Bonding of cable tray to to drop raceway

A

AmjadIzz

Member
Location
Denver
Occupation
Electrical Engineer
Hey,

So I have a switchboard with main breaker of 4000A feed coming from a transformer on the outside (bottom entry) (not utility transformer). I'm running cables within a cable tray inside the building. The largest feed I have on the cable tray is 2500AT (350kcmil EGC). I'm running a single 350kcmil on the tray to another switchgear. The problem is I'm dropping the cable to a RMC chimney nipple on the other switchgear. I'm running 7 parallel run of 500kcmil. I have 7 chimney raceway.
The problem is bonding these nipple/raceway with a bonding bushing to each one and the last one to the cable tray. Is there anyway I can reduce the size of these bonding jumper ? I'm using 250.122 for Equipment bonding jumpers. What can I do if I can't down size these jumpers?

I also bond the cable tray to the main switchgear with the largest breaker(4000A) to 500kcmil! I could use the largest circuit in the cable tray to size these but it would come down to 350kcmil which is still bigger than bonding bushing standards of max 250kcmil. I can't always use larger (4") chimney with 500kcmil!

Any help would be good!

Thank you,
 
You cannot reduce the size of the bonding conductor.

There are clamps that can be used to attach the nipples to the tray that would eliminate the need for a bonding jumper.

There are bonding bushings that will accept 350 to 500 kcmil conductors.
 
The thing is I'll not always be able to use 4" nipple to drop the cable to. The clamps seems like the better option. But I was wondering if I'll use the bonding bushings, can I bond these bushings to each other with smaller bonding jumper than the main one going to tray being 500kcmil?
 
And when should I use the raceway nipple to drop? when can I not use them and just punch a hole with plastic bushing on the inside of the cabinet?
 
AmjadIzz said:
The thing is I'll not always be able to use 4" nipple to drop the cable to. The clamps seems like the better option. But I was wondering if I'll use the bonding bushings, can I bond these bushings to each other with smaller bonding jumper than the main one going to tray being 500kcmil?
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No, with parallel installations, the EGC is always full size, based on the upstream OCPD, no matter how many you install.
 
Can I *not* bond the tray to the each nipples/or to one? and just bond the tray to the equipment itself through, for example, mechanical lug?
 
AmjadIzz said:
Can I *not* bond the tray to the each nipples/or to one? and just bond the tray to the equipment itself through, for example, mechanical lug?
Click to expand...
If the specification is for a wire type EGC from source to load, then you will need to splice / bond and distribute full sized EGCs based on 250.122 through each nipple / tray to the load switchboard.
If the specification allows you (or you are writing the specification) to use a raceway type EGC, then you can put a bonding bushing on the end of each nipple and tie the 350kcmil to each bushing and tray, and the conduit will act as the EGC.
 
To be clear, this is for the bonding cables bonding the chimney (nipples) on top of equipment to the tray. I'm thinking to just bond the tray to the equipment itself through mechanical lug instead of having bonding jumpers from the nipples to tray. Is that to code? If the equipment is bonded to tray through wire type bonding jumper, do I need to bond each nipple to tray? and there is separate wire EGC going to the equipment busbar ground.
 
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