Underground Electric Questions

[slughammer]

I'm prepping my site for a shop and was planning on running electric out to it. The shop is a pre fabbed shed that is on pressure treated 4x4 runners. I'm cutting out the sod and then I'll bring in probably 20tons of stone. Before I bring in the stone I need to trench from the shed site to the house for the electricity.

Does anyone have any experience what the NEC is for burried wire?

Any advice on the depth and protecting the wire before back filling?

I'm planning on a 220v line from the box in the house, what size breaker can I put in the house box to feed a smaller box in the shop? I need power for welding etc.

I have experience with residential electricity, but I've never run a line outside.

[crazy mark]

Believe it's 18" and best to protect it in PVC conduit. Depends on what size wiring. Don't have my 2005 NEC in front of me right now but if you run a 100 amp panel you will need at least #2 for ungrounded conductors and #3 for grounded conductor or larger wire and the grounding conductor should be #4 or you can drive another ground rod at your shop. They do make direct burial wire but I've always used conduit.

[carpintx]

A few years ago I ran a 220 volt line from a junction box that my electric dryer is attached to. I dug a trench as deep as possible (lots of rocks), put another breaker box on the outside shop wall, and called some electricans to install the wiring inside the shop. Now I run a 220 volt AC in summer and a small 220 volt heater in cold weather. I have not tried running any other 220 volt items at this point.

The electricans also provided for multiple 110 volt outlets throughout the shop when they wired the shop.

The wiring used in the underground trench was exactly the same feed wiring that the Electric Company ran from their pole to my meter box - no difference. I was advised by the electricans that it would last even longer underground than it does above ground.

Haven't had any problems at all. I usually make it a point that the electric dryer in the house isn't being used at the same time as any major machinery in the shop. It's just an option I decided on to avoid tripping any circuit breaker etc.

[crazy mark]

those figures are for alum wires. If using copper you can go one size smaller and be safe. I prefer copper wiring as it takes less maint like lug tightening every so often. Mark

[felix]

http://www.powerstream.com/Wire_Size.htm

[drinks]

Depends on distance as well, voltage drop can be a real problem if the run is very long, have been retired several years, but there is a formula to figure voltage drop, something like K x I x 2L x CM =VD, where K is the constant, different values for aluminum and copper, I is the current, 2L is twice the distance, CM is the circular mills of the wire and VD is the amount of voltage drop.
You need at least a 60A service, but bigger is always better.
UL , direct burial cable is available, required to be 18" deep. but I always run mine in PVC. I have been mad at the NEC since '68 when they came up with the sardine can wire fill tables and always use at least 2 sizes bigger conduit than the tables call for.
It is much cheaper to get it big enough the first time than to try to redo it later.
Also, a receptacle every 12 feet as required by code, is just about 1/3 enough and they should be at least 4' above the floor in any kind of shop.

[Edward429451]

Our new code says receptacles every 6 ft, not sure if thats a regional thing or the IEC right now. You might want to think about if you'll ever possibly add other loads to the shop and maybe go ahead and upsize the feed wiring now rather than later.

[StarMetal]

Crazy Mark,

I can show you a trick for tightening aluminum wire in service entrance applications where they won't come loose. Learning it from my Dad and he learned it from the Electric Co guy where he lived in SW Pa. Works great. I used to install service entrances and never had a problem with any nor a call back.

Joe

[versifier]

Run it in PVC conduit and put it at least 24" down. Put the plastic dig safe tape down about 12" to warn anyone who tries to dig there later. Use 3" conduit as it's a lot easier to fish through, especially if you've never done it before. Aluminum cable is much cheaper, you need a little tube of No-Corrode antioxidant for the connections. Put the proper torque on the lug screws and you won't have to worry about them coming loose - I installed several thousand service entrances over the years and I never had to come back and tighten one. You need to know how much the welder draws, like 30, 40, or 50 amps, and what else you want to be running, now and possibly in the future. Then you can figure out what size cable you need. If the run is less than 100 feet, you really don't have to worry about voltage drop, but if more, then you just need bigger cable.
So...
How big (and how old) is the service in the house, 60, 100, 150, 200amps?
Figure total possible load: welder + lights + 120v outlets = how many amps?
How long is the run between panel and remote?
Give us some numbers and we'll tell you exactly what you need to run.

[slughammer]

Give us some numbers and we'll tell you exactly what you need to run.

