250.122(B) - Upsizing of EGC and multiconductor cables

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Carultch

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Massachusetts
Consider this example.

You have a 100A circuit, which would ordinarily require #3 Cu wire with a #8 Cu EGC. You then upsize it to a #3/0 Cu circuit for voltage drop, which would require a #3 Cu egc. (KCMIL 3/0)/(KCMIL 3) * (KCMIL 8) = 51.6 kcmil = #3.

If you plan on using MC cable, the combination of #3/0 with #3 doesn't exist. Because the #3/0 Cu cable was built with a 200A circuit in mind, which would usually require #6 Cu EGC. This manufacturer planned a #4 Cu EGC to allow room for error.
https://www.encorewire.com/wp-content/uploads/EncoreWire-MC_SL-THHN.pdf

What course of action would you recommend, to work through this situation?
1. Have the AHJ make an exception to approve the #4 EGC and call it day, noting that the metal cladding and cable tray also carry the ground.
2. Change the breaker to 150A, so that 3/0 & #4 is now compliant. Note that equipment manufacturers may limit the maximum ocpd, and thus not allow this.
3. Run a #3 EGC externally, grouped with the cable, in the same cable tray if applicable, and bonded at every opportunity. Note that this will leave an "insufficinetly sized" EGC inside the cable.
 
charlie b

charlie b

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I have come across this situation before. Other options would be,
4. Don't use MC cable.
5. Don't connect the EGC that comes with the MC, and only use the cable tray as the EGC.
6. Pay the manufacturer for a special order to get MC cable with the EGC that you need.
7. Start with your suggestion #2. But terminate the MC cable on a 100 amp enclosed circuit breaker. The MC cable becomes a feeder, not a branch circuit.

Your suggestion #3 would not be code compliant.
 
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Carultch

Senior Member
Location
Massachusetts
charlie b said:
I have come across this situation before. Other options would be,
4. Don't use MC cable.
5. Don't connect the EGC that comes with the MC, and only use the cable tray as the EGC.
6. Pay the manufacturer for a special order to get MC cable with the EGC that you need.
7. Start with your suggestion #2. But terminate the MC cable on a 100 amp enclosed circuit breaker. The MC cable becomes a feeder, not a branch circuit.

Your suggestion #3 would not be code compliant.
Click to expand...


I like suggestion #5, but it does bring up a 300.3(B) issue, which requires the EGC to be in the same wiring structure as the circuit conductors. The new EGC would be inside the same "parent wiring structure" (cable tray), and just not within the same "child wiring structure" (cable).
 
charlie b

charlie b

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Lockport, IL
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Carultch said:
The new EGC would be inside the same "parent wiring structure" (cable tray), and just not within the same "child wiring structure" (cable).
Click to expand...
The words "cable tray" are included in a list within a sentence that contains the word "or." So anything on that list will satisfy the rule. The circuit conductors (i.e., the MC cable) and the EGC (i.e., the cable tray) are within the same cable tray. That is enough.

 
Smart $

Smart $

Esteemed Member
Location
Ohio
Carultch said:
I like suggestion #5, but it does bring up a 300.3(B) issue, which requires the EGC to be in the same wiring structure as the circuit conductors. The new EGC would be inside the same "parent wiring structure" (cable tray), and just not within the same "child wiring structure" (cable).
Click to expand...
That's been debated before with supporters on both sides. I'm on the same side as Charlie.

However, you will run into a problem if there's anywhere where the MC cable is not in cable tray, such as running it up from gear or down to the load. May have to strip off the armor and run that in conduit with a properly-sized EGC.
 
charlie b

charlie b

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Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I agree that David's #8 was different from my #6. But the OP's intent was to find ways of dealing with situations in which the standard product does not fit the requirements.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
I always assumed getting a cable I want made was out of the question as it would require massive quantity and/or huge money, but I recently discovered it is not that big a deal at all as long as you order a spool length (say around 1000' for a medium sized cable such as 4/4 or 4/5.)
 
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