mbrooke
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- United States
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- Technician
Does anyone know how the wire sizes in Table 250.122 were derived? Anyone know how the CMP agreed to these numbers?
Table 250.122
- Thread starter mbrooke
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
No, but it might be able to be gleaned from the information in the sections in the note.
mbrooke
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- United States
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- Technician
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- New Jersey
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- Journeyman Electrician
Once you get over a certain number of amps maybe they're calculated as a percentage of the OCPD size?
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
Don has tried to get this table dropped and just use 250.66 for both, I believe-- I hate to speak out of turn but I remember something like that...
mbrooke
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Possibly- but remember that conductor withstand is one thing, OCPD opening time is another.
mbrooke
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kwired
Electron manager
- Location
- NE Nebraska
Probably wanted to go with table in 250.102 for everything, which at one time information in there was once in 250.66 (or still about same thing in a different section if you go back far enough)
mbrooke
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Since we are on topic, any issues that need resolving with parallel EGCs? I know some complained about the intent...
kwired
Electron manager
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- NE Nebraska
There is no issues with parallel EGC's. By nature of being connected to all non current carrying surfaces they naturally will end up in parallel at times.
If you are talking EGC's for paralleled conductor sets - you do need an EGC in every separate raceway, sized to the OCPD not the conductors in the raceway, but do not need to be larger than the ungrounded conductor in said raceway either. Pretty sure you are not allowed to parallel conductors to create a larger conductor with EGC's either, and think that should not change.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I like simplicity.
You like to complain but not try and do anything about it. I know you don't like to hear it but if there is something that you feel needs change then make a proposal
mbrooke
Batteries Included
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Trust me, I am doing lots about it.
Sometimes change can only come from outside when it can't come from within.
mbrooke
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I know, I have no issue with them
Why is that though? I've never understood it. I'd think impedance would matter more down stream as the down stream segments have it more than the upstream segments- in addition to breaker opening time being more prudent in the prior.
- Location
- Bremerton, Washington
From what I recall in the IEEE green book, it was based on the Insulated Cable Mfgs data, so that the EGC would carry the current required to open the OCPD and its self not be overheated to cause damage to the adjacent condutors. Maybe one of our IEEE members would have the green book and can check
- Location
- Bremerton, Washington
the reason you need a full size EGC in each parallel raceway is that in a fault the current will feed from each end of the circuit.
mbrooke
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Thermal withstand would make sense. But I can't imagine the prior as EGC impedance is governed by the individual installation.
mbrooke
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- United States
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Usually yes... But you could still have a situation where a none paralleled EGC is destroyed from short circuit current.
- Location
- Illinois
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- retired electrician
I recall reading or hearing some where that the size of the EGC is based on a maximum voltage drop on the EGC under fault conditions of 40 volts but have never been able to confirm that.
In the first draft report for the 2020 Table 250.122 was based on the size of the ungrounded conductors and not on the rating of the OCPD. That reverted back to the one based on the OCPD, because there were too many unresolved issues with circuits where the ampacities of the conductors were less than the rating of the OCPD, like for motors and AC equipment as well as some other issues.
In the first draft report for the 2020 Table 250.122 was based on the size of the ungrounded conductors and not on the rating of the OCPD. That reverted back to the one based on the OCPD, because there were too many unresolved issues with circuits where the ampacities of the conductors were less than the rating of the OCPD, like for motors and AC equipment as well as some other issues.
mbrooke
Batteries Included
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- United States
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- Technician
kwired
Electron manager
- Location
- NE Nebraska
If they ever adopt what is in that table, current cables with 6 AWG conductors and 10 AWG EGC's would have to be made with 8 AWG EGC's. Then we are possibly stuck with needing to change out anything existing that had 10 AWG in some situations.
Might be other larger cables with similar issues.
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