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FAQ: When is a neutral a current carrying conductor?

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    FAQ: When is a neutral a current carrying conductor?

    I'd like to propose an addition to the FAQ.

    <B> When does a neutral count as a current carrying conductors for purposes of 310.15(B)(4)?</B>

    Code requires that the ampacity of conductors be adjusted if several conductors are bundled together, for example in a single raceway. The reason is _heat_. The rules of 310.15(B)(2) are there to prevent excessive temperatures caused by the heat production of several conductors bundled together.

    In general, <B> all </B> of the conductors in a circuit will carry some current, and this includes the neutral conductor. Thus, in general <B> all </B> conductors count when applying 310.15(B)(2).

    There are a few specific circumstances where the neutral doesn't 'count' for 310.15(B)(2); these circumstances are outlined in 310.15(B)(4).

    These rule can be better understood if the focus upon the neutral is ignored. Rather than asking 'does the neutral carry current', ask 'Given this set of 3 (or 4) conductors, how much heat is produced?'

    In general, the neutral will carry some current, even when 310.15(B)(4) tells us that we don't have to count it.

    There are _two_ specific situations where this applies.

    1) The neutral conductor of a 3 wire circuit derived from a _single_ phase system. In this case, even though there are _three_ conductors which potentially carry current, the _net_ heat produced by these three conductors is always less than or equal to that of _two_ fully loaded conductors.

    2) The neutral conductor of a 4 wire circuit derived from a three phase wye connected system where there the major portion of the load is linear. In this case, even though there are _four_ conductors which potentially carry current, the _net_ heat produced by these four conductors is always less than or equal to that of _three_ fully loaded conductors.

    In the following cases, the neutral must count for 310.15(B)(2)

    a) Single phase circuits with _dedicated_ neutrals. In this case, there are _two_ conductors which potentially carry current, and the net heat produced will be that of two conductors.

    b) three wire circuits consisting of two phase conductors and a neutral, derived from a three phase wye connected system. In this case, there are _three_ conductors which potentially carry current, and the net heat produced will be as much as that of _three_ fully loaded conductors.

    c) four wire circuits consisting of three phase conductors and a neutral, where the major portion of the load is non-linear. In this case, there are _four_ conductors which potentially carry current, and the net heat produced can exceed that of _three_ fully loaded conductors.

    Basically the implications of 310.15(B)(4) are that we only have to count a total number of current carrying conductors equal to the worst case heating expected for a given circuit.

    -Jon

    #2
    Sorry, but I cannot go along with your approach.

    Originally posted by winnie
    The reason is heat. The rules of 310.15(B)(2) are there to prevent excessive temperatures caused by the heat production of several conductors bundled together.


    That is true. It is the physics behind the rule. But it is not the wording of the rule.

    Originally posted by winnie
    Thus, in general all conductors count when applying 310.15(B)(2).








    Originally posted by winnie
    (2) The neutral conductor of a 4 wire circuit derived from a three phase wye connected system where there the major portion of the load is linear. In this case, even though there are four conductors which potentially carry current, the net heat produced by these four conductors is always less than or equal to that of three fully loaded conductors.




    I appreciate your attempt to help us here. But I tend to prefer to keep things as simple as possible. The simplest approach I can find on this topic is simply to read the rules, as written. They tell you whether to count the neutral in a number of scenarios. I think that is enough information for my purposes.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

    Comment


      #3
      Given the number of times people ask about when to count the neutral as a CCC, I think that this does need to be addressed with something more than read the code

      However I see your point about starting with the code itself, giving and giving the FAQ answer based upon the code. I'll mull on this and write something up later that gives the simple answer, possibly putting the physics in _after_ the answer.

      -Jon

      Comment


        #4
        Winnie,

        Something else to look at using in your explanation is how the current is balanced, because that's the language the code uses -- a neutral does not count as a current carrying conductor if all it is carrying is the unbalanced current.

        A MWBC derived from a single phase system balances the current between the ungrounded phase conductors (the same thing happens with 3 phase, but I digress). As I understand the code, the issue is if the circuits are fully loaded, do the ungrounded phase conductors "balance" the load between themselves such that no current is carried on the neutral.

        That's where I'd put the focus -- what does "balance" mean and how can you know if that's what's going on. THEN I'd explain why that's important using the physics.
        Julie in Austin

        Born to brew, forced to work ...

