Derating more than 3 current carrying conductors in a conduit

[jap]

Seems if it werent so confusing it wouldnt have taken 37 responses to try to explain it.

 

[jap]

But then again The NEC made a lot of sense to me before this forum made me start thinking about it,,,,,,,,,,,,,,thats why this site is so helpful.It makes you think about things from more than just your perspective.

 

[charlie b]

I would think that in actuality there is hardly ever a case where there is a completely balanced load where a neutral would be carrying no current whatsoever.

I once did a mathematical analysis (just for the fun of it :grin in which I compared the heat that would be generated in three phase conductors fully loaded, and balanced, such that the neutral carried no current, against the heat that would be generated in the four conductors (3 phase, 1 neutral), with the current unbalanced. I did not bring harmonics into the analysis. In no case was there more heat generated by four conductors in an unbalance situation than the heat generated by three balanced conductors. That led me to wonder why there was a concern over the neutral being a current-carrying conductor at all.

 

[skeshesh]

The following is a relatively old article that discusses, in the context of ampacity and circuit sizing, current-carrying conductors and neutral conductors. Take a look if you want to:

http://www.easy-share.com/1908107113/cable ampacity article.pdf

 

[jap]

I had a coworker tell me one time he went on a service call on a chicken house where there were (15) 100w incandescent Lampholders running the length of the south side of the house and (15) running the length of the North side on (2) different circuits with a shared neutral
He stated that he accidently disconnected the neutral at the panel for these (2) circuits and all of the Lamps remained lit.
I didnt know wether to believe him or not,but never had the chance to try this for myself.

 

[charlie b]

He stated that he accidently disconnected the neutral at the panel for these (2) circuits and all of the Lamps remained lit. I didn't know whether to believe him or not. . . .

Believe him. Even with the neutral connected, current will flow away from the panel on one phase conductor, through the one set of lights, then through the other set of lights back into the panel on the other phase conductor. When we say the neutral carries the unbalanced current, it also means that with a balanced load the neutral carries no current. So if you disconnect the neutral, it will not alter the current flow in any way.

The danger in this situation is that if the loads are not balanced, if for example one of the lights is a 75 watt, then the voltage will be different in the two sides. That will make one side burn brighter, and the other dimmer. In the extreme, with nearly all the load removed from one leg and no neutral wire, you could get almost all the 240 volts across one set of lights, and nearly 0 volts across the other. Expect some damage to take place, if you try this (which I don't recommend).

 

[jap]

I believe what he really had was (30) lamps wired in series at 240v when he disconnected the neutral.

 

[jap]

Sorry, I see you had already replied, our replies are getting out of order.

Thanks,

 

[LarryFine]

I would think that in actuality there is hardly ever a case where there is a completely balanced load where a neutral would be carrying no current whatsoever.

It doesn't matter. It is impossible for the neutral to carry any current unless that current is reduced in (i.e., subtracted from) one or more of the line conductors. There can never be more than three conductors' worth of current.


Note: harmonics not included.

 

[infinity]

I believe what he really had was (30) lamps wired in series at 240v when he disconnected the neutral.

If that were the case then nothing would happen. If the supply system were208Y/120 the lights would stay lit but they would all become dim due to the fact that the VD across each light would be 104 volts.

 

[LarryFine]

I didnt know wether to believe him or not,but never had the chance to try this for myself.

If you understand why a neutral isn't a CCC, you'd believe him.

I do, and I've never met the guy.

 

[LarryFine]

I believe what he really had was (30) lamps wired in series at 240v when he disconnected the neutral.

Actually, he had either 15 pairs of series-wired lights in parallel, or two 15-light paralleled strings in series. I tend to believe it was the latter, with a single neutral.

The more pairs of lights there are, the more stable the neutral-point voltage would be, and would be closer to zero in spite of bulb-to-bulb impedance differences.

 

[LarryFine]

If the supply system were208Y/120 the lights would stay lit but they would all become dim due to the fact that the VD across each light would be 104 volts.

Which is why the neutral is a CCC when derived from such supplies.

 

[jap]

When he disconnected the neutral in the panel, did he not send 240 volts across each lampholder less what voltage was lost across each filament?

 

[LarryFine]

When he disconnected the neutral in the panel, did he not send 240 volts across each lampholder less what voltage was lost across each filament?

No. There are 15 lights in parallel bewteen each line and the neutral, receiving 120v. When the neutral is lifted, the two sets of 15 lights are in series bewteen the two lines, receiving 240v. With the two groups in series, they split the voltage between them, and each sees 120v.

The now-floating neutral happens to stay close to the same voltage it was (i.e., zero) because the two halves of the load are equal. If he unscrewed a couple of bulbs on one side, the voltage would shift away from zero, and toward the voltage on the side with more bulbs working.

 

[jap]

I agree. Now that I've had time to think about it.

Thanks.

 

[flashlight]

I believe him !

Once had an unintentional experiment involving disconnected neutral on MWBC. The voltages did indeed run high on one end and low on the other.

 

[Juggernaut]

O.k. someone help me get clear. I have a multiwire branch circuit sharing a neutral. Black, red, blue, white. From a 3 phase 120/208 4 wire wye. Do I have to count this neutral as current carrying? From what I understand, only if its a 3 wire mwbc, using 2 phases. What about this 4 wire branch circuit using all 3 phases, is there any difference?

 

[infinity]

O.k. someone help me get clear. I have a multiwire branch circuit sharing a neutral. Black, red, blue, white. From a 3 phase 120/208 4 wire wye. Do I have to count this neutral as current carrying? From what I understand, only if its a 3 wire mwbc, using 2 phases. What about this 4 wire branch circuit using all 3 phases, is there any difference?

Both of the bold statements would be counted as 3 CCC's. So the answers would be, yes and then no.

 

[augie47]

To muddy the water just a bit, in the latter situation (4 wire branch circuit using all 3 phases) if the major portion of the load is non-linear harmonic currents would require the neutral to be counted.