Does 250.122(F)(1)(b) make any sense?

[JoeStillman]

250.122(F)(1)(b) requires us to up-size the equipment grounding conductor for parallel feeders, but only when the equipment grounding conductor is a wire-type. If the equipment grounding conductor is RGC, IMC or EMT, no upsizing is required.

So, if we run no ground wire at all, we have adequate grounding, but when we run a ground wire, it needs to be a bigger than the ground wire that would otherwise be more than adequate.

And how about 250.122(F)(2)? Under some circumstances, the jacket of AC and MC cable is permitted to be the grounding conductor. If a cable is listed according to 250.118(8) or (10)(b), does the grounding conductor inside have to be upsized for paralleling?

 

[Npstewart]

I’m assuming it’s because the conduit itself is disproportionately large relative to an equivalent grounding conductor.

I’ve never seen MC Cable Paralleled personally.


Sent from my iPhone using Tapatalk

 

[infinity]

If a cable is listed according to 250.118(8) or (10)(b), does the grounding conductor inside have to be upsized for paralleling

If there is a wire type EGC within the cable then it must be sized according to the OCPD ahead of the parallel conductors. This may mean that the parallel EGC's within the multiple cables are too small.

 

[JoeStillman]

I’ve never seen MC Cable Paralleled personally.

Actually, the MC cable question is where this puzzle comes from . A contractor VE'd our pipe and copper wire with AL MC cable for a couple of 800A feeders. All the RFI said was, "Hey, can we use AL?". They didn't ask us to size it.

 

[JoeStillman]

If there is a wire type EGC within the cable then it must be sized according to the OCPD ahead of the parallel conductors. This may mean that the parallel EGC's within the multiple cables are too small.

Inspector says he'll allow it if I sign off on it. Dare I?

 

[Npstewart]

Inspector says he'll allow it if I sign off on it. Dare I?

I absolutely hate when someone says “the inspector says it’s ok, he just needs a letter”. They seem to say this whether it’s actually OK or not and it shifts the burden on to you. Don’t sign off on anything that isn’t correct.


Sent from my iPhone using Tapatalk

 

[Dennis Alwon]

Joe, I presume because conduit is a much better ground than the copper wire the conduit does not have to be upsized--

For instance 1/2" emt is a much better ground than #10 copper wire. As the equipment grounding conductor get's larger so does the conduit thus it isn't an issue.

 

[Strathead]

I’ve never seen MC Cable Paralleled personally.


Sent from my iPhone using Tapatalk

I have done it, and Southwire makes feeder MC with different size grounds for this express purpose.

 

[Tulsa Electrician]

Food for thought. The EGC has to carry enough fault current to open the over current device without damage to the EGC or other conductors.
There's a calculation for this.
If too small it will damage the insulation of the conductors before the over current device can open based in it's design and operating parameters. One reason to use good bonding practice when installed in EMT,GRC etc. When installed in Pvc it has nothing else to help it.
For feeder MC cable it's very important to be sized correctly. I would not want the outer shell carrying full fault current. Not to say it can not if listed.

 

[electrofelon]

I have done it, and Southwire makes feeder MC with different size grounds for this express purpose.

Yeah me too. Did a 300A feeder few years ago with 2 parallel 2/0 AL MC. Doing a 400A here soon, actually already have the wire on site waiting, 2 runs of 250-4 AL MC.

 

[JoeStillman]

Food for thought. The EGC has to carry enough fault current to open the over current device without damage to the EGC or other conductors.
There's a calculation for this.
If too small it will damage the insulation of the conductors before the over current device can open based in it's design and operating parameters. One reason to use good bonding practice when installed in EMT,GRC etc. When installed in Pvc it has nothing else to help it.
For feeder MC cable it's very important to be sized correctly. I would not want the outer shell carrying full fault current. Not to say it can not if listed.

We plotted the breaker TCC curve vs. the cable damage curve. The right size ground looked just as wrong as the wrong size. They both fall to the left of magnetic pickup range. But that damage curve is for the insulation. What if it's a bare ground wire?

 

[don_resqcapt19]

We plotted the breaker TCC curve vs. the cable damage curve. The right size ground looked just as wrong as the wrong size. They both fall to the left of magnetic pickup range. But that damage curve is for the insulation. What if it's a bare ground wire?

You still have to use the insulated damage curve as the bare conductor will damage the insulation of the ungrounded conductors if the bare gets too hot.

 

[JoeStillman]

You still have to use the insulated damage curve as the bare conductor will damage the insulation of the ungrounded conductors if the bare gets too hot.

I wonder why the damage curve for the right-size ground wire cross the trip curve?

 

[jim dungar]

I wonder why the damage curve for the right-size ground wire cross the trip curve?

I can't remember ever seeing a TCC showing a UL489 listed breaker protecting a conductor with 600V insulation, even though the listing process proves that they do.

 

[JoeStillman]

I can't remember ever seeing a TCC showing a UL489 listed breaker protecting a conductor with 600V insulation, even though the listing process proves that they do.

Here's an SKM damage curve showing 1/0 copper 600V cable vs a 150A breaker (it's 75° OCP) and an 800A breaker (the one its an EGC for).

 

[jim dungar]

Here's an SKM damage curve showing 1/0 copper 600V cable vs a 150A breaker (it's 75° OCP) and an 800A breaker (the one its an EGC for).

There is a point where the cable damage curve is below that of the 150A breaker, but UL says it is protected when applied per the NEC.

IMO, actual testing is better than theoretical curves.

 

[Tulsa Electrician]

Seams like a long time to open under a ground fault looking at graph.
I would say not enough fault current or too much impedance or both.
Looking the 5 second withstand rating.
First pic if fault was in a single set of a two set feeder. 1- 1/0 @ 105600 cm
Second pic if ground fault was at load end of feeder 2-1/0 @ 211200 cm
Made an assumption of available fault current at 42 k
The clearing time may be increased if metal raceway is used and bonded correctly.
Other wise may even be a code violation based on Effective Ground Fault Current Path.
Note: 250.122 Table, Minimum size
250.122 (A) 2014 NEC"shall not be smaller than table 250.122 but in no case shall they be required to be larger than the circuit conductor supplying the equipment." Assume 2- sets 500 mcm cu and the round up rule in metal raceway 240.4(B) if applicable. NO increase for voltage drop.

Based on that if one new it was to small there required to increase the size so the OCD will open without damage to the conductor or equipment.
This is where I would rely on an engineer. I have seen this happen once and the equipment came apart. I did try to warn them.

I maybe missing something?
I like learning so please feel free to teach.
I find this interesting.

 

[synchro]

You still have to use the insulated damage curve as the bare conductor will damage the insulation of the ungrounded conductors if the bare gets too hot.

I wonder why the damage curve for the right-size ground wire cross the trip curve?

I think the damage curve establishes a maximum stress level which will not degrade the insulation significantly relative to the requirements for its voltage rating (e.g., a 600V rating). But such a conservative stress level would not be necessary for an EGC, and so this would allow the additional heating from the reduced size of an EGC. But as Don points out, there would still be a minimum EGC size in order to prevent it from damaging the insulation of the other conductors during a fault.
I assume that sufficient testing was done to verify the EGC requirements in the NEC, but I have no info on that..

 

[Another C10]

I absolutely hate when someone says “the inspector says it’s ok,

I always enjoy the scenario when the Inspector you work with says I'll approve it, then on final inspection a replacement Inspector comes in and shoots it down. been there ..

 

[romex jockey]

but UL says it is protected when applied per the NEC.

not by much......



~RJ~