Parallel TC Conductors

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KarlS

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Location
Houston, TX
I ran Parallel 3/0 TC rated conductors from a 400AT MCC breaker to a 400AT panelboard breaker. The cable is 2-#3/0 with #4 Gnd. I had an inspection this morning with an engineer who had his PE and he stated my ground conductors were wrong and should be sized larger. He mentioned 250-122 as the reason. I think he is wrong becasued 250-122 is a equipment gnd table. Also, the TC rated cable should come with a proper sized gnd. Parallel conductors should not matter. The TC rated cable was ran in cable tray with drops to the equipment. No conduit.

I need advice as this guy wants me to change the cable.

Thanks in advance

Karl
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
The engineer is correct. The conductor under discussion is, in fact, the Equipment Grounding Conductor,” and Table 250.122 does apply. The size of the EGC needs to be based on the rating of the overcurrent device. For a 400 amp OCPD, the minimum EGC size is #3 copper. If you have parallel runs that comprise a single circuit, then every EGC (i.e., in each cable or in each conduit, as applicable) needs to be a full size. Reference article 220.122(F)(1). Please note that 250.122(F) speaks of "raceway or cable."

A TC cable might have the right size EGC to match its ungrounded (i.e., phase) conductors. But that does not mean it will have the right size, when you run two or more in parallel to a single load (or to a single panel).

Sorry, but you will have to replace the cable, or find another way to satisfy the requirement.
 
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Rockyd

Senior Member
Location
Nevada
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Retired after 40 years as an electrician.
In regard to size of EGC, the exception for cable is very limited. Per 250.122(F)(1) & (2) is very explanatory. For each cable, the cable is only designed for, and of protection, to itself. When in parallel, you can't make the EGC bigger, but you can follow (F)(2) (1) and have ground fault protection for the purpose of protecting the EGC. That's the only NEC call-out I know of allowing an alternate to your problem.

Now it's matter of design/redesign acceptance (w/ the ground fault protection), cost analysis, and choice.

The engineer is correct unless you can do the above.
 

KarlS

Member
Location
Houston, TX
The table on 250.122 is labeled for equipment grounding conductors. This is the conductor ran from the equipment metal frame to the gnd bar. This would be the equipment frame. I would think we follow 250.66 for grounding electrode conductor-the cable from gnd bar on the MCC to gnd bar on the panelboard. THe gnd bar on the MCC will cable to the building pad and then to the grounding electrode in mother earth. On the otherside the Panelboard will cable from its gnd bar to the building gnd pad to the grounding electrode in mother earth.

Also, my cable tray is grounded on both sides with 4/0 green ground to the building ground pans. The alum tray is grounded.

I can't believe the TC manufactures would manufacture a product that would not work and require you to run another gnd.

Karl
 

charlie b

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Location
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Retired Electrical Engineer
Take a closer look at the definition of ?Grounding Conductor, Equipment,? in Article 100 of the NEC. You will discover that the conductor that goes from the ground bar of one panel to the ground bar of a downstream panel (I think that?s what you have here) is an Equipment Grounding Conductor. Thus, 250.122 applies, and not 250.66.

The only wires that get to be called ?Grounding Electrode Conductor? are the ones that go from some piece of metal stuck into planet Earth to the ground bus of the main panel. Anything further downstream is an EGC.

I do not know if you will be permitted to take credit for the cable tray, as part of the fault current path. If you try to use the tray as an EGC, then I am not familiar with the rules that you would need to follow.

The manufacturer of the TC cable will have sized the EGC correctly, and appropriately, for the size of its ungrounded conductors. But you are using it in a different way than its original intended purpose. If you put two in parallel, that does change the requirements for the size of the EGC. That is not the manufacturer?s problem. Rather, it is incumbent on the installer to make sure the EGC is the correct size.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Why can't he use the cable tray as the EGC?

<added>
110.54 (B) Equipment Grounding Conductors. An equipment
grounding conductor shall be run with circuit conductors
inside the metal raceway or inside the multiconductor cable
jacket. The equipment grounding conductor shall be permitted
to be insulated or bare.

Since the location of the EGC is optional, it does not appear that there is a requirement to actually have an EGC inside the cable, if it is in a raceway.
 
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Rockyd

Senior Member
Location
Nevada
Occupation
Retired after 40 years as an electrician.
I think he is wrong becasued 250-122 is a equipment gnd table.

What were the call-outs in the scope of the work? Most spec's have boilerproof fineprint covering all of this. Opinions don't count, You have been given the sections from the 2005 NEC. All of the aforementioned posts are supported by NEC. To further gain clarity, check it out in an NEC handbook on pages 232.

Also, the TC rated cable should come with a proper sized gnd.

No it doesn't when it comes to parallel runs. That is why 250.122(F)(1) & (2) (1)(2) and (3). were wrote.

Parallel conductors should not matter.

Did you read the code section we are discussing here?

It doesn?t say what you think it says, nor what you remember it to have said, nor what you were told that it says, and certainly not what you want it to say, and if by chance you are its author, it doesn?t say what you intended it to say. Then what does it say? It says what it says. So if you want to know what it says, stop trying to remember what it says, and don?t ask anyone else. Go back and read it, and pay attention as though you were reading it for the first time.

For the whole story go here for a great lesson in codeology.

The TC rated cable was ran in cable tray with drops to the equipment.

If your going to try and go 250.118(11) I'd say that is a serious stretch, again see comments on contractual language, and 90-4 has ruled against the present install, something is going to have change. See aforementioned and recommended changes.

No conduit.

250.122(F) in it's entirety addresses cable - go read it again and you'll see.


I need advice as this guy wants me to change the cable.

I think you've been given expert advice so far as opinion can only be offered, you alone have this problem, wherever this installation is taking place, and wants to receive financial consideration (paid). Perhaps others may weigh in, and have a different solution. I just try to go by what the NEC gives me to work with.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Rocky,
but you can follow (F)(2) (1) and have ground fault protection for the purpose of protecting the EGC.
The only problem is that, as far as I know, there are no ground fault protection devices listed for this purpose,
Don
 

Rockyd

Senior Member
Location
Nevada
Occupation
Retired after 40 years as an electrician.
Don,

The only problem is that, as far as I know, there are no ground fault protection devices listed for this purpose,
Don


Just going by what the NEC says. I would call Okonite cable company and ask them about solutions. Either the cable has to have a "super egc", or they will know of someone that makes such an animal that qualifys for ground fault protection. Then it is a matter of money, and design acceptance.
 

Smart $

Esteemed Member
Location
Ohio
petersonra said:
Why can't he use the cable tray as the EGC?

<added>
110.54 (B) Equipment Grounding Conductors. An equipment
grounding conductor shall be run with circuit conductors
inside the metal raceway or inside the multiconductor cable
jacket. The equipment grounding conductor shall be permitted
to be insulated or bare.

Since the location of the EGC is optional, it does not appear that there is a requirement to actually have an EGC inside the cable, if it is in a raceway.
Use as such is provisional...

http://www.cabletrays.com/techbl15.htm

...but in all likelihood, the best alternative to replacing the cables!
 
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