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MCC Ground bus sizes... When do you go up?

MCC Ground bus sizes... When do you go up?

MCC Ground bus sizes... When do you go up?

(OP)
I always specify a 300A Ground bus for MCCs. MCC manufacturers offer higher bus ratings as an option, ie 600A.

I'm just curious, when will this be required? How do you properly size the ground bus bar?

RE: MCC Ground bus sizes... When do you go up?

According to Siemens TechTopics No. 26:
"Such requirements illustrate a common misunderstanding about the function of a ground bus in switchgear.
In installations subject to the National Electrical Code (NEC) (ANSI/NFPA 70), the equipment ground conductor is not intended to carry normal load current. The ground bus is intended to carry current only when some sort of fault occurs that involves ground."
See: "Ground Bus Ratings"
http://www.energy.siemens.com/us/pool/us/services/power-transmission-distribution/medium-voltage-product-services/tech-topics-application-notes/techtopics26rev0.pdf
Also see:
"Rating of Ground Bus in Metal-Enclosed Switchgear"
http://www.netaworld.org/files/neta-journals/NWfall08-Bridger.pdf



       

 

RE: MCC Ground bus sizes... When do you go up?

(OP)
Thanks for the links. :)

"While a ground bus, like any conductor,will carry a certain amount of current continuously without damage, just how much and under what circumstances is not defined. Therefore, specifying a continuous current rating for ground bus has little or no meaning and should be avoided."
Source: Rating of Ground Bus in Metal-Enclosed Switchgear


Yeah that is what I figured... and I am wondering why most LV MCC manufacturers provide Ground bus current ratings of 300A, 500A, etc... with the option to upsize.  

RE: MCC Ground bus sizes... When do you go up?

I'd speculate that they size the earth bar according to fault rating and withstand time, but the bar size which results from such a calculation inherently has a thermal rating too and the manufacturers state it. I agree though that the thermal rating is actually irrelevant, the parameters of concern are fault withstand and fault duration and to calculate that requires other information such as support spacing.
  

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