Another high-leg delta problem

[wwhitney]

Wayne, I know you are not an electrician so maybe you havnt thought about it,

I haven't. : - )

but how do you think we can tell which phases make up the phantom winding (genuine question, not being snarky)?

I guess I assumed it would be determined at the time of the service installation and labeled.

You could apply a significant test load both A-B and separately B-C (I've been calling the high leg B). On an open delta, one of those will cause a voltage drop A-C, and one shouldn't. The one that causes the A-C voltage drop is across the phantom coil.

Cheers, Wayne

 

[electrofelon]

I haven't. : - )

I guess I assumed it would be determined at the time of the service installation and labeled.

You could apply a significant test load both A-B and separately B-C (I've been calling the high leg B). On an open delta, one of those will cause a voltage drop A-C, and one shouldn't. The one that causes the A-C voltage drop is across the phantom coil.

Cheers, Wayne

A few things. Pretty much everywhere I work, Delta are considered obsolete and not being installed new. I have certainly done build outs and renovations on Delta services, but never installed one from the ground up. I have never seen one marked indicating the open phases. If I was building a new service that was to be fed open Delta, I could certainly ask the linemen to let me know which phase is which and make note of it, but I probably wouldn't bother. I think it's kinda like rotation, there generally isn't any standard or coordination, it's just random.

A few buildings I frequently work on in Seattle are deltas. One is an OH bank so I can tell it's open. Another is fed by an UG vault so I don't know if it's open or closed. You have me curious tho on doing a load test on that open one, I'll do it at some point.

 

[ggunn]

Wayne, I know you are not an electrician so maybe you havnt thought about it, but how do you think we can tell which phases make up the phantom winding (genuine question, not being snarky)? That is not a thing that is normally coordinated or marked on equipment. Frequently we dont even know if its open or closed delta.

Is it not typical for open delta services to use two transformers (one center tapped for A and C phases and the other for the B phase) while closed delta services use three transformers?

 

[electrofelon]

Is it not typical for open delta services to use two transformers (one center tapped for A and C phases and the other for the B phase) while closed delta services use three transformers?

The number of transformers is typically, what is not is any coordination of which phases contains the open winding

 

[jaggedben]

Is it not typical for open delta services to use two transformers (one center tapped for A and C phases and the other for the B phase) while closed delta services use three transformers?

I might be able to look on the pole and see two transformers, but how do I trace the conductors to the weatherhead? (Assuming I can even trace them from the weatherhead to the service disco.)

 

[ggunn]

I might be able to look on the pole and see two transformers, but how do I trace the conductors to the weatherhead? (Assuming I can even trace them from the weatherhead to the service disco.)

Why would you need to? High leg to neutral is 208V and the other two are 120V. Am I missing something?

 

[jaggedben]

Why would you need to? High leg to neutral is 208V and the other two are 120V. Am I missing something?

Yes, you are. He wants to tell which phase is the one without a transformer. E.g. AB or BC where B is the high-leg. One has a winding, the other doesn't. Can't tell which by measuring unloaded voltage.

 

[tortuga]

I think we have the makings for a 2029 code change proposal to label which phase does not have a transformer. I am not sure how to word it but we got time LOL.
The way I have found the opens in the recent past is by accident, basically they overloaded the phantom and had the POCO out to check voltage issues.

Pretty much everywhere I work, Delta are considered obsolete and not being installed new. I have certainly done build outs and renovations on Delta services, but never installed one from the ground up.

If your designing new builds you should keep delta in your playbook, there are some advantages to having 240 instead of 208 in a building that does not use allot of 120, such as a salon with tanning beds that require 240.
For rural light manufacturing like a cabinet shop its a great fit and as kwired pointed out you can get someone 3 phase when single phase is the only other option.

 

[electrofelon]

I think we have the makings for a 2029 code change proposal to label which phase does not have a transformer. I am not sure how to word it but we got time LOL.
The way I have found the opens in the recent past is by accident, basically they overloaded the phantom and had the POCO out to check voltage issues.

If your designing new builds you should keep delta in your playbook, there are some advantages to having 240 instead of 208 in a building that does not use allot of 120, such as a salon with tanning beds that require 240.
For rural light manufacturing like a cabinet shop its a great fit and as kwired pointed out you can get someone 3 phase when single phase is the only other option.

Yeah I actually like 240 high legs, and kinda hate 208, but the pocos in all the territory I work do not offer them anymore.

 

[ggunn]

Yes, you are. He wants to tell which phase is the one without a transformer. E.g. AB or BC where B is the high-leg. One has a winding, the other doesn't. Can't tell which by measuring unloaded voltage.

