[hardwiredman]

First I want to thank those who have given such good advice on this forum.

I have a 900sqft detached garage. I am installing a 125amp sub-panel. I plan to install a 100amp main in this panel.

The house is 100ft away. My plan is to run 2" PVC sched 80 conduit from the house to the garage. I will burry it 24" deep. I will install 2 ground rods 6 ft. apart near the garage main, and run #4 copper from the rods to the ground bar in the new panel. The neutral and ground in the sub-panel will be isolated (no bonding screw)

I will run 4 wires from the house. Three #2 copper, and one #6 copper. The breaker in the house will be 100 amp as well.

My question is, do I have the wire size correct? If I were to run aluminum to save money, what size aluminum would be appropriate. Also what type of wire do I use. I've heard several different acronyms but not sure which one is correct for this underground application.

Also, if you can think of anything I should change... thanks!

[tkrussell]

#4 is allowed for a residential 100 amp feeder. The #2 you propose is fine as it helps greatly with voltage drop. With an 80 amp load, the Vd will be 2.43%.

To do voltage drop calculations, the amp load must be used. Assuming a 50 amp load, #2 aluminum will give a 2.45% Vd, the max recommended is 3% for a feeder.

If the load will be any greater, then you may want to consider using #1 Al.

Assuming the conduit will be a complete system, with fittings, from point to point, then you may use aluminum wire with XHHW insulation.

You can find copper wire in THHN/THWN, only for conduit, and XHHW, which also is only for conduit.

The acronyms you may have heard are USE and URD. USE is an insulation on individual wires that can be direct buried in earth. URD is Underground Residential Distribution, can be three or four wires twisted into a cable assembly, can be direct buried in earth.

[hardwiredman]

[QUOTE=tkrussell]#4 is allowed for a residential 100 amp feeder. The #2 you propose is fine as it helps greatly with voltage drop. With an 80 amp load, the Vd will be 2.43%.

The load will likely be closer to 80 than 50. I haven't done a load calculation yet, but some of the constants are big, like the water heater for radiant heat, dust collection, air compressor, air filtration system, plus the saws, and welder. So it sounds like I should use either #2 copper or #1 AL. I will do a load calculation first, and then a Vd.

You can find copper wire in THHN/THWN, only for conduit, and XHHW, which also is only for conduit.

What are the differences between THHN and XHHW? Any advantage to using one over the other?

[tkrussell]

Most THHN is dual rated THWN. THHN is for dry locations, THWN is allowed in wet locations, ie: an underground conduit is considered as a wet location.

XHHW is wet location rated. It actually is a fraction smaller in diameter than THHN/THWN.

While THHN is used the most, I find that THHN has a layer of material that will wrinkle and bind at times. XHHW is very smooth and slick.

Either thou can be used in a buried conduit. Compare prices if available.

[hardwiredman]

Thanks for the great information!