# 112.5 KVA secondary voltage transient



## Big John (May 23, 2010)

Are you measuring 133V to the actual equipment ground? What is your phase-to-neutral voltage?

Your voltage to ground is likely out of whack because you have an ungrounded transformer as it exists now. It's not code compliant to leave 120V wye ungrounded and needs to be fixed regardless.

However, it doesn't explain why your phase-to-phase voltages are high. If you really have stable 480V, my guess is that that taps on the transformer are also set wrong.


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## walkerj (May 13, 2007)

What is it with a load


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## Electrician102 (Aug 2, 2013)

is it possible to post a picture here so that you can understand better.. The voltages I am giving are with a load as this transformer controls critical machinery running all day


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## Electrician102 (Aug 2, 2013)

the secondary side ground is terminated at the frame. the bonding jumper is also terminated here. This is then jumped to the copper bus.... Not the XO. The ground from secondary panel, neutral, frame ground , and bonding jumper all need to be at XO. If they are configured this way, would I be getting the transient voltages?????? If i correct everything and make to code, would that bring my voltage down to 208. The taps are correct. The transformer itself is 20 years old


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## Electrician102 (Aug 2, 2013)

https://plus.google.com/u/0/photos?pid=5907316023932277794&oid=110737133546446917835


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## Big John (May 23, 2010)

Your secondary phase-to-phase voltage is held constant by the construction of the transformer windings and the primary voltage.

If your input voltage is actually a rock-solid 480V, then that only leaves the transformer windings that will change the voltage. Your ratio is off by 10%. That's a really significant error, and if it's equal and constant on all phases it's likely not a fault, but a wiring problem. Fixing the missing bonding-jumper will not solve this. I would double-check the taps.


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## Electrician102 (Aug 2, 2013)

Big john thanks for the input. I never gave the winding too much thought as the transformer has been in place for 20 years. I will double check them and get back tom.


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## IslandWire (Aug 3, 2013)

Oh, If it has been 20 years, check the taps if it has any. The POCO's have tended up in voltage over time. That transformer may be 460V and not 480V. If it is tapped too low and you apply a higher voltage to the primary, your secondary will be that ratio higher also.

Now on the the bonding. The 208 is a separately derived system. the X0 and Ground must be bonded on the secondary. On the primary, you must connect only the grounding conductor. If there is a Neutral conductor on the primary circuit, DO NOT connect it to anything, just insulate it and tie it back somewhere.


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