7.5 kva transformer

[gbarone]

I am running 2 #2 aluminum cables 800 feet from a 277/480volt source to a 277/480 volt 120/240volt 7.5 kva transformer. What is the largest breaker panel I can use off my secondaries? How many amps should I feel comfortable with drawing?

 

[augie47]

Some variables in there....

See 450.3(B)

If you are using the 480 primary of the transformer and you protect it with a 20 amp breaker, you can use as large a secondary breaker as you desire as long as you protect the wire accordingly.
(you would only have about 40 amps at 240v available so a larger breaker would be of little use,

 

[jbelectric777]

Watch voltage drop at 800 feet. The higher the load the more the VD.

 

[augie47]

I ran that. 480 v on #2 Al at 500 ft and 20 amps was within acceptable standards.

 

[gbarone]

Augie, your saying all I'm going to have is 20amp 230volts. If this is true then what would you suggest I do to get more than 20 amps?

 

[skeshesh]

You need to explain more clearly what you're exactly trying to do? What is the primary voltage you're using at the transformer (480V or 277V)? Is the load actually 7.5KVA or would you like to size for maximum load on the transformer anyway?

 

[augie47]

Augie, your saying all I'm going to have is 20amp 230volts. If this is true then what would you suggest I do to get more than 20 amps?

7500/480=15.625 amps at480 primary protection only 15.625 x 1.25 = 19.5=20 OCP

7500/240= 31.25 amps @ 240, If you loaded the transf to 125%= 39 amps

want more = bigger transformer

to maintain a 3% VD at 800 ft on #2 AL you would need to stay at 30 amps x 480 = 15 kw = 60 amps at 240
(round figures and assuming I made no math mistake)

 

[gbarone]

to maintain a 3% VD at 800 ft on #2 AL you would need to stay at 30 amps x 480 = 15 kw = 60 amps at 240
(round figures and assuming I made no math mistake)


If I understand you correctly I wll be able to install a 60 amp sub-panel and have 15kw available at 240v after the step down transformer.

 

[kwired]

to maintain a 3% VD at 800 ft on #2 AL you would need to stay at 30 amps x 480 = 15 kw = 60 amps at 240
(round figures and assuming I made no math mistake)


If I understand you correctly I wll be able to install a 60 amp sub-panel and have 15kw available at 240v after the step down transformer.

You can not gain power that was not input. In other words you can not put in 7.5 kva and get 15 kva out. (if you find a way to do that let me know right away, we will become very wealthy)

KVA is not what is transformed voltage is. With a change in voltage will come a proportional change in available amperage that still equates to same total volt-amps.

7.5 Kva (7500 VA) / 480 volts = 15.625 amps.

7.5 Kva (7500 VA) / 240 volts = 31.25 amps.

This will be the current on each side of the transformer if loaded to 7.5 KVA

 

[augie47]

You have used the term "secondaries" in you original post which leads me to think you may be using two transformers, one to step the voltage up at the source, and one to step it down at the end of the 800 ft run. If that is the case, the are particulars that need to be addressed in terms of OCP and grounding, but the bottom line is kwired's statement that you can't gain power.
KW is KW. One you determine the load you wish to supply in terms of KW, any and all transformers will need to have at least that same KW rating, but in your case, the wire size limits your load to approximatekly 15 kw.