External Parallel Grounding of Liquidtite Flexible Conduit

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Mavtech

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A lot of our products are shipped out of the Continental United States which results in a multitude of codes that must be adhered to, generating a lot of debate over what is proper and what is not as it applies to all the code requirements. One major topic that is constantly being reviewed is the use of flexible conduit sections to connect between rigid conduit and rotating/vibrating equipment. We have been using the flexible conduit connectors with the external grounding connection, but the one consistent argument that always come to head is what size conductor we are to use for the ground connection. These conduit runs are all instrumentation control, there is no power or voltage above 24 VDC. One group says that since the instrument control HART system is using 16 AWG stranded, then the ground connection should match this size, the other group states that if you do not match the current carrying capability of the internal metal core of the flexible conduit, then you a derating the parallel ground, basing this last argument on the OEM's data that 1/2" flex metal core is approximately the same as #12, 3/4" flex metal core is approximately the same as #10 in regards to the current carrying capability of the inner metal core. Most of this external flexible conduit grounding is for our CSA customers, but it is becoming very popular and we would like to standardize as much as possible. Any help, direction, or suggestions would greatly be appreciated.
 
NEC 250.118(5) allows flexible metal conduit to be used as an equipment ground under certain conditions. Since it is being used for rotating machinery then you must have an EGC. NEC 250.122 would apply in this case. Since it's low current then the EGC would correspond to a 15A maximum OCPD. The EGC conductor size for a 15A OCPD would be No 14 AWG copper.
 
what makes you think you need an EGC at all?

in any case, how is that you can ship products out of the US using US style flexible conduit?
 
petersonra said:
what makes you think you need an EGC at all?

in any case, how is that you can ship products out of the US using US style flexible conduit?
Click to expand...

NEC 250.118(5)(d) requires that an EGC be installed if the FMC is used to connect to equipment that vibrates i.e. rotating machinery.
 
petersonra said:
if you don't need an EGC at all why would you need an external one?
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When connecting where flexibility (for movement and/or vibration) is not necessary after the installation, you could get by with the LFMC qualifying as the EGC (for this case). However, where flexibility is necessary after installation, an EGC other than the LFMC is required.

The external grounding conductor may be a bonding jumper. Consider that if the metallic core of LFMC cannot be the EGC, then one can surmise it also cannot provide electrical continuity of the raceway. Yes, I know it seems overly redundant for 24V instrumentation wiring... but there are those that take it to that degree.
 
petersonra said:
if you don't need an EGC at all why would you need an external one?
Click to expand...

An EGC is needed because he is connecting to rotating machinery. NEC 250.118(5)(d) states:

If used to connect equipment where flexibility is necessary to minimize the transmission of vibration from equipment or to provide flexibility for equipment that requires movement after installation, an equipment grounding conductor shall be installed.

This was changed in the 2011 NEC. I'm looking at Mike Holt's book "Changes to the 2011 NEC". They mean an EGC of the wire type.
 
The question I have is whether the NEC even applies to your equipment. The NEC only applies to "field" wiring. If you as the manufacturer furnish the equipment assembled as one (listed?) piece of equipment, the NEC certainly does not apply.

I know zip regarding CSA standards...

Can you give a description of the equipment involved?
 
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