3 phase 208V wye to 480V delta 15kva transformer OCPD questions
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    Default 3 phase 208V wye to 480V delta 15kva transformer OCPD questions

    I have some questions on how to connect a 3 phase transformer in reverse. The transformer is rated at 15kva, 480V delta primary, and 208/120V wye secondary. If I backfeed this transformer with the three 120V legs coming from my panel in order to get 480V delta, what size and tyep OCPD should I use between my panel and the transformer? What about between the transformer and the machines that will be using it?

    Thanks

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    Just look up the 208V current rating of that transformer to base your selection of an OCPD, then follow the code. There is no special magic involved from that standpoint. I have a "cheap sheet" that says a 15kVA 3 phase transformer at 208V is capable of 41.6A FLA, yours may vary. NEC aricle 450 deals with the selection of the transformer OCPD, but you must also mind Article 240, which dictates the conductors and their protection. So what you use will depend on how you do it.

    Two other things you MUST do however:
    1) Do NOT connect the "X0" terminal on the 208V side to anything, let it float. So you are NOT connecting "120V" conductors to the transformer, you are connecting 208V.

    2) 99.9% of 3 phase transformers will be Delta on the 480V side, and that is NOT a good idea if you are feeding any electronics, i.e. VFDs or servo drives. All VFDs and servos are built based on expecting a Wye service, meaning no more than 277V referenced to ground. In a Delta service, the potential line reference voltage to ground is possibly capable of being the full 480V. A FEW brands of VFDs allow you to remove any protective devices that are referenced to ground, but that leaves your drives unprotected as well.

    If you don't have VFDs or servos to contend with, then the NEC requires that you have EITHER a continuous ground fault monitoring system on an ungrounded Delta service, or you must use a "corner grounded" Delta system, wherein you tie one leg of your 480V transformer output directly to ground. This freaks people out, but it is totally legit.

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    Quote Originally Posted by Jraef View Post
    Two other things you MUST do however:
    1) Do NOT connect the "X0" terminal on the 208V side to anything, let it float. So you are NOT connecting "120V" conductors to the transformer, you are connecting 208V.
    That is the ONE thing most everyone seems to agree on. One thing about this transformer though, besides the X0 lug and a ground lug on the enclosure itself, there is a metal strap that connects the support frame (where the coils are situated) to the enclosure. That metal strap stays, correct? It does not touch the X0 as best I can tell. I took some pictures if I need to post them up I will.

    Quote Originally Posted by Jraef View Post
    2) 99.9% of 3 phase transformers will be Delta on the 480V side, and that is NOT a good idea if you are feeding any electronics, i.e. VFDs or servo drives. All VFDs and servos are built based on expecting a Wye service, meaning no more than 277V referenced to ground. In a Delta service, the potential line reference voltage to ground is possibly capable of being the full 480V. A FEW brands of VFDs allow you to remove any protective devices that are referenced to ground, but that leaves your drives unprotected as well.
    This raises another good point. I noticed that most all were Delta on the high side, though I did manage to locate a step up transformer locally that is delta on the low, wye on the high side. It is model T20LH42-15 Federal Pacific - 15 KVA 3 Phase Transformer 208D-480Y/277

    What is 208 delta? My service is 208 wye. Is this something completely different, or is it simple as connecting all three 'hot' legs from my 208 wye service to this and presto I have 277/480?

    Quote Originally Posted by Jraef View Post
    If you don't have VFDs or servos to contend with, then the NEC requires that you have EITHER a continuous ground fault monitoring system on an ungrounded Delta service, or you must use a "corner grounded" Delta system, wherein you tie one leg of your 480V transformer output directly to ground. This freaks people out, but it is totally legit.
    I have read about the corner ground. Would it be up to code for me to ground it to it's own ground rod? Or does it need to be grounded to the same source/point as the buildings electric service? Does it matter which leg? I have H1, H2, and H3. Somewhere I read it needed to be B leg... is that H2?

    Thanks

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    A transformer is a "separately derived source", and needs to be re-grounded on the secondary side. The ground should be the same as the building ground, so that a fault (short) from primary to secondary produces a large current and opens protective devices.

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    Quote Originally Posted by JST View Post
    A transformer is a "separately derived source", and needs to be re-grounded on the secondary side. The ground should be the same as the building ground, so that a fault (short) from primary to secondary produces a large current and opens protective devices.
    Thank you!

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    Quote Originally Posted by iuctx View Post
    ... there is a metal strap that connects the support frame (where the coils are situated) to the enclosure. That metal strap stays, correct? It does not touch the X0 as best I can tell. I took some pictures if I need to post them up I will.
    Yes, that is the safety ground strap for the enclosure, it stays.

