My house has a 200 amp service. I have a new 30x40 building in back. I have a 100amp Square D load center for the building. I want to run a line from the house to power the building. Big items include, air comp 30 amps, welder 50 amps, other raidial arm saw, ban saw pipe saw 15-20 amps? It is 125 feet between boxes. My question is what size uf wire to run? I have heard anywhere from #4-3 with ground, #2-3 with ground or the same but ground at the box on the building??
thanks for any help

Can you bury a conduit and run #3's for your circuit, and #6 for the ground? You will need 4 wires, a separate grounding rod and a sub-panel with separate neutral and ground buss's... and the neutral must not be bonded to ground. Confused yet?

Does your four wires include the ground? Are you suggesting the following? From the 200 amp panel, run two hots and 1 neutral #4 wires to the shop and then at the shop run a ground #6 ground wire to a ground bar next to the buiding. Do not use the

Sorry I am learning how to use this web site. I think I responded to your answer as a disagree but I do not mean to disagree, just my inability with the computer at work...
What I was responding to your answer was if I need to run 4 wires from the house to the building (2 hot, 1 neutral, 1 grnd) or 3 wires from the house (2 hot, 1 neutral) and then ground the 100 amp box at the buiding.
And if I do not use the screw with the saure d box to bond the neutral bar to the box like is suggested by the paper work with the box and where to connect the ground to the 100 amp box?

You will need 4 wires from your main panel. You will also need to install a ground rod at the building and install a ground wire there. The neutral "bar" will be isolated from the box by insulators and the elimination of the bonding screw or strap. The ground "bar" will be bonded to the box, and the ground wire which will terminate at the new ground rod at the building. Only in the main panel are the neutral and ground bonded. (per NEC) And #4's isn't quite enough for a 100 amp sub panel at that distance.

Ok I think I have just a couple more questions. The 4 wires from the house are, 2 hot, 1 neutral, 1 ground. Does the ground wire from the house (#4 wire) and the ground wire from the ground rod at the building (#6 wire bare) both connect to the ground bar or strap on the new subpanel box?
Seoond, is running UF #4 wire direct buried less expensive and easier to do than running the #4 stranded in conduit for the 125 feet?

It is very important that we know both the anticpated connected load and the anticpated calculated load to properly size the feeder to the sub panel. If this is just a single branch circuit or a multiwire branch circuit it is not a problem. If you are going to a subpanel, it is a problem.

If you are planning for a full 100 amps you should be aware that the most you can load the breaker is to 80% of capacity or 80 amps. That should be more than adequate, in fact 60 amp would probably be fine.

The Grounding Electrode (conductor from the Ground Rod to the Ground Bus) can be a bare copper #6 AWG.

However, if the resistance between the ground rod and earth is greater than or equal to 25 Ohms, then a second ground rod must be installed and bonded to the first rod with #6 AWG copper conductor at a distance of no less than 6 ft. from the first ground rod.

This rod or the combination of the two will discharge any transient electrical hits (lightning for example). The ground that you bring over from the main panel will also connect to the ground bus at the sub panel.

Personally if I were doing this installation, I would trench to the proper depth and then use conduit and pull individual conductors.

My reasoning for using conduit is:

1) The conduit will protect the conductors from damage.

2) If I ever have to pull a new conductor because it failed, it is much easier to pull a conductor through conduit that to have to retrench and then repair or repace the conductor followed by backfilling the trech.

3) I would size the conduit for 1 size larger then I need. Easire pull and if I ever have to increase the conductor size I will not have to retrench to replace the conduit.

To amswer the ground wire question, yes... both connect to the ground bar or strap. Regarding the expense... you will have to do some research to find out the costs. As donf pointed out conduit has many advantages over direct burial, and unless I was feeding a single circuit (receptacle or yard light) I would never use the direct bury method. Any nick in the insulation or wire will create problems and you will dig up 125 of cable to find the problem. And again... I would use a #3 or even #2 wire at that distance. The voltage drop may become an issue at 125'

Keep asking the questions and we will keep answering them. Good luck!

Ok. It is 4 #3 copper THHN stranded wire to the building. First, not bonding the neutral bar at the 100 amp load center, second, attaching a separate ground bar to the box and running the (1) ground wire from the house, (2)one from a #6 wire attached to a ground rod in the earth next to the new building or possibly two since we do not know the ohms of the ground wire (per donf), and (3)the ground wires from the circuits in the building (which we are using #12 stranded in 1/2 inch emt conduit. The #3 wires from the house to the building will be in one and a half inch grey pvc conduit with an LB at each wall entering said buildings. With the help of a retired electrician next door I think we will be able to do this. Any other words of advice or cautions? Thanks for everyone's help.

Don't forget your Alarm, Intercomm, Network, Cable/Satellite, Phone or Video Conduits, It doesn't get easier or cheaper than now(a trench).

Sounds like you have a good understanding of what you want to do. As tempting as it may be, don't run any low voltage wires through that conduit (phone, TV, intercom... etc) with the service wires. But you may want to put in a smaller conduit for those items, in the trench with the service pipe. Again, good luck.