Attachment of Equipment Grounding Conductors to Metal Boxes

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StronerRich

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Pella, Iowa, USA
I work for a large manufacturing company in Iowa with 4 plants around the state and have gotten conflicting answers from electrical inspectors on this application. We install equipment (480 VAC/30 amps or less) using the following method. Coming off a 600 amp buss duct with a 30 amp fusible buss duct switch we'll run IMC or EMT conduit to a deep 4-square metal box using 10 gage wire. The buss duct is grounded to the power distribution system and the metal 4-square box is attached to a metal ceiling truss using metal fasteners. The metal truss is attach to cross beams and columns which are attached to the building's grounded grid system. In the 4-square box we splice the conductors and ground wires and change over to a rated flexible cord drop which we run to the machine disconnect. The question is whether we have to bond the ground wire to the box. I have had an inspector say yes because of Article 250.148 and I have had an inspector say no that since the circuit is above 277V and there are no ringed knockouts used Article 250.97 says that the bonding jumper is not required. My discussions with the inspectors revolves around this point, the metal conduit provides a ground connection to the 4-square box and the grounded grid building system provides a second grounding source for the box what added safety factor does the bonding jumper get us? We do this with larger conductors up to 250 MCM (not the soft drops), but using large Hoffman type boxes as pull boxes with conduit in and out that might have splices in them do we us a bonding jumper in that case? Thanks
 
Are you asking if a bonding jumper is required or if bonding the equipment grounding conductor to the box is required?

Imo you don't have to do the former but you must do the latter. Look at 250.97 for bonding circuits that are over 250V to ground
 
IMO, since your conductors are spliced within the box, any equipment grounding conductor must be connected to the box per 250.148.
As far as you installing a grounding conductor from the buss plug to the box, your metal conduit qualifies as an equipment ground per 250.118. Additional bonding would depend on your box being listed as noted in 250.97 Exception.
As far as your box being grounded by it's attachment to the structural steel, 250.136 (A) states that structural frame of a building shall not suffice as the required equipment ground.
 
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StronerRich

StronerRich

I have asked the NFPA to look into this. I understand that the building grounding system cannot be used for a grounding purpose according to the code, but in all reality, to that junction box I have two ground paths, what extra safety factor does one get by attaching a equipment grounding conductor to it? It seems that 250.148 was written more for residential or commercial applications. Thanks for your insight.
 
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