Running 100 amp underground wire

Old 11-30-04, 04:37 PM
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Running 100 amp underground wire

I am in the process of adding a new service box to a new pole barn.
The source will come from the house 200 amp service box. The main service box has room for the 100 amp breaker I will add. the new service box is a 100 amp unit with 20 breaker slots. I have W 2 AWG four strand underground cable for this 100 foot run. The run will start at the main service box in the basement and then into a crawl space. The ceiling is open in the basement ... thats to say the floor joists are easly accessable ( also in the crawl space ). It will then run outside into a trench that is now 42 in. deep
( other utilities will be going in there too ). My questions are:
* am I doing this right so far ?
* whats the best / easiest way to run this stranded cable ? Do I need to drill
holes in each floor joists like the rest of my household runs ? Or could it be
hung from the floor joist (tight to the bottom of them ) say with a hanger
like you would use with black pipe. Or is there another way I'm not
thinking of ?
I plan to use a pro to do the service panel hookups, but I'm justing trying to learn something here and do the meanal prep work.
Old 12-01-04, 11:27 AM
Join Date: Sep 2000
Location: Fayetteville, NY, USA
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It sounds like you're on the right track. Minimum burial depth under the most stringent cases, per NEC, is 30". Of course you can go deeper. Minimum depth in residential, where the area is NOT exposed to vehicular traffic, is 18". You may use direct-buried cable but it must be specifically rated for direct burial. Conduit is good but not required. If conduit, schedule 40 PVC is fine below grade, but where you come up and it is exposed above grade, many inspectors require transition to schedule 80. It's thicker and stronger and protects your wiring from "physical damage".

Drilling joists is OK. Drill in the center (top-to-bottom) of the joist, as this is the point of least stress on the joist.

If you go perpendicular across the bottom of joists, you must install a "runner board". This can be basically a 1 x 4 nailed to the bottom of the joists. Staple your cable to the bottom of this board. You are not permitted to run wire across the bottom of the joists without it, but the runner board method is legal. Conduit may be run directly across the bottom of a joist legally, without a runner board. (Black pipe is for gas & water, I think you meant conduit, either rigid galvanized, PVC, or "EMT".)

If you are using wire rated 75 or 90 degrees C, #2 is the right choice. For 60 degree C wire it is #1 AWG. (This is for 100 amps.)

If you are planning to have a pro do the final terminations, what you are doing is called "roughing in". Only note of caution is that if there is anything you do in your end of the work that is not code compliant, he will refuse to hook it up. Once he does this he is responsible for the entire job even though he didn't do all of it himself. Some electricians will advise you of what to correct and how, but some won't, and instead will want to charge you to correct your mistakes.

Another thing to note: if you are setting up the new 100 amp panel, fed from your main panel, all neutrals and grounds must be separate in the new panel, as it is considered a "sub-panel". This means that your "bonding screw" must be removed. This is a screw that passes through your neutral bus into the sheet metal of the new panel, effectively grounding the neutral, which is required in a main panel but forbidden in a sub-panel. It should be identified either with a label in the new panel, or in the installation instructions. You will have to purchase and install a ground bus. They are available in nearly all home centers for about $5 to $8. You must sand off all the paint on your panel's sheet metal underneath where you will install the ground bus so you get good electrical contact between the bus and the sheet metal your "box" is made of.

Hope that helps. Let us know if you have any more questions, or how it worked out.


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