When to install ground rods for utility pad mount transformer?

C

choppedslaw

Member
Location
Usa
Occupation
Journeyman
I’m installing utility pad-mount transformer underground.

Relevant details:

•Spec calls for (2) ¾” x 10’ copper-clad ground rods.
  • Note states: “Install two 3/4” x 10’ copper weld ground rods driven to a minimum depth of 9’-0”.”
  • There is a copper ground bus mounted in the transformer pad window (at finished grade elevation).
  • The rods connect to that ground bus inside the window.
Current site condition:
  • Excavation is approximately 4’ deep below finished grade.
  • Pad/window elevation is at finished grade.
  • Primary and secondary conduits are already installed and and stubbed up from the excavation.
My question is about sequencing and driving elevation:

If I drive the rod from the bottom of the 4’ excavation:
  • Driving 9’ would leave 1’ exposed at the ditch bottom.
  • That would put the rod top 3’ below the pad window.
  • I would not be able to reach the ground bus.
If I drive only 6’ from the ditch bottom so the rod top lands at grade, I would only have 6’ embedment, which does not meet the 9’ minimum driven requirement.

So it seems the only way to both:

  1. Achieve ≥9’ embedment, and
  2. Have the rod top land at the pad window for connection

Is to backfill/bring the area up to near grade and then drive the 10’ rod from that elevation.

Is that the correct interpretation of “driven to a minimum depth of 9’-0”” in this context?

For those who do utility pad-mount work regularly:

  • Do you typically drive rods from finished grade elevation once the pad base is built up?
  • Or is there another accepted method in this scenario?
  • I’m concerned if I backfill then drive rods I run the risk of hitting the installed underground conduits. Primary side is robroy sweeps. Secondary side are pvs 90s.
I want to make sure I meet embedment requirements while also keeping the rod top accessible for connection to the ground bus in the pad window.

Appreciate any insight from those familiar with utility primary installations.
 
maybe "drive/set" some 1"pvc ......if the rods will fit in that.....then you will know the other conduits wont get hit when driving the rods in later.
but I agree with Hal from post #4.
 
A couple things jump out at me. First, you don't have to drive the ground rods in the ditch and it may be easier to drive them beside it because backfill around the rods sticking up as Hbiss suggested may be difficult, not that he is wrong. Second, if your specs, etc. are written by the job Engineer and you have utility work by a separate utility company, find out what they want or need from you. Respectfully ignore the Engineer if the Utility requirements are different.
 
This is all "Greek" to me. Around here, we only dig for the transformer. POCO does their own ground rod driving & connecting to the transformer. They also run their own conduit & conductors, customer paid for of course.
 
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