EGC size with Parallel Multiconductor Cables

[leboot]

I'm working on a commercial project in an industrial complex.

Existing there are :

-two sets of 3/C #350 + #3GW (CVTC multiconductor cable) currently serving an existing piece of equipment (motor) that has been removed.
-Cable is routed in tray from gear and then transitions to rigid metal.
-a 800A feeder breaker that was set to 600A to serve demolished equipment
- system is 480V,3PH,3W

The owner would like to re-use these cables to feed a new distribution panel in the same place as the removed equipment. They initially wanted an 800A panel, but the cables aren't large enough to allow for that. Based on the #3 GW, my thought is that we could only have a 300A panel using one set of cables. The cable is rated for 310A and we only have a #3 GW. Using two sets of cable would require a larger GW bc of 250.122(B) where the undergrounded conductors are increased from minimum size, the EGC shall be increased proportionally.

The owner came back and said they want to use all the cables and set the breaker for 400A and implied that there are different interpretations of the NEC on this.

My question is: am I missing something? Is there something in the code that allows for smaller equipment grounding conductors? In looking at the previous installation with the 600A CB and #3GW, I think the #3GW is too small.

Maybe they were using tray and conduit as the EGC before?

Any thoughts would be helpful.

 

[petersonra]

The code does not require a wire at all as a grounding conductor. I don't see why you can't use the tray and rigid conduit assuming it's metal as the equipment grounding conductor if you have individual conductors but the code requires cables to have the grounding conductor wired with the cable

 

[Carultch]

The code does not require a wire at all as a grounding conductor. I don't see why you can't use the tray and rigid conduit assuming it's metal as the equipment grounding conductor if you have individual conductors but the code requires cables to have the grounding conductor wired with the cable

Not all trays have couplings that are listed for EGC continuity with the tray. Meaning you'd need an EGC-jumper bonding both segments to the tray, while the coupling kit is only there for the mechanical continuity. Some tray systems also have a maximum assembled length where the system is UL classified as an EGC, and therefore would require a wire EGC to reinforce the bonding of the tray at certain intervals.

 

[LibertyEngineering]

Assuming the conductors are copper?

I am not a fan of using conduit and tray for the ground. The ground is the single most important part (IMO)

My suggestion;


In that case the next standard size would be 350A & the #3 Ground would be appropriate.
Since there are 2 sets of cables you could add 2 separate 350A panels for 700A provided you have the breaker space in the gear.

ALSO - REMEMBER VOLTAGE DROP ACROSS THE 350's check it to be sure. You may need to limit the allowable ampacity to 300A panels especially since the conductors are 350's. There is the cost of the second panel to consider also but I would not use it for the 400A panel it would not comply with NEC..

 

[LibertyEngineering]

Oh... and one other thing. You may have indicated this to them already but if not ......
Make sure they understand there is no neutral conductor included which can limit what they can connect to that panel. Just in case they had a load with a neutral they wanted to use the panel for.
Good Luck

 

[augie47]

What about adding an appropriate sized equipment grounding conductor in the cable tray,

 

[LibertyEngineering]

What about adding an appropriate sized equipment grounding conductor in the cable tray,

It appears the way it is written the ground is included as part of the multi conductor cable. Also on another note the OP is correct that if the OCP was set to 600A the #3G was under sized should have been a #1.

 

[LibertyEngineering]

What about adding an appropriate sized equipment grounding conductor in the cable tray,

Sorry Augie47 I read your response again and I get what you were saying.... Yes that would be another option if OP used the paralleled conductors it would be adding appropriately size EGC's for each set if that is possible #1 (cu) for 600A or #1/0 (cu) for 800A and in that case they can feed an 800A panel because the 350's added together = 720A next standard size would be 800A. Again must consider voltage drop as a factor.