        Comment


          #5
          Originally posted by tallgirl
          As I understand the code, the issue is if the circuits are fully loaded, do the ungrounded phase conductors "balance" the load between themselves such that no current is carried on the neutral.

          Julie, I believe that you understand it correctly, but I am not sure you said it correctly. Perhaps what follows is essentially saying the same thing; perhaps not. You judge for yourself.

          It is not necessary that the circuits are fully loaded, nor that at any point in time the neutral carries no current. But if, at a given moment, the loads on the ungrounded conductors are balanced, and if at that moment the neutral carries no current, and if the neutral would only carry current when you change the balance of loads (e.g., turn something on or off on one of the phases), then in that particular configuration the neutral is not counted as a “current-carrying conductor.” I think this is essentially what you were trying to say.

          For example, in a MWBC consisting of two ungrounded conductors and one grounded conductor, all derived from a 120/208V three phase system, you could perfectly balance the loads on the two sides, and the neutral will still carry current. It doesn’t just get the unbalanced current, it gets it all.

          Let me go back to Winnie’s point. Consider a MWBC consisting of two ungrounded conductors and one grounded conductor, all derived from a 120/240V single phase system.

          CASE 1: Load Phases A and B fully and equally. Both conductors carry their maximum current, and both generate heat. The neutral carries no current and generates no heat.

          CASE 2: Now turn off half of the loads on Phase A only. Phase B has the same current and generates the same heat. Phase A has a lower current and generates less heat. The neutral now has current, so it generates some heat.

          RESULTS: The total heat in the second case must necessarily be equal to, or lower than, that in the first case. It can be proven with a bit of messy math. That is why it is OK to not count the neutral in this configuration. Nothing you can do with neutral current will give you more heat than you had in the balanced case.
          Charles E. Beck, P.E., Seattle
          Comments based on 2017 NEC unless otherwise noted.

          Comment


            #6
            To answer the OP`S question. On a 120 volt circuit the neutral is always counted a CCC.

            But when you get into bundling the same count happens. A 3 wire is counted as 3 CCC`s a 2 wire is counted as 2 CCC`s.You just have to derate what is there depending on the install.Every install has their own particulars.JMHO
            .

            Comment


              #7
              Originally posted by winnie
              I'll mull on this and write something up later that gives the simple answer, possibly putting the physics in _after_ the answer.
              How about writing the simple answer, and then we can edit in links to related threads (ie this one) at the bottom, to add in the rest of the story? For an example, check out this one. I wrote a brief explanation of the main point of each thread.

              I agree with including this in the FAQ, it's a good topic - and I appreciate you going through with it, Jon. I look forward to seeing your next draft. :cool: :cool:

              Comment


                #8
                Cough cough, I look forward to seeing your next draft.

                Man, I can be a nuisance...

                Comment


                  #9
                  There's 2 ways to go.
                  The long explanation and the short list.

                  You're working on the long explanation, I'd rather make the short list.

                  Don't count any of the following:

                  1] Non-current carrying conductors
                  . a] Equipment bonding/grounding conductor
                  . b] Travelers between 3ways or 4ways are never both current carrying conductors at the same time so don't count both. . Only count the pair as one.

                  2] Unbalanced current only conductors
                  . When all grounded hot conductors of the system are sharing a grounded conductor in a multiwire feeder or branch circuit and present in the wireway in question.

                  David
                  David
                  I get paid for doing stuff in Ohio

                  Comment


                    #10
                    Originally posted by winnie
                    Pushy, Pushy

                    Is the neutral a current carrying conductor? How do I apply 310.15(B)(4)?

                    First the silly answer: the neutral always carries current.

                    310.15(B)(4) tells us that we don't need to account for neutral conductors that only carry 'the unbalanced current from other conductors of the same circuit'. These conductors still carry some current, but we are permitted to ignore them in our accounting when applying the adjustment factors of table 310.15(B)(2)(a).

                    This is found in two common situations.

                    1) A single phase feeder or multi-wire branch circuit consisting of two 'hots' (ungrounded conductors) and a single 'neutral' (grounded conductor). In this case, the neutral carries only the unbalanced current of the two hot conductors, we would count a total of two current carrying conductors.

                    2) A three phase feeder or multi-wire branch circuit consisting of three 'hots' (ungrounded conductors) and a single 'neutral' (grounded conductor) where the major portion of the load is linear. In this case, the neutral carries only the unbalanced current of the three hot conductors, we would count a total of three current carrying conductors.

