Section 240.21(B) and 240.21(C)

[hpec]

These sections in the 2005 N.E.C.have been changed to say that you are not allowed to round up conductor ampacities as permitted in 240.4(B).

Does this mean that a 500 KCMIL conductor which has an ampacity rating of 380 amps cannot be tapped from a feeder or a transformer secondary and be protected by a 400 amp overcurrent device? Am I missing something?

Hpec

 

[ryan_618]

Re: Section 240.21(B) and 240.21(C)

That is exactly what it means.

 

[itasca_mn]

Re: Section 240.21(B) and 240.21(C)

These sections in the 2005 N.E.C.have been changed to say that you are not allowed to round up conductor ampacities as permitted in 240.4(B).

Did you really mean to say that? 240.4(B) has never allowed us to round up conductor ampacities, the conductors must always be sized to handle the calculated load. 240.4(B) allows us to round up to the next higher standard sized overcurrent device rating.

 

[bob]

Re: Section 240.21(B) and 240.21(C)

HPPEC
Does this mean that a 500 KCMIL conductor which has an ampacity rating of 380 amps cannot be tapped from a feeder or a transformer secondary and be protected by a 400 amp overcurrent device?Am I missing something?

itasca_mn
Did you really mean to say that? 240.4(B) has never allowed us to round up conductor ampacities, the conductors must always be sized to handle the calculated load. 240.4(B) allows us to round up to the next higher standard sized overcurrent device rating.

Secondary Conductors tapped to a transformer are covered under 241.21(C). 241.21(C)2,3,4 & 5
requires that the secondary conductor have a capacity NOT LESS than the OC device. Therefore #500 kcm can not be protected by a 400 amp breaker. I really do not understand this exception to 240.4B.

[ January 09, 2005, 03:31 PM: Message edited by: bob ]

 

[don_resqcapt19]

Re: Section 240.21(B) and 240.21(C)

This is not really a change. Many of the rules in the 2002 code said that the conductor ampacity must not be less than x. Where the code rules say that, you cannot use the rounding up rule in 240.4. The new wording in the 05 code is just to make this fact clear.
Don

 

[john m. caloggero]

Re: Section 240.21(B) and 240.21(C)

There may be a little confusion in understanding Section 240.21(B)and 240.4(B). The permision to go up to the next standard size applies to the overcurrent device. The ampacity of a conductor is based on table 310.16 and the application of any correction factors which will result in a new curren-carrying capacity. This has been the interpretation for many years. The change in the NEC 2005 edition in section 240.21(B)makes it clear that the overcurrent device ahead of the feeder that supplies the tap conductor does not have the same permision to be bumped up when it is applied to the size of the "TAP CONDUCTOR". This was a common violation by those who did not understand the meaning of ampacity.