# Center tap 3 phase



## don_resqcapt19 (Jul 18, 2010)

FranklinsApprentice said:


> Hey guys, have a customer that wants to install a 3 phase boiler and this is the question I got from the manufacturer when I asked if there boiler will work on a center tap 3 phase service. It's coming this way from the utility.
> 
> "Is the center tap coming out of a transformer on site?
> 
> ...


Does the system have a high leg? I have heard both the high leg delta and a wye system called a "center tap". Most often that term is used with a high leg delta system. 

As far as 3 phase motors, they don't care if they are supplied from a delta or wye system...they only care about getting the correct voltage.


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## FranklinsApprentice (Apr 15, 2012)

It's 240 3 phase. They all measure 240 across each. To ground only A and C measure 240. The center tap measures 0


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## Pete m. (Nov 19, 2011)

FranklinsApprentice said:


> It's 240 3 phase. They all measure 240 across each. To ground only A and C measure 240. The center tap measures 0


That sure sounds like a corner grounded, not center tapped, delta to me.

Pete


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## FranklinsApprentice (Apr 15, 2012)

But 3 phase motors will work on it right?


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## just the cowboy (Sep 4, 2013)

*Motors should but.*

It should work fine for motors but some invertors don't like it I think.


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## sparky278 (Apr 8, 2015)

delta is 240v, wye is 208v, some motors will only run on 240v, unless they have a SF 1.15 or mark as such


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## hardworkingstiff (Jan 22, 2007)

deleted


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## 51360 (Jun 9, 2014)

FranklinsApprentice said:


> It's 240 3 phase. They all measure 240 across each. *To ground *only A and C measure 240. *The center tap measures 0*


Sorry, misread it! Oops

Borgi


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## Electric_Light (Apr 6, 2010)

FranklinsApprentice said:


> But 3 phase motors will work on it right?


small ones, yeah. When you get into double digit horses, they generally become dedicated 230 or 200.


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## Southeast Power (Jan 18, 2009)

FranklinsApprentice said:


> It's 240 3 phase. They all measure 240 across each. To ground only A and C measure 240. The center tap measures 0


It's not really what we define as "center tapped" 
When we say center tapped, we are physically splitting one of the two transformers.
Typically you will see two transformers, one maybe larger than the other on an open delta system.
The larger transformer is the one that gets center tapped.
This will give you 120 from each side of that transformer to the center tap.

What I believe you have is two transformers. It could also be three but would work the same.
The connection between the two transformers is connected to ground.
Therefore, You will have no difference in potential between that connection and ground.
We call that, as mentioned above, "corner grounded"


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## micromind (Aug 11, 2007)

Any 3Ø motor rated for 230 volts will run just fine on a 240 volt corner grounded system. 

As noted, most VFDs will need the MOVs disconnected in order to work on a corner grounded system, but a basic motor controlled by a basic starter will do just fine.


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## MDShunk (Jan 7, 2007)

Chances are pretty good this boiler will have a 240x120 or 240x24 control transformer. If that's the case, you're cool. If they have you run a neutral to the thing also, like for 120v controls, you could be semi-hosed if they connected anything B-N in the control panel.


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## Southeast Power (Jan 18, 2009)

Yes, good word of caution there. I have also seen many times where they go phase to ground to get a control voltage.
This would kind of work on a WYE or a non corner grounded, center tapped delta so, be on the look out for that on a used machine.
Check each phase to ground on each leg of the machine for continuity. Not 100% conclusive but can give you an idea if they did something sneaky.


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## hardworkingstiff (Jan 22, 2007)

jrannis said:


> Yes, good word of caution there. I have also seen many times where they go phase to ground to get a control voltage.
> This would kind of work on a WYE or a non corner grounded, center tapped delta so, be on the look out for that on a used machine.
> Check each phase to ground on each leg of the machine for continuity. Not 100% conclusive but can give you an idea if they did something sneaky.


If the equipment has an approved listing I doubt it would be using the ground to power a control power power supply. If it doesn't have a listing, in NC you are not supposed to connect it.


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## Southeast Power (Jan 18, 2009)

hardworkingstiff said:


> If the equipment has an approved listing I doubt it would be using the ground to power a control power power supply. If it doesn't have a listing, in NC you are not supposed to connect it.


They start out UL but over the years some genius will figure out some work around to get the machine back on line and then never correct it.
Been there.


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