Find what amp isolator you need: enter the load current or power and supply voltage to get a current target and the next standard isolator rating.

Use line-to-line RMS voltage for three-phase AC (e.g. 400 V); single-phase is usually 230โ€“240 V (UK/EU/AU) or 120 V (US).

0.85 is a typical motor or mixed-load value; use 1 for resistive loads such as heaters and showers.

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Electrical Isolator Size Formula

The following formula gives a current-rating target for a modeled load current and a selected current multiplier. It does not establish complete device suitability.

Iโ‚› = Iโ‚— ร— Sf

Variables:

  • Is is the calculated current-rating target in amperes
  • Il is the maximum load current in amperes
  • Sf is the selected current multiplier

Multiply the load current by the selected current multiplier. Final device selection also depends on rated voltage, switching duty, installation conditions and coordinated protective devices. The example rating series is illustrative; a higher amp rating alone does not establish suitability.

What is an Electrical Isolator?

An electrical isolator, also known as a disconnector or isolating switch, is used to isolate part of an electrical circuit for maintenance or service. The device and its switching duty must suit the application. Safe isolation and verification require qualified personnel; an isolator is not a substitute for coordinated overload and short-circuit protection.

How to Calculate Electrical Isolator Size?

The following steps outline how to calculate the Electrical Isolator Size.

  1. First, determine the maximum load current (Il).
  2. Next, choose the current multiplier (Sf) appropriate to your planning assumption; 1.25 means 25% above the modeled load.
  3. Finally, calculate the current target using Is = Il ร— Sf. For power inputs, distinguish real electrical input from equipment output; output requires efficiency. Three-phase AC uses line-to-line voltage and a balanced-load model.
  4. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Maximum Load Current (Il) = 50 amperes

Current multiplier (Sf) = 1.25. The target is 50 ร— 1.25 = 62.5 A; the next listed example rating is 63 A. Device suitability still requires the checks above.