- Location
- Massachusetts
EGC size
- Thread starter infinity
- Start date
- Status
- Location
- Illinois
- Occupation
- retired electrician
It is a problem for cables and is one of the reasons behing the proposed change to T250.122 that will base the EGC on the size of the ungrouded conductors. This would mean that the EGC in a cable with #2 ungrounded conductors would be a legal EGC no matter what size OCPD supplied the cable.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
That was, in fact, the opening statement I used in my proposed revision for 250.122(B). And I do understand Rob's point of view here. But I still agree with Bob, and I think the words, as written, can push us to use a #6. I base that on my belief that a #12 was a legal and viable option for the ungrounded circuit conductors. A #12 would not have been a viable or legal option for the final terminations, but a way around that problem was already described above. You could have used a #12, but you ran #6 instead. That's an increase in size.
M. D.
Senior Member
So we are obliged to start from the minimum circuit ampacity for conductor sizing?? It says any reason ,.. what if the # 6 is simply what I have on hand ??? I hope they will see how poorly written this section is
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I wish that were the case. My proposed code revision would make it so.
Then you upsize the EGC according to the proportion by which the ungrounded conductors were upsized.
I hope so as well.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Okay, now that none of this is settled, can someone tell me why a 20 amp circuit with an ungrounded #6 wire needs a #6 EGC to trip the breaker. Why wouldn't a #12 EGC work or work as well as the #6 EGC?
- Location
- Illinois
- Occupation
- retired electrician
It is possible that the #6 could let enough current flow in a fault that the #12 EGC would be damaged before the OCPD opens the circuit. It is also possible that if the #6 was used to correct a voltage drop that the resistance of the #12 would limit the fault current to the point where the OCPD does not open or does not open as quick as it should.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Thanks Don. So theoretically if I had a 20 foot run of #6 on a 20 amp breaker there would probably not be an issue with using #12 as an EGC? I know it is not code compliant just curious.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
I can understand and agree with the intent of 250.122(B). As Don said if the #6 is used for a voltage drop issue the #12 EGC may not be large enough to open the OCPD in the case of a fault. But that isn't the real issue here. The issue is that the circuit conductors cannot be smaller than #6 due to the terminations (let's forget about splices and other scenarios). So when we read the actual words of that code section do we really know what the size of the EGC is supposed to be? I can see it both ways but I can't find the words to support that argument that the conductors have been up sized because the code does not tell us where to begin. Do we begin at #12 or #6?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I don?t know for certain. But here is how I look at it:
1. If you have 25 feet of #12 from the breaker to the load, and if the load has a short to its case, and if you have 25 feet of #12 EGC from the case back to the panel, there will be some amount of resistance in those two wires. Therefore, there will be some amount of current, and it will trip the breaker after some amount of time delay. Will that be fast enough to prevent an injury to a person who is touching the case at the moment of the fault? Almost certainly, for that is the very purpose of the EGC.
2. If you have 250 feet of #12 from the breaker to the load, and if the load has a short to its case, and if you have 250 feet of #12 EGC from the case back to the panel, there will be significantly more resistance in those two wires. Therefore, there will be significantly less fault current, and it will trip the breaker after a longer time delay. Will that be fast enough to prevent an injury to a person who is touching the case at the moment of the fault? Well, perhaps not.
3. If you replace the 250 feet of #12 AWG ungrounded conductor with a #6 AWG, but keep the #12 EGC, the fault current will be lower than Case #1, and higher than Case #2. The time delay before the breaker trips will be shorter than Case #2, but longer than Case #1. Will it be fast enough to prevent an injury to a person who is touching the case at the moment of the fault? Maybe, maybe not.
4. If you replace both the 250 feet of #12 AWG ungrounded conductor and the 250 feet of #12 EGC with #6 AWG conductors, the fault current will be higher than Case #3. Thus, the time delay before the breaker trips will be shorter than Case #3. Will it be fast enough to prevent an injury to a person who is touching the case at the moment of the fault? I think it will.
This reasoning process really applies to the upsizing due to voltage drop. But a similar line of reasoning could be used in the case of upsizing due to ambient temperature conditions. It should also work for upsizing due to multiple current-carrying conductors in a raceway, for the simple reason that we derate to avoid higher temperatures in the raceway.
The only thing I can grab at, to explain away the ?upsize for any reason? wording that is now in the code, is that it would be too hard to write an article that says upsize the EGC in these circumstances, but not in these other circumstances.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I agree that this is a weakness in the code's present wording.
- Location
- Massachusetts
That section used to say 'for reasons of voltage drop' I heard that did not work out as people would simply say 'I used it because I had it, not for voltage drop.' So it was changed. I will go hunting for some ROP info.
- Location
- Massachusetts
This was a change for the 2002 NEC
The bold and underlinded sections to be removed.
The bold and underlinded sections to be removed.
wireman3736
Senior Member
- Location
- Vermont/Mass.
I run a 40 amp circuit for a range, I use 8-3 nm w/g and its OK, Because of the distance I up it to 6-3 w/g. Now this is a violation because I didn't increase the size of the ground.
I remove the 40 amp breaker and install a 50 amp breaker and now it meets code.
doesn't make sense buts it not the fist time.
I remove the 40 amp breaker and install a 50 amp breaker and now it meets code.
doesn't make sense buts it not the fist time.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Here's another take on the word increased, disregard the EGC for a moment and tell me did the circuit conductors become increased in size when #6 is used as opposed to #8?
- Location
- Massachusetts
Rob, not one person has said the section is perfect as written. :smile:
- Location
- New Jersey
- Occupation
- Journeyman Electrician
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
That is a very valid argument. One would tend to think that the #6 was increased due to wire size but according to capacity it is not. Wow-- Another prime example of how difficult it is to write this section.
There is no question that this section needs help however I still cannot see your original post permitting a #12 egc. :smile:
- Location
- Illinois
- Occupation
- retired electrician
You would have to know the available fault current and do a conductor withstand calculation to know for sure what would happen. Part of this would be to calculate the fault current flowing in the circuit and look at the OCPD trip curve to find the trip time. Then with the trip time and fault current you can find out if the #12 would be damaged.
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