MWBC= more heat or Less heat?

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Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
Was on a light commercial job and noticed that the 1/2" emt was a bit warm to the touch. The conduit contained lighting circuits that were appropriately load-- about 13 amps per phase. The conduit had 6 conductors 3 phases and individual neutrals. Each neutral is obviously a CCC but my question is would there be less heat in the conduit if it were wired as a MWBC? I realize the extra neutrals take up more space so heat will not dissipate as well but I want to know if there would actual be less heat produced with a MWBC?
 

chris kennedy

Senior Member
Location
Miami Fla.
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60 yr old tool twisting electrician
If there are two less CCC's in the conduit and your one grounded conductor is carrying close to zero amps, then yes, less heat.
 

bob

Senior Member
Location
Alabama
Each neutral is obviously a CCC but my question is would there be less heat in the conduit if it were wired as a MWBC? I realize the extra neutrals take up more space so heat will not dissipate as well but I want to know if there would actual be less heat produced with a MWBC?
If you used 3 phases and one neutral with a balanced load there would be no neutral return current and thus no heat generated in the neutral. However if you load generated harmonics, they would add in the neutral and thus heat generated. So it depends.
 

Dennis Alwon

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I get all that but I am trying to understand why a load of 20 amps would create more heat in one situation. Your answers were as I thought but it seems 15 amps will only produce "X" amount of heat. Obviously not.
 

Dennis Alwon

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So in order to avoid 3 pole breakers under the 2008 code we will be increasing the heat in the conduits. Sounds like a bad deal all around.

I should have prefaced this with the fact that I stink at theory. I guess it is obvious now.... :grin:. Yes I am embarrassed but c'est la vie.....
 
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chris kennedy

Senior Member
Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
Here is the 3? neutral calc. For a balanced load plug in all the same values for your ungrounded conductors like 10's and see the result is zero. This of course would be for linear loads.

 

480sparky

Senior Member
Location
Iowegia
I recall Ed MacLarren posting the calcs for 2 circuits , and the Vd for a 3wire MWBC was less than for 2 circuits with seperate noodles.
This was what he posted:

 
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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
. . . I want to know if there would actual be less heat produced with a MWBC?
Yes, all other things being equal. Three wires carrying 13a each and a neutral carrying nothing would develop less heat than six conductors carrying 13a each. Same with 20a each.
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
Thanks all. I understand the balanced loads and the calcs for load on the neutral. I just don't get it..... If something draws 15 amps, in my mind, it should produce "x" amount of heat no matter what but it is obviously not true. Probably what I need to do is understand the reasoning WHY there is no current on the neutral and that's what I can't see. Formulas work and I accept it but don't get it...

I better go do something I am good at--- Oh yeah-- time to go dancing. That I can do.
 

ToddN45

Member
If your lighting is flourescent with electronic ballasts you have a non-linear load. It screws up the sine waves and creates "dirty power". A more likly solution to your problem is to oversize to neutral to handle to non-linear loading.
 

rfwells

Member
Location
PDX
I get all that but I am trying to understand why a load of 20 amps would create more heat in one situation. Your answers were as I thought but it seems 15 amps will only produce "X" amount of heat. Obviously not.
In your mind turn the 2 wire circuit into heat trace, 5W per foot. Now, run it back and forth 4 or 5 times in the same space (such as in a conduit). more heat or no?
 

PetrosA

Senior Member
Think of it this way - at the panel you have a neutral bar that equalizes all the current from the phases to the neutral feeding the panel. From that point back, the neutral is carrying only the remaining (not phased out) current. By using a shared neutral in a MWBC, you are in effect extending the neutral bar to the JB where the three phases (or two legs) split off. So you have only one conductor carrying the grounded current back to the neutral bar at the panel. If you run separate neutrals, that current has to travel through each wire back to the neutral bar to be equalized, thus each neutral is a current carrying conductor. I'd also venture to guess that the current being carried by the separate neutrals would really like to induct back together and achieve equalization as soon as possible, also creating a rise in temperature, but an engineer would have to comment on that theory :)
 

Dennis Alwon

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Do you grasp the concept in single-phase systems?
I understand how to calculate the load on the neutral but I never understood why it works that way. I see 15 amps going in on a 2 wire system and returning on the neutral. I get that both carry 15 amps. I guess what I don't see is why with the MWBC there isn't 30 amps coming back on the neutral with 2 hot conductors of different phases when the load is 15 amps on each. I intellectually know and can measure it but the concept isn't there. I never had a course in electricity and yet I have done well for 30 years runing a business not knowing the theory.

