Receptacle circuits common neutral

[JEFF MILLAR]

Circuits 2, 4 and 6 ( a b c ) Each circuit with the same number of duplex outlets .Question is can you use a common neutral on duplex receptacles ?
How do you balance the load on the panel ?
Seems logical that on lighting circuits when circuits 2 , 4 and 6. are all loaded the same a common neutral would be no problem.
In our modern world with computer circuits and harmonics, i belive it would be good design practise. to run a neutral with each receptacle circuits.
Please correct me if i am wrong as you most likely have figured out i work in design. And am interested in learning how to design the best systems.

 

[infinity]

As you've mentioned due to concerns about harmonics MWBC's aren't used that much anymore. Without harmonic components there is no problem using a MWBC with a shared neutral.

 

[Little Bill]

You would need a handle tie on the breakers so they can all be shut off at the same time. If this is a single phase service, I don't know that I ever saw a handle tie for three breakers. If its a 3Ø panel, you could use a 3-pole breaker or handle tie.

 

[electrofelon]

I dont think the harmonic thing is an issue 99.97% of the time. I run MWBC's whenever possible. Yes, three phase MWBC's where they are all receps, whenever possible.

 

[JEFF MILLAR]

Explain MBBC's is that. MAIN BREAKER BRANCH CIRCUITS ?
Did I understand. correctly ?
Your answer is yes / 20 amp Branch circuit Breakers for circuits. 2, 4 & 6. all loaded with 8 general purpose duplex receptacles, a common neutral is allowed.
How do you balance the loads on the all. Receptacle Panel.
All receptacle CIRCUITS. ONLY. ?

 

[ptonsparky]

Explain MBBC's is that. MAIN BREAKER BRANCH CIRCUITS ?
Did I understand. correctly ?
Your answer is yes / 20 amp Branch circuit Breakers for circuits. 2, 4 & 6. all loaded with 8 general purpose duplex receptacles, a common neutral is allowed.
How do you balance the loads on the all. Receptacle Panel.
All receptacle CIRCUITS. ONLY. ?

I am not familiar with MBBC. MWBC = Multi Wire Branch Circuit
You will not be able to balance the loads exactly with receptacles. It is load dependent. Mostly computers and electronic power supplies could possibly give you a large harmonic current on the neutral. This is where an oversized common neutral or separate neutrals would be used.

 

[Fred B]

Explain MBBC's is that. MAIN BREAKER BRANCH CIRCUITS ?

If you are referencing what others have posted already you mis-typed, it's MWBC - Multi Wire Branch Circuit.

Did I understand. correctly ?
Your answer is yes / 20 amp Branch circuit Breakers for circuits. 2, 4 & 6. all loaded with 8 general purpose duplex receptacles, a common neutral is allowed.
How do you balance the loads on the all. Receptacle Panel.
All receptacle CIRCUITS. ONLY. ?

As far as a general "Yes You Can" response I don't think we can give it 100%, there are a lot of variables to consider. What are the applications anticipated? Is it residential or commercial or industrial?
If each circuit of the MWBC can be expected to each reach near the capacity of the individual circuit, consideratation to sizing of the neutral of the MWBC and load capacity of it. For example if you have a large office complex, and have the suggested 2,4,6 MWBC, used on general purpose receptacles, you could have a cleaning crew that all run vacuums at same time, (depending on location of receptacles) those vacuum running individually on each branch would not overload the 20A breakers, but would overload the neutral if not upsized to carry the potential 9-12 Amps each of the vacuums.

AFA balancing the loads I don't see an issue nearly as much as I see an issue as already mentioned of neutral load, in general balance of loads is desired in relationship to panel loads, but with "general purpose" receptacle near impossible to acheive when user specific loads are applied. Also considerations to other loads to be potentially applied to panel and the locations within the panel and what phase each load is applied to, as I'm sure you're aware of.

I'm sure there are other considerations, and questions not asked or answered, that effect the answer as to use of or installation of a specific wireing method, and is why DIY instruction not given on this forum.

The short answer to your question would be "Maybe", yes you can potentially have MWBC on the 2,4,6" but the answer to all the "only IF questions" would need to be addressed. And as a designer (assuming you mean "electrical designer" not a "fashion designer") of a system you should have the knowledge of the all requirements of the system you are putting together, and the answer to your opening question can vary depending on how you answer the various requirements of the circuits you are designing, so I think it would be irresponsible to give a blanket "Yes".

 

[infinity]

As far as a general "Yes You Can" response I don't think we can give it 100%, there are a lot of variables to consider. What are the applications anticipated? Is it residential or commercial or industrial?
If each circuit of the MWBC can be expected to each reach near the capacity of the individual circuit, consideratation to sizing of the neutral of the MWBC and load capacity of it. For example if you have a large office complex, and have the suggested 2,4,6 MWBC, used on general purpose receptacles, you could have a cleaning crew that all run vacuums at same time, (depending on location of receptacles) those vacuum running individually on each branch would not overload the 20A breakers, but would overload the neutral if not upsized to carry the potential 9-12 Amps each of the vacuums.

