Transformer

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cowboyjwc

cowboyjwc

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OK, may seem like a silly question and if it is I'll take my licks. Looking at my cheater card and do not see a 3 phase 50KVA transformer, I see one for single phase. Do they make one?

This is for a plan check.
 
I did a quick search, and it seems to be a common transformer used on higher primary voltages, I can't remember ever installing one that size, as with Bob, 45 and 75 seem the most common.
 
Okey, poorly drawn plans. Looked at it again. Main panel 480/277, feeding 50 KVA transformer with two pole 125A OCP, trans is feeding existing 120/240 single phase 400A panel.

So 480 three phase in, 120/240 single phase out.

Either I've never seen it, I'm just not getting it or this engineer doesn't know what he's talking about.
 
Depending on the nature of the load, the engineer may be following a sound path.
As you know, a more common 3 phase transformer is 208Y/120. If your loads are designed for 240 volt he may have gone with the 240/120 single phase. A three-phase 240/120 transformer is available, but unless things have changed, they have a severe restriction on neutral loading, (5-10% of the kva rating) so if you have a significant 120v load, the engineer may have called for a single phase unit.
 
It could be a single phase 50 kva transformer fed from a 2 pole 480 volt breaker .....480 / 120/240 volts 50 kva single phase is approx 200amp output, Maybe?????
 
cowboyjwc said:
Okey, poorly drawn plans. Looked at it again. Main panel 480/277, feeding 50 KVA transformer with two pole 125A OCP, trans is feeding existing 120/240 single phase 400A panel.

So 480 three phase in, 120/240 single phase out.

Either I've never seen it, I'm just not getting it or this engineer doesn't know what he's talking about.
Click to expand...

480 single phase in, if it's a two pole 125A. :)
 
cowboyjwc said:
Okey, poorly drawn plans. Looked at it again. Main panel 480/277, feeding 50 KVA transformer with two pole 125A OCP, trans is feeding existing 120/240 single phase 400A panel.

So 480 three phase in, 120/240 single phase out.

Either I've never seen it, I'm just not getting it or this engineer doesn't know what he's talking about.
Click to expand...
It could be just a typo, but maybe the EE wanted three phases pulled for some other reason, like future expansion to three phase in the future. He indicated a two pole breaker so it should work fine as drawn.
 
cowboyjwc said:
Okey, poorly drawn plans. Looked at it again. Main panel 480/277, feeding 50 KVA transformer with two pole 125A OCP, trans is feeding existing 120/240 single phase 400A panel.

So 480 three phase in, 120/240 single phase out.

Either I've never seen it, I'm just not getting it or this engineer doesn't know what he's talking about.
Click to expand...
Just because there is three phase available does not mean you have to use all three phases does it? According to what you said it is feeding existing single phase panel - how do you propose to feed the existing single phase panel with three phase? Maybe I am missing something or you have not provided enough info.

Sounds to me like three phase supply - two phases utilized for single phase transformer to supply single phase panel- pretty common installation, although usually not for that large of capacity for the separately derived system.
 
kwired said:
Just because there is three phase available does not mean you have to use all three phases does it? According to what you said it is feeding existing single phase panel - how do you propose to feed the existing single phase panel with three phase? Maybe I am missing something or you have not provided enough info.

Sounds to me like three phase supply - two phases utilized for single phase transformer to supply single phase panel- pretty common installation, although usually not for that large of capacity for the separately derived system.
Click to expand...

No I'm with you. It's a building expansion (church actually, not that that matters) I guess I could have also called the utility and ask why if it was originally being fed single phase, where are they now getting the three phase from (it's in a residential neighborhood). Being a fairly small town here, usually if it's three phase it's three phase and if it's single phase, it's single phase.

Think I've got it guys. Thanks.
 
Well a little bit of an update. Just talked to POCO and they have never heard from anyone concerning this job and they don't have three phase availible in that area.:happyno:

So it may become a nonissue.
 
A 50kva transformer is 104 amps on the 480 side and 208 amps on the 240/120 side, feeding a 400 amp panel I would want a load calculation on that existing 400 amp panel, although I have seen it done before, but setting the primary OCPD at 125% sure looks like they can easily over load the transformer, and then you have the inrush to worry about?

My question is if 3-phase is not available why do they or will they even have 480? this whole thing doesn't add up, this engineer didn't do his home work,
 
If the person who designed the system is suborn and a 45kva is not doable there a transformer manufacturer who will build you a 5okva of any other odd rating that is desired. Often times more than not the designer may not hove a grasp of what the commonly available ratings are.
 
hurk27 said:
A 50kva transformer is 104 amps on the 480 side and 208 amps on the 240/120 side, feeding a 400 amp panel I would want a load calculation on that existing 400 amp panel, although I have seen it done before, but setting the primary OCPD at 125% sure looks like they can easily over load the transformer, and then you have the inrush to worry about?

My question is if 3-phase is not available why do they or will they even have 480? this whole thing doesn't add up, this engineer didn't do his home work,
Click to expand...

:thumbsup:
 
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