alternatives to trenching

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JoeNorm

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I have a client who wants a feeder run to their garage/shop which is 100' away. The ground is solid rock starting just a few inches down. He has a quote for the trench of $7K from a reputable excavation company.

Are there any other alternatives to consider? It's a driveway so running pipe above ground is not an option. The only other things I could think of is overhead which I have never heard of for a feeder or some type of cord and plug which I am almost certain would be illegal.

Any thoughts?
 
You could probably install one tall pole right in the middle and run a messenger supported cable between the two buildings. You would need to have the pole hole drilled out but it would probably be a whole lot less than $7,000.
 
Table 300.5 exception #5 allows for conduit at a lesser depth where solid rock prevents specified depth but requires it to be covered with 2 inches of concrete extending down to the rock.
Are what you are dealing with literally bedrock or just extremely rocky soil? The suggestion that an excavation company will dig for only $7K suggests to me only very rocky soil as you don't mention blasting or stone cutting, both of which would likely cost a lot more around here. If only rocky soil the exception would not be allowed.
Personally I would opt for concrete encased over blasting (while that would be exciting) or cutting a trench, as that generally creates a canal way for water to set in or ingress into you building depending on elevations differential of trenchway involved.
 
I would seek a horizontal-boring assessment. A local company should be familiar with the terrain.
 
Fred B said:
Table 300.5 exception #5 allows for conduit at a lesser depth where solid rock prevents specified depth but requires it to be covered with 2 inches of concrete extending down to the rock.
Are what you are dealing with literally bedrock or just extremely rocky soil? The suggestion that an excavation company will dig for only $7K suggests to me only very rocky soil as you don't mention blasting or stone cutting, both of which would likely cost a lot more around here. If only rocky soil the exception would not be allowed.
Personally I would opt for concrete encased over blasting (while that would be exciting) or cutting a trench, as that generally creates a canal way for water to set in or ingress into you building depending on elevations differential of trenchway involved.
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I have considered covering with concrete. Yes this will be a mix of solid and broken bedrock. Around here they use a rock hammer on the machine to hammer out trenches.
 
LarryFine said:
I would seek a horizontal-boring assessment. A local company should be familiar with the terrain.
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I would at least get a quote for horizontal-boring. That way you don't have to tear the drive way up and do any patching and that can save money.

Last time I was on a job like this with lots of rocks it was supposed to be a 4 hour job and ended up taking two days but it still was the best choice.
 
petersonra said:
Good question. I think a lot of residential electricians have little experience with running wires that way so shy away from it. just like many electricians shy away from aluminum feeders for no good reason.
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Yes, good question and good answer. I have never done it but sounds fun to try. Is there a code section that lays out the method? Does the support wire have to be listed? is there a max distance before needing another support to the ground?

Is it best to buy Quadplex? This is not readily available from my supplier. Would SER cable work? What's the method for Putting tension on the wire? How do i insure there are no hard spots that will wear over time in the wind?

We don't do any overhead service drops in my area, everything is underground, so I have zero experience with overhead.
 
JoeNorm said:
Yes, good question and good answer. I have never done it but sounds fun to try. Is there a code section that lays out the method? Does the support wire have to be listed? is there a max distance before needing another support to the ground?

Is it best to buy Quadplex? This is not readily available from my supplier. Would SER cable work? What's the method for Putting tension on the wire? How do i insure there are no hard spots that will wear over time in the wind?

We don't do any overhead service drops in my area, everything is underground, so I have zero experience with overhead.
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It is available at your local poco, also you could ask their engineers what span would require support. Did one last year. The original was installed in the 60’s, and was too low. They would lift it up to get a corn picker under it. Still wasn’t as high as I would like, but now clears the farm equipment being worked on.
 

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