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Question: I am bonding ladder rack sections (carrier grade stuff from Central Steel Fabricators). I normally use a #2 green wire jumper about 12" long, with hydraulically crimped 2 hole lugs. They are attached with 3/8" bolts and belville washers after grinding the paint off the steel rack

AHJ tells me the bonding jumper needs to be the same size as the rectifier buss output (4/0) Euipment in the rack below already has a 4/0 insulated ground that ties to the CGB. Any idea where in NFPA 70 this requirement is?

I don't want to try to interpret it but you might look at NEC 2008 392.7 "Grounding - Metalic Cable Trays" and Article 250 Part IV (250.80-250.86) and 250.96. Key question seems to be is this rack "supporting electrical conductors" per 392.7 and does the NEC apply. Article 250 seems to say grounding/bonding needs to "conduct safely any fault current imposed".

I suggest you check with you local inspector for an interpretation.

k2hz wrote:I don't want to try to interpret it but you might look at NEC 2008 392.7 "Grounding - Metalic Cable Trays" and Article 250 Part IV (250.80-250.86) and 250.96. Key question seems to be is this rack "supporting electrical conductors" per 392.7 and does the NEC apply. Article 250 seems to say grounding/bonding needs to "conduct safely any fault current imposed".

I suggest you check with you local inspector for an interpretation.

I'm not sure the local electrical inspector has jurisdiction in this case. I'll give him a call for an opinion though.

The -48V feed, return, and grounding conductor carried on the ladder rack are all 4/0. They are oversized for voltage drop issues.

In the end, changing the bonding jumpers to 4/0 is not a big deal, just was an unexpetced cost to the project. One of those thing where you want to know why for next time.

Per Mike Holt: (fair use excerpt)
Grounding - Grounding metal parts to the earth in premises wiring is only useful to provide a path for lightning, shunting high-frequency noise, or reducing static discharge.

Bonding - Bonding all metal parts together and then to the system winding is done to provide a low-impedance path to the source (system) to facilitate the opening of the circuit-protection device to remove dangerous voltage on metal parts. In addition, bonding the system to metal parts stabilizes the system voltage to the metal parts and it provides a zero system reference (to the metal parts).

As a side note, we have changed to using the Chasworth line of cable tray which is, itself, UL Listed as a grounding conductor.............

if this installation in a telco central office don't for get the BSP's

kt

The issue with bonding is fault current...

Fault current builds up... Even if your circuit is only feeding 100 amps for instance, fault current at discharge can be anywhere from 10,000 to 40,000 amps...

http://en.wikipedia.org/wiki/Fault_current

Another good read - MIL-STD-188-124B

https://acc.dau.mil/GetAttachment.aspx? ... &aid=26190