In a recent Mike Holt 2011 NEC Q&A, an example was provided for a 40 amp bkr with the ungrounded conductors increased from #8 to #6, the equip ground conductor (EGC) had to increase proportionately from #10 to #8. This is to the letter of the Code. Is there a reason why a 60 amp bkr using #6 conductors can use a #10 EGC (per table 250.122), while a 40 amp bkr with #6 conductors must use a #10 EGC?
2011 NEC 250.122(B), increase in EGC size
- Thread starter eesac
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- Chapel Hill, NC
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- Retired Electrical Contractor
There could be an issue depending on the distance of the circuit. Apparently on a very long circuit there could be an issue with the egc not being able to carry the fault current when the CCC are increased in size. Hopefully others can explain it better.
Although it is code I would be in many cases it would not matter esp on short runs and you use a larger wire for the CCC just because it is available.
Although it is code I would be in many cases it would not matter esp on short runs and you use a larger wire for the CCC just because it is available.
texie
Senior Member
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- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
It is my understanding that the reason for this rule is to account for the possibility that as the EGC gets too long there is the risk of the resistance becoming so high that it will not be a hard enough short to trip the OCPD. Maybe the PE's here could comment on the rationale of this requirement.
Texie,
That is the rationale. In the 1999 NEC the proportional increase in the EGC applied only where the circuit conductors were adjusted in size to compensate for voltage drop. Now the adjustment is required where the circuit conductors are adjusted in size for any reason. In many cases it makes no sense at all. I think it was primarily a rhetorical argument.
That is the rationale. In the 1999 NEC the proportional increase in the EGC applied only where the circuit conductors were adjusted in size to compensate for voltage drop. Now the adjustment is required where the circuit conductors are adjusted in size for any reason. In many cases it makes no sense at all. I think it was primarily a rhetorical argument.
I am a PE and I do not understand it. I posted it here to see if I am missing something and to gain another person's perspective. Maybe it is just poor code wording. In regards to the length of run, if fault current were to lessen due to the higher reactance due to a long run, it would be lesser whether it was a 40 amp breaker or a 60 amp breaker. Given faults of equal ampacity, conductor types, lengths and sizes, a lower amp breaker should trip "sooner" than a higher amp breaker, which is part of breaker coordination and indicated by breaker time-current curves.
Thank you for the replies and keep them coming.
Thank you for the replies and keep them coming.
texie
Senior Member
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- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
I think your example illustrates that this "one size fits all" rule does not always fit with the situation. Maybe it needs tweaking, get out the NEC change proposal paperwork!
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- New Jersey
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- Journeyman Electrician
That's likely the problem, finding code wording that would cover every single scenario. To date no one has come up with that wording.
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- Illinois
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- retired electrician
Just make table 250.122 work like 250.66. The size of the EGC would be based on the size of the ungrounded conductor and not on the size of the OCPD.That's likely the problem, finding code wording that would cover every single scenario. To date no one has come up with that wording.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
That would do it but it might be too simple and concise for the NEC. :roll:
I wonder if someone has ever submitted something of that nature as a proposal?
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