three phase 277/480 to 100 amp sub panel question...

[crfmario]

Hey guys my question is, I am adding power to a subpanel in a 40 ft cargo container at an asphalt plant (just for lights and plugs) but the way I have been told to do it concerns me because Im wondering if it is wrong. I need to come off of a 3phase 480v 50amp breaker to feed a basic single phase, 100amp 120/240 sub panel, but my boss told me to use a 30kva 3phase transformer that he had laying around his shop. If I bring three phase 480 to this disconnect then to the transformer, can I just come off of 2 of the secondary lugs (instead of using all three) to feed my single phase subpanel? I feel like I could get away with it but that it isnt right, and i understand that I could potentially throw the system off balance by doing this. Anyhow , hopefully I described my situation in enough detail to understand. I feel like its kind of an amateur question but its buggin me pretty bad. Any and all input is greatly appreciated

 

[iwire]

You do not have to use all three phases from the transformer.

 

[GoldDigger]

But you will probably end up with 120/208 instead of 120/240. If that is OK, go for it.

 

[kwired]

If you brought all three phases to a three phase panel but only fed two single pole circuits out of it you still only use two lines to actually power any loads.

Assuming the load here is pretty minimal - if you put all loads on one line to neutral (all 120 volt of course) - you still have about 10kVA capacity from the transformer on that one line, what size of single phase transformer would be used if you had to specify one?

 

[crfmario]

My mistake I meant I would be just bringing Single Phase 480 into the transformer and then out 120 / 208 on the secondary but I wasn't sure if this was a very Orthodox way of doing it or not so I wanted to run it by you guys here at the forum thanks for your responses

 

[crfmario]

Kwired, I'm not sure but I know 30 kcal seems pretty oversized for what they need my guess would be like a 10 kVA single phase, I'm pretty sure neither my boss or the customer wanted to spend any money on a new Transformer so they just used this one

 

[crfmario]

Typo kcal =kva

 

[GoldDigger]

My mistake I meant I would be just bringing Single Phase 480 into the transformer and then out 120 / 208 on the secondary but I wasn't sure if this was a very Orthodox way of doing it or not so I wanted to run it by you guys here at the forum thanks for your responses

If you bring single phase 480 to a three phase transformer having a 208Y/120 secondary all you will have coming out is one 120V line to neutral phase. No 208, no 240 If the output is high leg delta, then you could get single phase 120/240.

Tell us more about this left over three phase transformer please.

 

[ActionDave]

If you bring single phase 480 to a three phase transformer having a 208Y/120 secondary all you will have coming out is one 120V line to neutral phase. No 208, no 240 If the output is high leg delta, then you could get single phase 120/240.

Tell us more about this left over three phase transformer please.

Wring pulling grunt here. Would you have 120V line to neutral on X1, X2, and X3?

 

[GoldDigger]

Wring pulling grunt here. Would you have 120V line to neutral on X1, X2, and X3?

If the input is wye with one winding connected, I would expect 120 X1 to neutral and zero from both X2 and X3 to neutral. If two windings connected I would have two hot leads, both at 120. But a wye primary is unlikely.

If the input is delta, with two wires connected, I would expect 120 from X1 to N and 60 on each of X2 and X3 to N, with 120 between X2 and X3 (since the other two primary windings would be in series across 480, in parallel with the directly driven winding.)

 

[ActionDave]

....If the input is delta, with two wires connected, I would expect 120 from X1 to N and 60 on each of X2 and X3 to N, with 120 between X2 and X3 (since the other two primary windings would be in series across 480, in parallel with the directly driven winding.)

Let's focus on this one and draw a picture with crayons.

 

[GoldDigger]

Fresh out of crayons, sorry...
(I am graphically challenged.)
If you supply a labelled drawing I will talk about it.

 

[ActionDave]

(I am graphically challenged.)

I am too. Hold on to your thoughts though.

 

[kwired]

I am too. Hold on to your thoughts though.

Draw your delta triangle representing the primary windings. Now apply 480 volts across just one side.

Now look at how it is connected to the other two sides, they are 480 volt rated coils in series across the two input leads, each will have only 240 volts voltage drop across them - since that is half of normal rating the corresponding secondary coils will only put out half of their usual 120 volts.

I don't think you would have any rotation if you would try to connect a three phase motor.