The plant where I work at had a 1200A 480V 4W 3PH Panel installed. I got involved in shutting down the sub-station so the EC could tie-in the feeders. They ran 4-sets of 350MCM in 3” IMC. They had a 1/0 equipment ground in each conduit, but after checking with the code book it appears it should have been 3/0 according to Table 250-122. Is there some reason the EG was reduced, i.e. because IMC was used or is there some other reason for its reduction? Other then replacing the 1/0 with 3/0, what can be done to remedy this issue? Maybe something like parallel cables in 250-122(F) (2).

Something else that was kind of odd, they installed bonding bushings on all the conduits. But they didn’t run a bonding jumper from the bushings to the ground bar in the panel. Why would you install bonding bushings but not bother with the jumper isn’t that the function of a bonding bushing, to make sure your conduit is electrically bonded to the enclosure? I agree all that was need on IMC are two locknuts one on the outside of the box and one on the inside and then use a plastic bushing. Why go to the expense of bonding bushings and not use them. They also did the same thing at two junction/pull boxes, used bonding bushing at the conduit entries but didn’t bother to put jumpers or bond to the junction boxes.


[This message has been edited by bjwill (edited 07-03-2004).]

They did not have to pull a grounding conductor at all, the IMC is a suitable grounding conductor.

Once they decided to run or the job specifications required a grounding conductor it must be installed per code.

Table 250.122 requires a 3/0 cu for a 1200 amp OCPD and 250.122(F)(1) requires that size EGC in each raceway.

As far as the bonding bushing they may have been used just as the insulating bushing required by 300.4(F). Depending on the installation bonding bushings may not be needed.

If these are feeders and the IMC is not entering the enclosure through concentric KOs bonding is not required by NEC.

Bob

250.122(F)(2) can not be used for raceways, cables only.

Strangely enough the only code compliant remedy short of replacing the 1/0s with 3/0s would be to simply cut off the 1/0s and use the IMC as the equipment grounding conductor.

That of course is only OK if the IMC is electrically continuous and job specifications allow a steel EGC.

Iwire
I agree with your comments. It seems to me that the 1/0 could be bonded at both ends of the EMT and would be ok. Conforming to the letter of the code can produce strange results.
Bob

[This message has been edited by Bob (edited 07-03-2004).]

It is possible that specs may have called out insulating bushings with a 150°C rating as opposed to 105°C, and all that was reasonably available meeting that spec were metal bushings with nonmetallic-insulating inserts. [150°C parts are less susceptible to damage/cut-through by smaller-diameter rope/line during cable pulling.] Or—they could have been ‘surplus’ from another job.

On the lopped-off 1/0s, there may be objections to abandoned conductors left in new work.

Is there any possibility that 250.122(F)(2) was used to size the equipment ground? If the ground fault current is limited to less than 800A, then 1/0 equipment grounds are acceptable.

In addition, I wonder if individual OCPD on the parallel conductors would meet the requirements of 250.122(F)(1), for example if 'limiting lugs' were used so that the current on an individual phase conductor in an individual raceway was limited to less than 800A, would the 1/0 equipment ground be acceptable?

-Jon

Jon 250.122(F)(2) does not apply to raceways, only to multiconductor cables.

Bob

I believe that OCP on the parallel conductors would violate 240.8.

Bob

What is length? IF 250.4(A)(5) can be satisfied mathematically, this may be something to consider.

As an inspector, I don't use 90.4 to waive specific requirements very often, but if the math shows that an undersized EGC is installed in a metallic raceway is better than a full sized EGC installed in say PVC, I would consider OK'ing the installation.

Can you give us the length so that I could crunch numbers?

Bob,

Thanks for double checking those code references. 250.122(F) describes parallel conductors in raceways _or_ cables, and I didn't notice that 250.122(F)(2) further restricted to multiconductor cables only.

Regarding 240.8, I see how one could argue that this might or might not apply to the situation of using limiting lugs on the individual cables in a parallel set. Current flow in the conductor set clearly is in parallel to the final load, but each limiting lug is in series with a different conductor; so the limiting lugs are _not_ directly in parallel.

Ryan,

What happens if you have the undersized EGC in a metallic raceway, and you have a phase to insulated EGC fault that doesn't involve the raceway? It seems to me that the problem that 250.122(F) tries to address is all of the fault current flowing in a single EGC from the parallel set. I would suggest that if a variance were permitted, it would be better to stretch 250.122(F)(2) to include conductors in raceways, then to try to allow an undersized EGC on the basis of the _potentially_ parallel raceway.

Hmm, which leads to another silly 'what happens when you stretch the code' questions on this topic, relating 250.122(A) to conductors in parallel. Does 250.122(F) every force the EGC to be larger than the phase conductors, or does 250.122(A) prevent this? If a 600A service were run as 5 1/0s Al in parallel, how large should the EGC be

-Jon

John: Since the raceway and the EGC will be bonded (directly or indirectly) at each end, you basically have the pipe and the EGC in parallel with each other, creater a lower impedance EGC.

For example: You could install 3" IMC with 350's and a 1/0 for 540 feet and still get a 4800 amp fault.

If you had 3" PVC with 350's and a 3/0 EGC and were trying to get a 4800 amp fault, you could only go 131 feet.

These numbers are assuming a fault of 4 times the rating of the OCPD to initiate the OCPD under ground fault.

I think that is why there is a note to table 250.122 that tells you that following the table values doesn't allways achieve compliance with 250.4(A)(5).

A also don't think this type of allowance is really stretching the code, since 250.4 is main requirement of article 250, and 250.4 is in fact a performance based requirement, not a prescriptive one. That tells me that the AHJ should be on his/her "A" game when sizing EGC's.

[This message has been edited by Ryan_J (edited 07-06-2004).]

Ryan,

I am in agreement on the general point of calculating the fault current to see if it is sufficient, and presumably calculating the energy delivered to the conductors in a fault situation to make sure that the OCPD protected the conductors. However I believe that the point of this exercise is to size the _parallel_ EGCs so that they correctly function even when _not_ effectively in parallel.

In the example situation that started this thread, 4 parallel 1/0 conductors were used as the EGC for a 1200A panel. Table 250.122 calls for a single 3/0 conductor. The total cross section of the parallel 1/0 conductors is in excess of 2.5x the cross section of the 3/0 conductor; so _taken as a set_ the 4 1/0 conductors are clearly a better EGC than the single 3/0 conductor. Clearly then, the issue is 'What happens when these conductors are _not_ effectively in parallel?'

The situation that I was suggesting requires separate calculation is as follows:

Equipment ground bonded at _both_ ends of the metallic raceway.

Phase to ground fault _inside_ of the raceway.

Now you cannot consider the two conductors to be in parallel, but would instead have to consider the following two paths in parallel: the 'short' path through the EGC to the supply end of the raceway, and the 'long' path through the EGC to the load end of the raceway, and then back through the raceway to the supply end.

-Jon

Ryan_J, I believe the total lenght of this circuit is roughly 300 FT. The EC lead man is saying that 1/0 is fine and they could have gone with a smaller EGC. I think he's using 250-66, but that art. is for sizing the grounding electrode conductor, is there any way someone could use this table to size the equipment grounding conductor. Beside they are two different conductors. Does anyone believe this may be the case. I believe 3/0 is what is needed according to 250-122, can anyone disagree with that. Or how should I go about letting them know how they should size an equipment grounding conductor?