equipment grounding conductor size

[tower]

art. 250.122 (b). why is there a conflicting method in sizing a equipment grounding conductor. one way your instucted to use the circular mil area of the ungrounded conductors and the other way is the table 250.122 useing the rating or setting of overcurrent device. whats up with this ? can there be a dispute here ?

 

[bphgravity]

Re: equipment grounding conductor size

In my opinion, there really is no conflict. Section (B) is there for installations that the ungrounded conductors are increased for whatever reason, but the OCD is not. Table 250.122 is based on the rating of OCD of the circuit. If for example you increase the size of your ungrounded conductors for the reason of VD, you are not increasing the size of the OCD, so Table 250.122 will no longer apply. The method is to determine what size EG you orignialy required, and then increase it at the same ratio of cm increase of the ungrounded conductor.

[ May 04, 2003, 04:33 PM: Message edited by: bphgravity ]

 

[tower]

Re: equipment grounding conductor size

voltage drop is not a code issuse thou .It is a guide to better cuircit performance.can you think of any other reason why there could be two different ways to size equipment grounding conductors ? thankyou

[ May 04, 2003, 08:22 PM: Message edited by: tower ]

 

[bennie]

Re: equipment grounding conductor size

One calculation is for conductors, with protection, with wire size to match.

The other is to compensate for impedance.

A feeder with larger conductors than the overcurrent device needs a lower impedance equipment ground conductor to compensate for the voltage drop.

I made corrections, I was on the wrong page.

[ May 04, 2003, 10:27 PM: Message edited by: bennie ]

 

[bphgravity]

Re: equipment grounding conductor size

The VD example was just one example. It doesn't matter what the increase in the ungrounded conductor size was for. I have seen instances on this forum where people have used a wire size or two larger just because thats what they had in stock.
Read the section very carefully, and think what Bennie has given for information. The reason the EG needs to be increased is for total circuit impedance. If the ungrounded conductors are going to carry a larger fault current, than the return path also needs to be able to handle that greater fault current.
Remember that the EG has nothing to do with normal current-carrying operations. Its purpose is to EFFECTIVELY clear fault current. If the conductor is not increased, there is a chance that the conductor will become the fuse before the breaker trips and never clear the fault.
So there really isn't two ways to determine EG size. A size needs to be determined, and if need increased.

 

[dereckbc]

Re: equipment grounding conductor size

I agree with Bryan, there is no conflict of information presented in 250.122. You can use the table to select the minimum size EGC if the ungrounded conductor is the minimum size for the OCPD. If for any reason the ungrounded conductor is upsized, then the EGC must be upsized proportionately in size.

 

[roger]

Re: equipment grounding conductor size

Bryan and Dereck have hit the nail on the head in that "for whatever reason" there is an increase in ungrounded conductors, the EGC must be adjusted the same. This does not relate to the table after said increase.

Roger

 

[tom baker]

Re: equipment grounding conductor size

The 1999 NEC footnote to table 250.122 said the size may need to be increased, the note now in the 2002 NEC states the size (of the EGC) shall be increased whenever the grounded conductors are increased

 

[don_resqcapt19]

Re: equipment grounding conductor size

The big difference from '99 to '02 is that is the '99 code 250-122(b) only required an increase in the size of the EGC if the circuit conductors were increased to "compensate for voltage drop". In the '02 code any time the circuit conductors are larger than what is required by the ampacity tables for the size of OCPD, the EGC has to be increased proportionately to the increase in size of the circuit conductors. This change was made to close the "loophole" in the '99 code. If the inspector was told that the size increase was "just because that is the only size on the truck", he couldn't, under the '99 code, require an increase in size for the EGC. Now, under the '02 rules, any time the circuit conductors are larger than the minimum required size, the EGC must also be increased in size.
Don

 

[tower]

Re: equipment grounding conductor size

is there a table in the code book that you can proportionately size the eg for the increased size of the ungrounded conductors ?

 

[bphgravity]

Re: equipment grounding conductor size

No, but the calculation is fairly easy.

Example: Lets say you have a 100-amp breaker which is protecting #3 wire. Per 250.122, a #8 EG is required.

Now lets say you use a #1 instead for whatever reason. Per Table 8: #3 = 52620 cm
#1 = 83690 cm

The ratio is 83690 / 52620 = 1.6 (rounded)

Per Table 8 for the EG: #8 = 16510 cm.

To increase proportionately: 16510 cm X 1.6 = 26416 cm.

Per Table 8: The new EG should be #4

*Also, none of this would be an issue if you simply made all the conductors the same size.*

[ May 05, 2003, 07:49 PM: Message edited by: bphgravity ]

 

[dereckbc]

Re: equipment grounding conductor size

Tower, there is not a table to size the ECG if upsized. You got to do it the old fashion way by looking up the conductor size in mills in chapter 9, table 8 and perform a ratio.

 

[bennie]

Re: equipment grounding conductor size

Metal conduit fits all.

 

[tower]

Re: equipment grounding conductor size

thankyou.

 

[bphgravity]

Re: equipment grounding conductor size

Beenie, you make an interesting point. I guess that kind of blows my total Z theory right out of the water considering you are not required to change the size or type of conduit when used as the EG to meet the requirement of this section.

I guess I will fall back onto the idea that someone might come back later and install a OCD that is sized to the rating of the larger ungrounded conductors and not be required to change any of the wiring.

In that regard, doesn't this seem to be more of a design issue than an immediate safety issue? I find it strange that some sections of the code are indeed concerned with future events and activities after the intial installation, and other sections make no consideration.

 

[don_resqcapt19]

Re: equipment grounding conductor size

Actually, conduit doesn't "fit all". While few actually do the calculations, 250.4(A)(5) requires that where the conduit is used as the EGC, that it " ... creates a permanent, low-impedance circuit capable of safely carrying the maximum ground-fault current likely to be imposed on it ... ". You can download a calculator for evaluating steel conduit as an EGC from the Steel Tube Institute.
Don

 

[bphgravity]

Re: equipment grounding conductor size

Don

Great link. Thanks.

Bryan

 

[bennie]

Re: equipment grounding conductor size

As usual I read more into a situation than possibility exists.

When loop impedance, is an amount, that will limit fault current to the rating of the overcurrent device. Would this comply with the code?

 

[don_resqcapt19]

Re: equipment grounding conductor size

Bennie,
I don't think that we want to use the loop impedance to reduce the fault current. We need the fault current to be as high as possible to make the OCPD open as soon as possible. Reduced fault current can make the fault last longer and may result in more damage than what would be caused by a higher current, but shorter duration fault.
Don

 

[charlie]

Re: equipment grounding conductor size

Reduced fault current can make the fault last longer and may result in more damage than what would be caused by a higher current, but shorter duration fault.

Which is the problem that the proposal for 110.16 is trying to fix. The proposal wants the calculations affixed to the equipment instead of just the field marking to warn of a hazard. The problem is that the electric utility can not provide a figure for the amount of fault current available at X number of cycles since we all have dynamic systems. If we provide a number, it will be wrong by the time the calculations are completed. Our systems are being continually switched for load, trouble, planned outage, road construction, etc. Each time switching is done, the numbers change.

If this proposal is accepted by the Panel, you will be required to do the calculations. The electric utilities will not be able to supply accurate figures and the trial lawyers will have a field day.