600Amp underground grounding

[jcervone@santonelectric]

I ran 3 2" emt conduits for a 600 ampere feeder to a junction box than the conduits continued in 2" pvt underground to a 600 ampere distribution panel. The feeder is 120/208V., 3 phase, 4 wire. What size grounding conductor needs to be installed in the emt and pvc conduits.

 

[bphgravity]

Re: 600Amp underground grounding

The EGC must be sized per 250.122 of the NEC. The EMT per 250.118(4) can serve as the EGC if you wish. For a 600A feeder the EGC's must be not smaller than #1 Awg Copper, 2/0 AWG AL.

If the available fault current is known and exceeds the withstand rating of the EGC's sized per Table 250.122, they may need to be increased.

 

[pierre]

Re: 600Amp underground grounding

To add to Bryan's post, 250.122(F) requires the 1 AWG to be installed in each of the parallel raceways.


To stir this thread a little. Would he be able to use the EMT as the EGC, then in the Jbox, install lugs and transition over to the 1 AWG from that point on?
Or
Is he required to install the EGC the entire run?

 

[don_resqcapt19]

Re: 600Amp underground grounding

To stir this thread a little. Would he be able to use the EMT as the EGC, then in the Jbox, install lugs and transition over to the 1 AWG from that point on?

Why not? The code does not say that the EGC must be continuous or of the same type for the complete circuit.
Don

 

[charlie b]

Re: 600Amp underground grounding

Originally posted by bphgravity: If the available fault current is known and exceeds the withstand rating of the EGC's sized per Table 250.122, they may need to be increased.

I did not think that was a consideration. The Table does not say that, nor does the text that is associated with the Table. The only thing that would require us to increase the size of the EGC would be if we used a larger phase conductor than was necessary for ampacity purposes.

 

[bphgravity]

Re: 600Amp underground grounding

I'm referring to the Note to the Table which could include the consideration I expressed.

[ November 28, 2005, 02:48 PM: Message edited by: bphgravity ]

 

[charlie b]

Re: 600Amp underground grounding

The note to Table 250.122 says you may have to increase the EGC, if necessary to comply with two earlier articles. Both of those articles require a low-resistance path that will facilitate operation of the overcurrent device, in the event of a fault.

Suppose we design a facility, and give our service load calculation to the utility. Suppose that we have selected EGC sizes for the various feeders and branch circuits on the basis of Table 250.122. Suppose that we hear from the utility that they changed their initial plans, and have now chosen a service transformer that would give us a much higher "available fault current." Clearly, we need to verify our design choices for the fault current ratings of the main and other panels. But I don't see how this could have any possibility of forcing us to upsize any EGCs in the facility. If you do, could you let me know how you see it happening?

 

[don_resqcapt19]

Re: 600Amp underground grounding

Charlie,

But I don't see how this could have any possibility of forcing us to upsize any EGCs in the facility.

If the higher available fault condition could create a case where the EGC would fail without clearing the fault, then I think that 250.4(A)(5) would require an increase in the size of the EGC over what is required by Table 250.122.
Don

 

[charlie b]

Re: 600Amp underground grounding

If you do upsize the GEC, the fault path will have a lower resistance, resulting in an even higher fault current. So I surmise that the larger conductor must have a higher withstand rating. But where is the withstand rating of various size conductors tabulated or otherwise described in the NEC? How would a designer go about selecting an EGC on the basis of this consideration?

 

[bphgravity]

Re: 600Amp underground grounding

Originally posted by charlie b:
If you do upsize the GEC, the fault path will have a lower resistance, resulting in an even higher fault current. So I surmise that the larger conductor must have a higher withstand rating.

Yes this is true but that also means the duration of the fault should be less due to the increased tripping speed of the OCD at tht ehigher fault.

But where is the withstand rating of various size conductors tabulated or otherwise described in the NEC? How would a designer go about selecting an EGC on the basis of this consideration?

You have to use the Withstand Rating Formula, I?xT to dtermine how much current the egc can carry for the time (cycles) the OCD will trip at.

 

[pierre]

Re: 600Amp underground grounding

I can see where Bryan is going with this. The title to Table 250.122 is: Minimum Size...

The note at the bottom of the table gives us direction that there may be instances where this table would not provide the proper size conductor for the fault current that may develop.

That is where the Engineer will shine, when he/she finds those instances that this may occur.

 

[pierre]

Re: 600Amp underground grounding

I just read a White Paper put out by the IEEE. It is dated March, 2005. It states that there are quite a few installations where conductors size 3 AWG and smaller where the protection of the phase conductors and the EGC are not adequate. There are several solutions, of which two are; use current limiting devices, and use larger conductors, including a larger EGC to withstand the large fault current that may be available.
I found this by mistake, but thought it is appropriate for this thread.

 

[don_resqcapt19]

Re: 600Amp underground grounding

How would a designer go about selecting an EGC on the basis of this consideration?

There are withstand ratings available for cables. The general rule of thumb is that an insulated copper conductor can withstand one amp for five seconds for ever 42.25 circular mils. This number can be expressed by the term "ampere squared seconds. (I^2t). A #8 has an area of 16510 circular mils, so its 5 second ampacity is 16510/42.25 or 391 amps. Its I^2t value becomes 764,405 ampere squared seconds. If you know the clearing time you divide the I^2t value by the clearing time in seconds and take the square root of the result. This answer gives you the withstand amps for that clearing time. It we put our #8 on a circuit with an OCPD that will clear in 2 cycles (0.0333 seconds), we find that the number 8 can withstand 4791 amps for 2 cycles. This rule of thumb is based on keeping the copper from exceeding 150?C when the conductor is operating at 75?C. I understand that this short term temperature will not damage the insulation.
Don

 

[pierre]

Re: 600Amp underground grounding

Don
That is some pretty impressive info. My question is do we also know the temperature that the 8 AWG will rise to in the 2 cycles with 4791 amps applied?

I know that the testing has proven the insulation to withstand this temperature, I am interested to see what the value would be. Also how often could this occurance happen with the insulation retaining it's integrity? There are instances when this overcurrent/groundfault/shortcircuit could happen several times - such as someone trouble shooting the tripped OCPD and trying to reset it several times.

 

[don_resqcapt19]

Re: 600Amp underground grounding

Pierre,
My understanding is that if the conductor was operating at 75?C at the time of the fault that the copper will reach 150?C after two cycles at 4791 amps. You can find this information in the Soares Grounding book, at least some of the older editions.
Don

 

[bphgravity]

Re: 600Amp underground grounding

The problem is that at 150? and higher, the annealing of the conductor and its terminations begin, which can result in bad connections and damaged insulation.

 

[pierre]

Re: 600Amp underground grounding

One would think that more that 4000 amps, even for 2 cycles would generate more heat than 150 degrees Celcius. This heat would be transfered to the terminations as well. I wonder how the equipment terminations are design to handle this.