XFMR 208 3 phase primary 240/480V 1-phase secondary

[NEC User]

I spoke to Square D, the manufacturer we use for transformers, and I requested an transformer that would accept a 208 3-phase primary fed using 3 wires and then have a secondary single phase voltage of 480V using 2 wires.

They told me it is impossible because you can't have a 3 phase input and a single phase output...transformers can't funnel 3 phases into 2 phases. Also, they said I should not install transformers backwards.

I spoke to the Senior Electrician that works for my client and he told me we can use Square D transformer catalog number EE30T3H which is a 30 KVA transformer listed as a 480V delta primary and 208Y/120 secondary. He said we can install it backwards and use a 3-pole breaker so 3 wires from our 120/208 panel would feed the would be primary side of the transformer and then connect only 2 wires on the secondary side to feed to our load.


As a designer that uses a by the book approach using calculations and doesn't have any field experience...I'm confused to the point I barely even know what questions to ask.
Is installing a transformer backwards a common practice even though it isn't desired by the manufacturer?
Any consequences that can occur?
Since the 480V side of the XFMR is delta: Connecting 2 phases gives 480V single phase, and Connecting 3 phases gives 480V 3-phase?

 

[jap]

If you dont need a 3 phase secondary then you dont need a 3ph primary.
Why not a 1ph Delta primary 2 wire with a Single Phase Secondary?

You dont have to wire a transformer backwards, they make single phase transformers in this configuration.

 

[jap]

Disregard my last statement, I would have to check into see if that was an available secondary.

 

[jap]

I apologize, i was thinking of the Eaton V29M47T30EE. It's 3ph 208v Delta primary 480/277v secondary.

I guess you could set a 480/277v Panel with overcurrent protection on the secondary of it and then use a 480v 2p breaker or 2 phases of a 3p breaker to feed the load but I dont know what your circumstances are.

 

[jim dungar]

Since the 480V side of the XFMR is delta: Connecting 2 phases gives 480V single phase, and Connecting 3 phases gives 480V 3-phase?

I know it is a common practice, in our industry, but it often leads to confusion: we use the word 'phase' as if it had a single meaning. then, we often mix its use as both an adjective and a noun.

If the winding configuration is a delta, then you are correct; connecting two line conductors will provide (1) phase (line-line) voltage, while connecting 3 line conductors, will provide (3) phase (line-line) voltages.

Basically, your transformer winding configuration needs to match that of the load not the source.
If you need 480V 2-wire, then you only need a single phase transformer.

There are some really strange connections that may help balance a single phase load over 3-phases, but I can't imagine your system needs, or can afford, that complexity.

 

[david luchini]

I spoke to the Senior Electrician that works for my client and he told me we can use Square D transformer catalog number EE30T3H which is a 30 KVA transformer listed as a 480V delta primary and 208Y/120 secondary. He said we can install it backwards and use a 3-pole breaker so 3 wires from our 120/208 panel would feed the would be primary side of the transformer and then connect only 2 wires on the secondary side to feed to our load.

Why not just get a single phase transformer, 480-240/120 and connect it for 120-480?

 

[jap]

Why not just get a single phase transformer, 480-240/120 and connect it for 120-480?

What?

This would be an amazing transformer.

 

[jap]

I'm curious what size single pole 120v breaker you would need to feed a 120/480v single phase transformer to get 62.5 amps of
480 volts out of the secondary,,,,,,,,,,,,, somewhere between a 250 and 312 Amp single pole?

 

[iceworm]

I... we can use Square D transformer catalog number EE30T3H which is a 30 KVA transformer listed as a 480V delta primary and 208Y/120 secondary. ...

Why not just get a single phase transformer, 480-240/120 and connect it for 120-480?

I'm curious what size single pole 120v breaker you would need to feed a 120/480v single phase transformer to get 62.5 amps of
480 volts out of the secondary,,,,,,,,,,,,, somewhere between a 250 and 312 Amp single pole?

Malt -
It would be good to know much single phase 480 you need. (as in kva?) That alone will eliminate a lot of solutions.

My first guess is you need 10kva (1/3 of 30kva) - but I don't know that. Howecer, consider at 208V, plus 125%, that is 60A and 104A at 120V. Yuck.

I'd be looking at any way I could to use 3ph, 480V.

Yes ,you can hook up xfm backwards. but as already mentioned, why would you. Get a 208D - 480Y and leave the Y point ungrounded - or get a 208D - 480D

But Acme does make a 208/480 15kva.

ice

 

[templdl]

I worked for a transformer mfgr the could may one. The best thing about it is that you could pretty much balance the 1ph load on all 3 primary phases and not just one ph-ph.

 

[jap]

If I was going to use a 3 phase 208v delta primary to 480/277v secondary transformer to get 480v single phase out of the secondary, I'd go ahead and ground the Y connection , not float the neutral,and go ahead and bring the neutral to the first means of overcurrent protection.
You may not need it now for the single phase 480v load your currently serving, but there's no good reason not to extend the neutral to that point wether you use it or not.