transformer protection.

[stew]

I have 2 questions on transformers. The first is I just ran across an installation of about a 50 or so kve 480/120-208 v transformer feeding a panelboard. The primary has a 60 amp breake. the secondary has about 6 or 7 feet of flex down to the panelboard which then terminates in the panelboard 125 amp main breaker. There is no secondary protection at the transformer itself. Also they have only used # 4 cu for the feeder to the panelboard along with a #4 cu neutral. I was called out because the main breaker 125 a on the panelbord kept tripping. This is in a freight terminal operation so all thier phone,fax lites and computer equipment was down. I removed the 125 breaker and as I was removing found one primary ball lug stripped and 2 secondary bolt on connections so loos that the washes were arced. I put lugs on the feeders and bolted direct to the bus to get them going for the weekend. Does the 125 amp breaker serve as both the disconnecting means and the panelboard protection. Also why do you suppose the orginal installation then only had # 4 feeders. This is a three phase set up and has me somewhat puzzled.

 

[don_resqcapt19]

Re: transformer protection.

Stew,
If it is a 50kva, the primary full load amps will be about 60 amps so a 60 amp primary breaker will meet the rule in Table 450.3(B) for the transformer protection. Secondary protection for the transformer would not be required in this installation. Disconnects are not required for transformers. The secondary conductors require protection per 240.21(C). It looks like they tried to used 240.21(C)(2), but the secondary conductors would have to have an ampacity of at least 125 amps to be used with the 125 amp main breaker in the panel.
Don

 

[augie47]

Re: transformer protection.

Since this transformer is a

50 or so kve 480/120-208 v

the wire might be able to be a "#4 or so"

Does the 125 amp breaker serve as both the disconnecting means and the panelboard protection.

YES

Once you have determined the actual transformer size (with 3 phase its more than likely a 45kva and not a 50)then you can determine the secondary conductor and breaker. Assuming the panelboard is rated 125 and the transformer is a 45 kva you are set to go. Ignoring certain "exceptions" to the Code, your secondary conductor would need to be a #1.