I have a metal enclosure bolted to a metal casing of a motor which also is bonded to building steel. I am being told that I need to provide an external bond between the box and the casing. I am not seeing the necessity of this or what is gained. I understand that all noncurrent carrying conductive material shall be connected together in a manner that establishes an effective ground-fault current path. The (4) 1/2" bolts through the back of the enclosure holding it to the casing seems to provide an effective ground path and provide adequate mechanical connection. I am getting a resistance value of .2ohms from the enclosure to building steel. Am I off base on this, or is there something that is in the code requiring an external bond that I am missing?
Bond a metal enclosure bolted to metal casing
- Thread starter Hubl3r
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
Typical industrial spec, especially in classified areas, but not required by the NEC
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Your low impedance ground fault current path needs to be a conductor or approved raceway run with the circuit conductors back to the source. You can connect the motor to building steel if you want but it serves no NEC purpose.
It is more for the requirement to bond all non-current carrying metal parts than to serve as a EGC.
250.100 Bonding in Hazardous (Classified) Locations.
Regardless of the voltage of the electrical system, the electrical
continuity of non–current-carrying metal parts of equipment, raceways,
and other enclosures in any hazardous (classified) location,
as defined in 500.5, 505.5, and 506.5, shall be ensured by any of
the bonding methods specified in 250.92(B)(2) through (B)(4).
One or more of these bonding methods shall be used whether or
not equipment grounding conductors of the wire type are installed.
Informational Note: See 501.30, 502.30, 503.30, 505.25, or
506.25 for specific bonding requirements.
250.100 Bonding in Hazardous (Classified) Locations.
Regardless of the voltage of the electrical system, the electrical
continuity of non–current-carrying metal parts of equipment, raceways,
and other enclosures in any hazardous (classified) location,
as defined in 500.5, 505.5, and 506.5, shall be ensured by any of
the bonding methods specified in 250.92(B)(2) through (B)(4).
One or more of these bonding methods shall be used whether or
not equipment grounding conductors of the wire type are installed.
Informational Note: See 501.30, 502.30, 503.30, 505.25, or
506.25 for specific bonding requirements.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
OH ok, hazardous locations. Not very familiar with those requirements.
Dsg319
Senior Member
- Location
- West Virginia
- Occupation
- Wv Master “lectrician”
I second, very typical spec. Especially in oil and gas industry. Almost always will have a bonding wire and never rely on mechanical connections for bonding with most job sites I’ve seen.
I’ve seen pre fabricated skids with pressure transmitters mounted to a metal post that is mounted to the skid just to have a little green wire run from the skid to the transmitter.
Also very typical to have an external bond to the motor from the stations binding/grounding grid system.
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