Grounding conductors increased in size 250.122(B)

[NOV]

I have a situation where an engineer is upsizing a feeder due to voltage drop. Using 75 degree C, the upsized feeder is rated 200A and it is sourced via a 175A breaker. Article 250.122(B) tells me that I have to increase the grounding conductor proportionately according to the circular mil area of the ungrounded conductors. This article is typically pretty straight forward for me but what’s making me doubt the increase in this case is that the upsized feeder (200A - #3/0 ungrounded conductor) and the feeder required for the intended load (175A - #2/0 ungrounded conductor) both use the same size grounding conductor (#6AWG) as shown in Table 250.122. Do I increase the #6AWG grounding conductor by 126% to a #4AWG or is the #6 acceptable in this installation? My initial thought was to increase it but isn’t the reason for the increase of the grounding conductor to handle the additional fault current that the increased ungrounded conductors would allow? The grounding conductor is already rated for the 200A. This one has me a little puzzled.

Thanks in advance!

 

[Sparky2791]

If phase conductor is increased ground conductor must be increased if the calculations require it. I do not have a lot of time right now to completely answer your question however ground is increased proportionally based on increase in phase conductor. Occasionally the increase is not needed but you must do the math to determine this.
This article is very helpful with respect to how this is achieved.
https://www.ecmag.com/section/codes-standards/voltage-drop-sizing-equipment-grounding-conductors

 

[jaggedben]

Until you get to mcm designated sizes each AWG size is proportional the next. Increasing the ungrounded conductors from 2/0 to 3/0 is one size, so 250.122(B) would require increasing the EGC from 6 awg to 5awg. So you end up using 4awg because they don't make 5.

If you are on the 2020 NEC a 'qualified person' can provide calculations to show you have an effective ground fault current path for a smaller size. Don't try to make sense of it otherwise. See this recent thread: https://forums.mikeholt.com/threads/equipment-grounding-vs-voltage-drop.2569520/

 

[wwhitney]

An unhelpful comment: if you're worried that ambient temperature may hit 105F, and you decline to use 240.4(B), then a #6 EGC is sufficient. Or if you throw in 1 or 2 spare conductors (say #14s, enough to get the # of CCCs up to 4).

Cheers, Wayne

 

[TCTucker]

Whatever percent of wire size increase (e.g. 125%) was applied to upsize the phase conductors, that same percentage of increase must also be applied to the ground conductor. If the percentage of increase results in a non-standard wire size not shown in the NEC wire tables (e.g. #3.5AWG, or 375MCM), then you must move up to the next standard wire size.

Another way to say it: upsizing a ground conductor may "push" you into a standard wire size that is larger than the original percentage of increase applied to the phase conductors.

Edit: language

 

[Dennis Alwon]

I think this is one of those crazy things about increasing the equipment grounding conductor. If you increase the conductor to 3/0 with a 200 amp breaker then you can use #6

However, I would say that the 3/0 is .26 bigger than 2/0

#6 =26240* 1.26 = 33062 which means you would need a #4

If you can use a 200 amp overcurrent protective device then you wouldn't need to upsize the equipment grounding conductor.

 

[Dennis Alwon]

If this is a motor then you can use 200 amp as the feeder overcurrent protective device and then use a disconnect with 175 amp overcurrent protective device.

 

[tom baker]

This rule has changed frequently. Try using the GEMI software program to see what it recommends. Search Steel Tube Institute GEMI, it’s free and interesting to use

 

[NOV]

Thank you all!

Not a motor, a feeder to a sub panel with a 175A MCB. Point taken Dennis, but for whatever reason he has selected to source this panel via a 175A breaker in the distribution panel and put it on a 200A rated feeder.