I'm hooking a 100A service panel to my meter pan. There is the 100A disconnect panel on the right side that feeds the little house, I'm installing another panel on the left side to service my barn, an approximate 150' run. Feed to the barn is 3 conductor direct burial cable. Two of them look like 00 AWG, guessing, but the wire is a good solid 1/2 inch across. The 100A breaker terminal will fit, looks like, #4 at best. Is there is "reducing" type terminal that will allow me to tight fit the 00 AWG wire and then insert into the breaker?
Short answer - you don't. I'll echo what others have said - don't guess - find out what you have. Size and whether copper or Al. Find out if you can make splices in your panel per local code and (if allowed) splice down to the correct maximum size that the breaker will accept. Tester says it correctly - 4 wires needed -2 hots, a neutral and a ground, but the ground wire may be a size smaller than the required "hots". ALSO - once finished, will you have two separate services both feeding off the meter pan? It sorta reads that way. I do not believe you're allowed to "Piggy back" wires on the meter pan and in my locale there must be one main to turn off everything. So- one main panel to feed two sub-panels. Draw and post a diagram if you can take a picture of it. Indicate hots in red and black, neutral in white and ground in green. This is the level of detail you need to understand it, if you do this as a DIY. About a year ago, I took on a sub-panel and ran flex metal cable throughout a new shop. I researched the heck out of. It was not that hard to learn and obey all safety rules, whether or not you have code enforcement.
Good job! Finding out WHAT TO DO is first thing. Then finding out the correct way, e.g., how much wire to strip for placing under terminals, correct torque on lugs, etc. Do not forget about that extra cable needed to set up sub-panels and the ground rod (case ground) required at terminal ends near the sub-panels. Three wires will work but usually not code-compliant today. All sub-panels shall ground back to main panel and neutral and ground are kept separate (no tie in sub-panel). Any need to pass future inspection in this install? There's lots of things that will work but are not the best way in the event of a failure within the system. That's why the code changes as better info or devices become available. One method that folks have used is find a "friendly electrician" who will look at your plan, then look at your finished job for a minimal fee, with you doing the work. Don't expect anything in writing. There's lots of info on the internet about wiring a sub-panel.