How to calculate a low voltage transformer in volt amp rating

Written by pauline gill
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How to calculate a low voltage transformer in volt amp rating
Some low voltage transformers go directly into wall sockets. (Power Pack image by Cinneman from

Low voltage transformers are used in many domestic controls and personal electronics applications where less than 30 volts AC (alternating current) are required. They may take the form of completely enclosed wall-socket plug transformers as are common to power or charge some DC-powered tools, computers, or sound equipment. They may also be contained in a small metal enclosure for low-voltage outdoor lighting, sprinkler systems, or pool lights. You can easily calculate the required volt-amp rating for your application.

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  1. 1

    Define your low-voltage transformer application. In this example, a low-voltage lighting system is being installed around a home swimming pool and garden area including the walkways. It has 30 lights. If you know what the voltage and amp rating is for each light post, you can calculate the required transformer size and rating.

  2. 2

    Determine the electrical specifications for each user in the circuit. Each of the 12-volt bulbs consumes 18 watts. This means that each bulb requires 1.5 amperes of current (18 watts/12 volts). The entire system would require 45 amps of electricity (30 lights x 1.5 amps) at 12 volts.

    How to calculate a low voltage transformer in volt amp rating
    Domestic low-voltage transformers are small versions of these high-voltage units. (transformers 3 image by Kate Jones from
  3. 3

    Calculate the required low voltage transformer volt-amp rating. The output voltage of the transformer will be 12 volts AC, and the power output will be 12 volts x 45 amps, or 540 volt-amps of power. For non-reactive resistive loads (light bulbs versus motors) 540 volt-amps translates directly to 540 watts. This defines the secondary or output winding rating.

    How to calculate a low voltage transformer in volt amp rating
    Underwater pool lighting uses low-voltage transformers for safety. (handrails to the night illuminated swimming pool . image by Sergey Mostovoy from
  4. 4

    Calculate the primary winding's volt and amp values. A typical low-voltage transformer would use 120 volts AC at the input. If the transformer reduces the voltage by a factor of 10 (from 120 volts to 12 volts), then it increases the output amperage by a factor of 10 to keep the power constant at 540 volt-amps. This means the input amperage must be one-tenth the output value, or 45 amps/10 = 4.5 amps. All of these values would be plainly printed on the transformer's product label.

Tips and warnings

  • Always select a transformer with a 25- to 50-percent greater power capacity than calculated to keep it cool and operating in its efficiency zone.
  • Observe published voltage drop factors for long wiring runs with low voltage circuits to obtain the best wiring gauge size for your installation.
  • Always follow low-voltage transformer wiring instructions completely to avoid short circuits, overloads, fires, and personal injury from electrocution.

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