# What size wire do I need for a 100amp underground 300 ft run?

I want to put in a 100amp subpanel in my garage. The run will be underground and 300ft long.

I have a Siemens panel with breakers rated at 60/75 deg C.

What is the proper wire size I need? I was originally thinking I needed to put the wire in conduit, but is there a better way to go with a direct burial wire?

• This answer might be helpful. – Tester101 Jul 28 '14 at 13:56
• Yes, I read that post. That is for 60 ft run, does the 300 ft in my case make a difference? – Ryan Jul 28 '14 at 14:57
• Only if you care about power quality. 300' of #3 uncoated copper @ 100 amperes, will have a voltage drop of 14.7 volts. So if the panel is fully loaded (drawing 100 amperes), and you have 240 volts at the service equipment panel, you'll end up with 225.3 volts at the panel. And that's at the panel. You'll loose even more along the branch circuits. If the panel is only drawing 50% of the max (50 amperes), you'll only loose 7.35 volts. This is why load calculations are important, they allow you to figure out what wiring is required. – Tester101 Jul 28 '14 at 16:40
• Without knowing more about the equipment that will be connected to the panel, it's difficult to give an accurate answer. – Tester101 Jul 28 '14 at 16:41
• I used this link to figure out the capacity of the panel. Is this what you mean about load calculation? The highest single amp draw i have is a welder 240v / 30 amps. Simultaneous high draw tools is 23 amps (table saw + shop vac). Continuous draw amps is 15 (lights). The equation in the link gives me 76 amps and I bumped it up to 100. – Ryan Jul 28 '14 at 16:57