Table 250.102(C)(1) with parallel conductors

[philly]

If I have (4) sets of 4 1/C 500MCM conductors between transformer secondary and downstream panelboard how do I go about determining supply side bonding jumper (SSBJ) between transformer and panel?

Can I simply use the table and size a SSBJ for each of the (4) parallel runs? For example if I supply a 1/0 SSBJ with each run based on copper size shown in table for 350-500 kCMIL is that adequate? Or do I need to somehow calculate an equivalent area of the (4) parallel feeders and size a single SSBJ based on that equivalent size?

 

[don_resqcapt19]

That table is based on the ungrounded conductors in each raceway or cable, so you just look at the table for 500 kcmil and install that size in each raceway

 

[charlie b]

I agree with Don. But I admit I was briefly confused, because the analogous rule for EGCs reads much differently. I am guessing that you shared the same confusion, or so I gather from the way you worded the question.

 

[LarryFine]

Why isn't it that each EGC is sized as if it was the only one?

 

[Jpflex]

That table is based on the ungrounded conductors in each raceway or cable, so you just look at the table for 500 kcmil and install that size in each raceway

It looks like that’s what he said. He said put in a 1/0 AWG SSBJ for each parallel conduit containg 1 or more parallel ungrounded (phase) conductors.

The other method is to put all parallel ungrounded conductors in 1 big ass raceway and size the SSBJ to 12.5% of the total circular mill area of 1 set of phase parallel conductors

I don’t have my code book on hand so I don’t have the conductor diameter table but could someone check the difference between both option? I’m actually interested to see outcome

 

[infinity]

Can I simply use the table and size a SSBJ for each of the (4) parallel runs? For example if I supply a 1/0 SSBJ with each run based on copper size shown in table for 350-500 kCMIL is that adequate?

Yes that's how you would do it. For the SBJ you would use the 12.5% rule.

 

[wwhitney]

Why isn't it that each EGC is sized as if it was the only one?

I made some speculation in this thread: https://forums.mikeholt.com/threads/parallel-conductors-extreme-example-thought-experiment.2578790/

But that model doesn't explain why we aren't required to size each SSBJ based on the total of all the parallel sets, rather than just the conductors in one conduit. Unless we posit that SSBJs are always much shorter than EGCs may be.

Cheers, Wayne

 

[LarryFine]

So, the rules for SSBJs and for feeders are different? And is that what's being discussed here?

 

[infinity]

So, the rules for SSBJs and for feeders are different? And is that what's being discussed here?

Philly's question is about the "grounding" conductors run with the secondary conductors from a transformer. As he correctly states those conductors run in each parallel raceway would be SSBJ's. EGC's are not part of the discussion other than to say that the rules for EGC's are for some reason different than SSBJ's.

 

[augie47]

The rule is similar to grounded service conductors in two or more raceways, 250.24(C)(2)