Advice for hiring electrician to install manual transfer switch

[lkruper]

I am curious for my own information. I put what you refer to as MWBCs in my current house when I wired it.

Now if I powered only one half the circuits with a 120V generator, on L1 *OR* L2, that alone would not be unsafe, would it? Assume I have no 240V loads. Does code address this?

Are you assuming in your statement that the HO would bridge L1 and L2, creating an unsafe situation on the neutral of MWBC circuits?

This is a hypothetical question. I would not normally connect a generator in this fashion. But... I can see someone doing such a thing, and placing critical circuits to be powered by generator on one phase and leaving the other phase unpowered in a power outage. Note I am asking about a non-solar application, where grid and generator power are the only options. Thanks, if you understand my long winded question.

Now I am wondering about my setup too. Can an electrician easily tell from looking at the panel if I have any MWBCs? My cabin had 1950s electrical until 2010. The electrician added circuits but some of the old ones probably did not get changed completely to the new codes.

[Living Large]

Can an electrician easily tell from looking at the panel if I have any MWBCs?

I would say "yes". In a MWBC on a 230V service, one method is to use 3 conductor Romex w/ground, and the red wire is connected to L1, and black to L2, or the reverse. It may be that individual wires are run in conduit for a branch circuit - in which case an MWBC may not be as obvious to an observer - but code requires a simultaneous shutoff like a shared handle or two pole breaker, which is another tipoff.

[lkruper]

I would say "yes". In a MWBC on a 230V service, the white is connected to L1, and black to L2, or the reverse.

So this is only a problem for 230V circuits? I am not trying to power the two that I have, water heater and range. I am only going to power 120v circuits with my generator.

[Living Large]

So this is only a problem for 230V circuits? I am not trying to power the two that I have, water heater and range. I am only going to power 120v circuits with my generator.

I am going to let those with more experience and knowledge answer.

[inetdog]

I am curious for my own information. I put what you refer to as MWBCs in my current house when I wired it.

Now if I powered only one half the circuits with a 120V generator, on L1 *OR* L2, that alone would not be unsafe, would it? Assume I have no 240V loads. Does code address this?

Are you assuming in your statement that the HO would bridge L1 and L2, creating an unsafe situation on the neutral of MWBC circuits?

This is a hypothetical question. I would not normally connect a generator in this fashion. But... I can see someone doing such a thing, and placing critical circuits to be powered by generator on one phase and leaving the other phase unpowered in a power outage. Note I am asking about a non-solar application, where grid and generator power are the only options. Thanks, if you understand my long winded question.

Yes. The neutral in an MWBC is sized based on never carrying more current than you would get from one of the L circuits at a time.
When you put identical loads on both sides, the neutral current is zero. When you feed both L sides from a single 120V phase, the neutral current is twice the current in either L conductor alone. So with a 20A breaker you could get 40A on the neutral. Not good.

As long as you only power (or only load) one half of the MWBC, you will have no problems. (Although the voltage drop to your load may be higher than under normal conditions.)

[inetdog]

So this is only a problem for 230V circuits? I am not trying to power the two that I have, water heater and range. I am only going to power 120v circuits with my generator.

It is a problem with a pair of 120V circuits that share a common neutral (MWBCs), even if you do not have any 240V loads.
240V loads simply will not see any voltage at all.

[lkruper]

It is a problem with a pair of 120V circuits that share a common neutral (MWBCs), even if you do not have any 240V loads.
240V loads simply will not see any voltage at all.

Bear with me... still struggling to understand. If I have MWBCs.

1. Is this a problem if my generator puts out 220v?
2. Would this still be a problem if my 110v generator is supplying 110v to a 30 amp plug with hots bridged (both hot)?
3. Is this only a problem if my 110v cable only energizes on leg of the 220v?

[foo1bar]

I would say "yes". In a MWBC on a 230V service, the white is connected to L1, and black to L2, or the reverse. Edit This assumes a standard Romex cable is used - it may be that individual wires are run in conduit for a branch circuit - in which case an MWBC may not be as obvious to an observer - but code requires a simultaneous shutoff like a shared handle or two pole breaker, which is another tipoff.

