EGC Sizing / Parallel Sets

[EETD]

Help needed in sizing equipment grounding conductor when cable are installed in parallel, in cable tray.

Setup:
Ambient Temp Max. 30C
Seconday Unit-Substation
Pri OCPD: 4,160V Fused Switch, 300E Fuse
XF Pri Feeder: 3/C-250kcmil w/ #3 Gnd, MV-90
XF: 1500kVA, 480Y/277V, 5.75%, Dry
XF Sec Feeder: 6-3/C-500kcmil w/ #2 Gnd, Type TC
Sec OCPD: 480V LV MCC Main CB, 2000A
Raceway: Uncovered Cable Tray

Notes:
1) The transformer secondary conductors are installed in parallel.
2) No ground fault protection is included on the transformer primary, or at the MCC main.
3) Would prefer to use standard cable manufacturer's offerings for multiconductor power cable configurations.

Question:

Does the code (NEC) allow installation of "supplementary" equipment grounding conductors to meet
the intent of 250.122?

i.e. Could additional ground cabling be installed in addition to those already included with the phase conductor
cables to increase the overall, or "total" size of the EGC?

If so, how would the supplementary conductors be sized?

My initial thought it that the EGC should be equivalent in size to a 250kcmil per Table 250.122.
So, with the #2 already included in the multi-conductor cable, we could install another single #4/0 next to each cable.

#2 = 0.067in2
#4/0 = 0.219in2
250kmcil = 0.260in2

0.219 + 0.067 = 0.286


Thanks in advance for your suggestions and comments.

EETD

 

[dpc]

This is a long-standing NEC issue when using multi-conductor power cables in parallel. Since this is a separately-derived system, I'd probably use the sizing in 250.66 for grounding electrode conductors, but this will be up to the AHJ in the end.

From an NEC perspective, a metal cable tray by itself can be used as an equipment grounding conductor in many situation, so using that, in addition to the ground wires in the cables is probably adequate from an engineering perspective. But of course, passing inspection can have little to do with engineering.

The basic intent of the NEC is that the grounding conductor be adequately sized to carry the maximum fault current with damage and to allow enough fault current to trip the upstream overcurrent device.

When you have parallel cables in two separate steel conduits, it's a bigger concern because for a fault in one of the cables, essentially all of the ground current will flow in the ground conductor in that cable. With both cables in the same tray, it's not quite as bad.

 

[resqcapt19]

The code does not permit a supplementary ECG external to the cable. You have to use a cable where the metallic jacket qualifies as an EGC or you have to get a cable that has a 250kcmil EGC. One other option I have seen used is to use a 4 conductor cable and use the 4th full size conductor as the EGC.
Another possible issue in this installation is the required overcurrent protection for the secondary conductors. You need to take a look at 240.21(C) to make sure you are in compliance.
Don

 

[dpc]

I agree with Don that you may have problems convincing AHJ that supplemental grounding conductor is acceptable.

 

[EETD]

Thank you dpc and resqcapt19 for your input.

I like the option of using 4/C cables. We'll just have to make sure the incoming section at the MCC can handle the cabling.

And, I'll be sure to review 240.21(C)!

Thanks,

EETD