For single- and three-phase applications.

Calculating the size of a power conditioner is straightforward. The most challenging aspect is determining the amperage (or current).
Single-Phase Sizing

1. Determine input voltage for the equipment or circuit to be protected
2. Determine the rated amperage for the equipment or circuit to be protected
3. Multiply the voltage by the current and divide by 1,000 to obtain the size rating in kVA

Example

A single-phase device has a nameplate rating of 120 volts, 40 amps

The single-phase kVA size is then:

120 X 40 = 4,800 volt-amps

4,800 volt-amps ÷ 1,000 = 4.8 kilovolt-amps (approximately 5 kVA)

#### Three-Phase Sizing

1. Determine input voltage for the equipment or circuit to be protected
2. Determine rated amperage for the protected equipment or circuit
3. Multiply the voltage by the current by 1.732 and divide by 1,000 to obtain the size rating in kVA

Example

A three-phase device has a nameplate rating of 480 volts, 60 amps

The three-phase kVA size is then:

480 X 60 X 1.732 = 49,882 volt-amps

49,882 volt-amps ÷ 1,000 = 49.9 kilovolt-amps (approximately 50 kVA)

#### Amperage/ Inrush Current

AC amperage is a measure of the flow of current to a device or in a circuit. Electrical devices draw varying amounts of current depending upon their operating state or the amount of work they are doing. For example, the current flow into a three-phase electric motor goes from zero (off) to peak level (peak, locked rotor, starting, or inrush current) and drops down to an intermediate level (full-load or steady-state current). The starting current for a three-phase motor can be 5 to 10 times that of the full-load current. (See overload capacity.)

#### Amperage Calculation

Determining the amperage to use in the kVA size calculation depends upon the type of power conditioner to be used. For power conditioners with a high overload capacity, the steady state or full-load amperage is typically used. For power conditioners with low tolerance to overload conditions the starting or peak amperage is typically used. It is not unusual for a power conditioner with a high overload tolerance to be 20% to 50% smaller than their intolerant counterparts.

There are several ways to determine amperage.

The first way is obtaining amperages from the nameplate or documentation for each device. This method is fairly accurate and easy.

The second way is determining circuit breaker amperage ratings for circuits the power conditioner protects. This method tends to give values that are too high for overload-tolerant units and may be too low for intolerant units.

A third way is measuring the current for the devices or circuits to be protected. This method should only be undertaken by qualified technicians or professionals familiar with measurement method and safety procedures. This method often provides very accurate results, providing that the amperage measured accurately represents the expected maximum draw.

In all cases, it is prudent to ensure that there is some margin in the amperage calculation to ensure that the power conditioner is not undersized.