How does the neutral wire prevent MWBCs from operating at 240v?

[SG-1]

Grounding only one node of a circuit does not create a short circuit on any kind of circuit, anywhere, whatsoever. Grounding has nothing to do with phase angles, either.

We are in perfect agreement.

 

[SG-1]

The Experiment

The Experiment

Managed to run the experiment this afternoon. Here are my results.

Some notes I take away from this thread:


The MWBC is a series/parallel arrangement of sources & loads.

In a perfect world with two perfectly balanced loads & sources no electric current would be present in the neutral or parallel section of the circuit.

The idea that the currents cancelled each other out only applies in the mathematical model, in the physical world there is no electric current in the neutral or parallel portion of the circuit, no electric current enters the node.

With unbalanced loads the series path tries to operate as a voltage divider, but a proportional electric current in the neutral keeps the voltage across the loads even.

 

[FionaZuppa]

O, another MWBC debate? love these.

one thing i don't see to much about is how MWBC power is distributed. having all the loads used most often on one side of CT xfrmer is not good.

the issue with MWBC is if one load decides to run south, the load on the other side of MWBC can be damaged if both are on at the same time.

 

[kwired]

the issue with MWBC is if one load decides to run south, the load on the other side of MWBC can be damaged if both are on at the same time.

Please enlighten us on this.

What exactly do you mean by load running south? We have multiwire feeds to buildings nearly all the time, loads do go south, often not effecting anything else. Now if you meant if the neutral goes south, then I get that - though like I said we have multiwire feeds supplying buildings nearly all the time so same goes for the main supply as it does for a (multiwire) branch circuit.

 

[FionaZuppa]

Please enlighten us on this.

What exactly do you mean by load running south? We have multiwire feeds to buildings nearly all the time, loads do go south, often not effecting anything else. Now if you meant if the neutral goes south, then I get that - though like I said we have multiwire feeds supplying buildings nearly all the time so same goes for the main supply as it does for a (multiwire) branch circuit.

MWBC with loads that are not compromised, a lost N will not be an issue, as long as both loads are on at the same time, the loads turn on and work. w/o the N one load by itself will not turn on.

take two 120ohm loads, one on one side the other on the other side. 240v CT 1ph xfrmer.
both are 1amp on half the xfrmer when only one is on. both on at the same time is still 1amp across each load with is now across the whole xfrmer. so what happens if the one load goes "bad" and is now just 60ohms. when both on you'll get more amps on the good 120ohm load, which is bad, right?

next, take 20 MWBC's, all red hots on one pole, all black hots on the other pole. now, if all the most used loads are on the red hots then you'll be loading the xfrmer heavily on only half the full winding. one half of the CT xfrmer gets real hot, the other side stays cold, etc.

 

[LarryFine]

MWBC with loads that are not compromised, a lost N will not be an issue, as long as both loads are on at the same time, the loads turn on and work.

Only with equal loads; otherwise, the smaller (higher resistance) load will see higher voltage.

w/o the N one load by itself will not turn on.

If you mean if one load's switch is open (turned off) off, then none will be energized.

take two 120ohm loads, one on one side the other on the other side. 240v CT 1ph xfrmer.
both are 1amp on half the xfrmer when only one is on. both on at the same time is still 1amp across each load with is now across the whole xfrmer.

Agreed.

so what happens if the one load goes "bad" and is now just 60ohms. when both on you'll get more amps on the good 120ohm load, which is bad, right?

With an intact neutral, both loads still see 120 volts, but the "bad" 60-ohm load will pass twice the current, or 2 amps.

next, take 20 MWBC's, all red hots on one pole, all black hots on the other pole. now, if all the most used loads are on the red hots then you'll be loading the xfrmer heavily on only half the full winding. one half of the CT xfrmer gets real hot, the other side stays cold, etc.

If the center-tapped secondary was physically constructed that way, probably true.

 

[jaggedben]

MWBC with loads that are not compromised, a lost N will not be an issue, as long as both loads are on at the same time, the loads turn on and work. w/o the N one load by itself will not turn on.

There's enough additional information required for those statements to make any sense that, as stated, they end up being untrue. i.e. a lost neutral will almost always cause big problems. It will cause a higher resistance load to burn up and a lower resistance load to run weak when they share the same neutral. This has nothing to do with either load being 'compromised.' Even a neutral that develops significant resistance can do it. Look at the math (see post #80).

You're probably right about badly unbalanced loads not being good for transformer longevity, but I would wager that bad cases of that are few and far between.

 

[FionaZuppa]

If the center-tapped secondary was physically constructed that way, probably true.

how else does a 1ph CT xfrmer get constructed? the N comes off the center point of two secondary 120v coils (or literally tied to the center of the winding during the winding process). hence the 1ph 2pole CT xfrmer.

 

[LarryFine]

how else does a 1ph CT xfrmer get constructed? the N comes off the center point of two secondary 120v coils (or literally tied to the center of the winding during the winding process). hence the 1ph 2pole CT xfrmer.

