240.21(C)(1)

[augie47]

The provisions for two-wire & delta-delta protection tend to confuse me in regard to application of the ratio.
For instance, let's say I have a 480 to 240 delta-delta transformer with a 150 amp primary fuse.
The ratio is 2:1.
What minimum size conductor on the secondary side would be considered to be protected by the primary device ?

 

[Carultch]

The provisions for two-wire & delta-delta protection tend to confuse me in regard to application of the ratio.
For instance, let's say I have a 480 to 240 delta-delta transformer with a 150 amp primary fuse.
The ratio is 2:1.
What minimum size conductor on the secondary side would be considered to be protected by the primary device ?

Multiply the primary OCPD by the primary to secondary voltage ratio. That gets you the effective trip rating response to secondary current. Install at least that many amps worth of wire.

So in this case that would be at least 300A, which is 350 kcmil copper.

 

[augie47]

That was my thinking but somehow my mind likes to do that backasswards Thanks