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Old 03-26-2018, 07:31 PM   #1
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Default 120/240 3 phase delta high leg

H i all, I have been learning a lot here, some stuff I knew I did not know, and a lot that I did not know that I did not know.

So now I need to know if I know what I think I know or if it is something that once again I really did not know what I thought I knew.

The plant I work at has two buildings separated by a block. I work in the north plant. There is a 120/240 volt 3 phase delta panel in the south plant.

I had to do some work in the south plant and needed some 240 volt branch circuits. I found that they only used the high leg on their 3 pole breakers, there were mostly 2 pole breakers and they skipped all the high legs.

Is there a reason to skip the high leg with a 2 pole breaker?

I hope not. I moved and relabeled one whole side to make room for my circuits.

Thanks Steve

Last edited by Sblk55; 03-26-2018 at 08:07 PM. Reason: spelling
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Old 03-26-2018, 08:12 PM   #2
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Are they slash rated breakers? As in 120/240?
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Old 03-26-2018, 08:18 PM   #3
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I would check for the slash rating. If a breaker is slash rated 120/240. You cannot have more than 120v on each pole of the breaker (always go by the lower value).

The other reason would be if the 2 pole breakers are being used for multiwire branch circuits. You would not want to energize a 120 circuit with the high leg. Don’t just swap breakers unless you know what they are for.

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Old 03-26-2018, 08:48 PM   #4
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To your point about the multi-wire branch circuits, there are no neutrals puled with any of the 2 poll or 3 poll circuits, we do not pull circuits with shared neutrals.

Each circuit feeds one machine.

They have have built in transformers to create their own 24 volt control power. that is the one thing I failed to think of was how the hi leg may affect that.

But from resent experience most new power supplies have a very wide range of input voltages they can work with.

Steve
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Old 03-26-2018, 08:53 PM   #5
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The breakers on the moved circuits should be straight 240v breakers since one leg is now more then 120 to ground.
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Old 03-26-2018, 09:43 PM   #6
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Originally Posted by PaddyF924 View Post
The breakers on the moved circuits should be straight 240v breakers since one leg is now more then 120 to ground.
It's 240 volts phase to phase, no leg to neutral.
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Old 03-26-2018, 09:47 PM   #7
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Ah brain fart. i meant phase to phase
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Old 03-26-2018, 09:48 PM   #8
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Quote:
Originally Posted by Sblk55 View Post
To your point about the multi-wire branch circuits, there are no neutrals puled with any of the 2 poll or 3 poll circuits, we do not pull circuits with shared neutrals.

Each circuit feeds one machine.

They have have built in transformers to create their own 24 volt control power. that is the one thing I failed to think of was how the hi leg may affect that.

But from resent experience most new power supplies have a very wide range of input voltages they can work with.

Steve
Excellent understanding of your plant and a good catch with those transformers.
Yes, someone misunderstood how to use the hi leg or possibly they set it up not knowing if the equipment was sneaking a phase to ground for whatever reason.
It's not done these days but older equipment if loaded with crap like that.
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Old 03-26-2018, 09:54 PM   #9
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Excellent understanding of your plant and a good catch with those transformers.
Yes, someone misunderstood how to use the hi leg or possibly they set it up not knowing if the equipment was sneaking a phase to ground for whatever reason.
It's not done these days but older equipment if loaded with crap like that.
You should have posted under "NewElect85" instead.
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Old 03-26-2018, 09:58 PM   #10
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Not a lot of guys understand the high-leg system so they're afraid of it.

There's always some sort of a know-it-all who will forcefully state that the high leg is 'THREE PHASE ONLY'. thus confirming his lack of knowledge.

If any load, other than a VFD and maybe a few others, is 240 single phase and doesn't involve the neutral, it can be supplied by the high leg and one of the other phases.

As noted, the breaker is supposed to be marked 240, not 120/240 but the vast majority will use a basic 120/240 breaker.

When I'm working with a high-leg system, I will always use the high leg and another phase unless there's a valid reason not to.
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Old 03-26-2018, 09:58 PM   #11
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It's 240 volts phase to phase, no leg to neutral.
240.85
“A circuit breaker with a slash rating, such as 120/240v or 480/277v, shall be permitted to be applied in a solidly grounded circuit where the voltage of any conductor to ground does not exceed the lower of the two values of the ciruit breaker’s voltage rating and the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker’s voltage rating.”

It does apply because the high leg to gorund would be higher than the 120 rating.
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Old 03-26-2018, 10:02 PM   #12
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They have have built in transformers to create their own 24 volt control power. that is the one thing I failed to think of was how the hi leg may affect that.
The high leg has zero effect on a transformer. The transformer sees only phase to phase voltage, it doesn't care about phase to neutral/ground.
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Old 03-26-2018, 10:19 PM   #13
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The plant I work at has two buildings separated by a block. I work in the north plant. There is a 120/240 volt 3 phase delta panel in the south plant.

