Why use a delta tranformer in this case....why not a wye?

[Duncan8943]

I've got an assisted living facility which is classified as residential. Their are five 200A panels servicing the facility. Everything is single phase 120/240 VAC. Why would the previous electricians have requested that the utility transformer be a delta with a 208 VAC high leg? My line of thinking is that now I have a high leg that if I really wanted to take advantage of it then I would have to purchase 3 phase panels and pull additional wire and for what benefit. Not to mention the unused spaces in the panel that I would have liked to use for 120VAC circuits but cant because they would be 208VAC. Why wouldn't you get a 120/240 VAC 3 phase wye transformer?

 

[electrofelon]

I've got an assisted living facility which is classified as residential. Their are five 200A panels servicing the facility. Everything is single phase 120/240 VAC. Why would the previous electricians have requested that the utility transformer be a delta with a 208 VAC high leg? My line of thinking is that now I have a high leg that if I really wanted to take advantage of it then I would have to purchase 3 phase panels and pull additional wire and for what benefit. Not to mention the unused spaces in the panel that I would have liked to use for 120VAC circuits but cant because they would be 208VAC. Why wouldn't you get a 120/240 VAC 3 phase wye transformer?

When you say that "everything is single phase", do you mean the loads are all single phase but the service is three phase or both the loads and the service are single phase? I assume the former because you are talking delta wye and high leg. Also a 3 phase wye would be 120/208 no high leg, a delta would be 120/240 with a high leg. A delta is generally good where most loads are ine to line and there are limited line to neutral loads.

 

[ggunn]

I've got an assisted living facility which is classified as residential. Their are five 200A panels servicing the facility. Everything is single phase 120/240 VAC. Why would the previous electricians have requested that the utility transformer be a delta with a 208 VAC high leg? My line of thinking is that now I have a high leg that if I really wanted to take advantage of it then I would have to purchase 3 phase panels and pull additional wire and for what benefit. Not to mention the unused spaces in the panel that I would have liked to use for 120VAC circuits but cant because they would be 208VAC. Why wouldn't you get a 120/240 VAC 3 phase wye transformer?

Maybe because if the phase to phase voltage (three phase) is 240V, then the phase to neutral voltage in a wye configuration is 138.6V?

 

[Duncan8943]

Mistaken about wye tranformer type

Mistaken about wye tranformer type

I was mistaken earlier. The utility spec sheet had 240/120 V 3 Phase 4 wire. It did not call out the transformer type.

 

[Duncan8943]

When you say that "everything is single phase", do you mean the loads are all single phase but the service is three phase or both the loads and the service are single phase? I assume the former because you are talking delta wye and high leg. Also a 3 phase wye would be 120/208 no high leg, a delta would be 120/240 with a high leg. A delta is generally good where most loads are ine to line and there are limited line to neutral loads.

I would say that 80% of the loads are line to line. As I mentioned in another post, the utility spec sheet had 240V/120V Three phase 4 wire service. Am I mistaken in thinking that this means 120V between each phase and the neutral and 240V phase to phase?

 

[raider1]

I would say that 80% of the loads are line to line. As I mentioned in another post, the utility spec sheet had 240V/120V Three phase 4 wire service. Am I mistaken in thinking that this means 120V between each phase and the neutral and 240V phase to phase?

The only way to achieve a 240/120 volt 3 phase 4 wire service is with a delta configuration which will result in a high leg. A wye connection would result in a 208/120 volt 3 phase 4 wire system.

Chris

 

[jim dungar]

I would say that 80% of the loads are line to line. As I mentioned in another post, the utility spec sheet had 240V/120V Three phase 4 wire service. Am I mistaken in thinking that this means 120V between each phase and the neutral and 240V phase to phase?

The utility is providing a delta system with a high leg (i.e. 208V), not a wye.

 

[Duncan8943]

When you say that "everything is single phase", do you mean the loads are all single phase but the service is three phase or both the loads and the service are single phase? I assume the former because you are talking delta wye and high leg. Also a 3 phase wye would be 120/208 no high leg, a delta would be 120/240 with a high leg. A delta is generally good where most loads are ine to line and there are limited line to neutral loads.

All of the loads are single phase.

 

[GoldDigger]

All of the loads are single phase.

With a high-leg delta system, all 120 volt loads must be on the Line-to-line phase winding which contains the center point grounded neutral terminal. The high leg is only good for feeding three phase 240 volt delta loads. You cannot even use it for 208 volt single phase to "neutral" loads without drawing additional current through the 120/240 winding which it may not be able to handle.

 

[Duncan8943]

I'm glad you guys are clearing this up for me. I really appreciate it. One last question, If the previous contractor had specified 208/120VAC 3 Phase 4 wire--then how would he have powered the 240VAC loads?

 

[GoldDigger]

I'm glad you guys are clearing this up for me. I really appreciate it. One last question, If the previous contractor had specified 208/120VAC 3 Phase 4 wire--then how would he have powered the 240VAC loads?

If he really needed 240 would have used transformers, or if the loads were all resistive loads like cooking, water heating, air heating, etc., he might just have been operating them on 208 at a lower power rating.
Some motor loads would be OK with 208 also, although the motor would heat up more for a given power output and will draw more current.
Some light ballasts/drivers can also work on multiple input voltages.
But any loads that specifically needed 240 volt with a center neutral would require a transformer.
He could not have supplied 120/240 3-wire to the sub-panels with that service.

