Sizing the System Bonding Jumper per the NEC on a separately derived system

[bdn2004]

Attached is a sketch of a proposed system. I want to make sure I've got this system bonding conductor sized correctly.
As sketched there are 5-750kcmil conductors / phase feeding the panel. I'm using 12.5% rule to come up with this size.

I don't see it spelled out anywhere in the NEC or in the Handbook but I'm assuming this is the total conductor required - not the size required in each conduit.

 

[cuky2000]

The 1/0 AWG for the bonding conductor per the NEC Table 250.102(C)(1) looks fine to me.

GENERAL OBSERVATION:
a) The bonding jumpers are intended to carry fault current and its size increase as the size of the phase conductor conductors increases.
b) Grounding Electrode Conductor, it is not intended to carry fault current and never has to be larger than 3/0 AWG per Table 250.66.

For information, below is an excerpt of the new 12.5% Rule, from the NEC Table 250.102(C)(1)

 

[bdn2004]

I appreciate the answer...but look at the example I've attached from the 2017 NEC Handbook. I worked this the same way he did except I get we need 468.750 kcmil which would round up to a 500 kcmil cable.

My question is....

a) is that ONE conductor to be run separately?
b) OR can I divide by 5 and run a #1/0 in each conduit like I've proposed?
c) OR does that mean we need 5 separate 500 kcmil cables to one conductor run in each conduit?

 

[davidbeach]

c).

 

[dpc]

250.102 (2) seems to say that if you are running individual wires in each conduit, you can size each based on the size of the ungrounded conductor in the the conduit. This is what I would do.

 

[itsmoked]

I think I'd ask an inspector as a mistake will be phenomenally expensive.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[xnuke]

It appears to be your choice: either run one 500 kcmil Cu supply-side bonding jumper, or run five 2/0 AWG Cu supply-side bonding jumpers, one in each raceway, per Table 250.102(C)(1) for 750 kcmil ungrounded conductors. Definitely not c).

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[wroggent]

I think xnuke is right. Also, if 250.30(A)(2) Exception applies, I believe you can add another system bonding jumper at the transformer and eliminate the supply side jumper(s).

 

[itsmoked]

Keith Cress
kcress -

http://www.flaminsystems.com

 

[davidbeach]

It is a bit obtuse, but there’s a reason for the note about the ground not being required to be larger than the ungrounded conductors. Use the 12.5% rule to figure out how large each one is and run one per conduit. Eventually it gets as big as the phase conductor but doesn’t have to exceed the phase conductor. Fault in the conduit is worst case and a subdivided ground could melt open.

 

[bdn2004]

We called the NFPA and their guy over this part of the NEC. They said to run a 2/0 in each conduit. We asked about running a 500kcmil separately. He said running it separately does not protect against a fault inside of the conduit.

 

[dpc]

He's right - but yet the Code would seem to allow it, at least as I read it.

 

[xnuke]

Thank you for your feedback, bdn2004.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.