Receptacle circuits common neutral

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Fred B said:
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).
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Something to keep in mind about 'balancing' in MWBC systems: (in the below I am considering 3 phase systems, and ignoring harmonics and power factor)

The 'balancing effect' is always present, even if the loads are not perfectly balanced. This causes the net heating of the MWBC to always be less than the fully loaded balanced circuit.

Whenever the loads are not perfectly balanced, some current must flow in the neutral. But for this to be the case, less current must be flowing in one of the hots, so the total heat produced goes down.

Since derating for CCCs depends upon total heat produced, then for this situation the neutral doesn't count as a CCC.

Consider the worst case for heating, three circuits fully loaded. You have no current on the neutral and thus 3 CCC. Now consider the worst case imbalance, 2 circuits fully loaded but nothing on #3. The neutral gets fully loaded, but you still only have 3 fully loaded conductors, this 3 CCC. In any of the intermediate cases you have less total heating.

Harmonics can change this math, because certain harmonics don't balance on the neutral. Power factor of loads can also change this, and mixing L-N and L-L loads can create situations where a 3 phase MWBC has 4 current carrying conductors.

Jon
 
JEFF MILLAR said:
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.
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Not really, it wouldn't matter much if they are not the same.
JEFF MILLAR said:
Yes that is still standard design practise. On all other 3 phase / wire circuits i will consider the neutral as a current carrying conductor.
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Which is very rarely the case.
JEFF MILLAR said:
And all receptacle circuits will have their own neutral for each circuit.
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And even though it is the normal practice now it is not necessarily the best practice. As pointed out, damaging odd order harmonics is greatly exaggerated.

Roger
 
Fred B said:
Ok. So just not so readily available here, as when I've mentioned at supply house they didn't know of any. So these mean can start running MWBC in the resi settings again?
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There's no law against MWBCs, and I'm not against them. For me, it's just not any faster, easier or cheaper
 
Fred B said:
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).
Click to expand...
Why not single poles with a handle tie?
 
Office furniture manufacturers started using a #10 neutral with #12 phase conductors years ago before the handle tie requirements came in, but as others have said, I have not seen any overheated neutrals. Could be a problem in server rooms where circuits are loaded heavily, but with office furniture, the heavy loads were resistive space heaters under the desk! LOL!
 
JEFF MILLAR said:
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.
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But "lighting" no long means Thomas Edison resistive loads; it means LED's driven by switchmode power supplies.

That means non-sinusoidal loads, and that means harmonics.
 
Very fixture dependent Open Neutral. Might be one area where going with a Euro brand is smarter. Good ones are typically in the single digits which beats most fluorescent/HID options.

Seems these days almost everything is non-sinusoidal.
 
OK, COMMERCIAL, NON - RESIDENTAL . NEC. 220. 44
Regular. use. duplex receptacles ( 180 va ). installed on three circuits ( A , B , C. ie eg. 2 , 4 and 6 ). with a common neutral.
Your opinion is this is preferred. over. separate neutrals for each circuit.
I want to learn from you all.
 
It is only preferred from a cost savings point of view.

The most common MWBC neutral failure I have seen is the modular office furniture with the oversized neutral another poster mentioned. I'd blame the "fingers" on the connection (you would have to work with the stuff to understand). The second most common is stranded wire where electricians pre-twisted the wire.
 
JEFF MILLAR said:
OK, COMMERCIAL, NON - RESIDENTAL . NEC. 220. 44
Regular. use. duplex receptacles ( 180 va ). installed on three circuits ( A , B , C. ie eg. 2 , 4 and 6 ). with a common neutral.
Your opinion is this is preferred. over. separate neutrals for each circuit.
I want to learn from you all.
Click to expand...
Most of us that are regular participants here all worked in commercial before the addition of 210.4(B) ie the handle tie rule so all we ever did was run MWBC for everything. Because of current code requirements and a constant drum beat of anti-MWBC hysteria they have fallen out of favor. There is, however, a lot to like about them- greater efficiency because of lower voltage drop, more efficient use of materials as well can save thousands of dollars.

The fear of the open neutral is so overstated. There has to be millions of MWBC in service right now all of them functioning trouble free for decades.
 
ActionDave said:
Most of us that are regular participants here all worked in commercial before the addition of 210.4(B) ie the handle tie rule so all we ever did was run MWBC for everything. Because of current code requirements and a constant drum beat of anti-MWBC hysteria they have fallen out of favor. There is, however, a lot to like about them- greater efficiency because of lower voltage drop, more efficient use of materials as well can save thousands of dollars.

The fear of the open neutral is so overstated. There has to be millions of MWBC in service right now all of them functioning trouble free for decades.
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I agree. For decades we used MWBC's without buildings burning down. IMO it was the people who represented the copper industry who pushes this hysteria to sell more wire two decades ago. The handle tie rule did more to make MWBC's obsolete than the potential for neutrals being overloaded.
 
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