"Single Phase"" From a WYE

[Joethemechanic]

Lets say two legs and a neutral from a 120/240 wye feed a single phase panel for lighting and receptacles. It's commonly called "single phase" but it really isn't. Even if your 120 single phase loads are perfectly balanced you will have a neutral current.

It's really three phase with unequal vectors. Does it have a name? It's led to some confusion when I am trying to explain neutral current calculations to some of the younger guys

 

[LarryFine]

First of all, a 120v line-to-neutral wye will have 208v line-to-line, not 240v.

Second, a fully-balanced full wye has zero neutral current, and each phase's currents are otherwise independent. Then, as you reduce the current on one phase, the neutral current will rise by the identical amount. Finally, as that phase reaches zero current, the neutral will match the others.

"It's commonly called "single phase" but it really isn't." In a way, it really is: L1-N = 1ph; L2-N = 1ph; L1-L2=1ph.

As for the terminology, you said it best: "... two legs and a neutral from a 120/(208) wye ..."

 

[norcal]

If it was wye & 240V, wouldn't the L-N voltage be 139V?

 

[LarryFine]

If it was wye & 240V, wouldn't the L-N voltage be 139V?

 

[wwhitney]

"It's commonly called "single phase" but it really isn't." In a way, it really is: L1-N = 1ph; L2-N = 1ph; L1-L2=1ph.

I have to disagree with that, as that argument would work for any voltage system.

Any two wires will just have a single voltage waveform between them. Phases refer to the comparison of two waveforms, so you need two pairs to talk about phases. And there is a clear difference between 120/240V 3-wire and 120/208V 3-wire, as far as phases.

Cheers, Wayne

 

[LarryFine]

I have to disagree with that, as that argument would work for any voltage system.

Any two wires will just have a single voltage waveform between them.

Well, I did say, "In a way."

 

[Joethemechanic]

Sorry that was a typo, I meant to say 120/208. I'm working off my phone because I'm reloading the operating system on my PC.

 

[Joethemechanic]

Sure perfectly balanced off of all three legs will have zero neutral current. But the feeders for these panels are only connected to two phases and the neutral. So even if you perfectly balance the load on the panel you will have a neutral current. Calling these sub panels single phase kind of leads to confusion. People hear single phase and think balanced loads equals 0 neutral current.

 

[infinity]

So even if you perfectly balance the load on the panel you will have a neutral current. Calling these sub panels single phase kind of leads to confusion.

Yes and the neutral current will approximately equal to the phase current.

 

[Joethemechanic]

So calling it single phase derived from a y transformer is a little clumsy and confusing out in the field. I might as well just call it obtuse isosceles triangle juice

 

[winnie]

Two legs plus neutral from a wye system is not just commonly called single phase, it is officially called single phase.

IMHO this makes sense because such systems are used to supply exactly the same sort of loads as true single phase 3 wire systems. (Eg. Residential electrical services. )

However OP is quite correct, this is not a true single phase center tapped system in the physics sense, even if it is nominally a single phase system on the official paperwork.

Jon

 

[electrofelon]

Calling these sub panels single phase kind of leads to confusion. People hear single phase and think balanced loads equals 0 neutral current.

Does that ever really matter in the field 99% of the time? (no it doesnt).

 

[infinity]

Does that ever really matter in the field 99% of the time? (no it doesnt).

The only really matters if you need to count the neutral as a CCC when there are more than 3 CCC's in a raceway. Other than that it doesn't matter.

 

[electrofelon]

The only really matters if you need to count the neutral as a CCC when there are more than 3 CCC's in a raceway. Other than that it doesn't matter.

right, and most of the time this would be a feeder in its own raceway or cable so unlikely to result in derating.

 

[Besoeker3]

I'm glad we Brits keep it simple.....................

 

[Joethemechanic]

Yeah it does matter. What I'm trying to teach some young guys what's going on when they amp clamp a neutral and the numbers don't add up. And then that led to questions about why is it three phase when it's an open wye primary when you have a open Delta high leg transformers.

Some kids have a little interest in this electrical thing beyond monkey see monkey do

Actually if you wanted to you could turn that single phase 120/240 back into three phase using two transformers


So when they asked me why it's three phase when it's two phase conductors and a multi grounded neutral when it's on the pole, but when it's in a 120/208 panel it's single phase I guess I'll just tell them to shut up and do what the code book says

 

[electrofelon]

I would say that the open wye connection case is quite rare - I mean it's not rare in that I see it all the time on Seattle City lights distribution, but I don't think I have ever seen just the two phases and the MGN on the pole. So it's typically still three phases up on the pole but they are just only connecting to two of them.

I would just say it's somewhat of a special case where if you have two phases and a neutral from a three-phase system it's called single phase but you can use transformers to get three phase back, and this transformer connection would be extremely rare in the low volt world.

 

[Besoeker3]

Yeah it does matter. What I'm trying to teach some young guys what's going on when they amp clamp a neutral and the numbers don't add up. And then that led to questions about why is it three phase when it's an open wye primary when you have a open Delta high leg transformers.

Some kids have a little interest in this electrical thing beyond monkey see monkey do

Actually if you wanted to you could turn that single phase 120/240 back into three phase using two transformers


So when they asked me why it's three phase when it's two phase conductors and a multi grounded neutral when it's on the pole, but when it's in a 120/208 panel it's single phase I guess I'll just tell them to shut up and do what the code book says

How old are you if you don't mind me saying?

 

[Joethemechanic]

One month shy of 62

 

[Joethemechanic]

How old are you if you don't mind me saying?

One month shy of 62