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jim c 02-27-2007 08:29 PM

wire to a workshop
ok, i have done alot of wiring both in homes and marine but this is a new one for me. i am putting up a workshop behind my house. i want to run the panel in the shop as a subpanel off the main panel in the house. here is the problem. i need minimum 100 - 125 amp service in the shop, i need to burry the wire and the run from panel to panel is 480ft. not exactly what kind of wire i need to be using for this. the elec. co was going to do underground to my house last year when it was built, a 550ft run. thats a 200 amp service in the house and they use some type of direct burial cable. not to sure how to approch this situation. any recommendations on how, wire type, etc. after searching the net i found something called rhw-2. i have no idea if this is what i need and what gauge but it is direct burial.

jwhite 02-28-2007 05:15 AM

You could use 3/0 copper thwn in at least a 2 inch conduit or you could get some direct burry cable.

I used 100 amps at 480 feet to calculate VD. I worked till I got 3 percent to the panel, this should keep it under 5 percent to the load.

Enter the numbers in the form on this web site for your answer. the voltage phase is 240 single phase, and the total circuit lenght is both ways, so 1/2 that is the one way measurement.

The wire size is large because the distance you are going will have alot of voltage drop.

You also need to be sure that your existing service can handle another 100 amps. A load calc should be done on your home and new shop to be sure.

Stubbie 02-28-2007 12:00 PM

One other thing you should check is the distance from the transformer. You probably have your own serving just your home. You dont say where they set the transformer. If it is an underground service lateral, it is probably pad mounted close to the house but if it is 550 feet from the house then another 480 feet to the shop thats a ways (1030') from the transformer. This can be a big factor in the voltage drop.

Assuming all works out most panels of 100 or 125 amps will most likely not accept 3/0 copper at the lugs yet alone 250 kcmil aluminum. So you may have to splice in smaller wire... for instance if the lugs are rated for 2/0 you will have to splice the 3/0 copper to a short length of 2/0 copper. This is going to be a rather expensive venture at the cost of wire these days.


jim c 02-28-2007 09:52 PM

the main line back to the house is arial and the transformer is on the pole right next to it. the drop going to the house comes right from the transformer. yeah your probably right about the wire not fitting in the small subpanel. ok well i will probably go with the direct burial wire so here are a couple ideas, let me know if you see a problem with any of this. one, i am obviously not going to get heavy cable like that off a breaker in the main panel either so could i attach the shop feed to the main lugs just before the main breaker in the house panel and get another main panel for the shop. basically running 2 main panels. the lugs in the house panel are large enough for 2 wires. second question, as for the wire i would run a 3 conductor wire, 2 legs and neutral then i would supply the shop panel's ground through a ground rod...correct. third, if i wired this like above with 2 main panels would i keep the neutral and ground seperate in the shop panel like if i was installing a sub. i should note that even though the house panel is 200 amp and the shop is to be 100 - 125. i will never draw anything close to that total. the shop will never continously draw 125 amps. it might peak here and there when electric motors are starting like a compressor thats all. continous draw on occasion might reach 50-70amp. house is probably way less than that.

jwhite 03-01-2007 04:41 AM

I have heard of doing this at the meter before, but you need a meter with double lugs on the bottom. You would then treat the shop panel as a main panel (grounds and neutrals bonded etc.) and lable equipment accordingly.

Remember that just because the lugs are "big enough" does not mean that you can put two wires under them The lugs must be rated for the number and size wire you are putting under them.

jim c 03-01-2007 06:39 AM

ok, thanks for your help. most of my expierence is in dc systems, electronics, and some light residential stuff. when it comes to this larger main wiring i am a little unsure about the proper way and want to make sure its right. i guess the big problem here is getting this big wire connected properly. my meter socket does not have double lugs. if you were doing this, is this the way you would go about making the connection or you think i might go about it differently. does someone possible make some type of junction box i can install inline between the meter and panel.

now that i look at it i only have 3 conductor comming into the house so i assume 3 conductor back to the shop is fine. having it setup with the third conductor being neutral and having a ground rod near the shop panel and running to the ground bus.

