CCC, Nuetrals.....again

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jazer

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Gibsonia, Pa
I have such a hard time with this concept. Ok, from a 3 phase, 4 wire 120/208 volt panelboard, I have a 1 1/2" emt conduit that runs to j-box about 10' away. Inside of this conduit are 5 individual circuits, each designed to be 20 amp, 120 volt receptacle loads. The conductors are THHN #12. 5 blacks (hots) 5 whites (nuetrals) and a single EGC. Now, 310.15(B)(3)(a) and it's sub paragraphs talk about conductors that carry "unbalanced" current from other conductors of the same circuit shall not be required to be counted blah, blah, blah..... So, initially I want to believe that there are only 5 CCC's in the conduit, which would require an 80 percent derating. With THHN, that would still allow overcurrent protection to be 20 amps. However, since this is coming from a 3 phase system, the nuetrals of the first 3 circuits would not count, balancing each other out. How would the remaining 2 be counted? Seems to me that they should count as CCC's, bringing the total up to 7 total CCC's??? Drastically altering the calculation.....or am i overthinking this???

P.S. All other adjustment factors do not apply to this application.

Thanks
 

jumper

Senior Member
Rob/Infinity wrote this post which explains a few examples of when to count the neutral as a CCC or not.

Your ckts are described in A.

Here's some examples of when to count the neutral as a CCC:

208Y/120 volt system-different circuit types:

A)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C)- 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B)- In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C)- In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with one exception, *if the current is more than 50% nonlinear then the neutral would count as a CCC.

120/240 volt system-different circuit types:

D)- 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E)- 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's

Notes:
D)- A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E)- In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
 

charlie b

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However, since this is coming from a 3 phase system, the nuetrals of the first 3 circuits would not count, balancing each other out. How would the remaining 2 be counted?
The only way a neutral will carry unbalanced current from the ungrounded conductors is if a single neutal wire is shared with the three ungrounded conductors. That can be either a multi-wire branch circuit or a single load that needs three phase power and also needs a neutral.

In your situation:

  • The neutral of circuit #1 is carrying 100% of the current associated with ungrounded conductor #1, and
  • The neutral of circuit #2 is carrying 100% of the current associated with ungrounded conductor #2, and
  • The neutral of circuit #3 is carrying 100% of the current associated with ungrounded conductor #3, and
  • (Do I need to continue?)

 

infinity

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However, since this is coming from a 3 phase system, the nuetrals of the first 3 circuits would not count, balancing each other out. How would the remaining 2 be counted? Seems to me that they should count as CCC's, bringing the total up to 7 total CCC's???.

Thanks

Why wouldn't they count? Aren't you using a single neutral for each circuit? You could eliminate two neutrals and share one with three circuits and that one wouldn't count.
 

hurk27

Senior Member
Even better- 2 MWBCs (3H+1N) = 6H (CCCs)+2N (not CCCS), 8 (9 w/EGC) total conductors, less derating (70%) and an extra circuit to boot.

Whadda ya think?

Me thinks you got a little corn fused:slaphead:

3 MWBCs = 9H+3N = 9CCCs neutrals or grounds are not counted in CCC, total conductors though are 13 if you install the (not required EGC if in a metal (250.118) raceway)

Oh never mind I see what you were trying to do, I thought you were counting the neutrals and EGC in the derating, they would count for fill though.

Yep the magic number 9 CCC's where derating allows you to still use the conductor at its allowed ampacity, at least for 14awg and 12awg's for 10awg's the magic number is 8

With a WYE source it is tricky to understand that unlike single phase running just a 2-wire MWBC the neutral will still carry current as if a third phase circuit was ran but not used, this is why you have to count the neutral if you only run 2 hots and a neutral, so in your example you could remove 3 of the neutrals to make one full 3-phase MWBC, and one partial 2-hots/1-neutral MWBC, that would give you 6 CCC's but would also kick in the requirement of handle ties which if you got a local supplier that stocks them is not bad, but if you have to resort to 3-pole/two pole breakers then it could drive up the install cost.

but many IT workers and engineers don't like MWBC's because of the problem of lost neutrals and the damage that they can bring, so you get stuck running more conduits to the same place to avoid MWBS and derating problems, IT workers can be funny because they really don't understand the theory, I have had some think that a MWBC is more noisier, and or produces more stray voltages when both theory's are a myth in a NEC compliant installed circuit, but sometimes it's a battle you won't win.
As far as the lost neutral problem, this is where I think the NEC was dumb downed because a competent electrician should always verify the circuit they are working on before opening any neutrals, it's not that hard, what's next, all MWBC wire be marked to which breaker they are fed from, including the neutral? that is about the only thing they left out as they require the MWBC to be grouped at each access point now.
 

jazer

Senior Member
Location
Gibsonia, Pa
Why wouldn't they count? Aren't you using a single neutral for each circuit? You could eliminate two neutrals and share one with three circuits and that one wouldn't count.

Yes they would count, I understand my mistakes now. This is why I come to this place.....to get straightened out after a brain fart.

I typically don't share neutrals.....no good reason why......aside from having to use a multipole breaker and the potential to lose 3 circuits if only 1 is overloaded.......come to think of it, that is exactly why I typically don't use MWBC's.

Thanks for all of the great discussion, this is a topic that needs to be revisited often (at least for me) as a refresher on how critical it is to understand unbalanced current and how it needs to be safely transmitted.
 

roger

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I typically don't share neutrals.....no good reason why......aside from having to use a multipole breaker and the potential to lose 3 circuits if only 1 is overloaded.......come to think of it, that is exactly why I typically don't use MWBC's.
Actually there are a number of good reasons to use MWBC,s some being less material, less watts loss, less voltage drop, for a few.

The illustrations below are showing the losses in two single phase two wire circuits verses a MWBC.

Ed's3wireverses2wire.gif


Roger
 

jazer

Senior Member
Location
Gibsonia, Pa
Ok, I can't argue with that. However, those are some rather hefty circuits, looks more like a feeder. I think if I had 2 branch loads, one being 40 and the other 60, I would still run 2 seperate "single" circuits. Again, no good reason why, I just feel safer and more workman-like. I also can't think of time where I've seen such a large neutral branch circuit load.....which would create such a drastic difference in power losses. Also, the potential for inadvertant loss of the neutral and creating a 240 volt circuit across those 120 volt loads is ever present. Does the slight decrease in power loss outweigh the possible long term risks associated?
 

ActionDave

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Ok, I can't argue with that. However, those are some rather hefty circuits, looks more like a feeder. I think if I had 2 branch loads, one being 40 and the other 60, I would still run 2 seperate "single" circuits. Again, no good reason why, I just feel safer and more workman-like. I also can't think of time where I've seen such a large neutral branch circuit load.....which would create such a drastic difference in power losses. Also, the potential for inadvertant loss of the neutral and creating a 240 volt circuit across those 120 volt loads is ever present. Does the slight decrease in power loss outweigh the possible long term risks associated?
Since every service and feeder is a MWBC they are hard to avoid.
 

roger

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Does the slight decrease in power loss outweigh the possible long term risks associated?
I personally don't see any risk and if you look at it on a large scale in numbers of circuits (even small circuits) it certainly makes a difference.

Roger
 
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