Supply Side Bonding Jumper for Transformers

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1972Grady

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Tampa Fl USA
So I have a question about supply side bonding jumpers when used on the secondary side of a transformer. I'm bonding my system bonding jumper to the neutral in the transformer not in the first means of disconnect.
I'm having a hard time accepting the code allowing me to size the supply side bonding jumper smaller than my EGC. Here is an example.
I have a 500 KVA 480V to 208V step down transformer feeding a 1200A fused disconnect. Table 250.122 tells me that I need a 250 KCMIL AL equipment ground from the 1200A fused disconnect to the equipment downstream.
I'm coming off the secondary side of the transformer with (4) sets of 500 KCMIL AL. So I have 3 ungrounded 500 KCMIL and one grounded 500 KCMIL in each conduit. I don't want a single supply side bonding jumper. Instead I want a supply side bonding jumper in each of the 4 conduits. So I go to Table 250.102(C)(1) using the 500 KCMIL unground conductors i see I would need to use a 1/0 AL supply side bonding jumper in each conduit. Why can this 1/0 supply side bonding jumper be smaller than the 250 KCMIL ECG required by Table 250.122. I would usually see the supply side bonding jumper increase in size from the ECG but in this example the supply side bonding jumper decreases in size from the ECG.
Can I really install (4) 500 KCMIL and a 1/0 SSBJ in each raceway from the transformer to the disconnect then go from the disconnect to the downstream equipment with (4) 500 KCMIL and a 250 KCMIL ECG in each conduit? This just feels wrong to me.
Any help understanding why I can do this would be appreciated.
 

infinity

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New Jersey
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Can I really install (4) 500 KCMIL and a 1/0 SSBJ in each raceway from the transformer to the disconnect then go from the disconnect to the downstream equipment with (4) 500 KCMIL and a 250 KCMIL ECG in each conduit? This just feels wrong to me.
Any help understanding why I can do this would be appreciated.

You don't have a smaller SSBJ, you have 4-#1/0's in parallel which is significantly larger than a single 250 kcmil. EGC's must be full sized in each parallel raceway but a single raceway would require only one.
 
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don_resqcapt19

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Illinois
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retired electrician
You don't have a smaller SSBJ, you have 4-#1/0's in parallel which is significantly larger than a single 250 kcmil. EGC's must be full sized in each parallel raceway but a single raceway would require only one.
Just an example of why 250.122(F) is broken code. Between the transformer and the 1200 amp OCPD, the supply side bonding jumper is 1/0 in each raceway, but on the load side of the OCPD, with the same size ungrounded conductors, the EGC is 250 kcmil in each raceway. It is my opinion that the worst case for fault clearing is on the line side of the OCPD and that if a 1/0 in each raceway is a large enough fault clearing path on the line side of the OCPD, a 1/0 in each raceway would be large enough on the load side of the OCPD.

It appears that they may correct this and the 250.122(B) issue in the 2020 code.
 

infinity

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New Jersey
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The full size EGC in each parallel raceway is a waste of money. Maybe they will finally accept the proposal that Don has been trying to get into the NEC for a few code cycles. :thumbsup:
 
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