make sure your transistor is rated for the application.
what is your relay's winding resistance? take your bus voltage and divide by that and this is the "hold" current. now you need derating on top of that so.
device rating * 0.7 >= (bus voltage)/(relay resistance)
also when the relay first turns on there is an 'inrush' current which is called umm.. i forget now. but generally you can expect this spike of current to be almost 7 times your hold current!! so, your transistor better be rated for that too.
Mr.Cool