                    The rest of the time we have to count the neutral.

                    To understand this, remember that 310.15(B)(4) is all about accounting, not about reality. This is about figuring out the correct number from 310.15(B)(2)(a) to apply, not about the minute details of how many conductors actually have a bit of current flowing on them.

                    The main requirement is that the conductor carry only the unbalanced current of the other conductors in the same circuit. This tells us that there have to be other conductors, and that these other conductors have to be able to carry the _balanced_ current of the circuit. If the other conductors can possibly carry all of the balanced current, then only the unbalanced current is left for the neutral. This is the case when 310.15(B)(4) kicks in. The neutral may carry some current, but it doesn't count for 310.15(B)(2)(a).

                    But if the 'hot' conductors are not balanced, then some of the balanced current must flow on the neutral. This this case with any single 'line-neutral' circuit, and is also the case when you have two phases of a three phase wye system sharing a neutral. The neutral carries both unbalanced and balanced current, and must be counted for 310.15(B)(2)(a).

                    Harmonics confuse this a bit: the neutral has to carry the _balanced_ triplen harmonic current of the load. We are permitted some leeway, but when the major portion of the load is non-linear, the neutral is no longer considered to carry _only_ unbalanced current from the other conductors, and thus must be counted.

                    -Jon
                    Sold. :cool:
                    Last edited by George Stolz; 03-14-07, 08:41 PM.

                    Comment


                      #11
                      Originally posted by dnem

                      . When all grounded hot conductors of the system

                      David
                      Is this a typo or am I missing something?
                      Nothing sells convenience like inconvenience.

                      Comment


                        #12
                        That is a typo. David meant to type 'ungrounded hot conductors'.

                        I have to admit the value of the simple list that he presents; my personal style tends toward explaining the physics of what is going on rather than just giving the answers. George pushed me on that exact point.

                        I personally believe that the answer is not good enough without some of the background; this way new situations can be handled correctly. But too much background is not a good thing; if you want to actually get work done than it is important to actually _know_ the answer rather than have to figure it out each time

                        Pop quiz:

                        A 120/208 4 wire feeder from a wye source feeds a panel. The panel is fully loaded with a large heater connected from the A leg to the B leg, with 120V loads fed between the C leg and the neutral. Is the neutral a 'current carrying conductor'.?

                        -Jon

                        Comment


                          #13
                          Originally posted by winnie
                          Pop quiz:

                          A 120/208 4 wire feeder from a wye source feeds a panel. The panel is fully loaded with a large heater connected from the A leg to the B leg, with 120V loads fed between the C leg and the neutral. Is the neutral a 'current carrying conductor'.?

                          -Jon
                          Unless I'm missing something that's not a particular hard question and the answer is "Yes".

                          Which means I'm missing something and the answer is "No" ...
                          Julie in Austin

                          Born to brew, forced to work ...

                          Comment


                            #14
                            I see something tricky in the wording of the question. If that trickiness is intentional, it may be enough to turn the answer into a "no."

                            The question is poorly worded, in two ways.

                            First, it talks of the 120 loads in the plural, but talks of the neutral in the singular. So when it asks whether the neutral (singular) is (again, singular) a current-carrying conductor (and yet again, singular), it should ask whether each



                            So fess up, Winnie, what's your answer? What's your game?
                            Charles E. Beck, P.E., Seattle
                            Comments based on 2017 NEC unless otherwise noted.

                            Comment


                              #15
                              My bad. I am not trying to be tricky in the wording, but trying to describe a tricky situation.

                              In the scenario I described, I want to know if the neutral of the feeder to the panel is a current carrying conductor. Clearly the neutral(s) to the single phase 120V loads are current carrying conductors.

                              As I see it, we have a 'full boat' going to this panel. Three hot conductors and a neutral. However the panel has been set up to be quite imbalanced in a tricky way. The load on the phase conductors is the same, but some of the load doesn't return to neutral, and some does.

                              Seeing that full boat, and not having any non-linear loads specified, most of us would say that the neutral doesn't count as a CCC.

                              By the physics of this rather hypothetical situation, the neutral most definitely _is_ a CCC.

                              The question, and one that I don't know the answer to: Given how the code is written, are we required to count this neutral as a CCC?

                              -Jon

                              Comment

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