Transformer Calculator

Transformer Voltage Drop Calculations

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While formulas for determining transformer properties such as full load amps or infinite bus through-fault current are relatively simple, running the calculations takes time away from other activities. Sometimes being able to plug in the numbers and get the information you need is a valuable way to save you time while designing the electrical system around a transformer. This calculator will let you plug some numbers in and get a quick result that you can use to properly size the system.

Transformer Calculations
By knowing the capacity of the transformer as well as the primary and secondary voltages, you will be able to calculate the full load capacity (in Amps) of the transformer for both the primary and secondary sides. These values can then help you determine the appropriate primary and secondary overcurrent protection necessary to meet NEC requirements, a key component to designing the electrical system around a transformer.

Infinite Bus Calculations
When a fault occurs on the secondary of a transformer, its impedance limits the amount of fault current capable of being supplied through the transformer windings. By utilizing the transformer impedance, and assuming an infinite power source on the primary, you can determine the absolute maximum amount of through-fault current capable of being supplied across the transformer. This current alongside other fault contributors on the secondary side of the transformer (generators, large induction motors, etc.) can help you identify the maximum available fault current on the secondary of the transformer and determine the minimum allowable short circuit and interrupting capacity ratings you should be specifying for downstream equipment.

Through Fault Calculations
When the limitations of maximum available fault current are known on the primary of the transformer you are also able to determine the actual maximum available fault current on secondary of the transformer. This information may be useful but use of the infinite bus method explained above is recommended for determining short circuit and allowable interrupting ratings as the primary system may change and the maximum available fault current at the primary of the transformer may vary over time.

Transformer Voltage Drop
In many cases, the voltage drop across a transformer is not considered when assessing the voltage at a given load. By knowing the transformer X/R ratio and the power factor of the downstream load on a transformer, you can use this calculator and determine the voltage drop that can be experienced across a given transformer and determine if it's significant enough to consider in your system design. In many cases, this voltage drop can be accounted for by adjusting the voltage tap on the transformer to compensate for the difference in voltage.