Best way to get 120/240v 1ph from straight 240v source.

[Randall Flagg]

Hello everyone,

I have a question, I have a source that has changed from SCE and now I must utilize a 200 a 3ph 5th clip from a straight 240v source. The site needs 200a 120/240v 1ph to feed some cell sites. I have some 50KVA transformers could I use 2 to step it up and then down again to generate the neutral? Is there a better way or a special transformer? Do I need a detail wye or delta delta? I've never pulled from a straight 240v source before. Any recommendations would be much appreciated. Thank you in advance.

 

[qcroanoke]

Hello everyone,

I have a question, I have a source that has changed from SCE and now I must utilize a 200 a 3ph 5th clip from a straight 240v source. The site needs 200a 120/240v 1ph to feed some cell sites. I have some 50KVA transformers could I use 2 to step it up and then down again to generate the neutral? Is there a better way or a special transformer? Do I need a detail wye or delta delta? I've never pulled from a straight 240v source before. Any recommendations would be much appreciated. Thank you in advance.

What does SCE mean and 3 phase 5th clip? Havent heard those terms.
I would think from what you're posting a 240 volt delta primary to 120/240 wye secondary transformer is what you would want to use, But I'm not sure what you've got there....

 

[electrofelon]

Yeah, I've been racking my brain on the "5th clip". I have heard that refer to adding an additional jaw in a single phase meter socket, say when you need to meter the neutral on two phases of 208 wye.....

 

[Jraef]

What does SCE mean and 3 phase 5th clip? Havent heard those terms.
I would think from what you're posting a 240 volt delta primary to 120/240 wye secondary transformer is what you would want to use, But I'm not sure what you've got there....

I'm guessing Southern California Edison (the largest PoCo in SoCal) and a 5 jaw meter socket, because what he had before was a 240/120 3ph 4wire service, and now he is being fed 240V delta, no center tap.

Randall.
If all of your loads are single phase 120/240V, then you can get single phase transformers off the shelf with a 480/240V primary and a 120/240V secondary, they are the most basic of distribution transformers. On the primary side, connect it for 240V off of 2 phases of your 3 phase service, on the secondary side, use the connections that tie the two 120V windings together in the middle, then ground that connection to make a neutral. No need to go up and back down, just get the right thing to start with.

 

[ronaldrc]

Hi, If you don't need isolation to create that neutral you don't need to worry about that primary winding.
I not familiar with Transformer prices, I've been retired now for about 7 years.

Just look for the most reasonable priced 240/120 volt General power transformer the other winding could even be 24/12 volts, but don't use it just cap it off. Just make sure the KVA is right.

Wiring, I'm sure you know this but just hook Line #1 to line #1 on transformer Line #2 to Line #4 on transformer
tie #3 and #4 together to make your neutral for 120 volts from Line 1 and line 2.

source line 1 to Transformer line #1
source line 2 to Transformer line #4

Tie line #2 and #3 together at the transformer and this will be your created neutral for your 120 volt loads.
"Don't forget to ground this neutral conductor to your main Eq. Grounding system."





Might save you a few bucks to go this route?

 

[Electric-Light]

What is the kind of service from the utility?

corner ground-delta, high leg delta?

 

[JFletcher]

Hello everyone,

I have a question, I have a source that has changed from SCE and now I must utilize a 200 a 3ph 5th clip from a straight 240v source. The site needs 200a 120/240v 1ph to feed some cell sites. I have some 50KVA transformers could I use 2 to step it up and then down again to generate the neutral? Is there a better way or a special transformer? Do I need a detail wye or delta delta? I've never pulled from a straight 240v source before. Any recommendations would be much appreciated. Thank you in advance.

What jraef wrote. You need a 240 x 480V, 120/240 transformer, wired as below:

http://www.schneider-electric.us/en/faqs/FA174768/

The primary winding for 240V hi side is wired in parallel rather than series, and the secondary side will have your neutral at the midpoint of the winding, giving you your 120/240V 1ph 3W. No need to step up and down. The exact type of your 240V source (full delta, open delta, etc.) isnt relavent to hooking up the xfmr so long as there is 240V between the two wires connected to the primary of your xfmr, and it has the capacity you need.

If you already have transformers that can accomplish what you want, obviously that is the cheaper route than buying a new, say, 50KVA transformer.

I have a friend who does cell sites, I will ask him what they typically use, and what/how they tie into an on-site generator/ATS.

