Short-Circuit Current Calculator

Find the current that would flow in a short circuit and check it against the breaking capacity of a breaker or fuse.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

For a three-phase fault the short-circuit current is the line voltage divided by root three times the impedance per phase back to the source. For a single loop it is the voltage divided by the loop impedance.

Every breaker and fuse has a breaking capacity, the largest fault current it can safely interrupt. The prospective fault current must be below it.

Formulas used

  • I_{sc} = \dfrac{V_L}{\sqrt{3}\,Z_{sc}}
  • I_{sc} = \dfrac{V}{Z_{loop}}

Worked example

With the default numbers in the calculator above:

  1. Prospective short-circuit current: 1996.67 A
  2. In kiloamps: 2.00 kA
  3. The fault current as a share of the breaker rating: 33.28 %

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Where do I get the impedance?

The network operator can give the fault level at the supply point, a test meter can measure the loop impedance, or you can add up the transformer and cable impedances.

Is the fault always highest near the source?

Yes. The impedance of a cable reduces the fault current further from the source.

Related calculators

Further reading: Standard electrical engineering texts (short-circuit calculations). Protective devices must be chosen by a licensed person.

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