This is partially a real scenario and partially hypothetical.
On the secondary of customer owned 12kV-480V transformer, it goes to 4000A switchboard.
Let's say I'm doing 11 parallel sets of 500kcmil CU for ungrounded conductors between transformer and swbd, each set in own pvc conduit
This is separately derived system, and System Bonding Jumper (SBJ) is at transformer. I must then have Supply Side Bonding Jumper (SSBJ) in each conduit between transformer and swbd. 250.102 (C)(2) dictates that I only have to size the SSBJ according to the ungrounded conductors in each raceway or cable. So my SSBJ is then 2/0 CU.
Now let's take the same exact scenario, but let me add a 4000A fused disconnect between transformer and switchboard. Still going to do the 11 parallel sets on both sides of this disco, and on the transformer side of the disconnect, everything will be exactly the same as before. But between the disconnect and switchboard this is no longer SSBJ but is now an EGC and needs to be sized per 250.122 which would require 500kcmil CU for this EGC.
Two questions:
1. Is my interpretation and application of the code correct here?
2. If #1 is "yes", how in the world does this make sense? Not that I have a naive view that the NEC always makes perfect sense, but this one seems more off than usual. If I have a fused disconnect, I have to make the ground wire 500kcmil. If I don't have fused disconnect, I can get away with 2/0. Adding protection makes the ground wire size go up...
On the secondary of customer owned 12kV-480V transformer, it goes to 4000A switchboard.
Let's say I'm doing 11 parallel sets of 500kcmil CU for ungrounded conductors between transformer and swbd, each set in own pvc conduit
This is separately derived system, and System Bonding Jumper (SBJ) is at transformer. I must then have Supply Side Bonding Jumper (SSBJ) in each conduit between transformer and swbd. 250.102 (C)(2) dictates that I only have to size the SSBJ according to the ungrounded conductors in each raceway or cable. So my SSBJ is then 2/0 CU.
Now let's take the same exact scenario, but let me add a 4000A fused disconnect between transformer and switchboard. Still going to do the 11 parallel sets on both sides of this disco, and on the transformer side of the disconnect, everything will be exactly the same as before. But between the disconnect and switchboard this is no longer SSBJ but is now an EGC and needs to be sized per 250.122 which would require 500kcmil CU for this EGC.
Two questions:
1. Is my interpretation and application of the code correct here?
2. If #1 is "yes", how in the world does this make sense? Not that I have a naive view that the NEC always makes perfect sense, but this one seems more off than usual. If I have a fused disconnect, I have to make the ground wire 500kcmil. If I don't have fused disconnect, I can get away with 2/0. Adding protection makes the ground wire size go up...
