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Monday, March 31, 2014
Question:
3000 amp service
Charlie,
I have an existing condition and need a solution…..
3000 amp service 480/277 volt 3 phase 4 wire
6 – 4” PVC conduits underground from transformer to MDP … 60’ runs
Customer has tons of 500MCM THHN copper to use to make this work
How would you make this work with 6 – 4” PVC conduits
Thank you ,
Chris Roberts
A
Answer:
Hey Chris, how about this for a solution?
Assumptions:
These are service conductors.
The 500 kcmil conductors are also marked THWN or some insulation type that is suitable for a wet location.
The neutral current is balanced and the major portion is not a non-linear load.
The neutral load does not exceed the ampacity of 6 paralleled 500 kcmil conductors (1824 amperes based on THWN insulation and 6 current-carrying conductors in each conduit).
You already have the 500 kcmil copper conductors
Pull 7 -500 kcmil conductors in each conduit - 2 (A phase), 2 (B phase), 2 (C phase) & 1 grounded neutral conductor. The total phase conductors in parallel will be 12 with 6 parallel grounded neutral conductors. The minimum grounded neutral conductor size required in each conduit is 2/0 (based on 1000 kcmil equivalent area of the phase conductors), but you already have the 500 kcmil conductors.
Using 380 amperes X 80% (for 6 current-carrying conductors) X 12 (number of parallel conductors) = 3648 amperes. It would be 3192 if you count the neutral as current-carrying but the parallel grounded neutral conductors would then have an ampacity of 1596.
Make sure the AHJ agrees with this before you start.