Load Bank Calculations – Voltage, Current and KW

VOLTAGE

Low voltage AC systems are rated 120/240v, single-phase, 2 or 3 wire; 208-240/416-480/575-600vAC, 3 or 4 wire. Medium voltage systems are in the 5kV or 15kV class.

DC systems can range from battery voltages of 12/24/32v, 125/250v, or 350/700v, typically. Keep in mind that the AC load banks in this catalog are generally 3-phase, 3-wire loads, meaning that a neutral is not required and that loads are applied in 3-phase balanced steps.

CURRENT

Current, expressed in amperes, is found by the following formula:

current voltage

Current must be known in order to size connection cables. Keep in mind that connection cables have a certain small resistance, and that a small voltage drop will be seen across the cable set and at the terminals of the load bank. Therefore, although the generator instrumentation may indicate 480v, the load bank instrumentation
will measure the terminal voltage at the load bank, for example, 473v.

KW AS A FUNCTION OF VOLTAGE (De-Rating)

KW varies as the square of the voltage change and can be calculated using the following formula: KW output = Rated kW x (Applied Voltage/Rated Voltage)2. Therefore, in the preceding example, if the load bank terminal voltage is reduced to 473v, then the resultant KW will be 97% of the rated KW. Where did that power go? It is lost as heat in the connection cables.