Transformer Calculations

[Nsmith]

I am wiring up a 225KVA transformer to test 480V/460/440 equipment around the shop. We have a 208 service coming into the building and the transformer is 440v Prim 208 Sec. I will be wiring it in reverse in order to get the 440v need to test equipment. I am only going to be connecting one 100 amp disconnect So I need to make sure my calculatins are correct. Money is also an issue of course so I am going to try to make this cost effective too.

I am not sure if I should leave the transformer wired at a specific voltage since many of the machines I will be testing will have control transfromers in them and they may be wired at a specific voltage in order to ensure proper operation of the coils on the contactors. I am thinking of wiring it for 460v since that is middle of the road and should cover me.

There is a spare 200 amp breaker in the panel if I where to use this I would be able to get 86 amps on the secondary wired at 480v correct? if the xfrm was wired for 440 I would have 94 amps. 90amps @ 460v

If I have #4/0 75c wire coming from the 200 amp breaker in the main panel feed a fused 200amp disconnect then feed the xfmr prim with #4/0 then have #2 wire off the sec feed a 90 amp fused disconnect I should be ok?

 

[augie47]

since you say 208 secondary, I assume your transforme is 3 phase..
If you used the full 225 kva capacity your primary (208) current would be
over 600 amps. Your available secondary at full capacity, 460 volts would be 282 amps (not taking the 125% allowable into account).
Since your available primary breaker is 200 amps, your available capacity is roughly 75 kva so you 460 volt output would be limited to 94 amps.

(Hopefully my math is correct)

 

[Nsmith]

Yes the service here is 3 phase. I agree I got 94 amps when calculating with 440v and 90a when useing 460v. When test used machinery from all over soo each machine is diffferent and I dont know if I will have to ever get into the transformer again to change the taps. I want to account for every instance.

 

[jim dungar]

First, do not connect the system neutral to the X0 terminal of the transformer, when reverse feeding it.

Second, the inrush of the transformer does not care how little load is connected to it. A reverse connected transformer can have a drastically high inrush current (I usually estimate 12X). Most OCPD have around a 10X instantaneous trip point so I would not be surprised that you actually need an 800A device to feed it.

 

[Nsmith]

No need for a neutral 3 current carrying conductors and a ground is all I will be needing. They had the XFMR being useing be for on some CNC machines before on a 200 amp breaker and it didnt trip. The disconnect on the 208 side was fused for 125 amps the machine maybe drew 40 amps on the 440 side.

 

[augie47]

The potential for it to trip is definitely there, but with no load on the secondary you may get by as you have already experienced.
In regard to your original post, you are not required to have the primary fused disconnect and you need to address secondary grounding and decide if you want to corner ground the 460v secondary or leave it ungrounded.
If it is left ungrounded, you need to address 250.21(B)

 

[Nsmith]

Interesting, I have never used a ground detector before. This Might be a better option. In order to ground the transformer I would have to pull another wire from the main panel to the transformer and then ground to the chasis of the xfmr right? Then I can go from the xfmr ground out to my disconnect and any other equipment I hook up? Where the ground detector would just be wired off the line side of my disconnect? Would it know if there was a fault out on the equipment I am wired too?

 

[augie47]

A ground detector should notify of any phase to ground fault anywhere in the system when that portion is "on".

 

[kwired]

Interesting, I have never used a ground detector before. This Might be a better option. In order to ground the transformer I would have to pull another wire from the main panel to the transformer and then ground to the chasis of the xfmr right? Then I can go from the xfmr ground out to my disconnect and any other equipment I hook up? Where the ground detector would just be wired off the line side of my disconnect? Would it know if there was a fault out on the equipment I am wired too?

You also need to ground one of the derived phases otherwise you still have an ungrounded system - or is that what you are asking about?

You need an equipment grounding conductor from the primary source with both grounded and ungrounded secondary setups.

 

[ibew501ed]

Interesting, I have never used a ground detector before. This Might be a better option. In order to ground the transformer I would have to pull another wire from the main panel to the transformer and then ground to the chasis of the xfmr right? Then I can go from the xfmr ground out to my disconnect and any other equipment I hook up? Where the ground detector would just be wired off the line side of my disconnect? Would it know if there was a fault out on the equipment I am wired too?

I believe you would just run a grounding electrode conductor from (building steel) the electrode to the derived phase conductor of the 480 v delta transformer and then install a system bonding jumper from the derived phase to the transformer case
as kwired stated