Thanks, I'll get some exact numbers and post back tomorrow.

[StarMetal]

Versifier,

Torque isn't the answer, it's vibration that makes aluminum wire come loose. Here's what you do fellow electricians out there. You put your anti-corrosion grease on the wire and connector. Then you tighten your connection. they you grap the wire you just tightened and you wiggle or try to shake it. You will find by doing that you will be able to turn the connector bolt more. Then again...tighten more...keep on till you don't get anymore bolt movement. A Western Pennsylvania Power Company service man showed my Dad that and Dad passed it along to me. He said vibration along with expansion and contraction makes a connection loose esp with aluminum wire. I've seen alot of burned out connecters in meter bases. Like I've said my Dan nor I have never had a problem with aluminum wire on service entrances we installed. I believe that service mans advice to be good.

Joe

[felix]

I think I would run the largest copper cable you can find locally. Then figure the maximum current that can be had, and fuse accordingly. You always need more, right? I thought a full house 357 was enough at one time. ... felix

[454PB]

Three years ago I built and wired a shop. Since I have an air compressor, several welders, a lathe and milling machine, and heat my reloading room electrically, I installed a 200 amp service. It costs a little more, but is way cheaper that trying to change things a few years (and wall finishing) latter. I installed GFCI receptacles throughout, and have a receptacle every 6 feet in the 24' by 40' shop. I also installed three 50amp 220 volt receptacles and three 30 amp 220 volt. It was overkill, but again easier to do when building than latter. I might build a monster electric lead smelter someday.

State electrical codes may not match NEC codes, so check both before starting. The pad mount transformer that serves my house and shop is only about 15' from the shop, and the power company supplied the service wire. I had to supply the PVC conduit for the underground service (out of the ditch and into the meter base), and the entrance wire from the meter to the load center. Our state code says the service has to be 18" deep. I used 3/0 copper. Montana code requires two ground rods spaced no more than 10' apart, and #4 bare copper from each to the ground bus.

I spent my life in power plants and don't like aluminum wire. It is cheaper, but requires a larger diameter and conduit size. and I've seen many fires caused by loose and corroded connections.

[Duckiller]

Check with local electric co. and building officials for burial depth. So. Cal. requires 4 feet burial. Install a PVC pipe or two. Easier to replace wiring. To install a pull rope, tie a rag big enough to seal pipe to rope and blow it through the pipe with a compressor.Whole lot easier than trying to install rope while laying pipe. If where you live has joined the latter part of the 20th century any excavato( person who digs with power equipment) can find electrical lines with a metal detector. Plastic gas lines need locator tape. Duckiller

[crazy mark]

Joe,
I work for a major medical center and have a limited maintenance electrical and limited energy electrical licenses. We hate alum wire and replace it if it gets in. Had alum wire to 2 elevator disconnects that had a 125' run from the main panel to the elevator disconnects. Them wires jumped in the conduit every time the elevators started up. We had a terrible time pulling out that alum wire as it was work hardened. I know it can be properly tightened in the lugs if the right lugs and no=ox is used. Copper is so much easier to work with however.
Slughammer you can always replace your existing panel with a 300 or 400 amp panel. You will also have to redo your incoming service. Or just add another service and pay another meter fee. The NEC and IEC require outlets every 6' now and GFIC's on any wet/outdoor outlets. States/locals can go more strict than the NEC?IEC but not less restrictive. Mark

[StarMetal]

Crazy Mark,

Yeah I know what you mean about that elevator wire. That's some of what I meant about aluminum wire vibrates. I too hate aluminum wire. Remember when it was legal for interior residential wiring? My God, I'm glad they outlawed that.

I was just mentioning the tip about tightening it should someone that's not familiar with electrical work just tighten it down lightly. When you get all the slack out of it like the wiggling and shaking I mentioned there's less of it coming lose, at least for a long time. Too bad copper is so expensive, it's the route to go.

Joe

[woody1]

If you've ever dug up a direct burial wire that was chewed by gophers chasing the leak, you'll put your wire in conduit. Speaking from experience! Regards, Woody

[454PB]

Check with local electric co. and building officials for burial depth. So. Cal. requires 4 feet burial. Install a PVC pipe or two. Easier to replace wiring. To install a pull rope, tie a rag big enough to seal pipe to rope and blow it through the pipe with a compressor.Whole lot easier than trying to install rope while laying pipe. If where you live has joined the latter part of the 20th century any excavato( person who digs with power equipment) can find electrical lines with a metal detector. Plastic gas lines need locator tape. Duckiller
4 feet? what, are they afraid it's going to freeze?

