I require 1200A current capacity from the secondary of the service transformer to the main distribution panel.
Originally looking at this table 3 from the CEC C22.1-09 was used and found that 3 750kcmil per phase could be run to supply 1200A. However it was pointed out to that because the raceway was underground it was required to use diagram B4-4 in appendix B (detail 3, 3 cables per phase) along with table D15A (appendix D). From this I realized that 3 750kcmil per phase would only allow for a current rating of 1032A, which would not meet the breaker size requirement in the MDP.
The two options explained to me now are to pull out the aluminum conductors and replace with copper (may be issues with bending radius and space/constructability at MDP, not to mention cost) or to change the size of the breaker.
I am new to the construction industry and have two questions. The first is how will downsizing the breaker to 1000A solve anything? If the load still requires 1200A won't this just trip the breaker anyways. How is it different than continuing to use the 1200A breaker? Is it just for protection of the cabling from the transformer to the breaker (ie, does not draw to much current and damage cable)?
My second question is a little more theoretical. Logically thinking about cable rating it would make sense (to me) that the current capacity for a conductor underground would be higher than that of a conductor in free air? The ground would be cooler so therefore dissipate heat more easily than in free air (normally warmer). Or does it have something to do with the heat not being able to go anywhere in the raceway causing damage more easily? I thought that tables 3 and D15A were counterintuitive, any info on this topic would be greatly appreciated. First post so go easy on me! Thanks in advance.
Originally looking at this table 3 from the CEC C22.1-09 was used and found that 3 750kcmil per phase could be run to supply 1200A. However it was pointed out to that because the raceway was underground it was required to use diagram B4-4 in appendix B (detail 3, 3 cables per phase) along with table D15A (appendix D). From this I realized that 3 750kcmil per phase would only allow for a current rating of 1032A, which would not meet the breaker size requirement in the MDP.
The two options explained to me now are to pull out the aluminum conductors and replace with copper (may be issues with bending radius and space/constructability at MDP, not to mention cost) or to change the size of the breaker.
I am new to the construction industry and have two questions. The first is how will downsizing the breaker to 1000A solve anything? If the load still requires 1200A won't this just trip the breaker anyways. How is it different than continuing to use the 1200A breaker? Is it just for protection of the cabling from the transformer to the breaker (ie, does not draw to much current and damage cable)?
My second question is a little more theoretical. Logically thinking about cable rating it would make sense (to me) that the current capacity for a conductor underground would be higher than that of a conductor in free air? The ground would be cooler so therefore dissipate heat more easily than in free air (normally warmer). Or does it have something to do with the heat not being able to go anywhere in the raceway causing damage more easily? I thought that tables 3 and D15A were counterintuitive, any info on this topic would be greatly appreciated. First post so go easy on me! Thanks in advance.