Multiple Circuits for Industrial Control Panels -- MWBC?

[lakee911]

We typically call forl 3#12,1#12G to our PLC panels. The utility circuit and the PLC circuit share a common neutral.

We wouldn?t want to power this from a two-pole breaker because a fault on the utility circuit would de-energize the PLC circuit as well. So, that means that we would need two single-pole breakers.

What I am wondering though, is would this be considered a MWBC and require a tie-handle across the two breakers? The tie-handle would not prohibit independent tripping, but it would allow for a common means to disconnect the panel.

What are your thoughts?

Thanks,
Jason

 

[A/A Fuel GTX]

It's a MWBC in my opinion and needs simultaneous disconnection means. Why not just pull 2 neutrals in and keep the single poles?

 

[lakee911]

It's a MWBC in my opinion and needs simultaneous disconnection means. Why not just pull 2 neutrals in and keep the single poles?

Could do that ... wire costs more than a tie handle though.

Thanks.

 

[tom baker]

I wouldn't use a mwbc due to the hazard of open neutrals.

 

[A/A Fuel GTX]

Could do that ... wire costs more than a tie handle though.

Thanks.

What guarantee do you have that a fault wouldn't trip both breakers that are utilizing the handle tie? Wire isn't THAT expensive

 

[lakee911]

What guarantee do you have that a fault wouldn't trip both breakers that are utilizing the handle tie? Wire isn't THAT expensive

If you had a fault leg to leg, then yes it would trip. I would expect that if it were 2P or two 1P breakers. If either leg to ground faulted then it would trip that leg only. If nuetral faulted (?) to ground, then nothing happens either way.

I don't think I have seen a breaker w/o independent tripping. In otherwords, if you try to hold it closed on a fault, it will still trip and this is why you need to throw it off and then on when you have a trip...allows it to re-engage.

So, given the fact that one would have to momentarily open both circuits to reset a fault on either circuit, then it would make sense to provide 4#12 (i.e. two neutrals) and keep them completely independent. If this was done, other than derating does anything else need to be done? Seems silly that one wouldn't have to derate a common neutral but would have to derate to neutrals.

Thanks,
Jason

 

[A/A Fuel GTX]

You would have four current carrying conductors if you ran two neutrals so you would have to derate to 80% of Table 310.16 but you could use the 90 C column for derating purposes so you will still be fine using #12 THHN/THWN on a 20 amp breaker.

 

[don_resqcapt19]

...Seems silly that one wouldn't have to derate a common neutral but would have to derate to neutrals.

Thanks,
Jason

It doesn't really seem silly when you take a close look at it. The purpose of derating is to limit the heat. The heat is from the I^2R losses in the conductors. If you look at the currents in each of the condutors for a multiwire branch circuit, you can see why there is no reason to count the common neutral as a current carrying conductor for derating purposes. There is no combination of loads on a 3 multiwire branch circuit that will produce more heat than the heat that is produced when one hot if fully loaded.