    This raises another good point. I noticed that most all were Delta on the high side, though I did manage to locate a step up transformer locally that is delta on the low, wye on the high side. It is model T20LH42-15 Federal Pacific - 15 KVA 3 Phase Transformer 208D-480Y/277
    That is actually a good choice, because the secondary is a Wye which is what you want. You WOULD ground the X0 on the secondary side on that transformer.

    What is 208 delta? My service is 208 wye. Is this something completely different, or is it simple as connecting all three 'hot' legs from my 208 wye service to this and presto I have 277/480?
    208V Delta is referring to the configuration of the windings in that transformer. That is a separate issue from what is the configuration of your SERVICE. Your service is irrelevant to the transformer internal connections, other than the voltage level.

    I have read about the corner ground. Would it be up to code for me to ground it to it's own ground rod? Or does it need to be grounded to the same source/point as the buildings electric service? Does it matter which leg? I have H1, H2, and H3. Somewhere I read it needed to be B leg... is that H2?

    Thanks
    As JST said, yes, you use the same ground as your building ground. Technically it does not matter which leg you ground, but if the corner grounded Delta SDS is run into a PANELBOARD, then the grounded leg must be connected to B phase.

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    Quote Originally Posted by Jraef View Post
    That is actually a good choice, because the secondary is a Wye which is what you want. You WOULD ground the X0 on the secondary side on that transformer.


    208V Delta is referring to the configuration of the windings in that transformer. That is a separate issue from what is the configuration of your SERVICE. Your service is irrelevant to the transformer internal connections, other than the voltage level.
    OK, I ended up ordering the Federal Pacific Model T20LH42-15 transformer this afternoon. In an email exchange between my vendor and a manufacturer rep I posed the same questions. This was the response:

    “Concerning the T20LH42-15 Perfectly suitable for 208 input across x1,x2,x3 (no neutral input needed) and yes the transformer will generate a neutral on the secondary.That transformer does have a stud welded to the case for a ground, does not need to be separated from service ground as far as the transformer is concerned. That unit has taps +2,[email protected]%. So the output could technically vary up or down 5% with tap manipulation.”

    ...which is exactly what both of you said.

    ***Hopefully this thread is not too confusing for someone years down the road. I started off by asking for wiring information on a different transformer make and model... and reverse feeding it at that.***

    NOW THEN - With this 208D to 480/277Y STEP UP transformer this is what I plan to do. Let me know if it sounds correct-ish.

    1) Mount the transformer in my shop with about 12" of clearance from any walls. I will probably use some rubber pads underneath the supports when I anchor it down.

    2) Install a ?50A?, 3-pole breaker in the nearest sub-panel. Should I be using a larger breaker or a special type of breaker for inrush current?

    3) Bring all three phases from that ?50A? breaker AND the ground from that sub panel to a fused disconnect loaded with time delay 50A fuses. I know this part is redundant, but I don't know how else to feed it from the sub panel (or any of our existing panels) without using a breaker. I suppose the breaker would be enough, but I like having the big "shut 'er down!" handle nearby.

    4) From the disconnect I will continue on to the transformer with #4 wire for each phase and the ground, at which point they will terminate on X1, X2, X3, and the ground wire to the welded stud inside the case.

    5) On the high side, I will connect #4 wires to H0 for neutral, H1, H2, and H3 for the three phases, and a ground wire from the same welded stud in the case, right???

    6) Then land those FIVE wires in a small panel that is rated for 480/277.

    Before I ask any other questions, does that part sound good?

    Thanks again to both (all) of you for taking the time to respond and walk me through this process.

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    Finally got this installed.

    Thanks for everyone's input!

    Sent from my Pixel 2 XL using Tapatalk

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    Your H0 terminal also needs to be bonded to the case ground stud.

    As it sits connected, faults on the 480V side have no way to get back to the (H0) 480V winding of the transformer, to open the overcurrent device.

    Then, after a fault, in essence your system becomes a corner grounded system. Before a fault, your 480V side is ungrounded, never a good arrangement, for a standard shop.

    SAF Ω

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    Quote Originally Posted by SAF View Post
    Your H0 terminal also needs to be bonded to the case ground stud.

    As it sits connected, faults on the 480V side have no way to get back to the (H0) 480V winding of the transformer, to open the overcurrent device.

    Then, after a fault, in essence your system becomes a corner grounded system. Before a fault, your 480V side is ungrounded, never a good arrangement, for a standard shop.

    SAF Ω
    Whoa, I am glad you pointed that out. Thanks! See attached. after.jpg

    I am going to get a double lug for the H0 and a small terminal/bus bar for the grounds.


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