sizing of service conductors

[mkd]

i just did a commercial project and used a 4/0 alum. on a 200amp main breaker and got turned down. the inspector cited lug ratings at 75. c and ampacity of 180amps even though insulation was xhhw-2.i dont have the guts to ask about article 240-3,devices rated 800 amps or less. is it the consesus here that my application is acceptable? i normally do not wire ith alum. wire except residential and i seemed to have outsmarted myself with this application. the main breakers had to be changed from200 to 150 amp.

 

[Smart $]

i just did a commercial project and used a 4/0 alum. on a 200amp main breaker and got turned down. the inspector cited lug ratings at 75. c and ampacity of 180amps even though insulation was xhhw-2.i dont have the guts to ask about article 240-3,devices rated 800 amps or less. is it the consesus here that my application is acceptable? i normally do not wire ith alum. wire except residential and i seemed to have outsmarted myself with this application. the main breakers had to be changed from200 to 150 amp.

Don't know what the general consensus is here, but I'd say your application was acceptable prior to changing out the breaker. AFAIK feeder size is determined by load, not the ocpd. Then the ocpd is determined by the ampacity of the feeder, not the other way around. And this is the current (i.e. 2005 NEC) way. It's fairly obvious that your load only approached the conductor's ampacity if you replaced the 200A breaker with a 150 without a hitch.

I've never got a grasp of the rationale behind the "conductor ampacity by the lug" rule. Perhaps someone has a good explanation...?

 

[paul32]

I've never got a grasp of the rationale behind the "conductor ampacity by the lug" rule. Perhaps someone has a good explanation...?

You have to limit the temperature of the conductor to what the lug can handle. Heating the conductor to 90 degrees will make the lug 90.