I bought mine refurbished. They would not send the grounding part of the cord, just the box and small diameter cord. They said they had had a problem with them heating up when used as a unit.
A couple of times I have forgotten and plugged into an extension cord (heavy duty -12g) and OUCH too hot.
Using only the square plugged into the wall & connected with the magsafe I haven't had it get overly hot.