ground resizing on USE wire for VD

mjmike

Senior Member
We are working on a project involving the replacement of a pump control panel located in a remote vault. The load is quite small; about 15 amps at 480V 3-phase with a feeder breaker rated for 30A in the main building. The run is very long so the installed conductors have been upsized for voltage drop. Installation is mid 80's. The feeder consists of 3-#1/0 and 1-#2 ground. Aluminum wire type USE XLPE (USE only, not USE-2 or dual ratings)

We are trying to reuse the existing conductors and are checking their ratings and voltage drop.

Regarding the voltage drop, a 30A feeder would have needed #8 with a #8 ground (aluminum). Upsizing the phase conductors to 1/0 would have required a #1/0 ground due to 250.122.B. Therefore, it appears the ground is undersized.

Possible solution:

Since we are putting in a new pump control panel, we have the ability to customize the panel. The plan would be to change the main feeder breaker to a 50A breaker (instead of 30A) and have a 50A breaker main disconnect in the pump panel. The 50A feed would initially need #4 phase and #10 ground (aluminum). With that being the starting point, looking at the load of 15 amps, the #4 would still need upsized. Therefore, they have been upsized from #4 to #1/0. Checking the ground, the new ground would only need to be a #4 which the installed #2 would just be an improvement.

Here is the math for 50A feeder: circular mills:

(new conductors/base conductors) x base ground.

(105600/41740) x 10380 = 26261 CM for new ground = installed #2 meets this.

Does this approach appear viable and I guess my confusion is why with the lower ampacity (30A) VD correction a larger ground is required, but if I increase the circuit ampacity (50A), the VD corrected ground is smaller?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
you will generally get some strange numbers when using smaller conductors and upsizing as the EGC and circuit conductors are going to have to always be the same size.

I do not see a problem with changing the breaker out to a larger one. The math works and it meets code. presumably there is appropriate OCPD at the pump panel itself.

How did you decide a 50A feed needs #4 AL? is that a limit of USE?
 

mjmike

Senior Member
you will generally get some strange numbers when using smaller conductors and upsizing as the EGC and circuit conductors are going to have to always be the same size.

I do not see a problem with changing the breaker out to a larger one. The math works and it meets code. presumably there is appropriate OCPD at the pump panel itself.

How did you decide a 50A feed needs #4 AL? is that a limit of USE?
I based the #4 AL on the 60 degree column of ampacity tables (table 301.15.B.16). #4 is good for 55A. I am using the 60 degree column based on 110.14.C.1.a

I know the 1/0 wire size is good for the VD load. I know the ground I have to work with is #2. To make it work, I picked the smallest breaker that allowed the ground ration to work.

At the pump panel, they probably only need a 20A MB but since it is being custom built we will ask for a larger main which will also allow termination of the larger conductor size.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I based the #4 AL on the 60 degree column of ampacity tables (table 301.15.B.16). #4 is good for 55A. I am using the 60 degree column based on 110.14.C.1.a

I know the 1/0 wire size is good for the VD load. I know the ground I have to work with is #2. To make it work, I picked the smallest breaker that allowed the ground ration to work.

At the pump panel, they probably only need a 20A MB but since it is being custom built we will ask for a larger main which will also allow termination of the larger conductor size.
I think you did good.
 

Smart $

Esteemed Member
Location
Ohio
Why increase the pump panel main to 50A if all you need is 30A? Don't forget that with motor circuits you have a max ocpd limit. Going with 50A, you may need to provide additional lesser-rated OCPD for the motor SC/GFP, and you may have to increase the ampacity of any wiring ahead of that additional OCPD.

Does this approach appear viable and I guess my confusion is why with the lower ampacity (30A) VD correction a larger ground is required, but if I increase the circuit ampacity (50A), the VD corrected ground is smaller?
Many have brought up the same reasoning. I have a proposed change submitted for the 2017 cycle. Will have to wait and see how that goes over... :happyyes:
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
I believe the thinking is that if you have to upsize the conductors that generally means that there is a great distance to the equipment. The further distance you go the more impedance there is on the smaller ground therefore the need to upsize. The question still remains why does a larger breaker only require the smaller equipment grounding conductor?

I am guessing that there is no real good means to write this rule so they did an all encompassing rule and we are left with these idiosyncrasies. :eek:hmy:
 

mjmike

Senior Member
Why increase the pump panel main to 50A if all you need is 30A? Don't forget that with motor circuits you have a max ocpd limit. Going with 50A, you may need to provide additional lesser-rated OCPD for the motor SC/GFP, and you may have to increase the ampacity of any wiring ahead of that additional OCPD.

This is 2 fold. Not saying the new panel need 30A or 50A but most likely, 50A is more than it needs. Increasing it for 1 to match the upstream proposed 50A breaker. It needs to be 50A to allow the #2 ground to work out. 30A would require a larger ground than installed. The 2nd part, is the 50A in the pump panel would allow the termination of the larger conductors without a haircut. Since the pump panel will be built in the future, we can spec the requirements. They can always add other downstream components in the panel if needed.
 

Smart $

Esteemed Member
Location
Ohio
I believe the thinking is that if you have to upsize the conductors that generally means that there is a great distance to the equipment. The further distance you go the more impedance there is on the smaller ground therefore the need to upsize. The question still remains why does a larger breaker only require the smaller equipment grounding conductor?

I am guessing that there is no real good means to write this rule so they did an all encompassing rule and we are left with these idiosyncrasies. :eek:hmy:
Agree, if you HAVE TO upsize the ungrounded conductors. But Code makes you upsize EGC even if you don't have to upsize the ungrounded. On top of that, Code does not require upsized conductors for voltage drop, relegating the issue to informational notes.

IIRC, the requirement first appeared back in mid 70's Code and covered only upsizing for voltage drop and only where a wire-type EGC was required. Lasted until '99 edition where someone got the air-brained idea the EGC should be upsized for any reason, any circumstance the ungrounded conductors are oversized. Other than a few modifications, it has stuck until now.

What I propose as a compromise is to upsize the EGC according to the maximum size OCPD rating the ungrounded conductors would be protected. Under such, the OP's EGC would be required to be #4 no matter what the OCPD rating is (of course going too large an OCPD rating would be a violation in itself).

Also proposed is a informational note to the effect minimum EGC may not be sufficient where ungrounded conductors are upsized for voltage drop... explaining that Code has historically avoided any requirement for voltage drop and EGC sizing should be no different, especially when a wire-type EGC is not required (such as when an EGC-qualifying raceway or cable wiring method is used).
 
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