The problem with so much "Quick and easy" wiring these days is the punch down connectors that are likely inside those MOLEX boxes.  This is a metal blade that cuts through the insulation and makes a knife edge connection with the wire.
Folks argue with me because "I worked in the telephone industry for bla bla years and we never had a problem"  They also never had an amp of current. 
At 15 amps.. Those tend to MELT
An electric fireplace is 12-13 amps and they tend to MELT.
Also a fire hazard when they do.
At anything over 10 amps I like a nice stripped wire properly bent around a Screw and properly tightened (NOTE all the "Properly" in that if not done Properly see Fire hazard above)  I do not like the punch down at all.  Though I do use them... For Data cables (RJ-type connectors).