# Transformer



## JRaef (Mar 23, 2009)

Hotlegs said:


> I read you can wire a 480 volt primary / 208 volt secondary transformer backwards and use it as a step up transformer. Is this correct ? Also how do you size the circuit to supply a step up transformer? The customer has a 480 volt machine that draws 40amps, how many amps would this draw from the 208 volt panel that supplies power to the transformer? Thanks


Yes, you can wire a transformer up and it has no idea what is forward or backward, it just transforms from one voltage to another. But that does not mean there are no issues, there are. 

First off, 99% of the transformers you are going to find available will be 480V Delta - 208Y120. Most of the time if you are connecting a machine it will not be using a Neutral on the 480V side, that's pretty much just for lighting. But even so, you should wire up the X0 terminal to the Neutral of your 208V panel on the low voltage side, even though you are not going to get a Neutral out on the 480V side. That's because the 208V windings of the transformer are designed based on the voltage of any one winding not being more than 120V to ground. If you don't hook up the X0 terminal to a grounded neutral, a fault can cause damage to the transformer.

The current ratio is going to be the same as the voltage ratio. The voltage ratio is 2.3:1, so will the current change be 2.3:1 as well. This means that whatever the current requirement of the machine is at 480V, the requirement on the 208V side will be that times 2.3, so 92A. But transformers only come in certain sizes, so to get 92A minimum, you need at least a 37.5kVA transformer (104A), which is going to take conductors capable of 130A from your 208V service. 

Then you also have to deal with the starting current requirements of the 480V machine, because now you have added a transformer in there and it can saturate if it's too small, which may cause a voltage drop that will affect your machine and possibly not allow a motor to start. So if you have an AC motor, the rule of thumb is that if it starts Across-the-Line, you need a transformer at least 2X the FLC rating. So if that 40A is the FLC of a single AC motor on the machine, that means you may need a 75kVA transformer to start it, unless voltage drop is not an issue for the machine. 

So you see, it's just not as simple as mounting and wiring the parts, some thought and investigation needs to go into it.


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## 10492 (Jan 4, 2010)

Single phase?

480*40=19KW

19KW/208= 91 A


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## walkerj (May 13, 2007)

JRaef said:


> Yes, you can wire a transformer up and it has no idea what is forward or backward, it just transforms from one voltage to another. But that does not mean there are no issues, there are.
> 
> First off, 99% of the transformers you are going to find available will be 480V Delta - 208Y120. Most of the time if you are connecting a machine it will not be using a Neutral on the 480V side, that's pretty much just for lighting. But even so, you should wire up the X0 terminal to the Neutral of your 208V panel on the low voltage side, even though you are not going to get a Neutral out on the 480V side. That's because the 208V windings of the transformer are designed based on the voltage of any one winding not being more than 120V to ground. If you don't hook up the X0 terminal to a grounded neutral, a fault can cause damage to the transformer.
> 
> ...


You are a beast


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## Hotlegs (Oct 9, 2011)

Dnkldorf said:


> Single phase?
> 
> 480*40=19KW
> 
> 19KW/208= 91 A


Sorry, 3 phase equipment


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## Hotlegs (Oct 9, 2011)

I left some of the details out, been a long day. The customer has 5 pieces of equipment that are 480 volt / 3 phase. The building only has a 208/120 volt / 3 phase / 400 amp service they are moving in to. He says 1 of the 5 pieces of equipment will be used at one time and never will 2 or more be used. So does this mean I can supply the 5 pieces of equipment from a 200 amp panel connected to the transformer ? I guess if they did accidentally turn on 3 pieces of 480 volt equipment then the 200 amp breaker would just trip.

The landlord does not want to pay to increase the service from 400 - 800 amps if it can be avoided.


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## MollyHatchet29 (Jan 24, 2012)

Oh. My. God. JRaef... ::swoon::


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## mbednarik (Oct 10, 2011)

JRaef said:


> Yes, you can wire a transformer up and it has no idea what is forward or backward, it just transforms from one voltage to another. But that does not mean there are no issues, there are.
> 
> First off, 99% of the transformers you are going to find available will be 480V Delta - 208Y120. Most of the time if you are connecting a machine it will not be using a Neutral on the 480V side, that's pretty much just for lighting. But even so, you should wire up the X0 terminal to the Neutral of your 208V panel on the low voltage side, even though you are not going to get a Neutral out on the 480V side. That's because the 208V windings of the transformer are designed based on the voltage of any one winding not being more than 120V to ground. If you don't hook up the X0 terminal to a grounded neutral, a fault can cause damage to the transformer.
> 
> ...


I thought someone one here said that there would be some circulating currents if the N was connected to a trans used in this setup? Am I thinking of something else or just plain wrong?:icon_confused:


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## freeagnt54 (Aug 6, 2008)

I was under the impression that the xo should be left unhooked. (assuming he is going to ground a corner of the delta side.)


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## BBQ (Nov 16, 2010)

freeagnt54 said:


> I was under the impression that the xo should be left unhooked. (assuming he is going to ground a corner of the delta side.)


I agree, XO must remain unconnected when a delta wye is used in reverse.


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## John (Jan 22, 2007)

Check the with machine manufacturer. A lot of CNC machines have to have 3 phase wye power and will not work right using delta.


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## BBQ (Nov 16, 2010)

John said:


> Check the with machine manufacturer. A lot of CNC machines have to have 3 phase wye power and will not work right using delta.


If they don't use a neutral it should not make a differance.:blink:


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## John (Jan 22, 2007)

BBQ said:


> If they don't use a neutral it should not make a differance.:blink:


I know that and you know that but some equipment manufacturers are anal about it being delta or wye.:blink::blink:


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## JRaef (Mar 23, 2009)

BBQ said:


> I agree, XO must remain unconnected when a delta wye is used in reverse.


OK, OK, I concede. This is something that is generally accetped now, but was not always this way, plus I was thinking one transformer to one machine (and in fact to one motor). Sometimes I get stuck in a rut when I've been dealing with single motor applications for a while. If there is a chance of an unbalanced load on the 480V side, or harmonics (which is virtually all the time now), then you do NOT want to connect the X0 to the service Neutral, AND you should make sure the bonding jumper is disconnected inside, otherwise IT tries to become the X0.

But now that we know there are multiple machines and a 480V distribution system involved, there is more to this. The 480V side will become an ungrounded Delta, or you must set it up as a corner grounded Delta. In either case there are multiple code issues to deal with and more equipment considerations. A lot of circuit breakers used in OEM equipment will not have 480V Delta ratings, the cheaper ones, usually used by OEMs, are what are called "Slash Rated" breakers, meaning they can ONLY be connected to a 480/277V Wye system. Changing out one breaker in a machine is not a big deal, changing out ALL of the breakers in 5 machines might be. Plus the comment about the possibility of electronics such as CNC servos or VFDs is a good one too because some cannot be connected to anything but a Wye service. 

In either of those cases, it may be better to get a 208-480/277V transformer. It will have to be ordered, but you may be able to get someone to spit one out pretty quickly.


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