Can I connect 12 AWG wire to 10 AWG in a junction box?
You can't do that

You cannot come off a 30A breaker with #10 wire (so far so good) and then terminate it at 15-20A receptacles. (nope, nope, nope!) 

Unfortunately people sometimes become aware of the rule that allows 15 or 20A receptacles on 20A circuits, and they falsely generalize that to "Any receptacle &lt;= circuit capacity".  Nope.  The receptacle ampacity must be an exact match to the breaker:  15 to 15.  30 to 30.  50 to 50.  

Now there are two exceptions, but they are... exceptional. 


40A receptacles do not exist. NEMA, distributors/stockers, and electricians didn't want the burden of yet another socket type (there are like 40 varieties already when you consider every iteration of 3-phase, 208, 480, etc.) and oven manufacturers didn't want consumers stymied by a 40A range's plug not fitting an installed 50A socket.  So the (only) correct receptacle for a 40A circuit is 50A. 
UL requires all 15A receptacles to be internally rated for 20A.  As such, 15A receptacles are allowed on 20A circuits. 


And that's all, folks! 

Only a 30A receptacle is acceptable on a 30A breaker. 



"But I want to plug in common devices!"  Then you have 3 options. 

Change the breaker to 20A.

Now it becomes a 20A circuit (overbuilt with #10 wire - overbuilding is allowed).  You can fit 20A receptacles.  You can also fit 15A receptacles due to the exception mentioned above. 

How do you handle the pinchy wires?  I work in metal boxes which are annoyingly small and constantly run out of statutory cubic inches.  Obviously your first defense is go with a bigger box.  Or go with a box extensions or lids which give extra space.   Another option is to place 2 boxes next to each other joined with a nipple -- and in one box, splice from #10 solid to #10 stranded, and in the other box fit the receptacle.  Once you work with stranded wire, you won't want to go back! 

Install a Power Distribution Unit

PDUs are typically used with server racks.  They allow the rack to have a single power cord for the entire rack.  It has a NEMA 14-30 or 14-50 plug, and splits that out to a number of 120V receptacles.  Mind you, the PDU is a commercial unit that has been through the rigors of UL listing.  You cannot build one out of parts except this way: 

Install a Subpanel

This is conventional stuff, you bring L1, L2, N and G into a small service panel then hang any number of 120V/15A or 20A breakers off that panel, and those power your 15/20A 120V circuits.  In any subpanel, neutral and ground are kept separate.  No "main breaker" is needed if the subpanel is in the same building.