600A Service Conductor Sizing

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gk351

Senior Member
Location
IL
I'm trying to put some info together for a 600 Amp 120/208 3ph service. All this info pertains to the service wires which extend from the POCO pad mount XFMR to the metering cabinet, then to the OCPD. My plan is to run parallel runs of 500kcmil XHHW Alu. I have a few questions. Do I have to derate for 4 conductors? I guess I'm not 100% sure when to add the neutral conductors for derating. Let's say I'm required to derate, then I derate to 80% which gives me an allowable amperage of 280A ( 90 degree column = 350A x .80= 280A). With parallel sets that gives me a total of 560A. Am I able to use the rules from 240.4(B) which would allow me to go to the next size circuit breaker ( 600A) Any help would be appreciated. Thanks
 

pete m.

Senior Member
Location
Ohio
Chances are that the neutral conductor will not be a CCC unless the majority of the load served is non-linear. I think you can find it in 310.15(B)(4)'s neighborhood. (at least I think it was about there in the 08 NEC)

Pete
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
In all likelihood you won't have more than 50% of the service load nonlinear so the neutral will not count as a CCC. That would put you at 620 amps for parallel 500 kcmil Al conductors. Even if the neutral did count as a CCC you could still go up to the next standard size {600 amps} if your connected load were 560 amps or less.
 

gk351

Senior Member
Location
IL
So for most purposes, if the load is less than 50% is non-linear I won't need to count the neutral as a CCC?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector
In a vasty majority of service sitaution you are not required to count the neutral as a current-carrying conductor. 310.15(B)(4) {08} requires you count it only when a major portion of the load is non-linear with harmonic currents so they type load you have would be the deciding factor.


In sizing your conductors, keep in mind the conductor must 1st be sized to carry the load.
If you are feeding a single service overcurrent/dissconnect then you must size to that device keeping 240.4(B) in consideration.
Also keep in mind that 110.14 nornally limits your terminating ampacity to the 75? rating so (2) parallel runs of 500 kcmil AL in serarate conduits would have an amnpacity of 620 amps (assuming no deratiing)
If you do have to derate (non-linear load) your load would need to be no more than your 560 amp
calculation.
 
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