NO - White should *NEVER* be connected to L1 or L2.
The only exception is if you make it so it *isn't* white (black electrical tape, or even sharpie marker coloring it)

Typical MWBC is dishwasher and disposal.
See Fig 2 here:

http://ecmweb.com/code-basics/branch-circuits-part-one

Typically around here it'd be 12-3 wire from the 2-pole breaker to the outlet under the sink. The red wire would be for the dishwasher and the black for the disposal. (or vice-versa) The white wire would be the "common" as usual. If dishwasher is on, but not the disposal, the current goes from the panel through the red wire through the dw and back through the common. If they're both on you can think of it as the currents going through the black and red wires, then meeting at the white wire in the outlet. And since they are opposite they cancel each other out.
Since typical US wiring is 120V on L1 and opposite phase 120V on L2, this means that the current on the common wire is *reduced* when both legs have power flowing.
If you were to make it so that the phases were the same, NOT opposite, then when the dishwasher and disposal are running, the currents would ADD together rather than subtract. So if you had 15A for the dishwasher and 10A for the disposal, now you have 25A going through that common.

[foo1bar]

Bear with me... still struggling to understand. If I have MWBCs.

1. Is this a problem if my generator puts out 220v?
2. Would this still be a problem if my 110v generator is supplying 110v to a 30 amp plug with hots bridged (both hot)?
3. Is this only a problem if my 110v cable only energizes on leg of the 220v?

If you are powering both L1 and L2 of your residence, then you either need to
A> use same 240V system that POCO supplies (120V on each leg, but they are opposite phases)
or
B> Make sure that any MWBC are NOT powered. (and expect that any 220V appliance will also not function properly) I'd probably turn off all 2-pole breakers.
or
C> Only provide power to 1 leg of your house (works fine if all your critical things like refrigerator are on the same leg) - I'd still turn off all 2-pole breakers


option B I think is somewhat non-standard, so if you don't know what a MWBC is, you should learn before choosing that path.

[Living Large]

NO - White should *NEVER* be connected to L1 or L2.
The only exception is if you make it so it *isn't* white (black electrical tape, or even sharpie marker coloring it)

You are correct, 3 conductor with ground is used for the circuit I was thinking of, which I have wired back in the day. What I wrote makes no sense for at least two reasons. Brain blip for me - and I apologize for the error.

[lkruper]

If you are powering both L1 and L2 of your residence, then you either need to
A> use same 240V system that POCO supplies (120V on each leg, but they are opposite phases)
or
B> Make sure that any MWBC are NOT powered. (and expect that any 220V appliance will also not function properly) I'd probably turn off all 2-pole breakers.
or
C> Only provide power to 1 leg of your house (works fine if all your critical things like refrigerator are on the same leg) - I'd still turn off all 2-pole breakers


option B I think is somewhat non-standard, so if you don't know what a MWBC is, you should learn before choosing that path.

I am not planning on having the transfer switch power either of my two 220v circuits, the hot water heater and the electric range. Does this ensure I don't have problems?

[Sunking]

I am not planning on having the transfer switch power either of my two 220v circuits, the hot water heater and the electric range. Does this ensure I don't have problems?

Not the way it works. Your genny will have 4 wires: L1, L2, N, and G. It will supply 240/120 just like any standard electric service. All your 240 and 120 branch circuits will have power. The trick is to balance your 120 loads between L1 and L2.

[inetdog]

Not the way it works. Your genny will have 4 wires: L1, L2, N, and G. It will supply 240/120 just like any standard electric service. All your 240 and 120 branch circuits will have power. The trick is to balance your 120 loads between L1 and L2.

Except that the generator which the OP describes has only 120V output (one duplex receptacle, not split wired.)
If he had a generator with 240V output this whole discussion would interesting but not applicable.

[lkruper]

Except that the generator which the OP describes has only 120V output (one duplex receptacle, not split wired.)
If he had a generator with 240V output this whole discussion would interesting but not applicable.

That is correct, and if I find out that my portable generator causes a problem with my permanently installed transfer switch, I will purchase the appropriate generator. I just hope I do not need to.

[Sunking]

That is correct, and if I find out that my portable generator causes a problem with my permanently installed transfer switch, I will purchase the appropriate generator. I just hope I do not need to.

Don't take this wrong, but why not do it right?

If you only feed 1 leg, do you even know what 120 circuits are on that 1 leg? Things like a 240 water heater will do some strange things with only 1 leg hot.