240.4(B) (upsizing a CB size)

[kwired]

I am also confused about same scenario , I Have Load of 600 Amp (Load is Chiller Units , and at Nameplate it is mentioned that OCPD should be 800Amp) , The ampacity of Cable is 882 amp , but once i apply the de-rating factor the ampacity is reduced to 740 amp. Can i use 800 amp CB and is it accepted as per codes. If yes, please mention the Code reference.

What is MCA of the unit?

Lets assume it is 600 amps, and the nameplate ocpd is 800, then you have 740 amps of conductor (derating is proper, thing your actual ampacity is what is left afterward) which is greater than the required 600.

Next size up OCPD rule in 240.4(C) doesn't apply to art 430 and 440 applications.

If you use nameplate 600 amp or greater for conductors and 800 amps for OCPD you are compliant.

You may or may not have other details that could change the response, this assuming 600 is the MCA and 800 is the actual max OCPD stated on nameplate.

 

[mohsan514]

FLA of Unit is 600 amp..

Sent from my SM-J700F using Tapatalk

 

[jaggedben]

The specific reference to 240.4(B) was added to 240.21 a few cycles back because code users were not following the previous language. The previous language required that the tap or transformer secondary conductor have an ampacity not less than the OCPD at the end of that conductor. It is clear that the result was intended when they added the reference to 240.4(B), but not sure of why it needs to be that way.
There may be information in the ROP and ROC for that change that would say why this is a requirement.

Edit...I posted before I read that you had found the ROP...I guess you need to be looking to the TCRs and TCDs from when the rule was written that required the tap or secondary conductor to have an ampacity not less than the rating of the OCPD at the load end of that conductor. I have no idea of when that went into the code.

I'd wager that whoever originally came up with the original "limit the load to the ampacity of the tap conductors" was simply not thinking about 240.4(B) and was trying to stay concise and clear about other aspects. If they'd chosen something else - say, "protect the tap conductors from overload" - then this whole thing would probably have come down differently. "Protect from overload", while used for service conductors in 230, has a bit of a problem of being confusing with respect to a tap conductor (i.e. it's not protected upstream for a fault, only for a load current downstream). So they chose something clearer in that respect, and inadvertently cast doubt about 240.4(B).

 

[don_resqcapt19]

I'd wager that whoever originally came up with the original "limit the load to the ampacity of the tap conductors" was simply not thinking about 240.4(B) and was trying to stay concise and clear about other aspects. If they'd chosen something else - say, "protect the tap conductors from overload" - then this whole thing would probably have come down differently. "Protect from overload", while used for service conductors in 230, has a bit of a problem of being confusing with respect to a tap conductor (i.e. it's not protected upstream for a fault, only for a load current downstream). So they chose something clearer in that respect, and inadvertently cast doubt about 240.4(B).

I am not sure when either rule was put into the code, but since both rules are in the same article, I am sure that if 240.4(B) existed when the rules in 240.21(B) and (C) were added that those panel members would have been well aware of the rule.

 

[jaggedben]

I am not sure when either rule was put into the code, but since both rules are in the same article, I am sure that if 240.4(B) existed when the rules in 240.21(B) and (C) were added that those panel members would have been well aware of the rule.

What I found, if I recall correctly now, was that 240.21(B) and (C) as we know them were consolidated in the 1999 code cycle from several previously existing but less organized rules. But the actual language - e.g. 'limit the load to the ampacity of the conductors' - was at least partly copy-pasted from those earlier rules, and I don't know if that language predates 240.4(B) or not.

 

[don_resqcapt19]

What I found, if I recall correctly now, was that 240.21(B) and (C) as we know them were consolidated in the 1999 code cycle from several previously existing but less organized rules. But the actual language - e.g. 'limit the load to the ampacity of the conductors' - was at least partly copy-pasted from those earlier rules, and I don't know if that language predates 240.4(B) or not.

The equivalent to 240.4(B) was Exception #1 to 240-3 in the 1975 code, and the feeder tap rule required that those conductors to be connected to an OCPD on the load end that would 'limit the load to the ampacity of the conductors". There were no transformer secondary conductor rules other than the one for transformer feeder taps with primary plus secondary not over 25' long. All of these were written as exceptions to 240-21.
As I recall, at that time, you used the feeder tap rules for transformer secondary conductors, and at a later date, they gave the transformer secondary conductors their own rules.
So to find the answer we have to look at codes older than the 1975 one.

 

[kwired]

FLA of Unit is 600 amp..

Sent from my SM-J700F using Tapatalk

There is no MCA?

This is a unit with hermetic refrigerant compressors, and other loads isn't it?

 

[mohsan514]

Please find the attached Nameplate
MCA 596 amp

Sent from my SM-J700F using Tapatalk