Code Violation, Hazard or Neither (Transfer Switches)

Merry Christmas
Status
Not open for further replies.
busman

busman

Senior Member
Location
Northern Virginia
Occupation
Master Electrician / Electrical Engineer
I had a customer the other day ask me if he could attach his 3000W Honda 120V generator to a manual transfer switch. Yes, they do sell L5-30P to L14-30R adapters, but I'm not sure they are UL listed. Anyway, I was installing the switch and one of the circuits the HO selected was a MWBC. In the past when I've encountered this, I've been careful to move both hots to the transfer switch. I then realized that if that circuit was run on the generator, that the both legs would be in phase and overload the neutral. In this case, I told the HO that the MWBC could not go on the TS.

So, is this a NEC violation (I can't think of one) or a hazard not covered by the NEC or neither?

Thanks,

Mark
 
It is certainly a hazard to have both legs of a MWBC fed by the same phase/leg of a supply. As you surmise, you could overload the 'neutral' wire.

Depending on how the transfer is done and the specifics/requirements of the circuit, you _may_ be able to transfer part of the MWBC load to the generator. Eg you could transfer only one leg of the MWBC or you could transfer both legs to a single pole breaker, changing it (on generator) to a normal single pole circuit.

-Jon
 
I think it’s a non issue with this small of a generator. Your customer will need to limit what he is running anyway. Granted he could plug two 1500 watt milkhouse heaters into that MWBC and cause grief but he/she needs to know not to do it. Otherwise the cycling of typical lighting and small appliances are not likely to cause extended loading of that MWBC neutral.
 
The probability of getting the average person to comply with a rule saying not to put large loads on a MWBC while operating on a portable generator is close to zero.

Code violation? I can't think of a specific rule being violated, but the intent of the NEC is to assure a safe installation, no matter what the residents plug in.

A 3000-watt generator can deliver 30 amps, maybe 35-40 in the dead of winter. That's far from a non-issue for a 15-amp MWBC with 14 AWG wire, (if that's what it is) and is also problematic for a 20-amp with #12.

You could always hook up one half of the MWBC.
 
drcampbell said:
The probability of getting the average person to comply with a rule saying not to put large loads on a MWBC while operating on a portable generator is close to zero.

Code violation? I can't think of a specific rule being violated, but the intent of the NEC is to assure a safe installation, no matter what the residents plug in.

A 3000-watt generator can deliver 30 amps, maybe 35-40 in the dead of winter. That's far from a non-issue for a 15-amp MWBC with 14 AWG wire, (if that's what it is) and is also problematic for a 20-amp with #12.

You could always hook up one half of the MWBC.
Click to expand...

I thought about only hooking up half of the MWBC to the TS, but was concerned that you would have the same problem if one leg was running on generator and the other on utility. The phase relationship would be unknown at that point.

Thanks for the response.

Mark
 
Really worried? Set a 5 KVA single phase transformer, feed it 120v primary from the gen, secondary feeds your transfer at 120/240. Do your bonding etc, providing SC and OC protection. Or buy a bigger generator that can carry the load.

I don’t think a 3000 watt gen is going to provide 4800 watts for any length of time.
 
I'm still trying to figure out how your hooking the circuits to the transfer switch.

If this was a standard 2p transfer switch that was refeeding the entire panel you would only be energizing one of the phases and only one of the circuits of the Multi-Wire branch circuit would be energized in the transfer. Not both.

If you are feeding a subpanel that is strictly 120v with a 120v feed from the generator through a transfer switch , then the MWBC would have had to been separated before it was ever moved to the 120v generator panel.

Also, You cant have one leg running on utility and one on the generator as was indicated in a prior post,

So,

Can you explain exactly how you set this up?

JAP>
 
jap said:
Also, You cant have one leg running on utility and one on the generator as was indicated in a prior post,
Click to expand...
As I see it, with the generator connected as a non-SDS, there is no issue with running two circuits sharing a neutral from two different sources.
 
LarryFine said:
As I see it, with the generator connected as a non-SDS, there is no issue with running two circuits sharing a neutral from two different sources.
Click to expand...

The way I see it, if you lose utility power and your running on generator power with the transfer switch thrown to the generator input, and you have one side of a multi wire circuit running on the generator power,and the power comes back on and the other side of the multiverse branch circuit is energized by the utility at the same time, then you've defeated the whole purpose of the transfer switch.

Jap>
 
Thinking about this more.

I don't see how you can move only one side of a MWBC to the generator panel and leave the other one in the house panel.

There's the possibility that even though the transfer switch is thrown to the generator side, you could back feed 120 volts to the load side of the breaker of the circuit that was left in the house panel with the shared neutral.

JAP>
 
jap said:
The way I see it, if you lose utility power and your running on generator power with the transfer switch thrown to the generator input, and you have one side of a multi wire circuit running on the generator power,and the power comes back on and the other side of the multiverse branch circuit is energized by the utility at the same time, then you've defeated the whole purpose of the transfer switch.
Click to expand...
jap said:
Click to expand...

