Neutral as Current Carrying Conductor

[GoldDigger]

I see your problem. My opinion - take the total current of the neutral conductor in question and determine what percentage of it is from non linear loads, but I will consider other approach at this so fire away if you have one.

But by the time you can measure the neutral current it is a little late to change the wire size based on CCC count.

 

[don_resqcapt19]

But by the time you can measure the neutral current it is a little late to change the wire size based on CCC count.

I have not seen any real world case studies that show neutral harmonic current is an issue, other than data centers or places like that.

I have not seen any that showed issues based on non-linear lighting loads, or even a combination of lighting and computer loads like you find in an office building.

Sure there is a lot of theoretical information out there showing this may be a problem, but most of that information comes from sources that have a vested economic interest in helping you solve the "problem".

 

[Smart $]

But by the time you can measure the neutral current it is a little late to change the wire size based on CCC count.

True. I don't believe that's what he meant. But even considering the calculated maximum unbalanced neutral current would be incorrect. Code says major portion of the load on wye circuit. So neutral connected load and current has little bearing on the determination. You could have essentially all nonlinear neutral connected loads supplied by a feeder, but if it supplies a majority 3? and 1? L-L loads, the neutral doesn't count as a CCC... even if it is reduced in size.

 

[Rick Christopherson]

I only skimmed the thread, so I don't know if this was already pointed out. I mention this because I saw it on another forum from an electrician. The neutral is ALWAYS a current carrying conductor, but that is not the same as questioning whether it should be counted as one for thermal purposes. Not "counting" it as a current carrying conductor does not mean that it doesn't carry current. It is always a CCC, but it just doesn't get counted in some calculations.

 

[Dennis Alwon]

I only skimmed the thread, so I don't know if this was already pointed out. I mention this because I saw it on another forum from an electrician. The neutral is ALWAYS a current carrying conductor, but that is not the same as questioning whether it should be counted as one for thermal purposes. Not "counting" it as a current carrying conductor does not mean that it doesn't carry current. It is always a CCC, but it just doesn't get counted in some calculations.

That is correct. The neutral has to carry current in order to complete the circuit -- Hopefully that is a known fact.:thumbsup:

 

[iwire]

The neutral is ALWAYS a current carrying conductor,

I disagree.

In my opinion a neutral conductor may or may not be carrying current regardless of it carrying current or not the NEC tells us when to treat it as a current carrying conductor.

For example, a 120/240 MWBC supplying two identical resistive loads.

The neutral will not be carrying current.

 

[kwired]

For purposes of this thread topic - we are talking about whether or not the neutral is current carrying in respect to amount of heat it contributes to raceways or cables. Most discussions like this usually are similar circumstances.

OP:

I'm a little unclear of the exception to neutral counting as a current carrying conductor when derating. It says only when the neutral is carrying only the unbalanced load.....etc. What exactly does this mean.

Ex.: Adding a panel for a video game store off an existing meter stack. Does the feed from the meter stack to the 200 amp panel qualify for the this rule? Let's say it is 3 phase, 120/208 power. It's in an emt conduit. There are 3 hots, a neutral, and a ground. Are there 4 current carrying conductors in that pipe, or just 3?

Thanks

 

[Rick Christopherson]

I disagree.

In my opinion a neutral conductor may or may not be carrying current regardless of it carrying current or not the NEC tells us when to treat it as a current carrying conductor.

For example, a 120/240 MWBC supplying two identical resistive loads.

The neutral will not be carrying current.

Having zero current is not the same as not being a current carrying conductor. Nevertheless, you will be very hard pressed to ever find a MWBC that truly has 0.000 amps when the circuits are active.