Neutral as a Current Carrying Conductor

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Dennis Alwon

Dennis Alwon

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Am I correct that a 4 wire, 3 phase Delta system would never have a balanced neutral load because you would never have a multiwire branch circuit with 3 conductors and a neutral?

(5) Neutral Conductor.
(a) A neutral conductor that carries only the unbalanced
current from other conductors of the same circuit shall not be
required to be counted when applying the provisions of
310.15(B)(3)(a).
(b) In a 3-wire circuit consisting of two phase conductors
and the neutral conductor of a 4-wire, 3-phase, wye-connected
system, a common conductor carries approximately the same
current as the line-to-neutral load currents of the other
conductors and shall be counted when applying the provisions
of 310.15(B)(3)(a).
(c) On a 4-wire, 3-phase wye circuit where the major
portion of the load consists of nonlinear loads, harmonic
currents are present in the neutral conductor; the neutral
conductor shall therefore be considered a current-carrying
conductor.
Click to expand...
 
Dennis Alwon said:
Am I correct that a 4 wire, 3 phase Delta system would never have a balanced neutral load because you would never have a multiwire branch circuit with 3 conductors and a neutral?
Click to expand...
If the L-N loads on the two phases of the split-phase 120/240 portion of a 4-wire delta were balanced, then there would be no current in the neutral conductor. Any L-L or 3-phase loads will not cause any current in the neutral, no matter if these loads are balanced across the three phases or they are not.
 
Yes the neutral is the center tap of one coil of the transformer so it will carry the imbalance of those two legs. I guess that if the high leg used for a 208 volt circuit is used then it would carry some of that neutral current.
 
You're correct that the neutral would never carry "balancing" current among all three phases, but it still qualifies as a non-current-carrying conductor, because:

A 4-wire delta is a 120/240 1ph supply superimposed on a 3ph delta supply. The neutral behaves as it would on 1ph, and is not involved in delta loads.
 
Thank you guys.

So the only time we would count the neutral in a delta system is if it were a 2 wire load (L-N). Is that correct?
 
The definition of neutral point makes is clear that the neutral wire in a 4 wire high leg delta system is a neutral. Since it is a neutral, it is rarely required to be counted as a current carrying conductor.
Neutral Point. The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct-current system. (CMP-5)
Click to expand...
 
Dennis Alwon said:
So the only time we would count the neutral in a delta system is if it were a 2 wire load (L-N). Is that correct?
Click to expand...

In a delta high leg system, you have the following combinations:
L-L single phase loads (no neutral used, no neutral counted)
L-L-L three phase loads (no neutral used, no neutral counted)
L-N single phase loads (neutral used, neutral counted) (this is the one Dennis identified)
L-L-N MWBC (neutral used, neutral balanced, neutral not counted)
HL-N single phase loads (neutral used, neutral counted)
HL-L-L-N MWBC (neutral used, neutral unbalanced, neutral counted) (this would be a stupid install since you generally should not use the high leg to feed single phase loads, but I think it is an example of counting the neutral on a delta system)
HL-L-L-N three phase load with neutral (presumably 120V control for a 3 phase load, with no 208V being used, so current on N means less current on one of the L) (neutral used, neutral balanced, neutral not counted)

Did is miss a case?

-Jon
 
LarryFine said:
In my opinion, in all of your examples, the neutral need not be counted as a CCC.

I exclude your L-N example, because of course, the grounded conductor counts.
Click to expand...

In the situation of 3 single phase loads (2 120V and 1 208V) with a shared neutral, you can have a situation where all 4 conductors are carrying full current. This is because the grounded conductor of a high leg delta system is _not_ neutral of all three phases, but is neutral of the grounded coil.

Contrast this to a three phase wye system with 3 single phase loads sharing a neutral. In this case if you load all 3 phase conductors to full current, the neutral sees no current.

-Jon
 
winnie said:
In the situation of 3 single phase loads (2 120V and 1 208V) with a shared neutral, you can have a situation where all 4 conductors are carrying full current. This is because the grounded conductor of a high leg delta system is _not_ neutral of all three phases, but is neutral of the grounded coil.
Click to expand...
You're not supposed to supply high-leg-to-neutral loads.

I see no other situation where the neutral would carry anything but imbalance current.

The center-tapped secondary will function exactly as any center-tapped single-phase secondary.

The delta secondary group will function exactly as any three-phase delta secondary.

They function as two distinct, superimposed power supplies, and can be calculated as such.
 
wwhitney said:
What happens with a wye A-B-C-N with one load A-N and one load B-C?

Cheers, Wayne
Click to expand...

The heat generated among the four conductors still doesn't exceed what it would be, if the amps were balanced among the three conductors. That is why neutral doesn't count as a CCC, no matter what the degree of imbalance.

WYE systems containing three phases/four wires only require you to count the neutral if non-linear loads are significant enough for harmonics to make the heat generated on the neutral significant. Similarly with split-phase three wire systems.

WYE systems from which you derive single phase-three wire "open wye" circuits, do require you to count the neutral. Because it is a mandatory part of the return path of the circuit.
 
Carultch said:
The heat generated among the four conductors still doesn't exceed what it would be, if the amps were balanced among the three conductors.
Click to expand...
Really? Say you have a 208Y/120V system with a 3-pole 20 amp breaker, supplying two non-continuous loads, one 2,400 W heating element (120V @ 20A) connected A-N, and one 4,160W heating element (208V @ 20A) connected B-C. Aren't all four conductors carrying 20A, so you'd have 4 CCCs?

Cheers, Wayne
 
LarryFine said:
That's two 2-wire circuits, not a neutral-sharing MWBC.
Click to expand...
At the point the branch circuit(s) leave the panel, I don't think you can tell the difference. And one A-B-C-N circuit could serve A-N loads, B-N loads, C-N loads, and B-C loads, which leaves open the possibility I brought up.

Cheers, Wayne
 
oldsparky52 said:
But the service conductors feeding this are MW (not branch) circuits. So what would the currents look like on the service conductors?
Click to expand...
Answer: the neutral would not need to be counted as a CCC in feeder or service conductors.

The common neutral takes care of that as it does in any such supply.
 
wwhitney said:
At the point the branch circuit(s) leave the panel, I don't think you can tell the difference. And one A-B-C-N circuit could serve A-N loads, B-N loads, C-N loads, and B-C loads, which leaves open the possibility I brought up.
Click to expand...
I don't follow.
 
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