Can I use flexible metallic conduit in a run of EMT ?

[steve61]

I need to extend a run of 1 1/2 " EMT for a 460 3 phase circuit for an overhead crane in an industrial setting. Where I need to extend the EMT, there is not enough room to fit a piece of EMT where it is needed. My question is : Can I install a piece of flex in this area between two pieces of 1 1/2 " EMT to make the transition to where I can resume the rest of the run with EMT ? The piece of flex only needs to be about 4 feet long.
Thank You for any help. Steve Wiese.

 

[AC\DC]

No problem

 

[Dennis Alwon]

No problem

I didn't think flex is a suitable means of an equipment grounding conductor if conductor in the flex were larger than 20 amps

 

[jaggedben]

Yes, the only thing to keep in mind is that if your breaker is over 20A you must run a wire-type EGC. You can no longer rely on the raceway like you could with only EMT. Otherwise all fine, they make transition couplers for it.

 

[Dennis Alwon]

Yes, the only thing to keep in mind is that if your breaker is over 20A you must run a wire-type EGC. You can no longer rely on the raceway like you could with only EMT. Otherwise all fine, they make transition couplers for it.

Does this not seem odd to you. You are connected to emt at both ends which is an acceptable means of grounding yet it appears an equipment grounding conductor must be used. ??????????

 

[jaggedben]

Actually looking at 250.118(5) again, for 1-1/2" flex (because it's larger than 1-1/4") you have to run a wire-type EGC regardless.

 

[jaggedben]

Does this not seem odd to you. You are connected to emt at both ends which is an acceptable means of grounding yet it appears an equipment grounding conductor must be used. ??????????

No it doesn't seem odd to me. Imagine if the flex were PVC instead.

 

[jaggedben]

If one used boxes instead of transition couplers then one would only have to install the wire EGC from box to box.

 

[Dennis Alwon]

No it doesn't seem odd to me. Imagine if the flex were PVC instead.

That makes sense for keeping he ground thru to the pipe however if the equipment grounding conductor is only there to continue the grounding to the rest of the run then what is grounding the flex itself? I assumed the need for an equipment grounding conductor is because the flex isn't a good enough ground so how does running an equipment grounding conductor thru the flex make the flex an adequate ground.

 

[wwhitney]

If one used boxes instead of transition couplers then one would only have to install the wire EGC from box to box.

Even without boxes, surely you could just run a bonding jumper between the two sections of EMT around the FMC?

Cheers, Wayne

 

[LarryFine]

It doesn't make the flex more conductive, it bypasses it.

 

[wwhitney]

That makes sense for keeping he ground thru to the pipe however if the equipment grounding conductor is only there to continue the grounding to the rest of the run then what is grounding the flex itself?

The connection to the EMT bonds the FMC. But if the FMC is considered not to have sufficiently low impedance, you can't use the FMC to bond something else.

Cheers, Wayne

 

[jaggedben]

Even without boxes, surely you could just run a bonding jumper between the two sections of EMT around the FMC?

Cheers, Wayne

Hmm, I think the EGC has to be 'contained' within the same raceway. 300.3(B)

 

[wwhitney]

Hmm, I think the EGC has to be 'contained' within the same raceway. 300.3(B)

See 250.102(D)(2), which allows an equipment bonding jumper installed on the outside of a raceway in lengths up to 6'.

Cheers, Wayne

 

[Dennis Alwon]

The connection to the EMT bonds the FMC. But if the FMC is considered not to have sufficiently low impedance, you can't use the FMC to bond something else.

Cheers, Wayne

I get that but what is making the flex more conductive? In other words, what ensures that the overcurrent protective device will trip if there is a short to the flex.

 

[AC\DC]

I get that but what is making the flex more conductive? In other words, what ensures that the overcurrent protective device will trip if there is a short to the flex.

Nothing. If you run an egc it still has no effect on the flex continuity.

Pertaining to the flex
Relying on emt as ground then bonding both emt pieces together has same safety factor as just running egc.
Post 6 mentioned regardless you still need an egc for that size flex.
Post 8 has best solution.

 

[wwhitney]

I get that but what is making the flex more conductive? In other words, what ensures that the overcurrent protective device will trip if there is a short to the flex.

I would assume that in short lengths the flex isn't going to have such a high total resistance from middle to end (the worst case if it is solidly bonded at both ends) to interfere with the OCPD operation. It would just have a sufficiently elevated resistance compared to a typical EGC that you wouldn't want to rely on it as part of the fault clearing path for other downstream bonded parts.

But that does raise a question, suppose you have a 1000' run of FMC that contains a wire-type EGC inside. If you get a fault near the middle of the run, can we have confidence the fault will clear through 500' of FMC? If not, why don't we have a requirement to bond FMC every so often in long runs?

Cheers, Wayne

 

[jaggedben]

For all I know, the problem with flex as EGC might not be that it doesn't conduct enough current but rather that it blows apart under high fault conditions. Anybody know?

 

[wwhitney]

Isn't it the same problem as with old AC armor, the fault path is spiral and so much longer than the conductor length?

Cheers, Wayne

 

[Dennis Alwon]

I would assume that in short lengths the flex isn't going to have such a high total resistance from middle to end (the worst case if it is solidly bonded at both ends) to interfere with the OCPD operation. It would just have a sufficiently elevated resistance compared to a typical EGC that you wouldn't want to rely on it as part of the fault clearing path for other downstream bonded parts.

But that does raise a question, suppose you have a 1000' run of FMC that contains a wire-type EGC inside. If you get a fault near the middle of the run, can we have confidence the fault will clear through 500' of FMC? If not, why don't we have a requirement to bond FMC every so often in long runs?

Cheers, Wayne

That is my point. I wasn't concerned about a 4' piece of 1 1/2" but a long run changes the scenario, imo.