When and How to De-Rate Load Banks
Load banks carry ratings for voltage, power, frequency, and other parameters. Using load banks where parameters differ from rated values makes them useful in a wider range of circumstances, but requires users to understand how circuit characteristics impact their maximum capacity. This article provides guidance for de-rating load banks accordingly.
De-Rating Maximum Capacity for Actual Circuit Voltage
Load banks should never be operated above their maximum rated voltage, but can be used for testing at lower voltages. Doing so, however, will affect the maximum amount of load a unit can apply according to the following formula:
[(Applied Voltage)2/(Rated Voltage)2] x Rated Capacity
The maximum load that a 400-kilowatt 240-volt load bank could apply to a 208-volt circuit would thus be calculated as follows:
[(208 V)2/(240 V)2] x 400 kW = 300 kW
De-Rating For Smaller Load Steps
Most load banks can apply discrete amounts of load in stepwise fashion. Any load steps would require the same de-rating factor used for total capacity. In the application above, 5-kilowatt load step controls would provide actual steps of 3.75 kW.
This principle can be used to advantage where smaller load steps are needed. A 400-kilowatt 480-volt load bank with 5-kilowatt load step increments could be used on a 240-volt circuit as follows:
[(240 V)2/(480 V)2] x 5 kW = 1.25 kW Load Step Increment
Increasing the rated voltage of the load bank beyond the circuit voltage thus enables users to apply more precise amounts of load.
De-Rating Three-Phase Load Banks for Single-Phase Circuits
Three-phase load banks are typically wired in a delta configuration. They can be used to test single-phase power by connecting two of the load bank’s phase terminals as shown in the following figure, resulting in a capacity of two-thirds of the nameplate rating.
Because all applicable de-rating factors must be applied, using a 100-kilowatt, 240-volt, three-phase load bank for a 120-volt single-phase application would produce the following rating:
[(120 V)2/(240 V)2] x 0.67 x 100 kW = 16.75 kW
Again, any load step increments must also be adjusted accordingly.
De-Rating Resistive Load Banks for Frequency
The load elements of resistive load banks are not affected by variance in frequencies between 50 and 60 Hertz. However, motor loads are affected linearly by this change, which would cause a load bank’s 60 Hertz cooling fan to slow. In this instance, the ratio of 50 Hertz/60 Hertz reduces the unit’s maximum load to 0.83 of its 60 Hertz capacity, a reduction of 17 percent.
De-Rating Reactive Load Banks for Frequency
Unlike resistive units, reactive load banks (rated in kVAR) are affected by differences in source frequency. Both voltage and frequency must be considered according to the following formula:
[(Applied Voltage)2/(Rated Voltage)2] x (Applied Freq./Rated Freq.) x Rated kVAR
A 500-kVAR, 480-volt, 60 Hertz reactive load bank used to test 380 volts at 50 Hertz would be rated:
(380V2/480V2) x (50 Hz/60 Hz) x 500 kVAR = 260 kVAR
Environmental Ratings
Load bank ratings are valid only when the units operate in anticipated environmental conditions. For instance, maximum rated ambient temperatures are typically 120°F/50°C. Ambient air temperatures in a hot desert environment could approach or exceed this level. Locating a load bank in an outdoor unshaded setting would expose it to additional solar loading that could raise temperatures beyond rated values. Likewise, reduced air density at higher altitudes could reduce load bank capacity as well. Users should review environmental factors for each application and check with load bank manufacturers for guidance regarding any exceptional conditions.
Summary
While no load bank should ever be used under conditions exceeding its ratings, knowing how to de-rate units can make them useful in a wider range of applications. The information provided herein gives basic rationale for adjusting ratings for specific applications. However, users should obtain manufacturer guidance for de-rating practices for specific load banks and specific applications.
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