting the (2) 120 volt circuits on opposing phases with a shared neutral is a direct violation of 210.4(A)

Yes, but I didn't re-route the shared neutral. That remains firmly landed on the neutral buss in the MBP

I understand that and it is not a problem.

the MWBC circuit wires originate in the MBP irrespective of whether they are transferred over to the generator.

No, that is incorrect, when on generator one side of the split circuit originates from the transfer switch which would be "similar distribution equipment" as found in 210.4(A)

Remember, I'm not supplying utility power from the manual xfer switch panel.

I know that, but you are supplying power from transfer switch during a power failure and when the utility power comes back on you will have a MWBC that originates in two separate places.

I'm only supplying power from the generator in a power fail condition on a per circuit basis

.

Actually you feeding half a circuit and that is a problem.

If your statement is correct then any circuit transferred by this particular xfer switch panel would be in violation of 210.4(A)

NO! :smile:

210.4 ONLY applies to MWBCs, 'two wire' circuits are not an issue.

Here's where I think we're having a problem. Each transfer switch in the xfer switch panel has a "utility-off-generator" position. If we're only transferring one of the 120 volt circuits it is not possible to supply the circuit simultaneously from both sources nor is it possible to back-feed into the other circuit or back to the utility.

Phil, this is not about a backfeed.

Zero volts.

That is incorrect, the voltage between them would be anywhere from 0 to 240 volts and that will constantly be changing as the home generator is not in sync with the utility.

There would be no measurable reference between any phase leg of the generator to the utility phase legs unless you bring the neutral back into play.

The neutral from both soures are tied together and that is the common reference.

Now lets say this 20 amp MWBC that we split has 15 amps of load on each leg.

There was a utility failure and the generator has been activated, time goes by and the utility is now back on.

Now we have a active MWBC with one leg supplied from utility and one leg supplied by the un-synchronized generator.

The neutral is subject to overload as the current only balances out when the two sources of supply are in sync.

Every moment they are out of sync the current on the neutral rises, when the two sources 180 degrees off....and they will be at times....the neutral would be carrying 30 amps.

I really think the problem here is your interpretation of 'originate'.

 

[goldstar]

Rod,

No need to apologize. This is an open forum and we welcome all opinions.

A multi-wire branch circuit with two hots connected to opposite phases of the same system, and supplying equal (balanced) loads, will have no current flowing on the neutral.The two hots are 180 degrees out of phase, or is it 90?

180

anyway, they cancel each other out.

Agreed

If one of those loads is supplied by the utility, and the other is supplied by the generator, it's possible that the neutral could carry the sum of both loads, depending on the utility/generator phase relationship, which would be a violation of 210.23.

I can understand the possibility of this happaning. If someone as knowledgable as one of us were making the connections to this xfer panel and knew that the calculated loads of circuits A and B of a 15 amp MWBC were say 8 amps and 10 amps respectfully that would have a 2 amp draw on the neutral. It would behoove us to transfer both these circuits for the reason you mentioned. However, if circuit B was only drawing 3 amps and it wasn't a circuit we wanted on during an emergency why eat up a point in the xfer switch panel ?

Also, since one part of the circuit would originate from the utility supply, and the other would originate from the generator supply, it's clearly a violation of 210.4(A).

There is no wording in 210.4(A) that makes reference to utility or generator. It states "All conductors shall originate from the same panelboard". My argument is that the xfer switch panel is not considered a panelboard. There are no provisions for landing branch circuit wiring directly to terminals inside the xfer switch panel. Except for the generator connection itself, all connections to the xfer switch panel are made through the greenfield whip. Now, if you want to make the argument that once you remove a branch circuit wire from a breaker in the MBP and series it in to the wiring of the respective circuit of the xfer switch panel and that, in and of itself, creates a code violation making the entire xfer switch panel a code violation then we may come to some agreement.

 

[iwire]

However, if circuit B was only drawing 3 amps and it wasn't a circuit we wanted on during an emergency why eat up a point in the xfer switch panel ?

Because that is not how the code is written and you can not count on what the load will be down the road, the conductors must be protected at their ampacity regardless of the connected load.

There is no wording in 210.4(A) that makes reference to utility or generator. It states "All conductors shall originate from the same panelboard".

Phil keep reading that sentence "All conductors shall originate from the same panel board or similar distribution equipment"

Clearly the transfer panel is similar distribution equipment.,

For that matter considering it is an enclosure with over current protective devices in it I would consider it a panel board.

My argument is that the xfer switch panel is not considered a panelboard

IMO you are incorrect.

Take a look at the NEC definition of panel board in Article 100.

Now, if you want to make the argument that once you remove a branch circuit wire from a breaker in the MBP and series it in to the wiring of the respective circuit of the xfer switch panel and that, in and of itself, creates a code violation

No that is not a code violation.

 

[goldstar]

I really think the problem here is your interpretation of 'originate'.

Bob,

Sorry, I replied to Rod before I noticed your response. If this is the general consensus for the meaning of "originate" then I stand corrected. However, in all fairness, 210.4(A) should state "originate from the same source and/or panelboard" and not just from the same "panelboard".

I understand yours and Rod's arguments and reasoning for transferring both circuits of the MWBC and based on the scenarios you provided I agree. However, you both provided a "worst case" scenario and I provided a "least case" scenario. I think we can both be correct in some respect.

Anyway, as always, this has been both fun and informative. I'll go and eat my left-over crow now !!!

Regards,

Phil

 

[goldstar]

Phil keep reading that sentence "All conductors shall originate from the same panel board or similar distribution equipment"

You're correct and this is what I get from referencing my 2002 code book instead of my 2005.