There's the possibility that even though the transfer switch is thrown to the generator side, you could back feed 120 volts to the load side of the breaker of the circuit that was left in the house panel with the shared neutral.
Click to expand...
Only if the integrity of the neutral is compromised.
 
LarryFine said:


Only if the integrity of the neutral is compromised.
Click to expand...

I'm not sure what would happen if the integrity of the neutral was compromised if you were running one of the circuits off of utility and one off of a generator.


Do you?

JAP>
 
If you had lighting MWBC with a shared neutral , and, If the transfer switch was on the generator side, and , the breaker for the circuit that was left in the house panel was off, and you took a reading from the load side of that breaker to neutral, would you actually read zero voltage?

Not sure.

JAP>
 
A multiwire branch circuit by definition can not be supplied by a 120v generator. It begins and ends there. If you feed both ungrounded conductors with a single 120v supply, there is no voltage potential between them...no MWBC, so each circuit must have its own grounded conductor. If both ungrounded conductors are connected to the one 120v source, the common grounded conductor would be insufficient for the amperage, which would be an NEC violation. Of course the ungrounded conductor *could* be sized to handle the current, but that defeats the purpose of a MWBC.

IMHO, either use a 120/240v generator, or rewire the MWBC as two branch circuits.
 
Craigv said:
A multiwire branch circuit by definition can not be supplied by a 120v generator. It begins and ends there. If you feed both ungrounded conductors with a single 120v supply, there is no voltage potential between them...no MWBC, so each circuit must have its own grounded conductor. If both ungrounded conductors are connected to the one 120v source, the common grounded conductor would be insufficient for the amperage, which would be an NEC violation. Of course the ungrounded conductor *could* be sized to handle the current, but that defeats the purpose of a MWBC.

IMHO, either use a 120/240v generator, or rewire the MWBC as two branch circuits.
Click to expand...

We already know all that.
And I agree that's exactly what should be done.

But,

The question is what would be the outcome if only one side of a MWBC was tied to a generator
source and the other side and the shared neutral was left in the utility side or say the generator side?

To me that's a no no.

Jap>
 
I still haven't heard exactly how this install was hooked up seeing as how they seem to be using a 120v output generator.

Changing an existing MWBC wired in 12/3 to 2 separate circuits with their own neutral in an existing residential install is no small task.

Just curious.

Jap>
 
jap said:
We already know all that.
And I agree that's exactly what should be done.

But,

The question is what would be the outcome if only one side of a MWBC was tied to a generator
source and the other side and the shared neutral was left in the utility side or say the generator side?

To me that's a no no.

Jap>
Click to expand...

I wrote that it begins and ends with the definition of a MWBC. I think that also addresses your question:

You can't meet NEC requirements for a MWBC and also the requirement that all conductors of a circuit be grouped together, if the neutral of a MWBC that is split only sources from one panel or the other.

Having a neutral in both the utility-only-sourced half of the MWBC and another neutral in the generator-backed half would effectively mean no MWBC, OR it would force the entire system to be non-separately derived, as there would be a continuous neutral from utility panel to load to generator panel, bypassing the transfer switch's neutral switching. Obviously a non-SDS designed system could support this.

Would it work if the neutral is sourced from one panel or the other? Yes. In a non-SDS the neutral is continuous from utility to generator to load. In a SDS the switched neutral will allow the generator-backed half to work in either mode, and the non-backed half only on utility power.

So so it comes down to wiring to function, or wire to meet code, which has more requirements.

Which comes full circle to either wire separate circuits or use a 120/240 generator, making the issue moot.
 
Craigv said:
I wrote that it begins and ends with the definition of a MWBC. I think that also addresses your question:

You can't meet NEC requirements for a MWBC and also the requirement that all conductors of a circuit be grouped together, if the neutral of a MWBC that is split only sources from one panel or the other.

Having a neutral in both the utility-only-sourced half of the MWBC and another neutral in the generator-backed half would effectively mean no MWBC, OR it would force the entire system to be non-separately derived, as there would be a continuous neutral from utility panel to load to generator panel, bypassing the transfer switch's neutral switching. Obviously a non-SDS designed system could support this.

Would it work if the neutral is sourced from one panel or the other? Yes. In a non-SDS the neutral is continuous from utility to generator to load. In a SDS the switched neutral will allow the generator-backed half to work in either mode, and the non-backed half only on utility power.

So so it comes down to wiring to function, or wire to meet code, which has more requirements.

Which comes full circle to either wire separate circuits or use a 120/240 generator, making the issue moot.
Click to expand...

I agree.

This would have been a good post to place right after post #5.

JAP>
 
That along with the fact some were talking about only hooking up half of the MWBC. :happyno:

JAP>
 
Status
Not open for further replies.
Top