30 KVA transformer

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wmhwdh00

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In my classroom I need to convert 240 3 ph. to 208 3 ph. Our school has purchased a 30 kva xfmr (too large) I have 5 labvolt A frames that are going to be dedicated loads (nothing else will be using this xfmr) They will pull no more than 10 Amps a pc. And none of the units will prob. be on at the same time. ALL I have is a 60 Amp disconnect (3 ph.) Do you think it would be ok to feed this xfmr with the 60A disconnect and what size conductors would you use to feed it with?
Thanks
 
Re: 30 KVA transformer

Do you think it would be ok to feed this xfmr with the 60A disconnect and what size conductors would you use to feed it with?
Click to expand...
It sounds like you are doing this work yourself. Your profile doesn't list what type of instructor you are. If you happen to be something like a history teacher, don't you really think you should hire someone to do this?


Please don't be offended if you are really qualified to do this work. I just want to be sure I'm not compromising the students safety by giving advice to someone who wouldn't know all the relevent questions to ask.

Steve
 
Re: 30 KVA transformer

I am an elect. instruct. I do know how it should be done, but we have limited money. The state gave us a grant for a lot of equip. that I told would not work with our current delta high leg system. They bought us the equip. but now I have to figure out the cheapest way we can get power for the new equip. and there is NO more money. Right now it's only use for my class is a boat anchor...you can not imagine how frustrating this is for me...
 
Re: 30 KVA transformer

At 240 volts, a 30 KVA transformer has a primary side rated current of about 72 amps. You are permitted to feed it with a breaker much larger than 72 amps. But the code gives this as a maximum value. There is no minimum. So if you want to feed it with a 60 amp breaker, that is acceptable.

The principal function of a breaker is to protect the conductors that it feeds. In this special case, the breaker is also protecting the primary windings of the transformer. The only requirement here is that the conductors be capable of handling every bit of the 60 amps that the breaker will permit to flow on a continuous basis. Since this is lower than 100 amps, you have to use the 60C column of Table 310.16. Therefore, you need a minimum conductor size of #4 copper. If you have something larger on hand, then you can use that instead, provided only that it will fit into whatever terminal connections you have to use.
 
Re: 30 KVA transformer

I'd like to give you a heads up on this in that, just because you have a small load to begin with doesn't change the inrush magnetizing current should the transformer be fully loaded or partially loaded. Anticipate the inrush to be up to 19x the pri. FLA for a 150degC unit which you should consider with your 60a disconnect. You don't want to blow your pri. OCPD when the transformer is simply energized.
To add insult to injury if you were unlucky enough to get a 115 or 80 degC DTDT the inrush could be as high as 20-60% more than the 150degC.
 
Re: 30 KVA transformer

Good point. That, in fact, is one of the reasons that the NEC "allows" you to use a primary protection breaker much larger than the rated primary current. If just turning on the transformer keeps causing the breaker to trip, even if there is no connected load (not an impossible situation), then you will have to find a breaker with a higher trip point.
 
Re: 30 KVA transformer

Doesn't loading the secondary reduce inrush on the primary? Still it sounds like time delay fuses are a must in your disconnect.

What things couldn't run on 240 3 phase?

You might try ebay if you can't energize it w/o blowing the 60's.

Good luck,

Jim
 
Re: 30 KVA transformer

charlie b
Breakers larger than the common residential type have an instantaneous (magnetic) trip calibration of 10x the breaker rating with a calibration range of +-20%.
So, consider 72.2 FLA x a possible worst case of 19x for a 150degC DTDT = 1372a. Remember that 115 and 80degc DTDTs are even higher.
The question is if you had a 60a fused disconnect could you apply a fuse that would ride through the inrush. With secondary protection it is of my opinion that it would be possible to apply the 250% rule from table 450.3b 72.2x250%=180.5a or 175a OCPD depending upon the location of the location of the OCPDs.
 
Re: 30 KVA transformer

"that I told would not work with our current delta high leg system. They bought us the equip. but now I have to figure out the cheapest way we can get power for the new equip. and there is NO more money"

I am sorry, but this work is over your head. What kind of system? how about grounding?
location of this transformer in the room...with kids?????????

To entice an electrician, have your school name the room after him/her ;) :cool:

If you really want to save money and possibly someone's life, reach out to the electrical community. I am sure that you can find a generous soul to help with the kids.
Good Luck!!!
 
Re: 30 KVA transformer

Are we to understand that you have a 240 delta service? And that you have a 240v delta-208y/120, 30kva transformer and it won't work??
They would have to be more specific. Somebody may not want to go through the trouble of installing a feeder and the associated feeder breaker to the transformer.
 
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