Stubbie 03-01-2007 05:06 PM

Hi Jiim

Jw is correct you can install what is called a 320 meter base with double lugs. And in most cases inspectors will allow this in my experience by using the unlimited tap rule of NEC 240.92(c) 1-5 and NEC 240.21 (B)(5). The key here is the tap must be made outside the structure of the main dwelling in your case. So that pretty much leaves only the load side of the meter. I have seen split bolts used on the load side service entrance conductors but that to me just isnt the way to go.

You also have a another choice... you can install sub-feed lugs in your main panel. Here is a link to the installation of sub-feed lugs in a square d Homeline panel.

Remeber when you install these the feeder to the shop sub-panel must be rated equal in amps to the main disconnect in the house main panel. This is because the main panel disconnect (overcurrent device) is protecting the sub panel feeder at 200 amps. The sub-feed lugs are not an overcurrent device merely a tap device. You will also need a neutral lug kit to install on the neutral bar of the main panel to accomodate the neutral going to the shop. You may also need to install a ground lug it will just depend. This is generally done by installing a ground bar in the house panel (there are predrilled holes for this) in the panel back and terminating the ground wire to the shop (if required) to that ground bar or you can terminate it to the neutral bar since ground and neutral are bonded there. Most will take up to a #4 awg wire. In your case if your main breaker or disconnect is 200 amps at the house panel then you will need a minimum #6 awg insulated copper ground wire or a #4 awg aluminum ground wire.
Now the determination of whether you need a 3 wire feed or 4 wire feed depends on two things....what local code requires for a new installation and if any metalic paths other than the sub-panel feeder connect between the house structure and the shop.... ig...water lines , phone/ data line... etc. If any metallic paths will exist in addition to the feeder then a 4 wire feeder is required. then of course if local code requires a 4 wire feeder. If neither then you may run a three wire feeder.
So if a 3 wire (H-H-N) is run then the neutral and ground remain bonded in the sub-panel. This is done commonly with a green bonding screw that fastens through the neutral bar and threads into the metal can of the sub-panel bonding the metal. It will vary from manufacturer as to what they use to bond the metal of the enclosure to the grounded conductor (neutral).
If 4 wires (H-H-N-Grd) are required then the neutral and ground are seperated electrically in the sub-panel. You will generally have to purchase a seperate ground bar for the sub-panel and install it to the swaged pre-drilled holes in the panel back. You will not install the bonding means such as the green screw or other method. the feeder ground wire and all branch circuit ground wires will connect to the ground bar and all white neutrals will atttach to the neutral bar that came with the sub-panel. In some cases the panel will come with the extra ground bar. Just make sure the bonding means isn't installed.

Here is a link to some information on what you are the first part then scroll thru the pictorial examples till you see the installations for a 240 volt underground supply for 3 and 4 wire feeders. Hope this helps... there is a lot to know with your project as you can see.


As for the type of wire if going underground as a lateral feed in conduit then thwn rated copper or al as Jw mentioned. You can also buy USE-2 which is direct bury and can enter buildings. This use-2 is also rated rhw-2 and rhh-2 in most cases. Remember if you opt for aluminum it will be 2 sizes larger than copper to provide the same amps generally and this will also effect your conduit size that is required to stay at 40% fill. In your case it gets bigger for both copper and al because we are having to keep voltage drop limited at that distance.

