What size grounding conductor for 5000amp Feeder?

[mbrooke]

There was a provision for GFP, but it specified that the GFP had to be specifically listed for the purpose. The implied purpose was the protection of the EGC and the 2005 code specified that. There was never a GFP listed for that purpose.

List item 3 was changed in the 2005 code to read:

Thats the section I was thinking of. Wish standard GFP would have sufficed, see no reason why it wouldn't- or a blurb that the GFP must be set to X time on the time dial.

 

[MasterTheNEC]

While reviewing the proposed changes for the next NEC cycle, I see the committee has proposed some changes to 250.122 regarding paralleling of conductors. See attached. Not applicable now, but interesting to understand what may be expected in the next edition.

Greetings,

The proposed changes to T250.122 will likely not actually happen in the public comment stage. While the premise was good in terms of using valued based on UL 1569 and UL 1277 for cable construction and EGC's, we found that the new table concept was flawed when used in motor applications.

Anyway at this moment in time work will need to be done at the CMP meetings in San Diego in late October to try and salvage anything.

We will see how it goes I guess.

Paul W. Abernathy
Electrical Code Academy, Inc.
CMP #5 and #17

 

[mbrooke]

Greetings,

The proposed changes to T250.122 will likely not actually happen in the public comment stage. While the premise was good in terms of using valued based on UL 1569 and UL 1277 for cable construction and EGC's, we found that the new table concept was flawed when used in motor applications.

Anyway at this moment in time work will need to be done at the CMP meetings in San Diego in late October to try and salvage anything.

We will see how it goes I guess.

Paul W. Abernathy
Electrical Code Academy, Inc.
CMP #5 and #17

What was the snag in terms of motors?

BTW- great to see you back Paul!

 

[jtinge]

Greetings,

The proposed changes to T250.122 will likely not actually happen in the public comment stage. While the premise was good in terms of using valued based on UL 1569 and UL 1277 for cable construction and EGC's, we found that the new table concept was flawed when used in motor applications.

Anyway at this moment in time work will need to be done at the CMP meetings in San Diego in late October to try and salvage anything.

Thank you for the insight.

 

[wwhitney]

What was the snag in terms of motors?

It seems to me the issue is this:

Currently the EGC size is based on the size of the OCPD protecting the circuit, and is presumably determined by a desire for sufficient current during a bolted fault to reach the instantaneous trip region of the OCPD. Switching to sizing the EGC based on the size of the circuit conductors will work just as well when the OCPD is providing the overload protection for those conductors. In that situation there is basically a direct correspondence between conductor size and OCPD size.

But in some situations the overload protection is provided downstream of the conductors. One case is motors and another case is tap conductors. In those cases, a straight switch to sizing EGC based on circuit conductor size would lead to a reduction in EGC relative to what is currently required. As that is likely not the intention, some additional rules would be required.

One option would be just to retain the current table and indicate it is for use whenever the conductors are not being protected from overload by an upstream OCPD. Another would be to include language referencing the minimum circuit conductor size that would be protected from overload by the upstream OCPD.

Cheers, Wayne

 

[MasterTheNEC]

What was the snag in terms of motors?

BTW- great to see you back Paul!

A spreadsheet was prepared for the task group to compare the size of the equipment grounding conductor for motor branch circuits from existing Table 250.122 and from the proposed replacement Table that is derived from UL-1569 and UL-1277. It became obvious that while the equipment grounding conductor for smaller sizes of motors compares favorably, the EGCs for larger motor sizes are smaller. We have no documentation that the smaller size EGC in Table 6.2 is adequate to provide an effective ground-fault current return path.

 

[don_resqcapt19]

A spreadsheet was prepared for the task group to compare the size of the equipment grounding conductor for motor branch circuits from existing Table 250.122 and from the proposed replacement Table that is derived from UL-1569 and UL-1277. It became obvious that while the equipment grounding conductor for smaller sizes of motors compares favorably, the EGCs for larger motor sizes are smaller. We have no documentation that the smaller size EGC in Table 6.2 is adequate to provide an effective ground-fault current return path.

I believe there is also an issue with NM in some sizes where the new table would require a larger EGC than what is currently being used. That problem arises because the new table is based on the ungrounded conductors having a 75°C ampacity and NM is only permitted to use the 60°C ampacity.

I can see the issue with the motor circuit EGCs because of the permitted rating of the branch circuit short circuit and ground fault protective device can be approximately twice that of the conductor ampacity. That could be addressed with a change to 250.122(D) to require the motor circuit EGC be based on the smallest ungrounded conductor that is permitted to be used for "normal" circuits based on the rating of the fuse or thermal magnetic breaker that is used to provide the motor circuit branch circuit short circuit and ground fault protection. There would have to be changes to (D) no matter what to address the size of the EGC where an instantaneous trip device is used on the motor branch circuit.

 

[MasterTheNEC]

I believe there is also an issue with NM in some sizes where the new table would require a larger EGC than what is currently being used. That problem arises because the new table is based on the ungrounded conductors having a 75°C ampacity and NM is only permitted to use the 60°C ampacity.

I can see the issue with the motor circuit EGCs because of the permitted rating of the branch circuit short circuit and ground fault protective device can be approximately twice that of the conductor ampacity. That could be addressed with a change to 250.122(D) to require the motor circuit EGC be based on the smallest ungrounded conductor that is permitted to be used for "normal" circuits based on the rating of the fuse or thermal magnetic breaker that is used to provide the motor circuit branch circuit short circuit and ground fault protection. There would have to be changes to (D) no matter what to address the size of the EGC where an instantaneous trip device is used on the motor branch circuit.

Yep. I assume this will be one of our quests in San Diego come October.

Paul W. Abernathy
Electrical Code Academy, Inc.
CMP #5 and #17