Hello,what is heat decipent in the transformer?
Looking foward for your reply 
Heat dissipation in the transformer depends on
Magnetization current or hysterisis loss
Copper loss in primary and secondary winding
Magnetization current is the no load current and represents the power required to remove the residual magnetic field of the lamination leftover from every cycle of the AC wave form. This loss is independent of load. You can measure the primary current with nothing connected to the load.
P_m=I_{pnl}X230
Copper loss is the loss due to the resistance of the wire in the primary and the secondary. You can measure this resistance with a multimeter and obtain the loss with P_c=I_p^2R_p+I_s^2R_s
Total power loss P_{tot}=P_m+P_c
Estimating the temprature rise from the above is an extremely hard task. It depends on the thermal resistance of the transformer’s components i.e core thermal resistance, winding thermal resistance, insulation thermal resistance, and the convection air flow in the vicinity of the transformer.
In the case of small transformers that we use for powering gadgets It is far easier to connect the load and measure the temperature with a thermometer or temprature probe in situ.
