250.122(B) Increase in size - were did the change come from.

[Dennis Alwon]

I calculate that at about 210 feet (one way circuit length), but it might be OK even for a 420 foot (one way) circuit. By contrast, if you used #10 for both the ungrounded and the EGC, you might not have trouble clearing a fault unless the circuit length exceeded 540 feet. But of course, you might be dealing with equipment performance issues with a circuit that long anyway, if you only used #10 for the ungrounded conductor.

So in light of this one, limited example, there is no technical reason that we would need to upsize the EGC from #12 to #10, in that any circuit that is not so long that it causes VD issues at the end of the line would be able to clear a fault in a reasonable time, even with a #12 EGC. Did I mention already that I didn't like this rule? :happyyes:

Charlie would you mind sharing that calculation. I may not understand it but I would like to try

 

[don_resqcapt19]

I have made some suggestions but I need input from those in the field as well as practicing PE's, I am not going to make the same mistake that those before me have made. The CMP seems to be very narrow minded on this issue for a reason not identified as of yet. I seriously doubt that the CMP made this rule because no one was up sizing the ground in long runs for voltage drop. ...

In general the CMPs do not make code rules, they act on proposals and comments that are submitted to them as part of the code writing process.

I have not gone back and looked at the technical substantiation for the original rule that required that the EGC be upsized where the ungrounded conductor was upsized for voltage drop. That is where it all started. If that substation was not valid, then the rule should go away. That is where you need to start with this.

One thing as I read some ROP's is that one person had issue because cables out there do not have the ground sized for up sizing as a reason. I would think that a cable company would be interesting in this issue.

I won't deny that in some cases there appears to have been undue influence by people who stand to profit from a new rule, but have not heard anything to lead me to believe that was the case with this rule.

where is the engineering to support the current code or a change. I don't feel I m qualified to assert those figures.

I don't see a need for engineering support for the current change as the proposed change is only trying to define how the rule is to be applied.

As far as getting some one who is qualified to make the change proposal, maybe others don't feel this is as big of an issue as you do, or maybe it isn't a real issue because the inspectors are not enforcing it as it is written.

 

[charlie b]

Charlie would you mind sharing that calculation. I may not understand it but I would like to try

It?s a simple Ohm?s Law calculation.

• I assumed (without doing anything to verify this assumption - see Note below) that a breaker would trip fast enough to prevent injury if the current were 10 times higher than the rated current.
• 10 times 20 amps is 200 amps.
• Dividing that into a voltage of 120 gives you a resistance value of 0.6 ohms.
• Using the ?effective Z? values from Table 9, a #12 has an impedance of 1.7 ohms per 1000 feet. The value for #10 is 1.1.
• If I have the same length of #10 ungrounded going out to the load a #12 EGC coming back from the load, the impedance is 2.8 ohms per 1000 feet.
• 0.6 ohms divided by (2.8 ohms/1000 feet) gives you 214 feet.
• If you give credit for a fault of 5 times the breaker rating as being enough to trip the breaker in time, the distance becomes 418 feet.
• If you use #10 for both the ungrounded and the EGC, that 418 feet becomes 545 feet.

NOTE: Having gone through this exercise, I thought I might take a look a a breaker curve or two. I now see that a current of five times the breaker's rating might not give you a trip as quickly as you might wish. One curve has a trip at 2 seconds for a fault of 5 times rated. Its trip for a fault of 10 times rated was 0.4 seconds, and perhaps that is not as fast as one might wish. For that particular breaker, to get the trip into the "instantaneous range," the one-way length of a #10/#12 combination would be closer to 178 feet, and for the #10/#10 combination would be closer to 227 feet. Does this prove or disprove anything? I am not at all sure.

 

[dana1028]

A book of comments not interpetations , It is called a handbook , some consider such a item a manual, what a crock it's nothing more than glossy toilet paper.

As Don said, most of the information in the handbook is accurate. I would argue that most electricians and electrical inspectors don't have the level of expert knowledge of the code that you seem to think you have.

I am an inspector and our bldg. dept. library has 'handbooks' from every discipline [building, electrical, mechanical, plumbing, etc.] - these 'handbooks' are very useful 'guides' to better understand the intent of the code requirements. Most municipalities [in Calif.] have 'Combination' building inspectors. That is, we are required to be knowledgable on ALL the codes [in CA - building elect., mech, plumbing, energy, Green....that is over 5,000 pages of code] - we depend heavily on some form of guidance to better understand some of the intricasies of various code requirements. NOBODY is an expert on 5,000 pages of code. We look to numerous resources for guidance and understanding [those of us who care]; we evaluate those resources, discuss the merits, ask for feedback among those of us who do have greater experience and utlimately make a decision on who to enforce various code requirements...it is an imperfect world....deal with it.

I always tell contractors they have every right to dispute my correction notice, ask for another opinion from my supervisor. I carry my code books with me in the field so the contractor can read for himself what the code language is requiring. YES sometimes the code is confusing...then we look for further information [mfr. instructions, etc.].

As far as your earlier complaint about the 40A, 240v circuit wiring being a problem, I have never denied such an installation, and I don't know of any inspector who has....after all, we [as inspectors] still have to justify our calls to a supervisor when challenged - I don't know of any supervisor who would support such a call [i.e. requiring a large EGC on that 40A existing wiring].