 

[__dan]

Hi, If you don't need isolation to create that neutral you don't need to worry about that primary winding.
I not familiar with Transformer prices, I've been retired now for about 7 years.

Just look for the most reasonable priced 240/120 volt General power transformer the other winding could even be 24/12 volts, but don't use it just cap it off. Just make sure the KVA is right.

Wiring, I'm sure you know this but just hook Line #1 to line #1 on transformer Line #2 to Line #4 on transformer
tie #3 and #4 together to make your neutral for 120 volts from Line 1 and line 2.

source line 1 to Transformer line #1
source line 2 to Transformer line #4

Tie line #2 and #3 together at the transformer and this will be your created neutral for your 120 volt loads.
"Don't forget to ground this neutral conductor to your main Eq. Grounding system."





Might save you a few bucks to go this route?

I've used that same transformer to make 240 from 120 to run a well pump on 120 generator power (had a bunch in the basement).

I would not think of what you propose. The neutral is not neutral, it's 120 volts to ground. I would call it a common. Considering it may be something that gets left in place for a while and someone else may have to work on it, imo it's too non standard. Not something I would call a neat trick (like making the 240 from 120 the same way. I would use a separately derived transformer type, spending a bit more maybe to make it standard and easily recognized as expected, easier to work on.

 

[ronaldrc]

I've used that same transformer to make 240 from 120 to run a well pump on 120 generator power (had a bunch in the basement).

I would not think of what you propose. The neutral is not neutral, it's 120 volts to ground. I would call it a common. Considering it may be something that gets left in place for a while and someone else may have to work on it, imo it's too non standard. Not something I would call a neat trick (like making the 240 from 120 the same way. I would use a separately derived transformer type, spending a bit more maybe to make it standard and easily recognized as expected, easier to work on.

Thanks:


I thought it was a ungrounded system
If its a corner ground he can not do what I said anyway.

He would have to use a 240 to 240/120 for isolation.

 

[wwhitney]

Hi, If you don't need isolation to create that neutral you don't need to worry about that primary winding.

Wouldn't what you propose violate 210.9?

210.9 Circuits Derived from Autotransformers. Branch circuits shall not be derived from autotransformers unless the circuit supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer.

Cheers, Wayne

 

[ronaldrc]

Wouldn't what you propose violate 210.9?


Cheers, Wayne

I think calling using the transformer the way I said, would be strenghting it to call it a auto
Transformer.



When I proposed that I thought that the system was a floating Delta bank.

By doing what I said and Bonding the created Neutral to the Equipment grounding
system, would be like using the transformer as a zig zag Transformer for a floating
ungrounded delta system. And it would give you the same voltages, if it where the only
grounded conductor on this, 240 between all lines and 208 from ground to the high leg.

 

[ptonsparky]

There is some farm equipment for grain bins that uses a very similar method of Ron's to obtain 120v for a small gear reduction motor. The larger motors are 240/120 single phase wired at 240. 120 is taken from one Line to the center point of the windings from one motor. That made me scratch my head back when I first encountered it.

FYI just encase any of you ever run across an older Stirator.

 

[ronaldrc]

There is some farm equipment for grain bins that uses a very similar method of Ron's to obtain 120v for a small gear reduction motor. The larger motors are 240/120 single phase wired at 240. 120 is taken from one Line to the center point of the windings from one motor. That made me scratch my head back when I first encountered it.

FYI just encase any of you ever run across an older Stirator.

I would have never thought of that.

But I do recall working on some 240 volt motors that had a 120 vac brake that got its 120 volts way.

Thanks

 

[wwhitney]

I think calling using the transformer the way I said, would be [stretching] it to call it a auto Transformer.

The NEC doesn't define autotransformer, but I believe a common definition is a transformer with only one coil. As your proposal uses only one coil of an isolation transformer, it seems to me it uses the transformer as an autotransformer.

Also, Article 450 refers to grounding transformers as "grounding autotransformers."

Cheers, Wayne

 

[ronaldrc]

The NEC doesn't define autotransformer, but I believe a common definition is a transformer with only one coil. As your proposal uses only one coil of an isolation transformer, it seems to me it uses the transformer as an autotransformer.

Also, Article 450 refers to grounding transformers as "grounding autotransformers."

Cheers, Wayne

Yes technically it is.

But in post #9, I said it should not be used with a corner ground and if it was a high
leg we wouldn't need to do this.