xfmr question

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jap2525

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I had a customer call and said he had a 100 kva 480 delta x 120/208 y 3ph. transformer and wanted reassurance about wire sizing for primary and secondary to feed a machine.He didnt say wether the machine had an overcurrent protection device on it or not.I came up with 120 amps for the primary feeder and 277 amp output.
They will be feeding the xfmr from a 480v busstap.
I figured installing 150amp fuses in a 200amp buss tap and using the 75 degree wire column came up with 1/0 thhn for the pri conductors and 500mcm or parallel 2/0's thhn to feed the machine.He calculated #1 for the pri and 350 mcm for the secondary.I wasn't comfortable enough with my calculations to argue the fact and thought I could get a reply from all of you.Also it says the primary if secondary protection is used can be as much as 250% meaning 300amp fusing for the primary,In that case do the primary conductors have to be rated for 300 amps or am I looking at this wrong? If we size the pri at 250% to begin with we would need a 400 amp tap box.
 
Re: xfmr question

I agree Article 450.3 (B) recognizes the transformer as being protected if the pri OCPD is set not higher than 125% of Pri FLA 120A X 1.25 = 150A. The 250% is only applicable if the secondary winding is protected at not more than 125% of the Sec. FLA. Be careful not to mix transformer protection and conductor protection.

The secondary conductors should be installed in compliance with the secondary tap rules of 240.21.(C) and generally they should be terminated at overcurrent protection. That is for the secondary conductors not the transformer.

The secondary conductors only need to be sized for the load served. What does the utilization Eq require??

Charlie
 
Re: xfmr question

Come to find out the machine has a 350 amp 3pole breaker that serves as the ocpd and disconnecting means.
 
Re: xfmr question

At 350A that would be 125kVA + or -

Whats the name plate current does the mfg recoment a fuse or breaker size, or a minimum circuit ampacity??
 
Re: xfmr question

Also it says the primary if secondary protection is used can be as much as 250% meaning 300 amp fusing for the primary,In that case do the primary conductors have to be rated for 300 amps or am I looking at this wrong? If we size the pri at 250% to begin with we would need a 400 amp tap box.
Click to expand...
The primary current for a 100 kva transformer at 480 volts is 120 amps x 1.25 = 151 amps. 2/0 thhn 75C is required for the primary conductor. The secondary current is 278 amps x 1.25 = 348 amps.
500 thhn 75C is required for the secondary conductor.
You have a 350 amp breaker which is adequate for secondary protection.
Since the secondary is protected at 125% then the primary can be fused at a max of 250% You may not exceed the 250%. The conductor will still be 2/0 thhn. Check the tap rules 240.21b(2)and(3)and (4).
This is handled similar to motor protection in that the OC device can be larger than the conductor capacity provide the conditions are met.

[ October 21, 2004, 09:02 PM: Message edited by: bob ]
 
Re: xfmr question

JAP2525
Your profile is not allowing private messages
so I can not answer your question. Check it out and see if you can activate private messages.
 
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