Yes, it would be good to know the theory but not imperative . I am also one of those people who basically says if you can't see it then it's not there. :grin:
 

Dennis Alwon

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Location
Chapel Hill, NC
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From that point back, the neutral is carrying only the remaining (not phased out) current.
Now I am lost with the term phased out current but the rest I get.

rfwells said:
In your mind turn the 2 wire circuit into heat trace, 5W per foot. Now, run it back and forth 4 or 5 times in the same space (such as in a conduit). more heat or no?
This I get but does not help me with what actually goes on.
 

480sparky

Senior Member
Location
Iowegia
I understand how to calculate the load on the neutral but I never understood why it works that way. I see 15 amps going in on a 2 wire system and returning on the neutral. I get that both carry 15 amps. I guess what I don't see is why with the MWBC there isn't 30 amps coming back on the neutral with 2 hot conductors of different phases when the load is 15 amps on each. I intellectually know and can measure it but the concept isn't there. I never had a course in electricity and yet I have done well for 30 years runing a business not knowing the theory.

Yes, it would be good to know the theory but not imperative . I am also one of those people who basically says if you can't see it then it's not there. :grin:

If you understand that two 15a loads that share a neutral in a MWBC are truly 'balanced', then you know that the neutral will carry 0 amps, and can, for all intents and purposes, can be physically removed.
 

rt66electric

Senior Member
Location
Oklahoma
I recall Ed MacLarren posting the calcs for 2 circuits , and the Vd for a 3wire MWBC was less than for 2 circuits with seperate noodles.
This was what he posted:

[/QUOTE

In diagram "B" - Shouldn't the arrows next to the 60 and 20 be reversed??? I always thought that electrical waves reacted similar to waves behind your boat when you are turning tight circles.

Sometimes they double their size, and other times they cancel out (pulling youngsters on a tube is one of my favorite hobbies)_ I've always wanted to get three boat drivers going in a circle (like a three phase motor ) to see what would happen??? Then run the tuberider/victim through the apex of waves. My unofficial record in getting a fatboy-little brother- over 12ft high

back to the subject --- in diagram B -- at the neutral point after load "A" and load "B" where there would be a wire-nut, 20 amps would 'magically disappear' or cancel out???? Similar to water waves cancelling out at the point of impact???


Electrical theory 101


Chemistry 101--- electrons -- valance ion travel from atom to atom valance
simliar to asteroids being moved from one ring to the other on the rings of Saturn(s)

Electronics 101-- Direct current -electrons move from holes in one substrate to other holes in another substate --similar to marbles on a chinese checkers
game

Electrical tramsmission theory 101---"ELECTRICTY" moves along the outside(rim) of wires, The ring theory , it is similar tolooking at a bullseye the greater the amperage, the bigger the rings on the bullseye ( a tic-tracer detects at the amplitude (rings)of Electomagnetic force).

Electrician apprentice training 101--- voltage = hurts amperage=death

LIGHTNING theory 101--- Electricity travels from the sky to the Earth

Motor theory 101 --If you wind a copper wire around a metal shaft energy is tranfered to rotating motion??? (I still haven't figgured-out how a transformer doesn't blow up as soon as you energize it???) just because you wind copper around steel it becomes "magnetic" and prevents direct phase to phase contact-- don't ask-just accecpt it - it works??



After years of -education, practically knowledge, experience, multi-discipline studies, personal arc-flash studies, many budweisers later---------
I HAVE A ------ UNIFIED---- THEORY of explaining electricty



The world SUCKS
 

Cold Fusion

Senior Member
Location
way north
rt66electric;1070152---I HAVE A ------ UNIFIED---- THEORY of explaining electricty The world SUCKS[/quote said:
Some of us, after long hours of peer group conversation while sparked by many Alaskan Ambers, have formulated a unified theory of gravity that doesn't contain any electrical phenomena:
"There is no gravity, the earth just sucks."

cf
(credit to the unknown philosopher - definitely not a original with me)
 
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