no more networking?

[twinman]

Shared neutral

Shared neutral

Analyzing the question from a mathematical point of view, a 3 phase, 4 wire system ALWAYS has and ALWAYS will have the phases 120 degrees appart. Thus, any line to neutral current will always be 120 degrees apart.

A single phase service (3 wire) derived from a three phase service will also share the 120 degree lag. A single phase (3 wire) service derived from a center tap transformer will be 180 degrees out of phase. As a result, 120V from either phase to neutral, but 240V phase to phase.

If the requirement is in place tfor disconnecting the multiple phase simultaneously in a circuit with a shared neutral, the savings realized by not running separate circuits may be negated by the downtime of critical equipment. As such, the client may elect to eliminate shared neutrals, via specifications, from his/her project.

 

[infinity]

Analyzing the question from a mathematical point of view, a 3 phase, 4 wire system ALWAYS has and ALWAYS will have the phases 120 degrees appart. Thus, any line to neutral current will always be 120 degrees apart.

Twin, Is this statement accurate?

 

[sparky 134]

I was under the impression that in a 120/208v 3phase 4 wire system the three phases were 120 degrees apart. In a single phase 120/24volt system they are 180 degrees.

A long time ago I learned that if you had a three wire (2 hots, 1 neutral) MWBC on a 120/208volt system and each phase had a load of say 10amps, the neutral will also have a load of 10amps returning to the panel. But on a 120/240volt single phase system the neutral load would be zero.

 

[Rick Christopherson]

Analyzing the question from a mathematical point of view, a 3 phase, 4 wire system ALWAYS has and ALWAYS will have the phases 120 degrees appart. Thus, any line to neutral current will always be 120 degrees apart.

You forgot about that illusive powerfactor and how it can be different for each phase.

As for the original question, I can understand derating the neutral when imbalanced phases, powerfactors, or harmonics could be present; but pulling a separate neutral for each phase is a bit of overkill.

 

[mivey]

Twin, Is this statement accurate?

As you have surmised, he should have said for a wye system. His statement is not true for a delta with a center-tapped transformer (the 3-phase 4-wire with a high leg).

add: and take note of Rick's #24. The 120 degree separation is for generalization only. Different impedances across the phases can skew the 120 degree difference.

 

[mivey]

I was under the impression that in a 120/208v 3phase 4 wire system the three phases were 120 degrees apart. In a single phase 120/24volt system they are 180 degrees.

A long time ago I learned that if you had a three wire (2 hots, 1 neutral) MWBC on a 120/208volt system and each phase had a load of say 10amps, the neutral will also have a load of 10amps returning to the panel. But on a 120/240volt single phase system the neutral load would be zero.

That is correct.

 

[charlie b]

Thus, any line to neutral current will always be 120 degrees apart.

Twin, Is this statement accurate?

No, but only because it is incomplete. It is like applauding a good performance with one hand held behind your back. Nothing is 120 degrees apart from itself. You have to mention what it is 120 degrees apart from. But I see that has already been addressed. So I'll go back to work now. :smile:

 

[infinity]

No, but only because it is incomplete. It is like applauding a good performance with one hand held behind your back. Nothing is 120 degrees apart from itself. You have to mention what it is 120 degrees apart from. But I see that has already been addressed. So I'll go back to work now. :smile:

I'm having a mental block here, he said any line to neutral current will always be 120 degrees apart. Since that would always be across one winding of a 208Y/120 volt transformer why would there be any phase angle involved?

 

[mivey]

I'm having a mental block here, he said any line to neutral current will always be 120 degrees apart. Since that would always be across one winding of a 208Y/120 volt transformer why would there be any phase angle involved?

If you're having a mental block, I don't know what I'm having as I never noticed he said current. I was thinking voltage.

So his statement is completely untrue. The currents do not have to have a 120 degree difference as it is driven by the load impedance.

 

[mivey]

Since that would always be across one winding of a 208Y/120 volt transformer why would there be any phase angle involved?

A 3-wire 120/208 service uses two windings, not one.

If you compare L1-N to L2-N voltages, they have a 120 degree displacement.

 

[infinity]

A 3-wire 120/208 service uses two windings, not one.

If you compare L1-N to L2-N voltages, they have a 120 degree displacement.

I was asking about a 3 phase WYE system with three secondary windings (208Y/120). Any 120 volt measurement from one phase to neutral would go only across one winding so how does the 120 degrees come into play?

 

[mivey]

I was asking about a 3 phase WYE system with three secondary windings (208Y/120). Any 120 volt measurement from one phase to neutral would go only across one winding so how does the 120 degrees come into play?

For one reading it would not, and that would make no sense. I was trying to give some credit to the poster and fill in the missing text. He must have been talking about comparing two readings, but maybe I'm being too generous.