Transformer currents

[nod]

Hi everyone, I have a 10 Kva single phase transformer. 480v primary & 120v secondary. Its my understanding at 120v I would have approx 41.5v from x3 to x1 and 41.5v from x4 to x2. Not 82 amps on 1 circuit. If this is correct and I have combined 25amps on 120v, and 6 amps on 220. Could the 25 amps come off 1 side , lets say x3 to x1? then put another 25 or so amps on x4 to x2? This all came about when the 10 kva transformer was used to hook up to an internal plant office. Now they want to hook up another office to the same transformer with approx. the same load.

Any help appreciated!
Nod

 

[electrifire]

Yes, probably.
Your description indicates a 480-120/240 volt transformer connection. If this is correct and you need both 120 and 240 volts then the coils must be connected togther to form a neutral connection, this is typically x2 & x3. Power one officew from x1 to neutral and one office from x4 to neutral. Power for 240 volts is connected x1 to x4. It is important to place not over 5000va on each connection. The 25a on 120v is 3000va and the 6a on 240v is 1440va.

Watch primary and secondary protection to agree with NEC to assure adequate protection and code compliance. This is in article 450 and 240. also watch neutral and bonding requirements as you feed two offices via separately derived system rules in article 250.

 

[busbar]

My apologies—wasn’t able to sort out the requirements of the original post. If electrifire has interpreted your needs correctly, there is one other important consideration. Whereas in the original installation only 120V required, [a 2-wire-secondary arrangement] changing to a 120/240V 3-wire service you must install overcurrent protection on both ungrounded secondary leads of the 10kVA transformer, By NEC—60 amperes maximum, and limit continuous load to 41.7 amperes on each ungrounded lead.

 

[nod]

Thanks Electrifire & Busbar for your prompt response. The transformer is a 480 - 120 /240. I had planned on over current devices on both hot legs. However I didn't reliaze the continous full load amps would be 41.7. I was thinking the 80% rule applied so 33.4 would have been continous full load.

 

[Ares]

You may find usefull a free calculator from

http://members.rogers.com/g.cygalski/electric.htm

It also helps calculate the size of overcurent protection.