240.21(C) TRANSFORMER SECONDARY QUESTION

[Chrispacc]

I NEED help!!!! I Have a 480V panel feeding a 75KVA X-FMR on the primary side, on the secondary side I have an 80' run on 4/0THHN to a 225AMP MCB panel where it HAS overcurrent protection!!! This inspector is calling me out saying that I cannot go over 25' without a disconnect? I am insisting that that is just a TAP rule.... there is a difference between a tap rule and secondary feeders. check out the definition of TAPS in 240.2....let me know ASAP!

 

[jim dungar]

Your 'tap' installation falls under the rules of 240.21(C) Transformer Secondary Conductors.
Other than outside installations, it would be rare that you can go more than 25' without a protective device.

 

[augie47]

I see you both being correct most liklely. It is not a "tap" as defined in 240.2.
It is a transformer secondary conductor as covered by 240.21(C) under which there are various scenarios. Without details, I don;t know which covers your situation, but the 25ft rule is the most common. Note wording in 240.21(C)(1) as to whether the secondary conductors can be protected by the primary (you did not indicate delta or wye).

sorry, jim, I posted on top of you.

 

[Smart $]

240.91(C) may apply if a supervised industrial installation.

 

[charlie b]

I am insisting that that is just a TAP rule.... there is a difference between a tap rule and secondary feeders. check out the definition of TAPS in 240.2....let me know ASAP!

I checked out that definition. You do not have a tap. The definition mentions the presence of an overcurrent device upstream of the conductor. Your installation does not have one.

Start with a 400 amp breaker serving a wire with an ampacity of 400 amps. Somewhere along that wire, you connect a wire that only has an ampacity of 200 amps, and you don't install a 200 amp overcurrent device at that connection point. Thus, the 200 amp wire has an upstream overcurrent device that is not capable of protecting it at its ampacity. That is a tap situation.

Sorry, but I believe, based only on your description, that the inspector is right.

 

[charlie b]

. . . saying that I cannot go over 25' without a disconnect. . . .

More accurately, you cannot go more than 25' without an overcurrent device. An unfused disconnect would not be sufficient.

The overcurrent device you install at 25' (or closer) will protect the wire against an overcurrent situation caused by a problem further downstream. Nothing protects the wire fagainst an overcurrent situation caused by a problem further upstream. We accept that risk because we take some mitigating precautions. One is to protect the wire from physical damage. Another is to make the run short, so there are fewer places at which a problem could occur. That second item is the reason for the 25' limit.