EGC size

[bennie]

Re: EGC size

Charlie: One of my complaints about the NEC is the numerous experts fabricating an explanation for specific procedures. They don't research the true intent, they add their own spin.

This is a good example, you have done the math. I agree with your findings and can prove them with a loop impedance test and comparison.

I try to research every one of my statements, but some get unfiltered. The participants on this forum usually set me straight quickly when I go to outer space.

 

[tom baker]

Re: EGC size

I've mentioned this software before, but the Steel Tube Institute has a program called GEMI, that calculates the size of a EGC based on the raceway type and size, but not on the OCPD. It turns out the RMC needs to have the EGC increased in size after about 300 ft.
STI is at www.steeltubeinstitute.org, or 440-974-6990.
The program was developed by Gerogia Tech and is the result of more than three years of research, testing and developement.

 

[dana1028]

Re: EGC size

Don - I recently had a code class administered by NFPA (when a state adopts the NEC, NFPA will come out for free and train inspectors on the code changes).

We had (?) Sargent (forgot his first name) from NFPA headquarters.

He told us that if you upsized your conductors for ANY reason you had to upsize your EGC proportionately. He even gave an example of the electrician only having a larger size conductor on his truck and using that (eg. only having #10 on his truck and needing to run a #12 circuit).

He further explained a situation some inspectors were encountering: inspector sees #10 conductors on a 20A circuit and a #12 EGC. Inspector (believing #10 conductors were installed for VD) asks electrician why he didn't upsize the EGC as well....electrician says: I didn't install the #10 for V.D., I only did it because that was the only wire I had on the truck. [actually the scenario he gave was for a larger circuit with larger conductors but I don't remember the particulars].

[ June 13, 2003, 05:24 PM: Message edited by: dana1028 ]

 

[don_resqcapt19]

Re: EGC size

Dana,
Upsize from what? Where do we start? The only logical starting point is from the ampacity of the conductor. The derating rule in 310.15(B)(2) changes the conductor ampacity. In this application, the #10 is the smallest permitted conductor, it has not been upsized.

Inspector (believing #10 conductors were installed for VD) asks electrician why he didn't upsize the EGC as well....electrician says: I didn't install the #10 for V.D., I only did it because that was the only wire I had on the truck.

That example was the reason that the words 'voltage drop' were removed from this section in the 2002 code.
Don

 

[bphgravity]

Re: EGC size

I understand your point, Don, and it makes complete and perfect sense to me. However, 310.15(B) basically states that ampacities for conductors are as those specifies in 310.16-310.19.

Section (B)(2) goes on to provide Adjustment Factors.

When I see the word "adjustment", I assume that means changing a set value to a new one to meet the requirement of the section.

For the example of voltage drop, I start with an original that is sized for the rating of the circuit, than I "adjust" the size for VD considerations.

The same holds for temperature or mulitiple conductor runs. I start with what would be normally allowed for the rating of the circuit, and then I "adjust" the size to meet the requirement.

Back to Table 250.122, it appears that the size of the EGC is based on the size of the rating of the OCD. Is that really the case, or is it actually based on the size of the ungrounded conductor that the code knows is protected by the respective OCD for that ungrounded conductor?

Part (B) goes on to make a requirement based on conductor size and makes no mention of OCD rating to imply that the OCD rating never really matter, only the ungrounded conductor size matters.

It goes back to the idea that the whole reason for all this is total circuit impedance. Part (A) states the the EGC never has to be larger than the ungrounded conductors and part (B) wants the EGC increased with the ungrounded conductor increase.

So if any of this actually makes any sense, and if anyone actually takes the time to read it all, the point I am trying to make is that maybe the concern is not circuit ampacity at all and regardless if you plan on loading a 4/0 to 15-amps, the EGC still must be sized to the 4/0.

Sorry to beat a dead horse, but I am really interested in getting to the bottom of this grounding stuff. There are just to many conflicting ideas on grounding issues from very respectable people in the industry.

