Simple Question per 250.64 (B)

[scott thompson]

Got a simple question regarding the use of LFMC for the Raceway of a Grounding Electrode Conductor - either in a small "whip" form (small 12" length, connected to something like EMT), or a continuous length.

Was asked this by a colleague recently, and wanted to toss your way!

Say, for example, we have a Dry-Type Transformer, and we are bonding the Secondary side of it to a local Grounding Electrode System via a Grounding Electrode Conductor (sized per 250.66).
In an attempt to reduce the 60 Hz vibrating hum, all Conduits terminating to the Transformer's Enclosure have 12" long whips of LFMC (Metallic Sealtite) between the EMT and the Transformer's Enclosure.

Bonding Bushings are used on both ends of the GEC's raceway.

If I am reading 250.64 (B) correctly, the Article _Does Not_ include LFMC as an optional raceway for the GEC to be run in.
The Article includes GRC (Rigid Metal Conduit), IMC, EMT, PVC (Rigid Nonmetallic Conduit), or Armored Cable - but does not include LFMC or LFNC.

Is this a simple "forgot to include LFMC in the list" thing, or is it due to the "typical Ground Bonding Limitations" printed on the LFMC.
Example:
1/2" LFMC normally has printed on exterior: "Use Separate Grounding Conductor for Circuits Above 20 Amps"

Any thoughts???

Scott

 

[JohnJ0906]

The way I would read this is that idf the grounding electrode conducter is #6 or smaller, no. Are you useing the LFMC for pysical protection, per 250.64(B)? What size GEC?

 

[bphgravity]

250.64(B) is applicable only when you are trying to provide protection for the GEC. LFMC is not to be used where subject to physical damage therefore it is not suitable for providing protection to internal conductors.

Don't confuse the requirements for system grounding with equipment grounding which requires an effective fault path.

 

[scott thompson]

Getting back to the topic finally!!!

Getting back to the topic finally!!!

Sorry it took me so long to reply... Work... busy... you all know the drill

In response to the replies:

The way I would read this is that idf the grounding electrode conducter is #6 or smaller, no. Are you useing the LFMC for pysical protection, per 250.64(B)? What size GEC?

Let's just say, for the sake of debate, I am running one #8 cu for the GEC, from the Transformer to the GES.

The LFMC is a 12" flexible whip at the Transformer end, in order to reduce some of the annoying 60Hz - 360Hz hum (an issue in its self!).
The remainder of the raceway is EMT.

250.64(B) is applicable only when you are trying to provide protection for the GEC. LFMC is not to be used where subject to physical damage therefore it is not suitable for providing protection to internal conductors.

The LFMC will be used in an area where it _may_ subjected to physical damage, but likely not (kind of difficult to ram a fork lift into a 12" whip only, without messing up other things!).
Also, for this example, we plan to use #8 cu for the GEC, but let's also figure the Project Manual + Specs on the Plans require us to enclose _ANY_ GEC in a Raceway, and the end of that raceway (at the Transformer) to have a minimum 12" flexible connection, to attempt in reducing that lovely hum.

Don't confuse the requirements for system grounding with equipment grounding which requires an effective fault path.

I mentioned the EGC issue only because the LFMC has indications about separate bonding for OCPDs in excess of X amperes (like "Use separate EGC for circuits exceeding 20 amps" is printed on most of the 1/2" LFMC I have dealt with for many years).
Also, we will have effectively bonded each end of the GEC raceway with Ground Bonding Bushings &/or clamps with threaded hubs.

*** Personal Note ***

This scenario is really not an issue - as Flex (FMC) may be used in lieu of LFMC. It's really more of a "What's Up" kind of thing.

Scott

 

[George Stolz]

Hi, Scott. Haven't seen you around here in a while.

Is this a simple "forgot to include LFMC in the list" thing, or is it due to the "typical Ground Bonding Limitations" printed on the LFMC.

I believe the reason FMC and LFMC are not on the list, is because the CMP is going a step too far. All raceway articles say "not for use where exposed to physical damage", which is subject to interpretation by the inspector on site. The amount of damage a raceway is to be exposed to is determined for each circumstance.

In this case, the CMP is assuming a high amount of damage is expected right out of the gate, so they set the bar high. For what reason, I have no idea.

Here's a ROP from the 2004 that demonstrates what I found in looking at this.

5-149 Log #163 NEC-P05
(250-64(B) )
Final Action: Reject
Submitter: W. Creighton Schwan Hayward, CA

Recommendation:
250.64(B). In lines 10 through 15, add to both lists of raceways (for enclosing grounding electrode conductors), "flexible metal conduit", following "electrical metallic tubing".

Substantiation:
Flex is certainly the equivalent to cable armor, which is permitted, for protection of grounding electrode conductors against physical damage. Flex is commonly used for this purpose. If there is some technical reason for not including it, will the Panel please advise.

Panel Meeting Action: Reject

Panel Statement:
No substantiation was provided to add flexible metal conduit to this section. Flexible metal conduit is not permitted to be used where subject to physical damage as specified in 348.12(7). Armored grounding electrode conductors are recognized for this purpose.

Number Eligible to Vote: 16
Ballot Results: Affirmative: 15 Negative: 1
Explanation of Negative:
MELLO: The panel action should have been accept in principle as a minimum. The panel statement is incorrect in that there was substantiation to add flexible metal conduit to this section. It is true that flexible metal conduit is not permitted where subject to physical damage, but neither is electrical metallic tubing or non-metallic rigid conduit yet these raceways are on the permitted list. There is no reason that flexible metal conduit applied in the right situations would not be just as good as EMT or RNMC where properly applied in accordance with the applicable raceway articles. See also comment on proposal 5-152.