Ground Sizing 250.122

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davidb123

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Hi Guys,

I am a bit confused with 250.122. My understanding is that if I have a 1200A breaker feeding a MCC, underground with 3 x 500MCM in 3 PVC conduits. Based on NEC and 250.122, there should be a 3/0 EGC in each conduit. However, if the same MCC was to be fed with teck cable, in a tray, it doesn't seem to me that the ground conductor have to be a 3/0 in each cable but I can't find any rule/arcticle about that..

Thanks for the help!
 
250.122(F)(2)(b) permits a single full size EGC to be installed in the same tray as the cables. The single EGC in the tray will be connected in parallel with the cable EGCs. (note that "teck" cable, unless dual listed as one of the cable types in the NEC, cannot be used for NEC applications)
 
The AIA cable I plan to use is a 500MCM with a #2 GND. I have three runs of that cable in the same tray. My point is that the ground in the cable is smaller than the 3/0 required by 250.122. I must have something wrong here ?

Thanks!
 
davidb123 said:
The AIA cable I plan to use is a 500MCM with a #2 GND. I have three runs of that cable in the same tray. My point is that the ground in the cable is smaller than the 3/0 required by 250.122. I must have something wrong here ?

Thanks!
Click to expand...

I think what you are trying to get at is a point that you do have something wrong. If I, for example feed a 400 amp panel with 2 parallel 3/0 MC cables the paralleled hots are rated for 400 amps. However, I can't use standard MC cable that has a #6 ground, I have to buy special MC cable that has a #3 copper ground in EACH cable in order to comply with the code. If you look up Southwire MC cable you will see they offer different ground sizes in many of their conduits.
 
Strathead said:
I think what you are trying to get at is a point that you do have something wrong. If I, for example feed a 400 amp panel with 2 parallel 3/0 MC cables the paralleled hots are rated for 400 amps. However, I can't use standard MC cable that has a #6 ground, I have to buy special MC cable that has a #3 copper ground in EACH cable in order to comply with the code. If you look up Southwire MC cable you will see they offer different ground sizes in many of their conduits.
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Which code cycle are you using?

I am pretty sure the 2017 now allows parallel multi-conductor cables with a standard sized EGC to be used in raceways if a full sized EGC is installed also and all the EGCs are tied together.
 
Gents,

Thanks for the answers. So it looks like that I cannot use 3 runs of the standard 500MCM w/ 2AWG GND in the same cable tray to feed a 1200A MCC from a 1200A feeder breaker. You guys are confirming that I would need a 500MCM cable with a 3/0 GND instead ? I don't want to run a separate GND cable, I want to use the one that comes with the AIA cable.

Thanks
 
davidb123 said:
Gents,

Thanks for the answers. So it looks like that I cannot use 3 runs of the standard 500MCM w/ 2AWG GND in the same cable tray to feed a 1200A MCC from a 1200A feeder breaker. You guys are confirming that I would need a 500MCM cable with a 3/0 GND instead ? I don't want to run a separate GND cable, I want to use the one that comes with the AIA cable.

Thanks
Click to expand...

Compare costs, that one run of 3/0 may be cheaper.
 
davidb123 said:
Gents,

Thanks for the answers. So it looks like that I cannot use 3 runs of the standard 500MCM w/ 2AWG GND in the same cable tray to feed a 1200A MCC from a 1200A feeder breaker. You guys are confirming that I would need a 500MCM cable with a 3/0 GND instead ? I don't want to run a separate GND cable, I want to use the one that comes with the AIA cable.

Thanks
Click to expand...

You cold do this in cable tray without the separate EGC if the tray meets 392.60(B).
 
davidb123 said:
Gents,

Thanks for the answers. So it looks like that I cannot use 3 runs of the standard 500MCM w/ 2AWG GND in the same cable tray to feed a 1200A MCC from a 1200A feeder breaker. You guys are confirming that I would need a 500MCM cable with a 3/0 GND instead ? I don't want to run a separate GND cable, I want to use the one that comes with the AIA cable.

Thanks
Click to expand...

You can't use 3 sets of 500 kcmil for a 1200 amp circuit no matter how you install it.
 
jumper said:
...
I am pretty sure the 2017 now allows parallel multi-conductor cables with a standard sized EGC to be used in raceways if a full sized EGC is installed also and all the EGCs are tied together.
Click to expand...
Just to be clear, that would only apply where all of the sets of cables are in a single raceway. Not likely for the application in this thread. It also works for all of the sets in a single cable tray and that would be more likely for this application.
 
So if the cable tray meets 392.60(B), I'd be allowed to use the tray as a EGC ? Does that means the AIA cables wouldn't be required to have a ground in it ?

I thought there was some rules when you used cable tray like the ones for ampacity like for a 1200A you need a 3/0 or a parallel #2 or something like that.

Anyway, thanks for all the answers
 
davidb123 said:
So if the cable tray meets 392.60(B), I'd be allowed to use the tray as a EGC ? Does that means the AIA cables wouldn't be required to have a ground in it ?

I thought there was some rules when you used cable tray like the ones for ampacity like for a 1200A you need a 3/0 or a parallel #2 or something like that.

Anyway, thanks for all the answers
Click to expand...

Assuming your tray meets 392.60(B), 250.122(F)(2)(c) would allow you to forgo the separate EGC. Article 250.122(F)(2)(b) would still require "normal" sized EGCs to be present in the cable and used as well.
Hope you noted the previous comment that your 500's won't be large enough in most cases as they are only good for 380 amps @ 75 degree. Of course, there are some applications where this would be OK @ the 90 degree rating, but that is another subject.
 
don_resqcapt19 said:
You can't use 3 sets of 500 kcmil for a 1200 amp circuit no matter how you install it.
Click to expand...

This.

240.4 (C) would prohibit using three sets of 500 MCM for a 1200 amp breaker. 380a @75*C x 3 = 1140A. You would need 3 sets of 600 MCM, or, as I just mentioned in another thread, use Polaris connectors, which are rated at 90* C, at each end of the 500 MCM run, then use short pieces of 600 MCM from those connectors to the MCC and breaker terminals, which are rated 75*C.
 
240.90(B) allow me to use 3 x 500MCM for a 1200A breaker since I meet requirement for a supervised industrial installation. The real problem is the gnd size that comes with the 500MCM since is a #2AWG and 250.122 requires a 3/0 in each cable...

Thanks guys
 
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