Carultch
Senior Member
- Location
- Massachusetts
This link tells us that in NEC2014 intended for it to primarily apply to voltage drop reasons, and explains that the present language doesn't require derate factors to drive ground wire size.
http://www.jade1.com/jadecc/courses/UNIVERSAL/NEC05.php?imDif=679.0
In my opinion, overcurrent devices trip quickly compared to how much time it takes the wire heats up. An extra 20 Celsius of conduit air temperature due to conditions of use, doesn't make a ground wire unable to carry the fault current.
There might be a few other reasons than voltage drop, why you might have to upsize ground because of larger power conductors:
1. Manufacturer's instructions overriding the minimum requirements of the NEC. Ok, I can see that as being a reason.
2. A larger than necessary wire is specified, because the customer specified to plan for future expansion. IMO, certainly a reason to upsize ground. Upsize the ground along with the wire, to what it needs to be in the hypothetical future.
3. Using larger wire because you have it left over from a previous job. (IMO, not a reason to upsize)
4. Ignorance about (100A/less) terminations being rated otherwise for 75C, and thus basing 110.14(C) off of 60C. It ends up being that both are rated for 75C, and thus it is oversized. What becomes of the EGC now? (IMO, not a reason to upsize the EGC)
5. The minimum OCPD doesn't fit in your equipment, so you use the next larger OCPD that does, yet still in the same 250.122 table row for EGC sizing. OCPD drives wire size above what it would need to be for the needed ampacity, if you could install the default OCPD needed for the load. (Given the 2014 language, I think you can justify this as not a reason to upsize EGC)
http://www.jade1.com/jadecc/courses/UNIVERSAL/NEC05.php?imDif=679.0
In my opinion, overcurrent devices trip quickly compared to how much time it takes the wire heats up. An extra 20 Celsius of conduit air temperature due to conditions of use, doesn't make a ground wire unable to carry the fault current.
There might be a few other reasons than voltage drop, why you might have to upsize ground because of larger power conductors:
1. Manufacturer's instructions overriding the minimum requirements of the NEC. Ok, I can see that as being a reason.
2. A larger than necessary wire is specified, because the customer specified to plan for future expansion. IMO, certainly a reason to upsize ground. Upsize the ground along with the wire, to what it needs to be in the hypothetical future.
3. Using larger wire because you have it left over from a previous job. (IMO, not a reason to upsize)
4. Ignorance about (100A/less) terminations being rated otherwise for 75C, and thus basing 110.14(C) off of 60C. It ends up being that both are rated for 75C, and thus it is oversized. What becomes of the EGC now? (IMO, not a reason to upsize the EGC)
5. The minimum OCPD doesn't fit in your equipment, so you use the next larger OCPD that does, yet still in the same 250.122 table row for EGC sizing. OCPD drives wire size above what it would need to be for the needed ampacity, if you could install the default OCPD needed for the load. (Given the 2014 language, I think you can justify this as not a reason to upsize EGC)
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