Just a little clarification, in your three vacuums example unless the vacuums are causing harmonics the load on the neutral would be 0 amps if the current were the same on each circuit. So for non-harmonic loads the neutral can be the same size as the phase conductors without being overloaded.

 

[Fred B]

Just a little clarification, in your three vacuums example unless the vacuums are causing harmonics the load on the neutral would be 0 amps if the current were the same on each circuit. So for non-harmonic loads the neutral can be the same size as the phase conductors without being overloaded.

I stand corrected.
Thank you

 

[roger]

Explain MBBC's is that. MAIN BREAKER BRANCH CIRCUITS ?
Did I understand. correctly ?
Your answer is yes / 20 amp Branch circuit Breakers for circuits. 2, 4 & 6. all loaded with 8 general purpose duplex receptacles, a common neutral is allowed.
How do you balance the loads on the all. Receptacle Panel.
All receptacle CIRCUITS. ONLY. ?

You are talking about a 208Y/120 V system correct?

Roger

 

[ActionDave]

I dont think the harmonic thing is an issue 99.97% of the time. I run MWBC's whenever possible. Yes, three phase MWBC's where they are all receps, whenever possible.

I agree. I know the possibility of harmonics burning up a three phase neutral but I have yet to see it.

I work in a lot of old office buildings wired up in the 70s and 80s that have original MWBC wiring. The lights have been changed to electronic ballast or LED and computers and printers are plugged in almost everywhere possible and things are humming along just fine.

 

[electrofelon]

I agree. I know the possibility of harmonics burning up a three phase neutral but I have yet to see it.

I work in a lot of old office buildings wired up in the 70s and 80s that have original MWBC wiring. The lights have been changed to electronic ballast or LED and computers and printers are plugged in almost everywhere possible and things are humming along just fine.

I was waiting for you to back me up on this, and you came through!

 

[ptonsparky]

I tried real hard to find harmonic problems back when this became a buzzword. I don't remember that I ever did. Limited exposure here, for sure.

 

[LarryFine]

I still like MWBCs.

 

[James L]

If I'm pulling through pipe, I really like MWBCs

Pulling MC or NM, it really depends.
Sometimes the jumper and extra time making up doesn't benefit anything at all.

FWIW, I've worked on a University medical office building that had MWBCs EVERYWHERE, and computers plugged in all over the place, without any issues at all

 

[JEFF MILLAR]

Thank you all for your replies.
As a good electric design it is now my opinion that 3 phase / 4 wire circuits WITH THE COMMON NEUTRAL should only be used on lighting circuits where the loads are the same on each phase. Yes that is still standard design practise. On all other 3 phase / wire circuits i will consider the neutral as a current carrying conductor. And all receptacle circuits will have their own neutral for each circuit. As always your comments and experience much appreciated. My interest besides understanding the NEC is the best possible design, with safety and obviously the cost factor. But safety and good design standards rule.

 

[LarryFine]

There is no issue with unbalanced receptacle loads on MWBCs. Same with turning off some of the lights.

Balancing loads is desirable in the design phase (no pun intended), but not an operational requirement.

 

[infinity]

Thank you all for your replies.
As a good electric design it is now my opinion that 3 phase / 4 wire circuits WITH THE COMMON NEUTRAL should only be used on lighting circuits where the loads are the same on each phase. Yes that is still standard design practise. On all other 3 phase / wire circuits i will consider the neutral as a current carrying conductor. And all receptacle circuits will have their own neutral for each circuit. As always your comments and experience much appreciated. My interest besides understanding the NEC is the best possible design, with safety and obviously the cost factor. But safety and good design standards rule.

I see this as a standard practice even though it may have no good reason electrically for doing so. In the end it costs someone more money to run separate neutrals for each circuit but it does eliminate the simultaneous disconnect problem where you would need to turn off 3 circuits at a time to work on one of them.

 

[roger]

I see this as a standard practice even though it may have no good reason electrically for doing so. In the end it costs someone more money to run separate neutrals for each circuit but it does eliminate the simultaneous disconnect problem where you would need to turn off 3 circuits at a time to work on one of them.

Besides installation expense two wire circuits are not as efficient as MWBC's in daily operation.

Roger

 

[Fred B]

I see this as a standard practice even though it may have no good reason electrically for doing so. In the end it costs someone more money to run separate neutrals for each circuit but it does eliminate the simultaneous disconnect problem where you would need to turn off 3 circuits at a time to work on one of them.

It may become more an issue if they start pushing AFCI into non residential applications. I dont know of or haven't seen any 2 pole let alone 3 pole AFCI breakers yet.
(Just to throw a wrench into it).