current carrying conductor

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shortckt70005

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In a 3/4" conduit I have 3 phases (a,b,C) and a neutral. Is the neutral classified as a current carrying and do I have to derate the wires using table 310.15 B 2. I thought I was taught that yes it can be a current carrying conductor but was not counted when using this table.

thanks
Shortckt
 
In you example, unless the majority of the load is non-linear, the neutral would not be counted as a current carrying conductor as far as 310.15(B)(2)(a) is concerned.

Take a look at 310.15(B)(4)(a).

Chris
 
To Shortckt,

I'd also like to emphasize Chris' specific wording: "the neutral would not be counted as a current carrying conductor as far as 310.15(B)(2)(a) is concerned"

The neutral almost certainly carries current. No amount of redefinition will change that, and if you open that neutral under load you will very likely cause damage. You simply do not need to _count_ this conductor when working out the number of conductors for 310.15(B)(2)(a)

-Jon
 
On most situations, in which a conduit contains 3p and N, most likely the loads will be non-linear, (208-277 lighting). I know there could be times this isn't the norm as in feeders, but in the situation I think he's referring to, most likely it is, so yes count the N as the fourth current carrying conductor. While your at it, increase your N size by one wire size, because non-linear loads also introduces harmonics into the system, which can cause the N to have more current on them than the Line, branch, conductors. One good way to combat this is to always share one neutral between 3phases, (A,B,C,N-A,B,C,N-A,B,C,N and so on), that way the load stays balanced. Sometimes you cant do this but always need to keep it in mind. For those times that you cant,(A,B,N-B,C,N or C,A,N), that's when you need to up size your N. But always,(if loads are balanced), at least size the N the same size as line, branch, conductors.
 
DavisIMI said:
One good way to combat this is to always share one neutral between 3phases, (A,B,C,N-A,B,C,N-A,B,C,N and so on), that way the load stays balanced. Sometimes you cant do this but always need to keep it in mind. For those times that you cant,(A,B,N-B,C,N or C,A,N), that's when you need to up size your N. But always,(if loads are balanced), at least size the N the same size as line, branch, conductors.
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Davis, Why would sharing a neutral with 3 phases combat harmonic loading? Wouldn't the neutral current be more likely to become additive in a shared neutral with three circuits loaded with harmonic current?
 
infinity said:
Davis, Why would sharing a neutral with 3 phases combat harmonic loading? Wouldn't the neutral current be more likely to become additive in a shared neutral with three circuits loaded with harmonic current?
Click to expand...

I agree.

If you run a separate neutral with each phase conductor you would not have additive harmonic loading in the branch circuit.

Chris
 
DavisIMI said:
On most situations, in which a conduit contains 3p and N, most likely the loads will be non-linear, (208-277 lighting). I know there could be times this isn't the norm as in feeders, but in the situation I think he's referring to, most likely it is, so yes count the N as the fourth current carrying conductor.
Click to expand...

I would not go as far as to say most situations would have a majority of the load being non-linear.

Chris
 
DavisIMI said:
While your at it, increase your N size by one wire size, because non-linear loads also introduces harmonics into the system, which can cause the N to have more current on them than the Line, branch, conductors. One good way to combat this is to always share one neutral between 3phases, (A,B,C,N-A,B,C,N-A,B,C,N and so on), that way the load stays balanced. Sometimes you cant do this but always need to keep it in mind. For those times that you cant,(A,B,N-B,C,N or C,A,N), that's when you need to up size your N. But always,(if loads are balanced), at least size the N the same size as line, branch, conductors.
Click to expand...
Actually, the harmonics that cause the excessive neutral current are the strongest when all three phases are persent.

The normal current does not have the ability to cancel when two phases and the neutral are used, that much is true.
 
raider1 said:
I agree.

If you run a separate neutral with each phase conductor you would not have additive harmonic loading in the branch circuit.

Chris
Click to expand...

This is true, but each neutral becomes a current carrying conductor in that scenario
 
winnie said:
To Shortckt,

I'd also like to emphasize Chris' specific wording: "the neutral would not be counted as a current carrying conductor as far as 310.15(B)(2)(a) is concerned"

The neutral almost certainly carries current. No amount of redefinition will change that, and if you open that neutral under load you will very likely cause damage. You simply do not need to _count_ this conductor when working out the number of conductors for 310.15(B)(2)(a)

-Jon
Click to expand...


In response to this statement.

The neutral would carry current, the current carried is from the imbalance between the phases. Since this "imbalanced current" does not add any addition heat to the situation, the neutral conductor is not counted as a current carrying conductor in this instance.

The number of current carrying conductors per raceway or cable is based on the HEAT produced.
 
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