250.122(b) and sizing of ground conductors

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chriskthx

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I would like some clarification on 250.122(b) and the following situation:
240/480V, single phase with 30A breaker. The smallest wire size is a #4 and it increases progressively to a #1.

The client's preference is to match the ground conductor to the ungrounded conductors. However, when we get to the #1's, they do not meet conduit fill requirements. A possible solution is to use a #2 ground with the #1 ungrounded conductors.

How does 250.122(b) and Table 250.122 apply in this sense? While the ungrounded conductors do increase in size from #2 to #1 due to voltage drop, can the grounded conductors remain #2 since they appear to be oversized already?
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
If the original size conductors were #10 then a #10egc is required. At each increased of conductor size the egc must increase proportionately. Thus the last part must have the egc sized to the largest conductor. IMO, #1 is required.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I am wondering if you start with a higher size conductor for a longer part of the run perhaps the final run would not need to be #1.
 

kingpb

Senior Member
I know the NEC requires an increase in EGC based on increase of current carrying conductors.

In the case of increase due to VD, I'm looking for a technical reason why this would be necessary. The breaker in the OP example is still 30A. The impedance in the larger cable will be less allowing slightly more SC current to flow, but I wouldn't think it would make enough difference to have to increase the EGC since your still limited by the 30A breaker.

Any thoughts?
 

Smart $

Esteemed Member
Location
Ohio
Please clarify...

You said 240/480 single phase with 30A breaker in opening post. Is the breaker one pole or two pole, i.e are you running a two-wire (L-N-G or L-L-G) or 3-wire circuit (L-N-L-G)?

Additionally, where applicable, are we talking about a grounded neutral conductor (N), or the grounding conductor (G), or both?
 

Smart $

Esteemed Member
Location
Ohio
I know the NEC requires an increase in EGC based on increase of current carrying conductors.

In the case of increase due to VD, I'm looking for a technical reason why this would be necessary. The breaker in the OP example is still 30A. The impedance in the larger cable will be less allowing slightly more SC current to flow, but I wouldn't think it would make enough difference to have to increase the EGC since your still limited by the 30A breaker.

Any thoughts?
I believe the premise is increasing size for voltage drop is also allowing more current to flow under nominal conditions. If the EGC is not upsized proportionately, it would hinder current flow under a fault condition such that the ocpd would not trip as soon, or perhaps not at all.

This is similar to calculating at what length a metallic conduit ceases to be an adequate fault current path. Upsize the conduit when such is the case, and the conduit is again an adequate fault current path.

I agree that it makes only a little difference and mostly on a resistive fault as opposed to a bolted fault... but that little difference caused the CMP to require the upsizing and we're stuck with it... for now at least ;)
 

bob

Senior Member
Location
Alabama
Don't forgrt you can use the conduit as the EGC if you feel it is in adequate condition.

I believe the premise is increasing size for voltage drop is also allowing more current to flow under nominal conditions. If the EGC is not upsized proportionately, it would hinder current flow under a fault condition such that the ocpd would not trip as soon, or perhaps not at all.
Table 205.122 allows a #10 conductor as the EGC with out specifying distance.
If the #10 is adequate per the table then increasing the ckt to #4 would increase the fault current. #10 would reduce the increased fault but it would also reduce the fault if #10 was the feeder. There is no science behind this requirement. This should be an engineering decision rather that use a table.

This is similar to calculating at what length a metallic conduit ceases to be an adequate fault current path. Upsize the conduit when such is the case, and the conduit is again an adequate fault current path.
I do not think the code would want to take that calculation on.

I agree that it makes only a little difference and mostly on a resistive fault as opposed to a bolted fault... but that little difference caused the CMP to require the upsizing and we're stuck with it... for now at least ;)
Yes we are stuck with it. I had hoped for a change in the new code but it appears to be the same.
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
The smallest wire size is a #4 and it increases progressively to a #1.

The client's preference is to match the ground conductor to the ungrounded conductors. However, when we get to the #1's, they do not meet conduit fill requirements.
I would lean towards ditching the #4, making it larger for that portion of the run, and then decreasing the #1, and get the same overall effect without overfilling the conduit. Then you should be able to pull the full sized ground 250.122(B) requires.
 
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