I have two grounding books in front of me right now published by two of the leaders in electrical education. Both are experts in their own right and both have nearly equal credentials. However, there are some major, and I mean major, key differences in grounding philosophy between these two books! It makes it hard to determine what the true answer is!

 

[roger]

Re: EGC size

Bryan, I'm not going to come close to the length of your post, that made me tired reading it.

This is one area that can be argued to have a real theory reason as far as impedence, resistance, VD, etc... (all are relative)

But, I think there is basically an effort to provide some uniformity in the situation. (I'm probably wrong)

I think in your examples, if there was some way to insure all would (or could) be consistant, this would be a perfect area to address each installation on an individual basis. Ha Ha Ha Ha cough cough!!!

Roger

 

[bphgravity]

Re: EGC size

My greatest fear is that 5 years from now, I am am going to look back at these posts I make and realize how clueless I probably am and wonder how I ever made it in this industry without killing someone or burning down a building.

That or I will have a good laugh!

 

[don_resqcapt19]

Re: EGC size

Bryan,
A change in size of a conductor to compensate for voltage drop does not change the permitted ampacity. An adjustment for more than 3 current carrying conductors does change the permitted ampacity of the conductor.

310.15(B) Tables. Ampacities for conductors rated 0 to 2000 volts shall be as specified in the Allowable Ampacity Table 310.16 through Table 310.19 and Ampacity Table 310.20 through 310.23 as modified by (1) through (6).

310.15(A) permits us to use the tables or engineering calculations. 310.15(B) tells us how to use the tables and 310.15(C) tells us how to do the engineering calculations.

I have two grounding books in front of me right now published by two of the leaders in electrical education. Both are experts in their own right and both have nearly equal credentials. However, there are some major, and I mean major, key differences in grounding philosophy between these two books!

That is only because none of us really understand grounding!!!!
Don

[ June 13, 2003, 10:54 PM: Message edited by: don_resqcapt19 ]

 

[dereckbc]

Re: EGC size

BPH, here is some food for thought. It is accepted by engineers that the impedance of a faulted circuit should low enough to induce at least 6 times the amount of current of the OCPD to ensure the device operates in one second or less.

Now lets take an imaginary branch circuit that consist of a 20-amp breaker, with 12 AWG copper phase, neutral, and EGC. The circuit distance is 250 feet and terminated to a receptacle. Now imagine if we were to short out phase to EGC at the receptacle. Would this be a NEC conforming circuit, and safe to operate? Answer; yes. The fault would produce 120 amps, which would operate the OCPD in one second or less.

Take the same circuit and increase the phase and neutral conductor to 10 AWG because we ran 5 other circuits with it, but leave the EGC at 12 AWG. Although it may or may not meet code, is it still safe? Answer; yes. The fault current would rise to about 150 amps due to the decrease in impedance in the phase conductor. But there is no danger of damaging the 12 AWG EGC conductor.

Now to the other extreme. 20-amp branch circuit, all 12 AWG conductors, length 3-feet. This is a code compliant circuit. Now induce a fault at the receptacle. What is the current, and could there be thermal damage??????

[ June 13, 2003, 11:30 PM: Message edited by: dereckbc ]

 

[bennie]

Re: EGC size

The requirement to increase the size of the equipment ground conductor, based on the increase of the load conductors for voltage drop is logical. Increasing the size for other reasons is illogical.

All code requirements, with a financial impact, should include engineering data for substantiation, like the calculations by Dereck, not someones personal opinion.

In politics, over wiring is called "pork".

[ June 14, 2003, 05:36 PM: Message edited by: bennie ]

 

[tom baker]

Re: EGC size

bhgravity: regarding your comment
"Back to Table 250.122, it appears that the size of the EGC is based on the size of the rating of the OCD. Is that really the case, or is it actually based on the size of the ungrounded conductor that the code knows is protected by the respective OCD for that ungrounded conductor?"

I was recently at a generator grounding presentation by Chuck Mello, who is member of CMP 5 (art 250). He stated that table 250.122 is based on 12.5% of the size of the ungrounded conductors, once you get above 10 AWG.

 

[bphgravity]

Re: EGC size

Thanks Tom!