You're picturing a number of turns of wire wrapped around a core, a tap attached, and then the same number of turns wrapped again. That means the first half of the winding is physically closer to the core than the second half, allowing for greater electromagnetic coupling. If two completely separately-wound coils are placed side-by-side, one coil could run even hotter than the other by being more physically separated.

Many transformers are constructed by winding with two parallel wires (called bi-filar winding), so both halves are identically coupled to the core. These two windings can then be placed in series for a center-tapped configuration, or in parallel for a single-voltage output with twice the current capacity, which describes a typical dual-voltage transformer. Of course, primaries as well as secondaries can be constructed this way.

 

[Besoeker]

MWBC with loads that are not compromised, a lost N will not be an issue, as long as both loads are on at the same time,

And are equal.

 

[kwired]

MWBC with loads that are not compromised, a lost N will not be an issue, as long as both loads are on at the same time, the loads turn on and work. w/o the N one load by itself will not turn on.

take two 120ohm loads, one on one side the other on the other side. 240v CT 1ph xfrmer.
both are 1amp on half the xfrmer when only one is on. both on at the same time is still 1amp across each load with is now across the whole xfrmer. so what happens if the one load goes "bad" and is now just 60ohms. when both on you'll get more amps on the good 120ohm load, which is bad, right?

next, take 20 MWBC's, all red hots on one pole, all black hots on the other pole. now, if all the most used loads are on the red hots then you'll be loading the xfrmer heavily on only half the full winding. one half of the CT xfrmer gets real hot, the other side stays cold, etc.

Good design is to try to achieve best balance possible. Sometimes load isn't all that predictible and this happens anyway. The more branch circuits you have the less likely you will unbalance it too severely as a general rule, but some applications can be easier to predict how loading will work out.

You're probably right about badly unbalanced loads not being good for transformer longevity, but I would wager that bad cases of that are few and far between.

Absolutely.

Heard a story one time about a new school building - three phase wye supplied. Every classroom had three rows of lights, each one was wired the same - A was front row, B was middle row, C was back row - or something like that.

They did end up having issues with the transformer because it was common to find nearly all the rooms with just the front row of lights turned on from what I was told as this was placing the majority of load on same phase.

 

[LarryFine]

Heard a story one time about a new school building - three phase wye supplied. Every classroom had three rows of lights, each one was wired the same - A was front row, B was middle row, C was back row - or something like that.

They did end up having issues with the transformer because it was common to find nearly all the rooms with just the front row of lights turned on from what I was told as this was placing the majority of load on same phase.

Good reasoning for rotating circuits room-to-room.

 

[Besoeker]

Good design is to try to achieve best balance possible. Sometimes load isn't all that predictible and this happens anyway. The more branch circuits you have the less likely you will unbalance it too severely as a general rule, but some applications can be easier to predict how loading will work out.

Absolutely.

Heard a story one time about a new school building - three phase wye supplied. Every classroom had three rows of lights, each one was wired the same - A was front row, B was middle row, C was back row - or something like that.

Is that permitted? It would mean there would be 208V between rows of lighting.

 

[kwired]

Is that permitted? It would mean there would be 208V between rows of lighting.

I don't know if they ran MWBC's in the example I mentioned, but it is permitted.

 

[Besoeker]

I don't know if they ran MWBC's in the example I mentioned, but it is permitted.

I don't think it is here but then we would have 400V in a room.

 

[kwired]

I don't think it is here but then we would have 400V in a room.

If the lighting were 277 volt lighting (pretty common here when the service is 480/277 wye) then you would have 480 volts between each leg of the MWBC.

 

[Besoeker]

If the lighting were 277 volt lighting (pretty common here when the service is 480/277 wye) then you would have 480 volts between each leg of the MWBC.

Can you have a MWBC from a 277V L-N wye connection?

 

[kwired]

Can you have a MWBC from a 277V L-N wye connection?

sure hope so -it is done a lot

Need handle ties on single pole or multi-pole breakers as the disconnecting means to assure you disconnect all ungrounded conductors of the circuit when you operate the disconnecting means - but that rule only was put in code maybe around 15 - 20 years ago.

 

[Jamesco]

sure hope so -it is done a lot

Need handle ties on single pole or multi-pole breakers as the disconnecting means to assure you disconnect all ungrounded conductors of the circuit when you operate the disconnecting means - but that rule only was put in code maybe around 15 - 20 years ago.

but that rule only was put in code maybe around 15 - 20 years ago.

Actually only 10 years ago. New for NEC 2008, 210.4(B).
And worth noting it may not have been adopted by AHJs 1, 2, 3, years or more after 2008.


.

 

[kwired]

Actually only 10 years ago. New for NEC 2008, 210.4(B).
And worth noting it may not have been adopted by AHJs 1, 2, 3, years or more after 2008.


.

Thanks. My guess would have been at least 2005.