5 Parallel runs of 350kcm MC Cable?

[iwire]

But if metallic raceways are properly installed and large enough to handle the ground fault current, the internal EGC is not required by the NEC because the conduit is the EGC. Again, 250.122(F) requires that an individual EGC be sized per Table 250.122, where used.

In the case of a metal conduit run I agree with you.

I can certainly leave the EGC out of the conduit and .122(F) does not have any consequence.

If I was to install and connect an EGC in this conduit run than .122(F) comes in.

But this thread is not about a conduit run, this thread is about an MC cable assembly.

IMO with a cable assembly 300.3(B) forces us to use the EGC internal to the MC. Now that it is used .122(F) comes in and forces the use of EGCs based on the OCPD.

Certainly the cable tray can be used in addition to the MCs internal EGC but not in place of the internal EGC.

Perhaps I am wrong about the NEC requirements here, as 300.3(B) has wiggle room, it would be interesting to see some CMP comments about this section.

 

[Pierre C Belarge]

With the uncertainty of whether the cable installed with the 3 AWG EGC and the cable tray as an additional equipment ground path, I would say that staying with 300.3(B) is important. Lets remember we are talking about the ability to open an OCPD.

Maybe someone has tested this scenario, I am not aware of it. Without knowing that answer, and in my history, having seen the failure of too many effective ground fault current paths not being very effective, I would have to error on the side of installing the proper size conductors within the MC cable installed in cable trays.

 

[dlhoule]

With the uncertainty of whether the cable installed with the 3 AWG EGC and the cable tray as an additional equipment ground path, I would say that staying with 300.3(B) is important. Lets remember we are talking about the ability to open an OCPD.

Maybe someone has tested this scenario, I am not aware of it. Without knowing that answer, and in my history, having seen the failure of too many effective ground fault current paths not being very effective, I would have to error on the side of installing the proper size conductors within the MC cable installed in cable trays.

Pierre, I thought MC cable came with conductors installed. The OP has the MC cable already installed. However, I agree that 3 AWG EGC is not sufficient for this application.

 

[charlie b]

The words "where used" are clearly included in 250.122(F). If they don't mean the use of the internal cable EGC is optional, then what do they mean?

I think they mean runs that employ (1) single conductors, or (2) multi-conductor cables that do not include an EGC.

 

[charlie b]

As I said I believe that the reason for this requirement was to reduce impedance during fault conditions.

I think there is a better reason. Look at the last sentence of 250.122(F). It tells us that the reason you put a full sized EGC in each conduit is to ensure the EGC is protected. I think this is talking about the possibility of a short circuit within one conduit (or cable) in a set of parallel conduits (or cables). The EGC within that conduit (or cable) will see the fault current from all of the parallel circuit conductors. Therefore, each EGC must be rated to carry the full fault current.

Using only the cable tray as the EGC and having the circuit conductors isolated from the tray by a continuous metal (likely steel) cable armor seems to my mind to be a bad design.

I'm afraid you lost me here. You want the circuit conductors isolated from the tray, else there would be an explosive short circuit. What does a steel armor bring to the party, other than physical protection? Where's the "badness" of the design?

Would this not create the same problem (often referred to as a 'choke') as running an GEC in a metal raceway without bonding it at each end?

It would not. The choke effect has to do with the magnetic field generated by the fault current becoming magnetically coupled with a (magnetically permeable) metal surrounding the conductor (i.e., the conduit). A current generated within the conduit will create its own magnetic field, and the orientation of the field will be such that it opposes the field from the conductor. That essentially adds impedance to the ground fault path, and therefore limits the fault current. Not good. It will delay the trip of the breaker.

However, you only get this magnetic coupling if the conductor is completely surrounded by the conduit, and if the conduit is "close to" the conductor. What I mean is that the effect is less evident in a 6 inch conduit than it would be in a 1 inch conduit. But the tray does not completely surround the conductor. Even if you put a cover on the tray, the effect would not be as severe, because of the size of the tray.

IMO what it says is not what it means.

We must always keep in mind that this will happen. Yes, it says what it says. But it was written by imperfect humans who might not have said what they meant.