conductor size for 600amp

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lx2077

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happy new year all!!! i need a second opinion im setting up a standby generator rated for 600a i am thinking twin 250mcm in paralell,let me know what you think thanks
 
A 250 MCM Copper conductor is good for no more than 255 amps. Two of them will not handle a 600 amp load. You need to use the 75C column of Table 310.16, not the 90C column, even if the insulation system is rated for 90C. That is because of the temperature limitations of the terminations. Reference 110.14(C). Also, even looking at the 90C rating of a 250 MCM conductor, you only have 290 amps. You can't use 580 amps of conductor, if the load is over 580 amps.

Welcome to the forum.
 
600a

600a

thanks for the fast reply does it make a diffrence its 3phase? and what size conductors would you suggest for this and what size neutral thank you so much
 
lx2077 said:
. . . does it make a diffrence its 3 phase?
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No.
lx2077 said:
. .. what size conductors would you suggest for this and what size neutral thank you so much
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My reading of Table 310.16 tells me that a pair of parallel 350 MCM copper conductors will have and ampacity of at least 600 amps. That presumes, of course, that they are not all in the same conduit, and that the ambient temperature is not higher than 30C. As to the neutral size, I usually specify that it match the ungrounded conductors. But if you want to make it smaller, then you will need to do a service calculation that includes calculation of the neutral load.
 
600a

600a

hey charlie thanks so much for your consult its always nice when 2 electrians can come togehter like 2 doctors i work for the NYCTA and we have a standby generator in case of an emergency and the generator can pull no more than 450a 3 phase however it has an 600a breaker and was a little baffled as to why it has a 600a breaker when it can only pull 450a possibly overcurrent maby....who knows in addition when the time comes it will be free air no more than a 25ft run si in closing i gave my imput to my boss to decide 250-vs-350....thanks again for the consult
 
lx2077 said:
when the time comes it will be free air no more than a 25ft run si in closing i gave my imput to my boss to decide 250-vs-350....thanks again for the consult
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You better go back to your boss again, for a Free Air application see Table 310.17

Roger
 
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