 

[480sparky]

Not having read the entire thread, the language of 250.122(B) goes back to 1975 (250-95). While there is no vertical bar to indicate a change in language, I cannot locate any similar language prior to that year.

 

[Sierrasparky]

As Don said, most of the information in the handbook is accurate. I would argue that most electricians and electrical inspectors don't have the level of expert knowledge of the code that you seem to think you have.

I am an inspector and our bldg. dept. library has 'handbooks' from every discipline [building, electrical, mechanical, plumbing, etc.] - these 'handbooks' are very useful 'guides' to better understand the intent of the code requirements. Most municipalities [in Calif.] have 'Combination' building inspectors. That is, we are required to be knowledgable on ALL the codes [in CA - building elect., mech, plumbing, energy, Green....that is over 5,000 pages of code] - we depend heavily on some form of guidance to better understand some of the intricasies of various code requirements. NOBODY is an expert on 5,000 pages of code. We look to numerous resources for guidance and understanding [those of us who care]; we evaluate those resources, discuss the merits, ask for feedback among those of us who do have greater experience and utlimately make a decision on who to enforce various code requirements...it is an imperfect world....deal with it.

I always tell contractors they have every right to dispute my correction notice, ask for another opinion from my supervisor. I carry my code books with me in the field so the contractor can read for himself what the code language is requiring. YES sometimes the code is confusing...then we look for further information [mfr. instructions, etc.].

As far as your earlier complaint about the 40A, 240v circuit wiring being a problem, I have never denied such an installation, and I don't know of any inspector who has....after all, we [as inspectors] still have to justify our calls to a supervisor when challenged - I don't know of any supervisor who would support such a call [i.e. requiring a large EGC on that 40A existing wiring].

Thanks very much for the reply. I am just getting tired of these rediculous rules and interpetations. I learn new things all the time. I know my scnario might not be an issue for some inspectors, However it appears to be not compliant. I would prefer not doing things wrong. In this case A change is needed. In my opinion.

 

[480sparky]

My 1975 and 1978 Handbooks make no mention of why the change was required.

 

[Sierrasparky]

Not having read the entire thread, the language of 250.122(B) goes back to 1975 (250-95). While there is no vertical bar to indicate a change in language, I cannot locate any similar language prior to that year.

The past versions of the code 250-95 specifically state " to compensate for voltage drop"
250,122 B) leaves out voltage drop.
The new proposed change wants to place words increase is any size larger than the minimum Ampacity necessary for the circuit.

Thanks

 

[hmspe]

One area I would like to see clarified is how to deal with 240.4(B). Should we use #6 or #4 copper as the base size for a 60A breaker? #2 or #1 copper for a 100A breaker? I vote for #4 AND #1, but I've had plans redlined because the reviewer believed that #6 and #2 were the smallest acceptable conductor sizes, so #4 and #1 were considered oversized and needed to have the E.G.C. increased proportionally.

 

[Sierrasparky]

It?s a simple Ohm?s Law calculation.

• I assumed (without doing anything to verify this assumption - see Note below) that a breaker would trip fast enough to prevent injury if the current were 10 times higher than the rated current.
• 10 times 20 amps is 200 amps.
• Dividing that into a voltage of 120 gives you a resistance value of 0.6 ohms.
• Using the ?effective Z? values from Table 9, a #12 has an impedance of 1.7 ohms per 1000 feet. The value for #10 is 1.1.
• If I have the same length of #10 ungrounded going out to the load a #12 EGC coming back from the load, the impedance is 2.8 ohms per 1000 feet.
• 0.6 ohms divided by (2.8 ohms/1000 feet) gives you 214 feet.
• If you give credit for a fault of 5 times the breaker rating as being enough to trip the breaker in time, the distance becomes 418 feet.
• If you use #10 for both the ungrounded and the EGC, that 418 feet becomes 545 feet.

NOTE: Having gone through this exercise, I thought I might take a look a a breaker curve or two. I now see that a current of five times the breaker's rating might not give you a trip as quickly as you might wish. One curve has a trip at 2 seconds for a fault of 5 times rated. Its trip for a fault of 10 times rated was 0.4 seconds, and perhaps that is not as fast as one might wish. For that particular breaker, to get the trip into the "instantaneous range," the one-way length of a #10/#12 combination would be closer to 178 feet, and for the #10/#10 combination would be closer to 227 feet. Does this prove or disprove anything? I am not at all sure.

Thanks Charlie,

 

[480sparky]

The past versions of the code 250-95 specifically state " to compensate for voltage drop"
250,122 B) leaves out voltage drop.
The new proposed change wants to place words increase is any size larger than the minimum Ampacity necessary for the circuit.

Thanks

The language of an Article is always subject to change. I was simply responding to the thread title.

 

[copperlap]

I agree with a previous post where the egc should be the appropriate size for the size of the ungrounded conductor. I have a situation where the inspector is requiring a #4 egc for a 2/0 feeder to a motor because of voltage drop. The original feeder size should have been a #3 with a #8 egc. Since the feeder increase size(kcmil proportion) is 2.5 times, that makes the egc #4 instead of the standard #6. Granted the cost difference between the two is minimal, a standard 2/0 multiconductor cable comes with a #6 egc. I agree with the word "increase" but not according to "proportional kcmil size" rather according to the cable size. #3 with #8gnd to 2/0 with #6gnd