Continue to ask your questions I'm sure there will be plenty.


jim c 03-01-2007 09:13 PM

holy crap stubbie, that perfect. every question answered in one article....i think. i thank both you guys for giving me an education on this stuff. great appreciated. the sub feed lugs look like the way to go to me. my main panel box is GE so i'll have to find out if they make such an animal. i'd rather not get involved in changing my meter base out. with my luck i'm sure the different one wouldn't be the same size and that would mess with the siding on the house. i will have nothing metal connecting house to the shop so if i read it right i will only need 3 conductor and a ground rod at the shop.

looked them up and found the ge parts i need for my panel with the part #'s and everything. the sub feed lugs and the extra ground lug...nice!!! one question though. i did a search for wire. most of what i found the 2 phased conductors are slightly larger than the neutral. i know the ground can be smaller but is that pretty much normal or do i need to keep the gauges the same. most of what i found is aluminum 4/0 4/0 2/0 ud wire use-2. sound right ??

Stubbie 03-02-2007 01:56 AM

Jim....glad you feel you got your moneys worth:thumbsup:

Yep GE makes them just a little different design. It is quite normal for the neutral to be downsized along with the equipment ground wire. Use-2 is a good choice. The wire you mention is fine if local code allows a 3 wire feed. An aluminum 4 wire will be 4/0 4/0 2/0 4.

Most sub-feed lugs for a 200 amp panel will take up to 250 kcmil (next size larger than 4/0.

Here is my favorite link to calculate the different things needed for a job like yours......

Your trench for direct bury needs to be 24 inches deep. 18 inches if sch. 40 pvc. conduit.

4/0 aluminum for the hots in reality will probably be just fine. The thing to remember is the more amps ( actual) needed the greater the voltage drop. When I calculate it at 90 amps at 480 feet one way I get 3% drop which is the industry standard for a feeder. Play around with the vd calculator on the site given and you will get a feel for whats going on in that regard.

When you exit the ground with your direct bury put the wires in a sleeve of conduit to protect them going to the sub-panel.

Be sure you need 100 amps at the shop (thats a lot of power). If 70 or 80 amps would do you could run a lot smaller wire and use a double pole breaker in the house panel. What are you going to have in the shop that is a big amp draw?

yes... you will need a ground rod.... 5/8" Dia by 8' L . You might need two.


jwhite 03-02-2007 04:43 AM


Originally Posted by jim c (Post 35282)
one question though. i did a search for wire. most of what i found the 2 phased conductors are slightly larger than the neutral. i know the ground can be smaller but is that pretty much normal or do i need to keep the gauges the same.

The slightly smaller neutral will be fine, most of the time. When you breaker out your panel, some of the loads will be 240v without a neutral, and on others a neutral will become shared when two circuits on opposite phases are drawing current at the same time. So, the neutral should carry less current than the two hots.

jim c 03-02-2007 06:25 AM

i have a 7.5 hp compressor that on startup draws about 56 amp then about 38 running. i have a spray booth with a 3hp motor and a crap load of flourescent fixtures plus the 12 or 14 ho fixtures in the shop and the occasional tool plugged into the receptacles. overall if i managed to turn on every piece of equiptment in the shop plus plugged in a bunch of things i could overload a 100-125 anp service but i'll never be able to do that. i figure when i a spraying i would be drawing the most with the booth, all the lights and compressor. thanks guys for all the help i think i'm pretty much on track now.

Stubbie 03-02-2007 03:41 PM

Hey Jim.... just wanted to point what JW mentioned about the tap at the meter.... the advantage to this is that you do not subject the loads at the workshop on the house panel. Just in case that makes a difference to you. I figure you will know whats best. Good luck on your project... sometime down the road come back and give us an update.

One other thing is the wire is large and is not friendly to work with. The square d sub feed lugs are very secure when installed. The ge lugs not as secure but pretty good. Bending this wire can be a pain if you have to get a 90 degree bend on it before connecting to the lugs. So try to get a knockout in the side of the main panel that lines up with the sub lugs. This makes things a lot easier because it is a straight shot to the lugs. If the panel is recessed between studs you can still do this, carefully use a hole saw and make a hole in the stud where it will expose the knockout. Then connect your conduit sleeve and bring the wires in. This may require some drywall removal but thats really not a very big deal to repair. If you need to bend the stuff i find the hole in the handle of a cresent wrench pretty handy to do that.


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