In a purely resistive AC circuit, voltage and current waves are in step (or in phase) with each other. To calculate current this formula can be used:
Current = Power/Voltage I = P/V
In a purely resistive system the Power factor is 1
When an AC circuit contains loads such as inductors or capacitors, a phase shift will occur between the current and voltage waves. Both these waves are not in step (in phase) anymore. Looking at the waves, if you calculate the power you will see that the True power (W) is less than the apparent power (VA).
When the power factor is known the apparent power can be calculated.
W x Power factor = V x A True power x Power factor = Apparent power
On average a residential AC circuit has an average power factor of 0.8. So, for general calculations it is okay to use 0.8 as power factor.
5.4 AC wiring In a house installation the incoming electricity is divided into groups, usually on a distribution board. The diameter of the electrical wiring for each AC circuit (group) needs to be matched to the size of the expected maximum current in that circuit. This is to protect the connected loads and the electrical wiring. Voltage drop and heating of cables can also occur in AC circuits. Voltage drops can lead to damage of the connected appliance and heating up of cables can lead to house fires. It is also important to make good cable connections. Bad cable connections can also lead to voltage drop and heating. Use the guidelines as already described in the chapter 5: “cable connections”. For wiring calculation use the same calculations as we used for DC wiring as mentioned in chapter 5: “select the right cable”. But be aware that the previous mentioned rule of thumb cannot be used. For wiring for voltages from 100 to 400 Vac use this rule of thumb:
Nominal current / 8 = core diameter in mm Add 1 mm2 for each 5 meters of cable length
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