I fully agree on the conduit....it can save a lot of headaches down the road. The power company uses direct burial wire because it's cheap and quick. It's not their problem when they have to tear your yard up to replace it.

[Firebird]

I replaced my overhead 100 Amp electric service with a 200 Amp underground service last Thanksgiving, and the local power company (Xcel) had very specific requirements for depth and size of conduit - which made no sense at all.
They said I had to bury the 4" conduit 24 inches below grade, and had to use 36" radius 90 degree bends at either end, and have 6" of straight 4" conduit to slip fit the conduit to the new meter box at my house. Well 36" radius bends leave 12" of curved conduit sticking up above grade when the main line is only 24" below grade, and this caused the guys that pulled the new wire and hooked everything up to the meter problems. But they agreed that what I had done in putting in the conduit followed the instuctions I was given. I had double-checked with Xcel since the instructions didn't make sense, asking if they really wanted the conduit 42" deep, or if the bends should be 36" diameter (18" radius) so that the end of the 90 degree elbow would be 6" below grade and need that piece of straight conduit to get to ground level. But everyone at Xcel said no, put it only 24" down in the ground and use 36" radius bends to make the wire easier to pull. But like I said earlier, this caused problems with making the slip joint in the conduit that hooks to the meter box on the side of my house.

[Duckiller]

4 feet avoids Firebird's problems and gets it below most idiots with Ditch Witchs. There are lots of not too smart people in the Peoples Repubik.

[slughammer]

The main breaker in the house is 200 amp. The house is 13 years old.
The run from the breaker box to the sub box in the shed will be 90 feet.

The biggest load in the house is the backup forced hot air on the furnace with a 90 amp breaker (but that is backup only). The heatpump is IIRC 30 amp. (does this seem right?)

I don't know what the largest size breaker is that I can hook into my box, but I do know they go up to 90.
What I'm thinking is to get another 90 amp breaker and run my shed line on that. The heaviest load I'll ever run is the welder. All of the larger load items in the shop require attention when running; so I doubt I'll be welding and running the mill, lathe and air compressor at the same time.

I talked to the code guy and he said I need to hook it up with a floating neutral and I need a ground rod at the shed. He asked how deep and I said as deep as the rocks will allow me. He said 18" deep and in conduit because I'm running 220v. He won't be coming out to inspect the depth though. He'll be out to inspect the final hook up.

This is a 14 x 28 prefab unit. Keeps me under 400 sq ft. It doesn't require a foundation. If I needed a foundation, it would be a "garage" and I would have to be 25 ft from my property line instead of 10ft. It's not 40 x 40,with 10ft ceilings, but the price is right and the little wood stove I have should keep it nice and warm in the winter.

[slughammer]

OK, the trench is 82ft long. I still need to make a LH turn and dig to the house, but that will be by hand (pick and shovel for Carpetman).

I was at the Lowes and I found that they have 4 gage copper stranded for $.59/ft when you buy 500'. I think I need to run three strands, 2 hot and 1 neutral, with a ground rod at the shed.

The PVC comes in 10' sections; 1" for $3.38, 1.25 for $5.57 and 1.5 for $6.36. I think the 1" may be marginal for the (3) 4gage wires.

Does it look like I'm heading in the right direction?

[DOUBLEJK]

Just did an underground service to our new house and pulled or pushed 2 #3 and 1 #4 copper wires thru 2" conduit and it was a mighty hard pull....especially at them dang 90's...go bigger if you can...I found it was easier to lay the 3 wires taped together about every 4' in the ditch and thread each piece of conduit on to the allready layed out wire than to pull the dang wire thru the conduit...YMMV

[454PB]

You never did say what kind of welder or amperage rating. If it's a 180 amp buzzbox, you should be okay, and wire feeds generally pull less. If your compressor is 220 volt, check the amp rating. Some of those 5 HP 220 volt motors pull a lot on starting. A run of less than 100 feet shouldn't be a problem. I'd go with the 1 1/2" conduit, but 1 1/4" would work. Get some wire pulling soap if you use the 1 1/4", whatever you don't use for this can be used as sizing lube!

