Strage comment from POCO re: MWBC

[Sierrasparky]

Monday I had a call where the POCO lost one leg of an underground service. Due to the logistics and private ownership of the right of way that the underground was run they could not fix it that day. Well the Poco told me they were going to just tie the two hots of the customer together until they could restore the missing hot leg. I told them I would need to turn of one breaker of each MWBC to avoid overloading the neutral . Well the POCO service man insisted that was wrong of me and that they do it all the time. He further states only the 220v stuff wont work and won't hurt them. I could have argued with this guy till I was blue in the face and was not going to get any where. I opened the customers panel and remove one wire from each MWBC to be safe as I had no indication as to when it would be fixed.

 

[iwire]

Well IMO you are both right.

If you had not changed anything there would have been a potential to overload a branch circuit neutral but it was not very likely.

If I had changed anything it would have been to move both conductors of the same MWBC to the same breaker so nothing would be dead.

 

[charlie b]

You took an appropriate precaution.

It should be noted, however, that it is possible, even likely, that nothing bad would have happened, even if you left it alone. Most circuits are not fully loaded, and it would take both halves of a MWBC to be fully loaded, before the neutral would be overheated. Keep in mind that we usually use 90C rated #12 conductors on 20 amp circuits, and those have an ampacity of 30 amps. Also, we are supposed to limit the continuous load to 16 amps. So if all is installed correctly, the most you will get on the neutral for the one day it takes the POCO to restore their damaged leg is 32 amps on a conductor with an ampacity of 30. That is not likely to damage the insulation in less than one day, and is not likely to cause a fire.

So when the utility person claims that they do it all the time, what they are really saying is that nobody informed them of any problems, so they infer that it is not possible for there to be a problem. You were right; they were wrong. But it is not as big a deal as you might imagine.

 

[gary]

Well IMO you are both right.
If I had changed anything it would have been to move both conductors of the same MWBC to the same breaker so nothing would be dead.

How is this any different than what the POCO already did?

 

[iwire]

How is this any different than what the POCO already did?

The way the power company left it you could overload a branch circuit neutral as one neutral would be the return from two breakers in parallel

The way I suggested to change it would limit the current on the neutral to just one breaker.

 

[charlie b]

The other difference is that both halves of the MWBC would have power. The way the OP did it took power away from one half of each MWBC.

 

[Sierrasparky]

This was an old home and the potential was great that the circuit was overloaded. Yes I could have doubled one breaker of the MWBC and probably been ok. Many times these old homes have well more than 10 amps on a 15 amp circuit. that places more than 20 amps on the Nuetral. What you dont want to happen is create burned up connections at back stabbed outlets. ( unless you want a further service call)

The main reason for the post is that the POCO thinks it was ok in the first place. The POCO claims they do this all the time.

 

[LarryFine]

Yes I could have doubled one breaker of the MWBC and probably been ok. Many times these old homes have well more than 10 amps on a 15 amp circuit. that places more than 20 amps on the Nuetral.

If both legs of a MWBC are placed on a single breaker, the neutral is still protected by the breaker.