Grounding an old subpanel

[Martinelli]

I was called out to a job on a farm. The home owner wanted me to ground a sub panel at his home that he had installed 15 years ago. The panel had a 200 amp 3 phase sub feed A, B, C and nuetral ran with direct burial wire, the run is over 500 feet. the main service is grounded but there is no ground ran with the sub feed. I installed a ground rod at the sub panel and bonded the nuetral. Is there a better way to handle this situation?

 

[Dennis Alwon]

I was called out to a job on a farm. The home owner wanted me to ground a sub panel at his home that he had installed 15 years ago. The panel had a 200 amp 3 phase sub feed A, B, C and nuetral ran with direct burial wire, the run is over 500 feet. the main service is grounded but there is no ground ran with the sub feed. I installed a ground rod at the sub panel and bonded the nuetral. Is there a better way to handle this situation?

That was a legal install then if there were no metallic pathways between the 2 buildings. Two ground rods should be installed now.

You did good short of digging it all up.

 

[mcclary's electrical]

If there is a water pipe or a phone line going out there(any metallic path) then it's not legal,,,,,,but if that's the only thing going to it, I would drive one more rod and you're good

 

[erickench]

Well I don't want to tell you to use the neutral as a fault current return path because alot of people don't like it. The ground rod will protect the subpanel from lightning but what if there's a fault? Is there a continuous metal raceway?:roll:

 

[winnie]

Using the neutral as the fault current path for outside feeders was legal until recently. (With the noted limitation of no bonded metallic paths other than the neutral between the two structures.)

However you mention that this is on a farm. Such installations have been implicated in 'stray voltage' issues on farms, so in my mind an open question is just why the customer wanted this subpanel 'grounded'. I would try to dig the why out of the customer. Any voltage drop on the neutral will show up as a voltage applied to the grounding electrodes at the subpanel. If there is significant neutral voltage, then this can show up as 'step potentials' (voltage from one patch of soil to a nearby patch of soil) around the grounding electrodes at the subpanel.

If you actually want to add an EGC, there are few sensible options beyond digging and adding an EGC.

You might be able to re-allocate some of the conductors, eg. making one of the phase conductors into the EGC and going to a three phase service, or making the grounded conductor into the EGC, and eliminating all line-neutral loads.

For example, (assuming that this is a 208/120 feeder) you could change the feeder from A,B,C,N to A,B,C,G, supply only 208V loads, and then use a transformer to supply any 120V loads. Somewhat more esoteric (and I am not sure about being legal), you could change the feeder to A,B,C,G and then use a 'zig-zag autotransformer' to derive a local neutral for 120V loads.

-Jon

 

[erickench]

Using the neutral as the fault current path for outside feeders was legal until recently. (With the noted limitation of no bonded metallic paths other than the neutral between the two structures.)

I'm just curious. When was the last NEC that allowed the neutral as a return path?

 

[Dennis Alwon]

I'm just curious. When was the last NEC that allowed the neutral as a return path?

The 2005 NEC 250.32(B)(2)

 

[erickench]

Well it looks like the rule is still in use in existing separate buildings but a new exception discouraged it from being used in new designs. I agree with this but the rule should apply to all existing buildings and not just separate ones. I mean there are still alot of places where there is no ground at all.

 

[Martinelli]

I think I am going to turn the C phase into the EGC. I believe the C phase is a stinger leg and not currently being used.
I am also going to ask the customer why he decided to ground the sub panel after all these years. Thank you for the advive