# count the neutral as current carrying



## farm (Oct 1, 2014)

In a wire way with ser cable running horizontal through it, does the 30 current carrying limitation include the neutral conductor for the derating factor. Do you count the neutral in the ser cables as a current carrying conductor? These are 125 amp panel feeder cables.




Thanks for your input, Mark.


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## RIVETER (Sep 26, 2009)

farm said:


> In a wire way with ser cable running horizontal through it, does the 30 current carrying limitation include the neutral conductor for the derating factor. Do you count the neutral in the ser cables as a current carrying conductor? These are 125 amp panel feeder cables.
> 
> 
> 
> ...


Without reading any farther I will just say that the Neutral...carrying current will dissipate heat just as an ungrounded conductor would. I believe it should be taken into consideration. What you think?:thumbsup:


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## don_resqcapt19 (Jul 18, 2010)

What is the system voltage?


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## farm (Oct 1, 2014)

120/208v single phase


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## RIVETER (Sep 26, 2009)

don_resqcapt19 said:


> What is the system voltage?


Why would that matter?


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## bkmichael65 (Mar 25, 2013)

Check out 310.15(B)(4)(b). The neutral isn't carrying the unbalance from all 3 phases so it needs to be counted as a CCC


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## don_resqcapt19 (Jul 18, 2010)

If you have two ungrounded conductors and and a neutral on a 120/240 volts system with both ungrounded conductors carrying 20 amps there will be no current on the neutral. 

The derating is based on the amount of heat and since power equals I²R, the more current you have flowing the more heat you will produce in the conductors. 

For a 120/240 single phase system, the most heat will be when either one or both of the ungrounded conductors are carrying 20 amps. If we are using the same size wire for all of the conductors, the resistance of the conductors will be equal. Each of the two conductors carrying 20 amps will produce 400R watts of heat. (R being the resistance of the conductor) That would give us a total of 800R watts of heat for the circuit. 

Any other combination of loads will produce less heat. For example if one ungrounded conductor is carrying 20 amps, and the other 19 amps, there will be 1 amp on the neutral. One ungrounded conductor would produce 400R watts of heat, the other 361R watts of heat and the neutral 1R watts of heat, for a total of 762R watts of heat.

This is exactly why you don't have to count the neutral of a 120/240 volt circuit as a current carrying conductor.

If you have a single phase 3 wire system fed from a 208Y/120 volt system, if both ungrounded conductors are carrying 20 amps, the neutral will also be carrying 20 amps and the total heat for that circuit would be 1200R watts of heat. 50% more heat than if the system was 120/240. 

With the 3 wire circuit that has been derived from a 208Y/120 volt system, if you have 20 amps on one ungrounded conductor and 19 on the other, you would have 19.5 amps on the neutral. 

There is not the reduction in the total amount of heat as we see in the single phase 120/240 volt system and the neutral conductor from the 120/208 volt 3 wire system has to be counted as a current carrying conductor.


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## Carultch (May 14, 2013)

farm said:


> In a wire way with ser cable running horizontal through it, does the 30 current carrying limitation include the neutral conductor for the derating factor. Do you count the neutral in the ser cables as a current carrying conductor? These are 125 amp panel feeder cables.
> 
> 
> Thanks for your input, Mark.



I've had numerous discussions to understand what counts as a C.C.C. for ampacity adjustments due to bundling.

A wire counts as a CCC, if:
1. it is an ungrounded conductor that carries current. (duh!)
2. it is a spare conductor that will eventually carry current, if used in its intended future application.
3. it is a neutral conductor that is guaranteed to carry the same current as there is in the ungrounded conductors, in order to provide a complete path for the circuit.
4. it is a neutral conductor carrying the harmonics of extremely non-linear loads

A wire does not count as a CCC, if:
1. it is the grounding (green) wire. EGC or GEC.
2. it is a communication wire, carrying only milliamps of current.
3. it is part of a set of wires that by the nature of the circuit, cannot be energized at the same time.
4. it is a neutral wire that is only used for instrumentation purposes
5. it is the neutral wire, only carrying the imbalance of loads from the ungrounded conductors.


To elaborate a bit on some of the YES points:
Yes #2: Spare conductors are not carrying current today, but the intention is that they will tomorrow. The entire reason you pulled spare wires along with the active wires, is so you don't need to remove and reinstall any wires when you eventually need them. Therefore we're going to count the spare wires as active today, and account for the ampacity adjustments accordingly.

Yes #3: One example would be a single live and a single return neutral wire, of a single phase circuit. Any current going out on the live wire, must return on the neutral.

Another example would be a using two phases and a neutral of a 3-phase WYE system, to supply two identical loads from each phase. The current of just the two phases will not add up to zero and complete the circuit. Both loads depend on the neutral being there, in order to carry the return current. It will thus be equal in magnitude to either of the two phase currents. Add them up as vectors, to see how this works.

Yes #4: Unlike the fundamental waveform that cancels out among the three phases of a balanced circuit, some harmonics will add up onto the neutral, instead of cancel. Especially the 3rd harmonic for 3-phase, which is 180 Hz for a 60 Hz circuit. If the amperage of the fundamental is 30A, and it has a 3rd harmonic at 10A, then the neutral current will be the full 30A. And just as continuously as the phase conductors.

To elaborate a bit on some of the NO points:
No #3: As an example, wiring of a travelling light circuit. A 3-way switch. The two travellers are never going to be simultaneously carrying current, because of the nature of the way that they are switched. Therefore, only one need to be counted as a CCC.

No #4: Examples would be voltage measurement, and effective grounding. Effective grounding means that the neutral is used, for the purposes of making sure that the three phase voltages are centered.

No #5: As counter-intuitive as it may seem, mathematically, you can prove that with linear loads alone, it is not a problem for 4 wires to be carrying current due to the neutral carrying the imbalance. The heat dissipated among all 4 wires with some imbalance being carried back on the neutral, will never exceed the heat dissipated in exclusively the three phase wires carrying a full balanced load alone.


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