240.4(B) "Round-up Rule" for transformer secondary

[dummycheck]

I'm working on a project where I'm going to be serving a 800A service entrance with a new transformer. I was planning on using (2) sets of 500kcmil that would get me to 760A (actual load will be less than 760A), use 240.4(B) to round-up to 800A OCPD and call it a day. But as I was looking through the tap rules I noticed that 240.21(C) specifically states that transformer secondary conductors shall not be permitted to use 240.4(B).

I'm sure the intention is for step-down transformers, but does this also apply to service transformers and service entrance conductors? I know for a fact that I've seen 800A service entrances with (2) sets of 500kcmil in existing installs.

What is the thought behind not allowing the 240.4(B) on the transformer secondary?

 

[jim dungar]

Utility (service) transformers and their secondary conductors are not subject to the NEC, so we ignore them and their sizing. At the service point the NEC applies to your conductors.

 

[don_resqcapt19]

Utility (service) transformers and their secondary conductors are not subject to the NEC, so we ignore them and their sizing. At the service point the NEC applies to your conductors.

That is very rapidly changing in many areas. The NEC starts at the service point and in this area the service point for all commercial installations is the secondary terminals of the utility transformer.
This is really just a function of the utilities passing more costs off to their customers.

 

[don_resqcapt19]

I'm working on a project where I'm going to be serving a 800A service entrance with a new transformer. I was planning on using (2) sets of 500kcmil that would get me to 760A (actual load will be less than 760A), use 240.4(B) to round-up to 800A OCPD and call it a day. But as I was looking through the tap rules I noticed that 240.21(C) specifically states that transformer secondary conductors shall not be permitted to use 240.4(B).

I'm sure the intention is for step-down transformers, but does this also apply to service transformers and service entrance conductors? I know for a fact that I've seen 800A service entrances with (2) sets of 500kcmil in existing installs.

What is the thought behind not allowing the 240.4(B) on the transformer secondary?

Even before they added the specific language in 240.21, that was the rule.

The previous language said the conductors had to have an ampacity not less than the OCPD at the load end. Many code users were violating that rule, because they assumed that 240.4(B) changed the ampacity of the conductors. It does no such thing....it only permits the conductors to have an OCPD with a rating that exceeds the ampacity of the conductors.

Any time the code has a rule that requires a conductor to have a specific ampacity, 240.4(B) does not apply unless that code rule says it does.