Table 250.122

mbrooke

Batteries Not Included
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United States
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*
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.jpg
 

infinity

Moderator
Staff member
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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?
 

Dennis Alwon

Moderator
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Location
Chapel Hill, NC
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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...
 

kwired

Electron manager
Location
NE Nebraska
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...
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
Location
NE Nebraska
Since we are on topic, any issues that need resolving with parallel EGCs? I know some complained about the intent...
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.
 

mbrooke

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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
Trust me, I am doing lots about it. (y) Sometimes change can only come from outside when it can't come from within.
 

mbrooke

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United States
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]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.
I know, I have no issue with them :)

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.
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.
 

tom baker

First Chief Moderator
Staff member
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
 

tom baker

First Chief Moderator
Staff member
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.
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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*
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
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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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.
Usually yes... But you could still have a situation where a none paralleled EGC is destroyed from short circuit current.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
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.
 

mbrooke

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United States
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*
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.
Thanks for the link-

 
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