That is very interesting.

 

[bphgravity]

Re: EGC size

Just out of curiosity, I decided to see what the ratio is between the ungrounded and Grounding Electrode Conductor per 250.66.

As it turns out, the ungrounded conductor is exactly 4 times larger than the required electrode conductor. This is based on the larger of the two ungrounded conductor size on each line.

I dont know what that means exactly, but Im sure there is a reason for that.

I cant seem to make the 12.5% value work for 250.122, but Im going keep working on it until I figure it out.

 

[dereckbc]

Re: EGC size

I worked the percentages from 15 to 800 amp on 250.122, results:

15, 20, 30 amp @ 100%
40-amp @ 62%
60-amp @ 40%
100-amp @ 31%
200-amp @ 15.6%
400-amp @ 8.8%
500-amp @ 13%
600-amp @ 11.9%
800-amp @ 8.8%

When I run 250.66 percentages I get a range from 12.1% on the low end to 39% on the high end.

[ June 15, 2003, 12:53 PM: Message edited by: dereckbc ]

 

[tom baker]

Re: EGC size

Here is Mike Holts authors comment in his 2002 Grounding & Bonding text on section 250.122(B)
"ungrounded circuit conductors could be increased in size to accomidate voltage drop, harmonic currents, fault currents, etc. For more information on sizing conductors for voltage drop see www.mikeholt.com/studies/vd.htm"

However, more interesting is his comments following 250.122:

WARNING: Equipment grounding (bonding) conduc-
tors shall be capable of safely conducting any
ground-fault current likely to be imposed on them to ensure that the overcurrent protection device will quickly clear the ground fault [(250.4(A)5]. " the equipment grounding (bonding) conductor is not sized to withstand the ground-fault currents, the conductor may bum clear before the protective
device responds. For more information on this
subject, http://www. NECcode.com/ Newsletters/7 -
21-99.htm.
---" AUTHOR'S COMMENT: A factor that must be
considered when sizing equipment grounding (bonding)
conductors is the terminal contact resistance. When an equipment bonding conductor carries fault current, the contact resistance of the conductor in the terminal might be less after the fault clears and the terminal cools. To
ensure proper conductor contact resistance, bonding conductors should be sized so that when they carry fault current, the temperature of the conductor and the terminal will not rise above 250?C (point at which cop-
per softens). To ensure that equipment grounding
(bonding) conductors can maintain proper contact
resistance, they should be sized so that their circular mil cross-sectional area is not smaller than the available ground-fault current1 times the following multiplier.

Protection Type Rating Multiplier
Breakers/Class H fuses -Opens
1/2 cycle 100A 1.56
Circuit breakers -Opens in 1 cycle 200A 1 .70
Circuit breakers -Opens in 2 cycle 400A 2.40
Circuit breakers -Opens in 3 cycle 1,200A 2.94
1 An Excel or Lotus spreadsheet to determine ground Fault current is available for free athttp://www.mikeholt.com/free/free.htm .

 

[bphgravity]

Re: EGC size

Originally posted by tom baker:


Protection Type Rating Multiplier
Breakers/Class H fuses -Opens
1/2 cycle 100A 1.56
Circuit breakers -Opens in 1 cycle 200A 1 .70
Circuit breakers -Opens in 2 cycle 400A 2.40
Circuit breakers -Opens in 3 cycle 1,200A 2.94
1 An Excel or Lotus spreadsheet to determine ground Fault current is available for free athttp://www.mikeholt.com/free/free.htm .

So, basically this means that 250.122 is not sufficient if the available fault current exceeds the fault current-carrying ability of the EGC sized off the OCD.

Do you know where these multiplier vaules come from?

Am I understanding this correctly?

100-amp breaker with 20,000-amp available ground fault current.

20,000 X 1.56 = 31,200 circular mils

Per Table 8, 31,200 = #4 verses #8 per 250.122

Very Interesting!

 

[bennie]

Re: EGC size

The change in direct current resistance, of the various sizes, appear to follow the percentages as shown by Derecks calculations.