I had to do some work in the south plant and needed some 240 volt branch circuits. I found that they only used the high leg on their 3 pole breakers, there were mostly 2 pole breakers and they skipped all the high legs.

Is there a reason to skip the high leg with a 2 pole breaker?

I hope not. I moved and relabeled one whole side to make room for my circuits.

Thanks Steve

Some loads are MWBC due some of the circuits do use phase to neutral loads so that why it skip the third breakers for 120 volts loads.

All the 240 volts loads ( single or three phase ) is not a issue.

Just becarefull with breaker slection if you have single phase load with slash rating you are ok ( as long you have 120 volt circuit or phase to ground less than 150 volts ) but once you use wild leg side that you need solid rated breaker. typically most I will say most three phase breakers are rated for that purpose. ( again look at the breaker specs for that )
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Old 03-26-2018, 11:03 PM   #14
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The typical scheme uses an under-sized stinger transformer to generate the high voltage. ( 208VAC L-N )

It's actually a BAD idea to tap 240 from the stinger to the other phases. Quicker than you can imagine, you end up with weird imbalances... 'cause the stinger is so often pitifully sized compared to the center-tapped transformer.

I wouldn't mess with it unless an EE signs off on the calculations. It's just not worth it.

The original intent was to size the stinger just for 3-phase motor loads. PERIOD.

If you insist on tapping the stinger for 240 single-phase L-L loads -- you'd best be sure that they are all balanced around -- and that the breakers carry the correct rating. ( 240VAC )

Don't attempt to use the stinger for 208VAC L-N loads as you're actually sending current right through the A or B phase and then to the center-tapped neutral. Restated, you're loading up the center-tapped Big Transformer all the same. That's not how the EE sized the gear.
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Old 03-27-2018, 06:04 AM   #15
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I have constantly, 30 plus years, designed and worked with open delta 3phase systems and haven't seem a single engineered drawing calling out a slash rated breaker or an inspector asking for such a thing.
These are usually small services under 400 amps. The largest one I can remember building was 1,600 amps.
We never use high leg to neutral and always use 2 pole breakers on a high leg for motors, etc. Never for MWBC involving a neutral of course.
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Old 03-27-2018, 11:38 AM   #16
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Originally Posted by telsa View Post
The typical scheme uses an under-sized stinger transformer to generate the high voltage. ( 208VAC L-N )

It's actually a BAD idea to tap 240 from the stinger to the other phases. Quicker than you can imagine, you end up with weird imbalances... 'cause the stinger is so often pitifully sized compared to the center-tapped transformer.

I wouldn't mess with it unless an EE signs off on the calculations. It's just not worth it.

The original intent was to size the stinger just for 3-phase motor loads. PERIOD.

If you insist on tapping the stinger for 240 single-phase L-L loads -- you'd best be sure that they are all balanced around -- and that the breakers carry the correct rating. ( 240VAC )

Don't attempt to use the stinger for 208VAC L-N loads as you're actually sending current right through the A or B phase and then to the center-tapped neutral. Restated, you're loading up the center-tapped Big Transformer all the same. That's not how the EE sized the gear.
Why do you need an engineer? Look at the transformers and see the rating OR call the utility and get the rating simple math will give you the answer.
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Old 03-27-2018, 08:39 PM   #17
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Thanks all. I put it all back the way it was. The new equipment we are getting is from Europe and was told that it can work ether with 240 2 pole or Hi leg 216 and neutral. Strange stuff they build over there.

Steve
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Old 03-27-2018, 10:44 PM   #18
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Thanks all. I put it all back the way it was. The new equipment we are getting is from Europe and was told that it can work ether with 240 2 pole or Hi leg 216 and neutral. Strange stuff they build over there.

Steve
Most of European system typically are wired for 230 volts line to netural and 380 to 415 volts phase to phase in wye format which it is very common over there but yes there is delta system there but kinda restricted to industrial area.

Almost everything in European side run on 50 HZ as well.

I live in Philippines currentally., and we do have 240 volts line to netural and 400 to 415 volts phase to phase.

Yes we do have 480 volts in both wye and delta system as well and all of it is on 60 HZ
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Old 03-28-2018, 04:57 AM   #19
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Why do you need an engineer? Look at the transformers and see the rating OR call the utility and get the rating simple math will give you the answer.
I live in a world of 'tards -- and blame shifting is my game.

I agree with Jrannis, these are pretty small Services.

My Poco's have been phasing out these schemes -- for decades.

They now only want to see 480Y277 or 208Y120.

This is stated right in their Green Book.

Nothing subtle about it at all.
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Old 03-28-2018, 06:19 AM   #20
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Here open Delta services go up every single day.
FPL rules here.


Also, very little natural gas and just about zero residential solar.
Typical electric bill here trying to keep a 2,000 sf house at 72 degrees when it's in the upper 80s low 90s is just over $200 per month.
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