wiring small detached building with 120/240

hello,
this is my first post here.
I have asked some of these questions on http://www.homewiringandmore.com/ and gotten good answers
maybe i can get a few opinions here.
I am wiring a building with 120/240, it is about 125' total wire run. The sub panel I bought to install in the building is rated for 125amp, but the 6awg wires (four) will be supplied with just 50 amps 240v from the main service panel.
I will have two 15amp 120v branches for lights (2) and GFCI plugs (2). I will have one 30amp 120v for an RV plug. In the future i will install a 30 or 40 amp 240v branch for a MIG welder.
Can i use the sub panel, even though it is rated for a higher amperage? I liked the layout and usability of this panel much more than the lower rated (cheaper) panels.
any other suggestions are appreciated,

thanks

 

Yes, you can use a 125-amp subpanel at any amperage up to and including 125.

60 amps sounds like enough for your needs, and 125 feet is not so far as to concern yourself very much with voltage drop (especially since it unlikely that you'll ever draw the full 60 amps anyway).

Make sure what you run what you posting name suggests.

 

Sure, you can do that. The wire is protected by the first breaker. However, why fool around, especially at that distance. Use a BIGGER wire. Then even if you only use a 60 a breaker it wont voltace drop if you load it up a little. I actually often use number 2 alum for those and use 60 a breaker.

 

Sorry John, we post at about the same time. Is it allowed to derate the neutral on a detatched building like that? Such as 4-4-6-10 I get a little paranoid about wire size. It seems after the fact out comes an air compressor and then wants to hook a welding machine welding machine on. I am probably over kill but after 50 ft I go up a wire size.