Transformer Secondary Feeder Tap

[quentman]

We have a 480-120/208 transformer located on a mezzanine that has no permanent access ladder. The primary overcurrent fuse size is 250a. The secondary conductors consist of (2) sets of 4, 250kcml cu thhn conductors. If the secondary overcurrent device is a 600amp circuit breaker in a panelboard and the conductors are in conduit, does this meet the requirements of 240.21.C.6,for the conductors to be up to 25' in lenght? If not what would be required to make this comply? I know if I install an overcurrent device at the transformer on the mezzanine (to stay within the 10') it would not be considered "readily accessible".

 

[templdl]

Re: Transformer Secondary Feeder Tap

I think you have a classic example of how you can correctly apply that NEC article. It appears that you and complied with all three points. Technically the 25' is actually measured from the pri. OCPD to the sec. OCPD.

 

[don_resqcapt19]

Re: Transformer Secondary Feeder Tap

templdl

Technically the 25' is actually measured from the pri. OCPD to the sec. OCPD.

That would be correct if applying 240.21(B)(3), but we are using 240.21(C)(6) in this case. Under (C)(6) the 25' length applies only to the secondary conductors.
However, using 240.21(C)(6) the 600 amp OCPD is too big. The "round-up" rule in 240.4(B) does not apply and the maximum permitted OCPD for the two sets of 250 kcmil would be 510 amps.
Don

 

[quentman]

Re: Transformer Secondary Feeder Tap

Don,
Then if I increased the secondary conductor size to (2) sets of 350kcml, this should satisfy the requirements of the article.

 

[don_resqcapt19]

Re: Transformer Secondary Feeder Tap

quentman,
Yes, two sets of 350 would give you 620 amps and it appears that your installation would be in compliance with the NEC.
Don