Bonding LFMC

[Steelhead]

If I were to run 2" LFMC from a panel with concentric knockouts to a machine panel where I would punch out holes in the panel would I need a bonding bushing on one or both sides of the LFMC? The circuit is 480V 3ph with an internal EGC in the LFMC.

 

[Dennis Alwon]

art 250.97 is for bonding of circuits over 250V to ground

 

[Cow]

I believe most concentrics/eccentrics are rated for grounding now. You'd have to look to be sure though. That said I always use reducing washers around concentrics, especially with flex for two reasons:

1. I've seen too many flex connectors pulled out and just hanging.
2. They're cheaper than bond bushings, and you don't waste time running a ground wire through them.

 

[augie47]

With one end of your LFMC conduit terminated without concentric or eccentric knockouts the LFNC itself would be bonded.
Assuming your circuit is greater than 60 amps, 250.118(7) does not allow LFMC to be used as a grounding means so an actual equipment grounding conductor would have to be installed. Sine the LFNC is not an approved ground path, I see no reason for a bonding means.

 

[Smart $]

I believe most concentrics/eccentrics are rated for grounding now. You'd have to look to be sure though. That said I always use reducing washers around concentrics, especially with flex for two reasons:

1. I've seen too many flex connectors pulled out and just hanging.
2. They're cheaper than bond bushings, and you don't waste time running a ground wire through them.

Under 2011 NEC, we are required to bond around reducing washers for over 250V. See 250.92(B) general statement, via 250.97.

 

[A/A Fuel GTX]

So it sounds like a mechanical lug in each enclosure, landed to the enclosure with threaded screw with the EGC connected to each lug, would fit the bill? Which brings me to another question, I see a good number of lugs landed to metallic junction boxes via a 1/4"- 20 nut and bolt. Doesn't seem like a very good connection that would pass fault current.

 

[infinity]

Bonding bushing are not required.

 

[Steelhead]

So it sounds like a mechanical lug in each enclosure, landed to the enclosure with threaded screw with the EGC connected to each lug, would fit the bill? Which brings me to another question, I see a good number of lugs landed to metallic junction boxes via a 1/4"- 20 nut and bolt. Doesn't seem like a very good connection that would pass fault current.

I install a bonding bushing on each LFMC connector and run the EGC through the lay-in lug and then to the equipment grounding lug or terminal in each panel. This is the way I always do it, but I wanted to know what the minimum requirement was.

 

[infinity]

I install a bonding bushing on each LFMC connector and run the EGC through the lay-in lug and then to the equipment grounding lug or terminal in each panel. This is the way I always do it, but I wanted to know what the minimum requirement was.

There is certainly nothing wrong with that but since it's not required IMO it's overkill and a waste of someones money.

 

[augie47]

Under 2011 NEC, we are required to bond around reducing washers for over 250V. See 250.92(B) general statement, via 250.97.

I don't believe I can go along with that:

UL White Book QCRV:

GROUNDING
Metal reducing washers are considered suitable for grounding for use in
circuits over and under 250 V and where installed in accordance with
ANSI/NFPA 70, ‘‘National Electrical Code.’’ Reducing washers are
intended for use with metal enclosures having a minimum thickness of
0.053 in. for non-service conductors only. Reducing washers may be
installed in enclosures provided with concentric or eccentric knockouts,
only after all of the concentric and eccentric rings have been removed.
However, those enclosures containing concentric and eccentric knockouts
that have been Listed for bonding purposes may be used with reducing
washers without all knockouts being removed.

That's one of the sections where an inspector can have a field day

 

[augie47]

I don't believe I can go along with that:

UL White Book QCRV:

GROUNDING
Metal reducing washers are considered suitable for grounding for use in
circuits over and under 250 V and where installed in accordance with
ANSI/NFPA 70, ??National Electrical Code.?? Reducing washers are
intended for use with metal enclosures having a minimum thickness of
0.053 in. for non-service conductors only. Reducing washers may be
installed in enclosures provided with concentric or eccentric knockouts,
only after all of the concentric and eccentric rings have been removed.
However, those enclosures containing concentric and eccentric knockouts
that have been Listed for bonding purposes may be used with reducing
washers without all knockouts being removed.

That's one of the sections where an inspector can have a field day

So as not to impart bad info, I contacted UL to discuss this and they did add one important stipulation. It does not appear in the above, but in the actual UL standard there is a requirement that the surface to which the washer connects must be UNPAINTED, so the contact surface would have to have any paint removed. He also emphasized that all remaining concentric and eccentric knockouts would need to be removed also.

