250.122

[JasonWrenn]

In regards to the Equipment Grounding Conductors, I have parallel runs of conduit that stub out of a switchgear and stop before connecting to cable tray. I say that the equipment bonding jumpers that go from the conduit grounding bushings to the cable tray need to be sized based off of Table 250.122. It is a 1600amp frame breaker, 1200amp trip setting installed in the breaker, and it has adjustable GFCI protection set at 300amps. I believe the size of the EGC should be a 4/0 based off the frame size. I have an engineer here that thinks since this is an "engineered system" that the EGC can be sized from the GFCI setting at 300amps, and since Table 250.122 says "Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)", that it can be sized off the GFCI "setting".

I have never thought of sizing EGC like this and have been task to show the engineer otherwise, PLEASE HELP.

 

[chris kennedy]

"Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)",

Tell the engineer that says setting of OCPD not setting of GFP.

 

[kwired]

Interesting question, I was going to side with Chris, and still am leaning that direction, but first think we need to confirm whether or not the GFP qualifies as art 240 overcurrent protection in any way at all. I was going to mention there can still be a line to line fault with very high current and the GFP won't care - but in that case the EGC in question isn't carrying any current anyway....

Still not decided but am still leaning toward what Chris mentioned though.

 

[mayanees]

My practice now is to size the egc using the trip rating of 1200 amps, and not the frame rating, which would put it at 3/0 AWG.
But I'm watching the thread closely as well!

 

[cuba_pete]

OCPD

OCPD

Interesting question, I was going to side with Chris, and still am leaning that direction, but first think we need to confirm whether or not the GFP qualifies as art 240 overcurrent protection in any way at all. I was going to mention there can still be a line to line fault with very high current and the GFP won't care - but in that case the EGC in question isn't carrying any current anyway....

Still not decided but am still leaning toward what Chris mentioned though.

I would lean all of the way.

An engineer should understand that GFI is not Over-current protection and the scheme is completely different. Conductor sizing is base on wholly different data.

 

[augie47]

My practice now is to size the egc using the trip rating of 1200 amps, and not the frame rating, which would put it at 3/0 AWG.
But I'm watching the thread closely as well!

You did not mention the size of your conductors. Normally I would look for 3/0 as noted above based on the actual overcurrent setting of the device. IF you oversized your conductors for any reason, including frame size, then the EGC would need to be increased proportionally.

As a side note, from your picture, having not seen the other end of the conduits I am unsure as to why you have the bonding bushings on the conduit, but the equipment bonding jumper for those would need to be sized the same as the one being discussed. From the picture it appears to be undersized.

 

[petersonra]

I think that if you are going to put the bonding jumper in place that it needs to be based on the setting of the OCPD, not the setting of the GFP device.

Whether it is even necessary is something else.

 

[JasonWrenn]

Tell the engineer that says setting of OCPD not setting of GFP.

I agree completely, my struggle is showing him the proof that it is done that way. The GFP is in built in the OCPD.....not sure how to convince him otherwise.

 

[GoldDigger]

I agree completely, my struggle is showing him the proof that it is done that way. The GFP is in built in the OCPD.....not sure how to convince him otherwise.

I can sort of see his point. The only fault in the circuit that could put current through the EGC as a fault clearing path will be a ground fault. And that fault will first be interrupted by the GFP setting.
Unfortunately the NEC generally does not recognize that type of logic except where there is a specific provision for engineering supervision.

 

[don_resqcapt19]

The 2005 NEC had the following wording in the section about EGCs for parallel runs.

250.122(F)(2) Where ground-fault protection of equipment is installed, each parallel equipment grounding conductor in a multiconductor cable shall be permitted to be sized in accordance with Table 250.122 on the basis of the trip rating of the ground-fault protection where the following conditions are met:
(1) Conditions of maintenance and supervision ensure that only qualified persons will service the installation.
(2) The ground-fault protection equipment is set to trip at not more than the ampacity of a single ungrounded conductor of one of the cables in parallel.
(3) The ground-fault protection is listed for the purpose of protecting the equipment grounding conductor.

This was removed in the 2008 code as no GFP has ever been listed for the purpose of protecting the equipment grounding conductor.

 

[Smart $]

... I have an engineer here that thinks since this is an "engineered system" that the EGC can be sized from the GFCI setting at 300amps, and since Table 250.122 says "Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes)", that it can be sized off the GFCI "setting"...

The definition of "Ground-Fault Protection of Equipment" in Article 100 differentiates GFP from the supply circuit overcurrent device. That's all the "evidence" you need.

 

[Canton]

On top of what everyone has said......you would still need at least a #4 bonding jumper for that 300 amp ocpd/gfp. With that logic than every feeder in the building would not need any thing bigger than a #4....??????

 

[kwired]

On top of what everyone has said......you would still need at least a #4 bonding jumper for that 300 amp ocpd/gfp. With that logic than every feeder in the building would not need any thing bigger than a #4....??????

You have a point there, if the GFP setting is determined to be acceptable basis - 6 AWG is still not large enough EGC.

 

[charlie b]

Am I to understand that this engineer believes that designing an "engineered system" allows the engineer to do anything he or she wishes? If so, then this engineer needs to go back to school. We need to follow the codes, just like everyone else does. We also need to understand the language of our profession. "Ground Fault Protection for Equipment" (it's GFPE, not GFCI) is not a mechanism for overcurrent protection.