Current carring conductors
- Thread starter Big Guns
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Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
That is actually a good question. As I was getting ready to type the stock answer of, when it is carrying harmonics, I realized that the harmonics of concern are triple harmonics, so I am guessing the answer is never for purposes of article 310.15. I will await the answers of some more expert persons on this forum.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I think the correct answer is "never." (What, never? No never. What, never? Well, hardly ever! :lol: ) The neutral would only be carrying the unbalanced current from the two phase conductors, and so it would not be counted.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
To clarify, Roger's answer and my answer are not really different :happyno:, as they are both correct answers to different questions :happyyes:. He interpreted the "system" word to mean that you are talking about any and all circuits fed from a 120/240V source. I interpreted the question as refering to a 120/240 volt feeder to a panel.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What about a three wire cable with two switch legs and a neutral?
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
First, I learned something new today. The NEC 310.15B no longer requires an Engineer to do diversity calculations.
So, to answer the question, if you actually mean switch legs and a neutral from the same circuit, then using diversity calculations, you would have the equivalent of no more than 2 current carrying conductors, because even without doing the load diversity calculations you would have less than 51% of the circuit current on each switch leg and then the neutral as a current carrying conductor. If you, in fact, mean a switch feed, a switch return and a neutral, they would all be current carrying conductors.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I have no idea what you mean by "diversity calculations." Also, I don't see anything in the 2008 or 2011 that would have required them, or required an engineer to do them. Can you explain what you meant?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Here's what I meant:
-12/3 cable
-same circuit
-Switch A w/6 amp load
-Switch B w/6 amp load
-Neutral 12 amp load
-all 3 wires in the 12/3 cable are CCC's.
-12/3 cable
-same circuit
-Switch A w/6 amp load
-Switch B w/6 amp load
-Neutral 12 amp load
-all 3 wires in the 12/3 cable are CCC's.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
If you had a three wire MWBC from a 120/240 volt system the neutral would only carry the imbalance between the two phase conductors so I would not count as a CCC. Take a look at 310.15(B)(4).
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
On a single phase 120/240 v system the neutral carries current on a 2 wire circuit. If it is a MWBC then it does carry current - the difference between the two phases- but it does not count as a CCC because there is never more than 40 amps total on all three wire. If phase A had 20 amps and phase B had 0 amps then the neutral has 20 amps also. That is the worse case scenario.
On a 3 phase system the neutral is counted when we have a 2 wire cir or a 3 wire mwbc. If we have 3 hots and one neutral then the neutral does not count as a CCC even though it may carry current.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
I used shorthand, but what it is referring to is called "load diversity." It is now dealt with in annex B. Basically, it is recognition in the code that, if the wires in the conduit are not carrying full load at the same time then the heat generation is less and the number of "current carrying conductors can be increased."
Regarding the code reference year, it could have been 12 years ago, for all I know, because I don't remember the last time I looked and I am getting old. There use to be a fine print note, that made a statement something about a load diversity of 50% calculated "under engineering supervision" cauld be used to increase the number of current carrying conductors.
Sorry for the confusion.
renosteinke
Senior Member
- Location
- NE Arkansas
I am going to stick my neck out, and go completely against the flow here .... and assert that a neutral is ALWAYS counted as a current-carrying conductor- even if it only rarely carries incidental amounts of current.
I say this, first, because the distinction only matters when we are talking about derating conductors. Specifically, when the 'current carrying' wire count exceeds three. Let's look at an example to understand the situation ....
Assume a pipe has two circuits in it. We'll do it once as a MWBC, and once as separate neutrals.
As an MWBC, you've already reduced your wire count by 'sharing' the neutral. That neutral can carry as much as half the combined rating of the 'hots.' There's no doubt that it's carrying the full current at least some of the time. In any event, this 'shared' neutral isn't going to affect the derating calculation, counted or not, as the derating starts at four conductors.
As separate neutrals, we have both neutrals carrying the full amount of current all the time. If anything, this will result in additional heating - now, isn't that a contradiction: more copper = more resistance! So, adding that fourth wire kicks in the requirement to derate.
Compare this to the role of the ground wire, which carrys current only when something goes wrong. Ordinarily, it only takes up space - so we count it for wire fill, but not for heating derating.
I say this, first, because the distinction only matters when we are talking about derating conductors. Specifically, when the 'current carrying' wire count exceeds three. Let's look at an example to understand the situation ....
Assume a pipe has two circuits in it. We'll do it once as a MWBC, and once as separate neutrals.
As an MWBC, you've already reduced your wire count by 'sharing' the neutral. That neutral can carry as much as half the combined rating of the 'hots.' There's no doubt that it's carrying the full current at least some of the time. In any event, this 'shared' neutral isn't going to affect the derating calculation, counted or not, as the derating starts at four conductors.
As separate neutrals, we have both neutrals carrying the full amount of current all the time. If anything, this will result in additional heating - now, isn't that a contradiction: more copper = more resistance! So, adding that fourth wire kicks in the requirement to derate.
Compare this to the role of the ground wire, which carrys current only when something goes wrong. Ordinarily, it only takes up space - so we count it for wire fill, but not for heating derating.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Yes, the neutral carrys current but the NEC is very specific as to when we need to count it when applying derating. In your example with a single MWBC in a conduit it won't matter whether you count it or not. But Install 4 of those MWBC's in a conduit and it will change the final amapcity of those conductors.
What about Part A?
What about Part A?
310.15 B 4 says that a neutral conductor that carries only the unbalanced current from other conductors of the same circuit need not be counted? if this is not a branch circuit with a neutral then what is it? For example if we pull four 20 amp circuits each with their own neutral is that 4 ccc's or 8? It makes a big difference when you get to the derating. Any inspectors out there have an opinion? I assume that since this is in the code there must be a situation where the neutral is not considered a ccc otherwise the code would just say that it is a ccc 100% of the time.
What about Part A?
310.15 B 4 says that a neutral conductor that carries only the unbalanced current from other conductors of the same circuit need not be counted? if this is not a branch circuit with a neutral then what is it? For example if we pull four 20 amp circuits each with their own neutral is that 4 ccc's or 8? It makes a big difference when you get to the derating. Any inspectors out there have an opinion? I assume that since this is in the code there must be a situation where the neutral is not considered a ccc otherwise the code would just say that it is a ccc 100% of the time.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
That would be 8 CCC's. Post #11 has the reference.
No disrespect to the inspectors but, why would it matter what their opinion may be, it is spelled out in the NEC and the laws of physics, you would follow that.
In a two wire circuit both conductors carry the same current, In a single phase MWBC the neutral only carries the imbalance and would not be a CCC. In a three wire MWBC of a wye system, the neutral would carry the same as the ungrounded conductors and would be a CCC. In a four wire MWBC of a wye system the neutral would carry the imbalance and would not be a CCC.
The above is not taking any additive harmonics into consideration which is pretty much overblown anyways.
Roger
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