 

[Smart $]

All of the loads are single phase.

Was the service spec'd by the electricians or EE?

Could have been spec'd that way because all the line-to-line equipment was intended to operate on 240V 1?... but it would have been a more efficient design (IMO) to spec the heavier load equipment to operate 240V 3?.

One advantage of 240V delta over 208 wye is simply the current difference, approximately 13% less.

With 80% of the loads 240V, empty spaces due to 120V loads in a 3? panel shouldn't be a problem. Where it would be a problem, a 1? panel is usually installed in addition to 3? panel.

 

[Smart $]

I'm glad you guys are clearing this up for me. I really appreciate it. One last question, If the previous contractor had specified 208/120VAC 3 Phase 4 wire--then how would he have powered the 240VAC loads?

In addition to what GoldDigger said, many 240V loads nowadays are designed to also operate on 208V. Whether true or not can usually be gleaned from the nameplate having a 240/208V rating.

 

[texie]

I've got an assisted living facility which is classified as residential. Their are five 200A panels servicing the facility. Everything is single phase 120/240 VAC. Why would the previous electricians have requested that the utility transformer be a delta with a 208 VAC high leg? My line of thinking is that now I have a high leg that if I really wanted to take advantage of it then I would have to purchase 3 phase panels and pull additional wire and for what benefit. Not to mention the unused spaces in the panel that I would have liked to use for 120VAC circuits but cant because they would be 208VAC. Why wouldn't you get a 120/240 VAC 3 phase wye transformer?

This is a very common application for a 240/120 3 phase 4 wire service where the vast majority of the load is 120/240 single phase but you have a few pieces of equipment that require 3 phase. Typically the feeders and panels to most loads are 120/240 single phase. Then maybe a single feeder that is 240/120 3 phase to feed the area that has the 3 phase requirements.

 

[texie]

I should have also mentioned in my previous post that in some cases like yours and are less than, say, about 150 KW of billing demand many POCO's won't give you a 208Y/120 service due to capital cost vs revenue. Also if you are in an area that the POCO does not have all 3 primary phases available this may be your only option if you need at least some 3 phase.

 

[meternerd]

I've got an assisted living facility which is classified as residential. Their are five 200A panels servicing the facility. Everything is single phase 120/240 VAC. Why would the previous electricians have requested that the utility transformer be a delta with a 208 VAC high leg? My line of thinking is that now I have a high leg that if I really wanted to take advantage of it then I would have to purchase 3 phase panels and pull additional wire and for what benefit. Not to mention the unused spaces in the panel that I would have liked to use for 120VAC circuits but cant because they would be 208VAC. Why wouldn't you get a 120/240 VAC 3 phase wye transformer?

If everything is single phase 120/240, then it would not make sense to install 3 phase 4 wire service. The cost of 3 phase power facilities are signifigantly higher and as you stated, the 208 leg could not be used.

 

[ggunn]

I would say that 80% of the loads are line to line. As I mentioned in another post, the utility spec sheet had 240V/120V Three phase 4 wire service. Am I mistaken in thinking that this means 120V between each phase and the neutral and 240V phase to phase?

240V three phase true wye is not a legit service anywhere that I could find. Phase to neutral would not be 120V but 138V and some change, like I said earlier. 120V phase to neutral in a wye has to be 208V phase to phase; there is no way around it.

 

[Smart $]

240V three phase true wye is not a legit service anywhere that I could find. Phase to neutral would not be 120V but 138V and some change, like I said earlier. 120V phase to neutral in a wye has to be 208V phase to phase; there is no way around it.

Well actually there is... but not a common configuration... and likely will not get POCO to cooperate... would have to be customer owned. The configuration would be the equivalent of three 120/240 1? secondaries with grounded and common center taps, each primary on a different phase. Essentially the output could be considered 208YY/120/240 6? 7W. Three phase loads would be limited to 208V, but all phases to neutral would be 120V, and 240V 1? across each secondary.

 

[GoldDigger]

Well actually there is... but not a common configuration... and likely will not get POCO to cooperate... would have to be customer owned. The configuration would be the equivalent of three 120/240 1? secondaries with grounded and common center taps, each primary on a different phase. Essentially the output could be considered 208YY/120/240 6? 7W. Three phase loads would be limited to 208V, but all phases to neutral would be 120V, and 240V 1? across each secondary.

That was also mentioned in another thread.
If anybody needed to run a rectifier to produce DC, the six-phase 120v input (7-wire) would be very interesting. But would leave a lot of confused electricians.

Motor loads could also be connected as 120v wye, but there would be little point in that.

Balancing three-phase loads would require distributing them over both 208-delta (or 120-wye) conductor sets.

 

[ggunn]

Well actually there is... but not a common configuration... and likely will not get POCO to cooperate... would have to be customer owned. The configuration would be the equivalent of three 120/240 1? secondaries with grounded and common center taps, each primary on a different phase. Essentially the output could be considered 208YY/120/240 6? 7W. Three phase loads would be limited to 208V, but all phases to neutral would be 120V, and 240V 1? across each secondary.

Yes, but that's not a true wye configuration. The OP was asking why he couldn't just mount a 240V wye transformer and get three 120V sources, phase to neutral.