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Old 06-14-2017, 05:12 PM   #1
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I have a Westinghouse 75kva 480 primary/208-120v secondary transformer that I wish to reverse wire to feed (3) 3phase motors. An engineer at Westinghouse said it's possible but my calc's will be in reverse. My FLA for the motors are 36A, 25A and 18A; I assume I should use the standard formula to configure my motor branch circuit protection on the secondary side (which end up being 90A and (2) 70A breakers). The secondary side has an output of of only 90 amps and my primary protection is ending up with a 225 amp breaker, if my math is correct. The problem with that is my main panel feeding the xfmr is a 225A!

Ideas? Help? Comments? all appreciated.
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Old 06-14-2017, 05:34 PM   #2
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NM, delete thread, please.
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Old 06-14-2017, 05:59 PM   #3
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What else is on the main panel? What other power is available?
75kva.\ 480 = 156a. sec. x 80% =124a.
75kva.\208 = 360a. pri. x 80% =288a.
You need a load center.
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Old 06-14-2017, 06:26 PM   #4
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Originally Posted by BuzzKill View Post
I have a Westinghouse 75kva 480 primary/208-120v secondary transformer that I wish to reverse wire to feed (3) 3phase motors. An engineer at Westinghouse said it's possible but my calc's will be in reverse. My FLA for the motors are 36A, 25A and 18A; I assume I should use the standard formula to configure my motor branch circuit protection on the secondary side (which end up being 90A and (2) 70A breakers). The secondary side has an output of of only 90 amps and my primary protection is ending up with a 225 amp breaker, if my math is correct. The problem with that is my main panel feeding the xfmr is a 225A!

Ideas? Help? Comments? all appreciated.
36x1.25 + 25 + 18 = 88



You're taking 100% of the power available from that panel.

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Old 06-14-2017, 06:30 PM   #5
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What else is on the main panel? What other power is available?
75kva.\ 480 = 156a. sec. x 80% =124a.
75kva.\208 = 360a. pri. x 80% =288a.
You need a load center.
That calculation is for single phase.
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Old 06-14-2017, 08:15 PM   #6
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That calculation is for single phase.
Yeah, whoops. Sometimes it pays to read things twice.
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Old 06-14-2017, 08:19 PM   #7
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Quote:
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I have a Westinghouse 75kva 480 primary/208-120v secondary transformer that I wish to reverse wire to feed (3) 3phase motors. An engineer at Westinghouse said ....
Wait, what??? Westinghouse has not existed as a transformer mfr for over 20 years now, selling that off to Eaton in 1994 before their total demise in 1999.
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Old 06-14-2017, 08:22 PM   #8
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Buzzkill.,,

Do you have other power source that you can use those larger three phase motor ?

I know the smaller one ya can able reconnected to run on 208 if the nameplate say so.,

how big the service you have in that place ? it may make the differnce on how ya can get it run correct.,,

Oh yah.,, reversed D-Y transfomer is done pretty often but be aware of inrush current in reversed mode it may trip the breaker if you are not aware of it.,

on 480 secondary you will just run like conventail delta but corner ground it ( useally B phase ) and go from there.,
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Old 06-14-2017, 08:51 PM   #9
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Don't connect the wye point (X0) to anything. Leave it floating.
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Old 06-15-2017, 08:47 AM   #10
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Your idea is not workable.

Backfeeding a 75 kva transformer will give you a useable connected load of about 85 amps. ( 100% is 90 amps @208, minus 6% additional losses due to reverse connection of the transformer.)

While your unadjusted connected load would be 79 amps, the STARTING LOAD would be the problem. If these motors are on across the line starters, and all started at once, the inrush current would be over 700 amps. a transformer will not compensate for this and will cause a severe voltage drop damaging equipment and/or tripping over current devices.

If all of these motors are on VFDs, the required VFD input current would be more than available from the transformer.

What is the service size to this building? If you are adding more equipment, it could be financially feasible to to upgrade the service to 480 volt and transformer down for the existing 208 loads.
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