I want to add a 125 amp service to a free standing workshop about 70 feet from my home. There is already some 15 amp circuits to the building but I blow those on a regular basis.

I've measured and found I'll need 95 to 100 feet of aluminum cable to go from an existing 200 service panel in my garage. I'm going to run the cable underground in 2" PVC conduit. I'm thinking that I can connect to the existing panel in the garage with a 125 amp breaker, then through the walls to conduit to the new panel in the workshop. I also plan to put in a grounding rod in the workshop side near that service panel.

I checked with the local hardware stores and the clerks ask questions I can't answer.
I gather I need 2/0 wire but would that be 3 wires and a ground or 4 wires and a ground.? What size & gauge is the ground for the grounding rod?

I guess I want to know what to say when I ask them to order the parts.

I ask for "100 feet of alum cable" , They ask "2/0-2/0-2/0-1 SER or what?"
I ask for "a grounding rod" , They ask "What size Grounding Rod?"
I ask for " 10 Ft of Ground Wire for the Rod", They ask "What Gauge?"

I respond to each, "I don't know, do you?" and they grin and say " you have to tell me"
I feel stupid and walk out with nothing but a red face.

Okay,

I'll take a stab at this one. But please consider calling an electrician as I believe you will have to be working on a the live Service wires.

I believe that you will have to at lugs to the incoming service posts within the panel, therefore you will probably have to pull the meter unless there is a service cut off between the residence and the meter. If there is not, you will have to pull the meter and I believe you must be a licensed electrician.

As to the cable spec. 2/0 (pronounced two aught) aluminum is fine This. Means that each 2/0 conductor can carry 125 amps. I believe that the only color the insulation comes in is black, so you will have to re-identify one cable as Neutral, use white tape on the insulation to designate the conductor as Neutral. The ground conductor should be no less that #4 AWG.

Run your cables to the shed and then use #4 AWG for your EGS. You must use a 125 Amp breaker in the shed inside a power panel.

Please wait for some of our resident electricians to verify the information I've just given you. Also, please consider that you are working with voltages and current that is quite capable of killing or maiming you.