When it comes to electricity, do not make assumptions!  Two outlets in the same room may be running or the same circuit or maybe not.  The only way to find out how your house/apt is circuited is to plug in a light bulb into an outlet and turn off the circuit breaker that you think controls that outlet.  If the light turns off, then you know what amps that outlet is running (since amps are written on the breaker switch).  You can calculate wattage by Watts=Amps x Volts and make sure you do not use more than 80% of the max wattage for continuous load.  How long was your friend mining for before the fire happened?  Also, when you say fire do you mean a burnt GPU or an actual house/apt fire?