Selecting Power Conditioning Transformers

In a perfect world, Power Conditioning Transformers should be operated between 40% and 100% of their Volt-Ampere rating. The life of the transformer is maximized when it operates at 100% of its rating because it runs coolest under this condition.

In order to select the proper size transformer, it is necessary to know the total load for the worst case scenario. Then choose the next larger available size, in Volt-Amperes.

Example:

On a golf course with a single power source, the following information is collected:

1. There are 18 controllers being supplied by the transformer at 120 VAC.
2. From the controller manufacturer’s catalog, we find that each controller has a current draw of 0.19 amperes when it is at rest (no load.) “At rest” means that the controller is not activating any of the solenoid valve(s).

The power required by the controller electronics (at stand by) is:

Controllers’ Power Needed = Volts X Amps X Number of controllers
Controllers’ Requirement = 120 X 0.19 X 18 = 410.4 VA

It is known that each controller will simultaneously activate up to 3 solenoid valves when the irrigation cycle begins. This means that 54 solenoid valves (3 valves per controller X 18 controllers) will be activated at the same time, as a worst case scenario. We learn from the valve manufacturer’s catalog that the valves operate at 24 VAC with an inrush current of 0.40 amps.

The power required to operate the solenoids is:

Power needed by solenoids = Volts X Amps X Number of solenoids
Solenoids’ Requirement = 24 X 0.40 X 54 = 518.4 VA

The total power supplied by the transformer, at maximum load, is:

Transformer Supplied Power = Controller Requirement + Solenoid Requirement
Transformer Supplied Power = 410.4 + 518.4 = 928.8 VA

From the table below, choose the size transformer just larger than 928.8 VA, which is Paige Electric part number 250040, rated at 1000 Volt-Ampere.

Paige Part No.	SOLA Part No.	Voltage		Output Capacity (Volt-Amperes)
		Output	Input
250000	63-23-112-4	120	120, 208, 240	120
250010	63-23-125-4	120	120, 208, 240	250
250020	63-23-150-8	120, 208, 240	120, 208, 240	500
250030	63-23-175-8	120, 208, 240	120, 208, 240	750
250040	63-23-210-8	120, 208, 240	120, 208, 240	1000
250050	63-23-215-8	120, 208, 240	120, 208, 240	1500
250060	63-23-220-8	120, 208, 240	120, 208, 240	2000
250070	63-23-230-8	120, 208, 240	120, 208, 240	3000
250080	63-23-250-8	120, 208, 240	120, 208, 240	5000

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