Make sure you identify each phase, you could ring them out latter, but it's easier to mark them first. If you're buying a 500' roll, it's obviously all the same color. Code says the neutral should be white. You are going to have to get that ground from the ground rod into the box somehow, are you using a seperate conduit for that at the load?

I hate to say it, but the best way to do this is a sub box mounted next to the 200 amp load center and fed from the service entrance. What you are doing will work.

[slughammer]

Just a normal 220v welder, no heavy equipment repair going on at my house. Right now I only have a 120v Wire Feed, good for most things, but I need to upgrade in power and quality. I would say 40-50 amp circuit should be plenty. I'm more concerned with the quality of welds and I don't do production work, so I may get a tig, depending on cost. The air compressor is IIRC only fused at 20-30 amp, (5hp x 746watts)/240v = 15amp.

Sounds like a reasonable idea to use the ground wire in a conduit also. Makes for a nice clean job.

I found my Residential Electricity book last night, I'll see what it says about this type of service. I guess with the extra box fed from the service entrance I'd be able to run 200 amp out to the shed, but then I'd be getting the electric company involved and perhaps they require me to hire a PRO? (I know they required it when my parents went from overhead to underground). I hate to turn a $500 job into a $1500 job, especially if the $500 job is safe and sensible.

[454PB]

What I was getting at with the welder and air compressor is this: You are doing a welding project, and the air compressor auto starts. Let's say your welder pulls 40 amps, your 5 HP air compressor pulls 40 amps on startup.....doesn't leave much leeway on a 60 to 90 amp breaker. What I meant about the sub box was to mount a small box containing a breaker along side the existing 200 amp house load center, then feed that box with some parallel wire under the existing lugs on the load side of the main breakers. Doing it this way, you would not be adding another big breaker to the existing load center, you would have an entire seperate feed.

Where I live, code says the ground wire has to be in conduit from ground level to the attachment point. In addition, the neutral and ground cannot be bonded (connected together). As long as the work is done to code, you don't have to be licenced......but you can't do work on another person's structure without a master electrician's license.

Again, I'm not trying to confuse or discourage you.....what you are doing will work.

[slughammer]

What I meant about the sub box was to mount a small box containing a breaker along side the existing 200 amp house load center, then feed that box with some parallel wire under the existing lugs on the LOAD side of the main breakers. Doing it this way, you would not be adding another big breaker to the existing load center, you would have an entire seperate feed.
......
Again, I'm not trying to confuse or discourage you.....what you are doing will work.

Did you mean load side or where the service comes into the 200 amp breaker? A pig tail off of the service side of the breaker? Did you do that? Is that to code?

I appreciate all the help I'm getting.

[454PB]

I meant shut off the 200 amp main breaker for your house, loosen the set screws on the load side of those, (these wires usually come in the top of the load center) then slip your new line side wire under the set screws, retighten, and turn the breaker on again. The house would be without power for maybe 5 minutes, and all the preparatory work can done ahead of time. You would then have a seperate sub box for the shop. No confusion about which breaker de-energizes the shop, and you're not using up the room in the house load center. It meets code if the wire is in conduit between the original 200 amp load center main breaker and the new sub box. Depending on your situation, that might be a few inches to a few feet. It depends on how you want to position the new sub box for convenience. You could even put it outside with the proper weather proof installation.

My shop has it's own entire system, meter base and all. The shop and the house are feed by the same pad mount transformer, but by completely different wires. It's kind of funny, I had the same option available that you have. The former owners of my house had a spa in the backyard, so they had installed the very same sub box that I'm describing, and I thought about using it to supply my shop. In the end, I decided that an entire new service would better serve my needs, and it saved me having to dig wire in 100' or so from the house sub box to the shop

[crazy mark]

You can use the "tap rule" and drill and tap the buss bars before the breakers and run to a 100 amp fused or breaker disconnect as long as it is no more than 6' away. Have to run the wire in conduit. If you put a panel at your building you are supposed to put a grounding rod for it and most places require 2 ground rods at least 6' apart if I remember right. The grounded conductor and grounding conductor actually will be bonded as the grounding conductor goes to a lug inside the box and the grounded conductor buss bar is bonded to the box. I wouldn't add wires under a breaker lug unless it was approved for that. I would suggest you read the IEC and the NEC and talk with your local inspector as to what is considered to be "code legal" as the "authority having jurisdiction" has the final say. I've seen to many home owners have to re-do their work several times. Mark