I'm running wire from the house to feed power to a 100 amp service in the new garage. It will run in thinwall pvc buried in the ground. The run will be 180' +. I would like to know what size and or type of cable/wire I need for this length of run. Not being very good at electrical work I am getting the needed supplies for a friend that is close lipped about what I need. I have the panel already along with assorted boxes and outlets but the high student help I recieved at Menards wasn't clear on the power feeding cable/wire I need for this length. Any help would be greatly appreciated.

 

Min #2 Copper

 

A 100 Amp service normally requires #3 AWG wire. Since you have a long run, I agree that you should use #2. You will have less voltage drop in the mains when drawing high amps (the lights won't dim so much) if you use #2.

Electrical code requirements differ from place to place, but you have a few options for how you run the service out to the shop. You can buy a direct-burial cable with the conductors you need (Type NMWU here in Canada) and simply bury it. You can also buy individual wires and pull them through buried PVC or metal conduit. Or you can buy cable and lay it in PVC sewer pipe or non-perforated corrugated black plastic agricultural drain tubing.

You'll also need to find out how many conductors you NEED by code. a 220V service is basically only two wires. You measure 220V between them, but measure 110V to ground on each one. In you main disconnect where the service enters your house, the 110V "neutral" (white) wire is connected to the ground bar ("grounded neutral"). A green or bare ground wire then runs either to a ground stake driven in outside your house, or to your water service if the entrance piping from the municipal water main is copper (or even steel). The neutral/white will need to be the same gauge as the mains (#2), but you may be permitted to use a lighter gauge for the ground. Where I live, #6 is generally used to ground a 100A panel - a main breaker or fuse will pop long before #6 burns up. For a 180 foot run, I'd consider upping the ground to #4.

Your local code requirements may allow you to drive in an additional (new) ground stake at the outbuilding, or it may require you to run the ground and neutral conductors back to the main service entrance. If you can drive in a new ground stake, you will save the cost of 180 feet of ground and neutral wire. Just remember to put the cable clamp on the stake before you drive it in - it takes a lot of pounding to drive in a 10 foot stake and the end will mushroom over a lot. It's easier to put the clamp on first than to cut/file/grind the end of the stake after it's driven in - TRUST ME! If you live in a rocky area, you may be permitted to bury a ground plate rather than drive in a ground stake.

180 feet is a long conduit pull, and that much cable is heavier than you'd believe. If you pull, make sure you get some pulling lube - it's basically a soap that is safe to use on the wire/cable insulation. Ask for pulling lube, "Yellow 77" or just "yellow snot" at an electrical supply house and they'll know what you're after. Alternatively, you can lay out the bare wires or cable in the trench and slide the lengths of conduit down the wires. You'll just need to be careful to keep dirt out so you get good glue action at the joints.

The depth you bury the cable/conduit/pipe depends on whether or not you install any mechanical protection for the cable, and whether the ground above is likely to see any vehicle traffic that will cause the ground to shift - especially the backfill. The "mechanical protection" can take various forms: galvanized or aluminum conduit, PVC conduit, PVC pipe, a layer of concrete spaced above the cable/conduit (2" thick is customary, I think & it's generally colored red by the concrete company). It's been several years since I looked into the code requirements where I live, but it used to be 12" clear fill above protected conduits and 24" above unprotected cables if no vehicle traffic was likely. The trench bottom and fill should not have sharp stones, sharp gravel or hard lumps that may damage cables.

Conduit or PVC pipe will stop a shovel from slicing the cable, but won't offer any warning to a backhoe operator. Concrete will alert a backhoe operator, but won't stop the bucket from ripping out the cable. An electrical supply house will have red flagging tape that you can bury shallow above the cable. It's the same stuff as caution tape, but it's red and is printed with warnings about electrical cables buried below. The idea is to warn the people digging before it's too late.

If you have a 180 foot run, I am assuming you will enlist the services of a backhoe or trencher, so depth shouldn't be too big of an issue. If you are planning on digging by hand you have my sympathies.

If I was doing this job for myself, I would lay the wiring in PVC conduit in the bottom of the trench. I would then lay in a run of 3" or 4" PVC pipe or black corrugated tubing properly sloped for drainage or condensation. I would fish at least one length of poly twine through the pipe as I laid it in the trench. This second pipe would be used for running phone cable, TV cable, ethernet, black polyethylene water piping, etc. to the shop. Whenever the twine is used to pull in something new, a new piece of twine must also get pulled into the pipe. I would trench at least 30" deep to protect against shovels, stakes, irrigation trenching, etc. and to provide freeze protection for water piping (whether installed now or later). I would bury the red flagging tape about 12" below the surface - it's cheap insurance.

There may also be code requirements on how the conduit is arranged at the buildings on both ends. The general concerns are free drainage of water from the conduit, preventing the wires/cable from being strained due to frost heave in the ground and mechanical protection of exposed cables where they enter the building above ground level.

The best source of advice you can get for what the code requirements are for your area is your local electrical inspector. Although you may have to pay for their advice time, they are the ones who will be giving you the approval on your completed work, so it's best to do it the way they want it than to have to change things later. There are likely details in the electrical code that they do not enforce, which can save you some money. You will also need to find out what the inspection schedule is, so you don't backfill something before the inspector gets a chance to look at it. Plus you will stroke their ego when you ask how they would like to see it done, which can only help you out later.

Good luck with your project.

Cheers,
Eric

 

Thanks for all of your help eehoepp. All this info sure will help and lead me to the correct supplies I need. The inspectors in my particular area aren't. We have no building inspectors or any other inspectors. The only inspection that was done when we built the home some 17 years ago was the septic system. It hasn't changed and I doubt it will in the near future. Kind of scary isn't it. It's not to say we get away with anything, but these things should be checked to prevent future problems. Thanks again for your response.

 

In ( N.C.) instead of driving a 10 foot ground rod with a 10 pound hammer,
stand the ground rod up beside a 8ft ladder if you can borrow a big
hammer drill (the kind you drill half inch holes in concrete) with no bit in it slip
the end of hammer drill over the ground rod it will drive that ground rod very fast!.

P.S. It doesnt mushroom the end , thought this could make life easy!

 

mick460 i would still run a ground rod and a connecting wire to the main panel. With that much space between the two panels the ground potential could change greatly. By running the extra ground rod you will equallize the ground potential. Most states use the NEC for guidelines but they and local goverments can change from it. If you follow the NEC you should be safe. You should be able to get one from a library.

Pro-street/StateTk that is a really great idea, i'll have to try that the next time.

 

Thanks for the tips. The folks at Home Depot helped me out with the info from eehoepp. I felt better talking to the guy there now that I had an idea what I needed and how it would be used. Thanks all. We will have power in the garage to keep the refreshments cool and the tools working.