Duct Bank Ground

[In-spector]

I have an underground duct bank with (12) 3 1/2" pvc conduits, each with (4) 500 kcmil and (1) 250 kcmil EGC, 480 vac, feeding an MCC in a facility, encased in concrete. The engineer is requiring an additional single #4/0 bare ground to be run in the concrete duct bank along with the pvc conduits. I cannot find anywhere where the #4/0 is a code requirement. It is not on the drawing and the contractor wants an extra because it is not a code requirement. Is this in fact a code requirement, or just a good practice?

 

[roger]

No NEC requirement however, it is a good practice for the copper industry and other than that it is a waste of natural resources.

What did the plans show it connected to?

Roger

 

[charlie b]

I agree that this is not an NEC requirement, nor is it a "good practice." However, I am concerned about the 250 kcmil EGC. If all 12 sets of 500 kcmil are feeding the same MCC, then I infer that the MCC has a rating at or above 4000 amps. That would require an EGC of at least 500 kcmil. Can you clarify your description of the installation?

 

[infinity]

Sounds like an attempt to make a CEE.

 

[In-spector]

My mistake gentleman. Sorry for the confusion....There are nine 3-1/2" conduits total. Three conduits are spare. Six conduits with (4) 500's and (1) 3/0 ground (not 250). Of the six current carrying conduits, four will feed a 480 vac, 1000 amp circuit, two remaining conduits feed a 480 vac, 500 amp circuit.

The 4/0 duct bank ground was called for in an RFI from the engineer, and he wants it to be terminated with the EGC in the MCC or with the site ground grid.

 

[roger]

The 4/0 duct bank ground was called for in an RFI from the engineer, and he wants it to be terminated with the EGC in the MCC or with the site ground grid.

Still a waste of energy and copper. Has he stated what he think he's achieving?

Roger

 

[roger]

Is this MCC in a separate building from the facilities service equipment?

Roger

 

[don_resqcapt19]

That is a common industrial installation in my area...each conduit with its own code required EGC and an additional bare conductor in the concrete encasement for the duct bank.

 

[ghorwood]

This is a ground ring of sorts. We do it all the time here. Typical engineering requirement in ductbanks. Remember the NEC is a minimum. If you are paid to do this, there should be no problem. If it isn't an NEC requirement and it isn't on the plans or specs, then charged them for it.

 

[Ingenieur]

Seems common
looked at 4 or 5 specs: u of mich, u of penn, city of seattle, etc

tied to rods at each end
provides a low resistance grounding system
a lot of area

seems like good practice

 

[Smart $]

That is a common industrial installation in my area...each conduit with its own code required EGC and an additional bare conductor in the concrete encasement for the duct bank.

Same here.

 

[Fulthrotl]

This is a ground ring of sorts. We do it all the time here. Typical engineering requirement in ductbanks. Remember the NEC is a minimum. If you are paid to do this, there should be no problem. If it isn't an NEC requirement and it isn't on the plans or specs, then charged them for it.

a +1 here as well. it'd done on all duct banks at LADWP
in their switchyards, etc.

a bare 4/0 runs the length of the bank *on top* of the
concrete. but... poco's have more wire in the ground
carrying nothing than ever carry anything. i did a double
switch rack in long beach, and averaged seventy-five
cadweld shots a day there on 4/0 and 500mcm....
for over three months. yeps, they are big on grounding.