 

[Smart $]

I don't believe I can go along with that:

UL White Book QCRV:

GROUNDING
Metal reducing washers are considered suitable for grounding for use in
circuits over and under 250 V and where installed in accordance with
ANSI/NFPA 70, ??National Electrical Code.?? Reducing washers are
intended for use with metal enclosures having a minimum thickness of
0.053 in. for non-service conductors only. Reducing washers may be
installed in enclosures provided with concentric or eccentric knockouts,
only after all of the concentric and eccentric rings have been removed.
However, those enclosures containing concentric and eccentric knockouts
that have been Listed for bonding purposes may be used with reducing
washers without all knockouts being removed.

That's one of the sections where an inspector can have a field day

I highlighted the key phrase in red. Couple that with...

250.97 Bonding for Over 250 Volts. For circuits of over
250 volts to ground, the electrical continuity of metal raceways
and cables with metal sheaths that contain any conductor
other than service conductors shall be ensured by
one or more of the methods specified for services in
250.92(B), except for (B)(1).

(B) Method of Bonding at the Service. Bonding jumpers
meeting the requirements of this article shall be used
around impaired connections, such as reducing washers or
oversized, concentric, or eccentric knockouts. Standard
locknuts or bushings shall not be the only means for the bonding
required by this section but shall be permitted to be installed
to make a mechanical connection of the raceway(s).

 

[augie47]

But, you conveniently left out the exception to 250.97


250.97 Bonding for Over 250 Volts. For circuits of over
250 volts to ground, the electrical continuity of metal raceways
and cables with metal sheaths that contain any conductor
other than service conductors shall be ensured by
one or more of the methods specified for services in
250.92(B), except for (B)(1).
Exception: Where oversized, concentric, or eccentric
knockouts are not encountered, or where a box or enclosure
with concentric or eccentric knockouts is listed to provide a
reliable bonding connection, the following methods shall be
permitted:
(1) Threadless couplings and connectors for cables with
metal sheaths
(2) Two locknuts, on rigid metal conduit or intermediate
metal conduit, one inside and one outside of boxes and
cabinets
(3) Fittings with shoulders that seat firmly against the box
or cabinet, such as electrical metallic tubing connectors,
flexible metal conduit connectors, and cable connectors,
with one locknut on the inside of boxes and
cabinets
(4) Listed fittings

Which is why the UL listing requires all concentric and eccentric rings to be removed.

You will note the UL listing specifies "for non-service" installations as 250.92 does not contain the same exception as 250.97

As the OP's installation does not involve service conductors he can use 250.97 including the exception provided he complies with all the stipulations.

 

[Smart $]

But, you conveniently left out the exception to 250.97


250.97 Bonding for Over 250 Volts. For circuits of over
250 volts to ground, the electrical continuity of metal raceways
and cables with metal sheaths that contain any conductor
other than service conductors shall be ensured by
one or more of the methods specified for services in
250.92(B), except for (B)(1).
Exception: Where oversized, concentric, or eccentric
knockouts are not encountered, or where a box or enclosure
with concentric or eccentric knockouts is listed to provide a
reliable bonding connection, the following methods shall be
permitted:
(1) Threadless couplings and connectors for cables with
metal sheaths
(2) Two locknuts, on rigid metal conduit or intermediate
metal conduit, one inside and one outside of boxes and
cabinets
(3) Fittings with shoulders that seat firmly against the box
or cabinet, such as electrical metallic tubing connectors,
flexible metal conduit connectors, and cable connectors,
with one locknut on the inside of boxes and
cabinets
(4) Listed fittings

Which is why the UL listing requires all concentric and eccentric rings to be removed.

You will note the UL listing specifies "for non-service" installations as 250.92 does not contain the same exception as 250.97

As the OP's installation does not involve service conductors he can use 250.97 including the exception provided he complies with all the stipulations.

But once you remove all 'centric rings, and use reducing washers, you have an oversized KO. The exception says where oversized is not encountered.

 

[augie47]

From what I understand, the CMP looked at that and started to include the following:
FPN: An example of an oversized knockout is a field-fabricated knockout
where minor tool misalignment or tool drift during fabrication results in an
enclosure or box hole larger than that permitted by the product Listing standard
for a factory-fabricated knockout in Listed equipment.
In the end, the panel decided there might be other causes for "oversized knockouts" and elected not to include the FPN in the final draft.
As with many sections of the NEC the wording is left with room for interpretation, but, I am advised that the intent was not to consider all factory rings removed as an "oversized" knockout.
Clarification on the CMP's decision is no doubt in order.