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Install Underground Electrical Wiring for a Building

home-improvement-electrical-projects Electrical Question Summary : We're applying for a permit for a 20x30 enclosed storage building. It is approximately 260 ft from main breaker box.

Underground electrical wiring for a storage building

Electrical Question:
We're applying for a permit for a 20x30 enclosed storage building.

It is approximately 260 ft from main breaker box. Our plan includes 4 internal receptacles, 1 fan, 2 shop lights, 2 outdoor wall lights, 1 window unit, 1 exterior outlet, 3 single pole switches, 1 70 amp sub panel.

To run wire underground-what size and type? In conduit or not?
Grounding rod at sub panel?
Size breaker at main panel?

Underground Wiring to Storage Building

Thanks for the reply Eric.

I basically went with 70 amps because I thought you may want extra power available for future, and I don't have the specs of the devices you will be using, specifically the window unit which may be one of your biggest loads, and I must say that given the distance of 260 feet that you may indeed have a problem if you install a #10 size wire.

Double check and make sure the inspector understands the distance, and you may want to find out the exact specs, if not get in touch with a representative of the window unit and pass this info to them and get their recommendations.

Just so you know, my calculations show that a 240 volt 20 amp load at a distance of 260 feet calls for a minimum #6 copper wire. I really don't believe the inspector realizes the voltage drop potential at 260 feet. Here's another scenario - lets say the window unit requires a 120 volt 15 amp circuit - at 260 feet the minimum wire size is #4. Are you not able to get any deeper than 8 inches?

I'm guessing not. Hopefully the inspector will allow a schedule 80 PVC over rigid pipe. Also - understand that if you use schedule 80 pvc the inside diameter is slightly smaller.

Underground Wiring to Storage Building

He actually gave me 20 minutes of his time on the phone-I was shocked.
He checked his books and came up with running No 10 wire in rigid conduit 8 inches underground with the tape 2 inches below the surface.
40 amp breaker and 2 20 amp circuits at building.

I'm going to call him back Monday and see if we can use schedule 80 rigid conduit instead of the metal-probably wishful thinking at only 8 inches.

Hi Eric,
I just wanted to follow up with you - how did the meeting go with the inspector, and their issue with the ground rod?

Another thing I wanted to mention is that if you use direct burial USE Cable that the PVC that is used to protect the wiring as it comes out of the trench should have a Plastic Bushing on the pipe where the cable enters the PVC.

The bushing is typically screwed onto a male adapter which is then glued onto the PVC pipe.

Thanks very much Dave , the only concern is the ground rod, some say no way-hook ground and neutral to neutral at main panel then at new box separate them, neutral to neutral, ground to ground bar-no ground rod.

I'll check with an inspector today and either way I'll let you know what he says, again, Thank you.

Details of Installing a Electrical Service to a Storage Building

Base on the distance of 260 feet here are your options:
To run wire underground-what size and type?
Copper Wire
Installing a minimum 1- one half inch or 2 inch PVC Schedule 40 Conduit at a depth of 18 inches.
Copper wire size = 3 - #2 THW or THWN

Aluminum Wire
Install 2 inch PVC Schedule 40 Conduit at a depth of 18 inches.

Aluminum wire size = 3 - 2/0 RHW, THW or THWN

An N-30 pull box should be installed mid way to make the pull easier, typically every 100 feet is best.
If pull boxes are in a road location then traffic rated boxes and lids are required.

The size of the pvc will depend on how many bends there will be. Use long radius sweeps for your 90 degree bends.
Sweeps into the pull boxes can be 45 degree to minimize the number of radius bends.

Wire pulling lubricant should be used generously before and during the pull.
The use of a pulling sock will be very helpful.

A flat mule tape type pulling rope is highly recommended above a nylon rope which can create hot spots in the pvc radius areas and create friction during the pull.
A shop vacuum can be used to install the mule tape tied to a small rag with a knot.

Wire In Conduit or not?
For Direct Burial Cable Install 3-2/0 USE (Underground Service Entrance) Aluminum at a depth of 24 inches.
Lay in the cable loose, not tight.
Protect the cable with clean river sand or screen out gravel and rocks for the bed and initial covering.
Protect the cable using schedule 80 PVC from 18 inches below grade and up to the enclosure locations.

Grounding Rod at Sub Panel?
Drive a 10 foot Copper Clad Ground Rod at the storage building below the service panel.
Install a #6 solid copper ground wire from the panel Ground Terminal Bus to the Ground Rod and attach using an Acorn Clamp.

Bond the Neutral Terminal bar to the frame of the panel enclosure. Typically a green bonding screw is provided for this.

Size Circuit Breaker at the Main Panel?
70 Amp 2-Pole 240 Volt Circuit Breaker at the main panel and a 2-Pole 70 Amp Main Breaker Disconnect in the panel at the storage building.

I personally prefer using pvc to avoid problems with direct burial aluminum cable.
An Underground Trace Ribbon may be required to be installed 12 inches above the cable or PVC.

Be sure to identify each cable using electrical phasing tape - red for one hot, white for the neutral.

Consult your local building department for local code compliance's and inspections before covering up the trench.

Be Careful and Be Safe - Never Work on Energized Circuits!
Dave Rongey

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