Sub-panel questins

Hello, I'm getting ready to install a sub-panel next to the main panel. Probably be using #6 wire to feed into it. Couple of questions:
Do I use 50a or 60a breaker feeding the sub?
I wanted to connect both boxes with 3/4" pipe. Can I use Pvc or does it need to be EMT?
Can I fit 4- #6 wires in 3/4" pipe?
Any recommendations for subs ? ( square D, GE, Eaton, Murray). The main panel is a Westinghouse.
Thanks
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In typed:

I don't know the answers to any of the above questions, but I do remember (almost) that there is something that needs to be kept in mind when adding a subpanel. If I remember correctly, I think it is that the neutral bar and the ground bar in the subpanel have to be insulated/separated from each other -- meaning that the ground and neutral cannot be combined in the subpanel the way that they can in a main panel. I forget the exact details.
You probably already know about that, but I thought that I would mention it just in case.
And, to be honest, I never really completely understood the explanation of why that is even though a number of people here tried to explain it. But, I do remember that it was something that I had to keep in mind with subpanels.
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On Mon, 29 Sep 2014 09:46:15 -0700 (PDT), Mikepier

You need 1" for 3 or 4 #6s. Gray PVC is fine and easier to work with. You can use a #8 for the ground tho.
If you are using THHN (individual conductors) you can use a 60a if you want. It sounds like you already know about the separate ground bus.
For the other poster, the reason you do this is because you do not want them on one bus is you do not want circuit current on your <safety> grounding conductor after you leave the connection to the grounding electrode. Any voltage imposed on that wire (voltage drop) would show up on the case of any grounded equipment in reference to true ground, like a water pipe or a concrete floor.
My only recommendation is you match your main panel so you only need to have one brand of breakers hanging around. Your Seimens/Murray is a good product. If you had a Zinsco or something I would have another opinion.
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