How to Size a Low Voltage Transformer

Written by richard asmus
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How to Size a Low Voltage Transformer
For consumer products, low voltage usually means under 120 volts. (power supply image by pmphoto from

The arbitrary term "low voltage transformer" means under 500 volts to a power company and under 120 volts to many consumer equipment manufacturers. But for any use---whether sizing a transformer for the power input to a house, for a garden lighting project, for a doorbell transformer or for a furnace control system--the sizing parameters remain the same. Size transformers by an input or primary voltage, an output or secondary voltage and a current or power rating. Express the size of the input and output in AC (alternating current) volts, and express the current or power size in amps or watts.

Skill level:


  1. 1

    Determine the voltage input for your transformer. For plugging into a standard house wall socket, the size or input is 120 volts AC. However, for a dryer or stove socket, the voltage is 240. The voltage source dictates the input voltage size to the transformer.

    How to Size a Low Voltage Transformer
    For a power company, low voltage usually means under 500 volts. (transformer image by palms from
  2. 2

    Determine the output for your transformer. A total house usually needs 240 volts, but could run on 120. A doorbell could run on 10 volts, but chimes run on 16. Your furnace needs 24 volts. Your lighting system should tell you on the bulbs the voltage they need (usually 12 volts). The equipment you power dictates the output voltage size.

  3. 3

    Determine the current or power requirement, expressed in amps or watts. For an entire house, add up the watts of all the electrical appliances you could possibly use at the same time. All appliances have a tag showing their power requirements. Transformers such as those used for low voltage lighting also have power options. Add up the watts of all the bulbs you will use. Ten 18-watt bulbs need 180 watts. Add 12 10-watt bulbs and you now need 300 watts. Such transformers come in options ranging from 100 to 900 watts.

  4. 4

    Express the size in terms describing the input and output voltage and current or power requirement as follows: "A 440 to 220 volt, 400 amp power line transformer," or "a 120 to 12 volt, 900 watt low-voltage lighting transformer."

Tips and warnings

  • All electrical appliances have a tag showing their power requirements, expressed in voltage and amps, watts or VA.
  • The term "VA" means "volt amp" and is the same as watts.
  • To convert, use the formula "power in watts equals voltage times current in amps (P=IE)."
  • You can describe some low voltage transformers for common use with only a partial description: "a 16 volt doorbell transformer" (doorbell transformers have a 10 or 16 volt output option, but only come with 120 volt inputs. All can operate one or two doorbells.) or "a (brand name) HVAC transformer" (all have 120 volt inputs, 24 volt outputs and a current rating suitable for the control circuits of a furnace). But if your situation requires nonstandard voltage or current, you will need all the data.
  • Always use the correct input and output voltages.
  • For current or power, you can use a transformer of a higher rating, but never a lower rating. For example, you can run 100 watts of garden lighting with a 900-watt transformer, but you can't run 120 watts with a 100-watt transformer. Never exceed the power rating of a transformer.

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