Motors are always considered to run at full load, this however, is rarely the case. When selecting a motor to drive equipment the mechanical designers add in margins and allowances and then check the starting torque required by their equipment, their final step is to select the nearest standard size motor above their calculated requirements. Thus the actual motor load will invariably be less than its stated output, but its starting current will be the full load current multiplied by the factor for the starting method given in section 02.09.05.00 ‘motor starting current’ of this article. It is necessary to use the full load current in the electrical system design unless the mechanical designers can be persuaded to give the actual absorbed load of their equipment. If the absorbed load is known it will allow the electrical system designer to introduce the utilisation factor which is the absorbed output divided by the rated output multiplied by one hundred, to give the percentage utilisation.
The utilisation factor is then used to determine the actual electrical loads on a site, the motor rating plate output will determine the size of the starter.
There are therefore cost implications to over sizing a motor a point for mechanical designers and engineers to note.