Now I am confused. Don't all three lines have at least one transformer connected to them? In open delta drawings I have seen, A and B each are connected to one transformer and C is connected to two.

 

[electrofelon]

Now I am confused. Don't all three lines have at least one transformer connected to it? In open delta drawings I have seen, A and B each are connected to one transformer and C is connected to two.

Yes, but without access to the transformer wiring and/or color coding , how do you know which two legs are the ones connected to only one transformer

 

[ggunn]

Yes, but without access to the transformer wiring and/or color coding , how do you know which two legs are the ones connected to only one transformer

I see, I guess. Whichever phase is 208 to neutral will only connect to one, but of the other two phases which is the phase connected to two? Is that the question?

 

[wwhitney]

I see, I guess. Whichever phase is 208 to neutral will only connect to one, but of the other two phases which is the phase connected to two? Is that the question?

Yes. Because connecting a 2-wire 240V load across the "missing" transformer will cause the load current to go through two transformers, rather than just one. The means less efficient transformer utilization and greater voltage drop from transformer impedance.

Cheers, Wayne

 

[LarryFine]

 

[ggunn]

Yes. Because connecting a 2-wire 240V load across the "missing" transformer will cause the load current to go through two transformers, rather than just one. The means less efficient transformer utilization and greater voltage drop from transformer impedance.

Cheers, Wayne

Makes sense. Thanks

 

[don_resqcapt19]

In the typical open delta high leg installed around here, one of the transformers is larger and that transformer always supplies the 120/240 volt loads. Our utility calls that transformer the "lighter" and the small one that supplies the high leg the "kicker".
No matter what size the transformers are, one will be center tapped and supply the 120/240 volt loads.

 

[tortuga]

In the typical open delta high leg installed around here, one of the transformers is larger and that transformer always supplies the 120/240 volt loads. Our utility calls that transformer the "lighter" and the small one that supplies the high leg the "kicker".
No matter what size the transformers are, one will be center tapped and supply the 120/240 volt loads.

There is weight/size limit for pole mounted transformers that works out to around 300KVA per bank.
On a big upgrade we usually run primary conduits down a pole, set a vault and provide a padmount ontop. Utility does the HV work.
All the padmount 240/120 deltas I have seen are 'full' delta.
There are also more costly below sidewalk options.
Here is an example of a new style 240 'closed' delta padmount transformer:

https://www.larsonelectronics.com/product/284148/150-kva-pad-mount-transformer-4800v-delta-primary-240v-120-delta-center-tap-secondary-copper-knan-bell-green

Now with all the new LED lighting being 100-277V on a 'closed' delta I use 2 or 3 wire (240v L-L) lighting circuits, so the 120V load is vastly reduced.
The lighting load is spread across all 3 windings. (I suppose you always could do that with ballasts but people never did.)

 

[wwhitney]

In the typical open delta high leg installed around here, one of the transformers is larger and that transformer always supplies the 120/240 volt loads. Our utility calls that transformer the "lighter" and the small one that supplies the high leg the "kicker".
No matter what size the transformers are, one will be center tapped and supply the 120/240 volt loads.

OK, but suppose you have a 25 kVA (100A capacity, rounding) kicker transformer and a 50 kVA (200A) lighter transformer, and your 220.87 load study (so realistic numbers) gives you a maximum load of 190A on the lighter, and a maximum 3-phase load of 50A line current (20.8 kVA). And you want to add a 40A 240V 9.6 kVA 2-wire load. There's not enough headroom left on the lighter, but the kicker is only 50% utilized.

So you can add your 40A 240V load A-B (B = high leg) as long as you're sure that's the kicker transformer. If you're wrong, and you add it to the "phantom" transformer, then that 40A is going to appear on both transformers, and you'll overload the lighter.

Maybe not the best example, as the utility is free to undersize its transformers, but you get the idea. Unless utility regulations typically specify that all single phase loads have to be installed on the lighter and that the kicker only be used for 3 phase loads?

Cheers, Wayne

 

[tortuga]

If you're wrong, and you add it to the "phantom" transformer, then that 40A is going to appear on both transformers, and you'll overload the lighter.

The code should require the 'phantom' to be identified so that hopefully can be avoided. Nothing in the code prohibits an open delta on the customer side so same issue. I wonder if anyone ever proposed such a proposal? I suppose it would go in 110 somewhere.

 

[Birken Vogt]

My shop is fed by open delta and I have large load banks in said shop. 25+50 KVA transformers. Only problem is the main panel is Zinsco braided bus and I don't want to mess with it. But it already has a 70 amp 3 phase circuit that I could connect a load bank to.

Wayne, if you want to explain in very small words how I would go about finding the phantom pot, I would love to do this test. If I can find the time.