Neutral as CCC

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res1323

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Does neutral counts as a current carrying conductor on a 3 wire branch circuit?
 
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Here is a previous post from Rob that helps:
Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:

3Ø- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1Ø- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
 
GoldDigger said:
Just for clarification, in the 120/240 3 phase four wire, your L2 is the high leg, right?
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The modern NEC standard is that the B-phase or Line 2 is the high leg.

It used to be that the standard was for the C-phase to the high leg. One place you may find it on older systems. Another place you may find it, is with utility meters, where the utility didn't update their standards. So you'd end up with the high leg connected to the meter's C-phase, and then the B-phase everywhere else.
 
The "neutral" is a CCC in a high leg delta if you have a feeder which uses only a high leg to neutral load.
If you have a full balanced delta load there will be no current in the 120/240 neutral.
In any other kind of full delta there is no neutral.
 
For ampacity adjustment purposes, the neutral conductor can sometimes be considered as a non-current-carrying conductor.But if it comes to the definition and nature of neutral conductor, it is important to remember that the neutral conductor is always a current-carrying conductor and is not safe to work on while energized.
 
George Stolz said:
While I concur that this is true as far as code is concerned, I must confess I am having a hard time understanding why the neutral is not a CCC in a delta system. :blink:
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Feeder- ABC+N = Not

Branch Circuit- ABC+ not much chance of a N being run in this circuit = Not

Branch Circuit- AC + N = Not

Branch Circuit- AB + N or BC + N = Not, I think, maybe the N is a CCC in this case, I'm not sure, but this circuit is a really bad idea anyway, so why even consider it.
 
Let me explain: when teaching why the neutral is a CCC in a wye system when only two phases of a three phase system are employed, I have a fancy (yet simplistic) animation drawn up that shows half the electrons leaving A seeking B, and half leaving A seeking C. If B is not included in the circuit, the only way the electrons seeking B find their way there is by traveling through the windings of the serving transformer.

This may be wildly incorrect, I don't know; but I find it gets the concept across to count the neutral as a CCC when only A and C are employed in a wye system.

That being said, if my analogy is close to correct, why would a delta system be immune? I haven't had opportunity to witness how a delta with A and C loaded appears on a neutral, but I had always thought it did the same thing because it is a three phase transformer, not a single phase. So help me out. :)
 
George Stolz said:
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That being said, if my analogy is close to correct, why would a delta system be immune? I haven't had opportunity to witness how a delta with A and C loaded appears on a neutral, but I had always thought it did the same thing because it is a three phase transformer, not a single phase. So help me out. :)
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You don't typically have high leg to neutral loads on a 4 wire delta system. The line to neutral loads are across a single transformer winding, exactly like they are on a single phase center tapped transformer. The two lines across the center tapped winding of the 4 wire delta system are on the opposite ends of a single sine wave (often called 180° our of phase) and the neutral load cancels just like on a single phase system.
 
George Stolz said:
While I concur that this is true as far as code is concerned, I must confess I am having a hard time understanding why the neutral is not a CCC in a delta system. :blink:
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Harmonic loads of three phase don't accumulate onto the neutral, like they do in a WYE system.

The neutral carries the imbalanced return current, of the two 120V lines. And on net, the heat generated by A + C + N, does not exceed the heat generated by A+C alone for balanced loads.


So unless you have B-N loads, which is very rare, you don't need to count neutral on a high leg delta.
 
I find it interesting that when you take only two lines and a neutral from a three phase wye (say 208Y/120) to a panel the resulting voltages and currents are identical to a 120V corner grounded delta. :)
 
GoldDigger said:
I find it interesting that when you take only two lines and a neutral from a three phase wye (say 208Y/120) to a panel the resulting voltages and currents are identical to a 120V corner grounded delta. :)
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How does a 120 volt corner grounded delta have 208 line to line?
 
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