Calculate shunt current from rated voltage drop and measured voltage, size a shunt by current and voltage drop, or subtract branch currents. Wattage multipliers are illustrative; actual ratings require manufacturer thermal and electrical limits.

Use the voltage measured directly across the rated shunt.

Ideal resistive/DC estimate. Verify actual shunt, meter, wiring and thermal ratings; a wattage multiplier alone does not establish safe operation.

Negative voltage gives signed reverse current.

Shunt Current Formula

The branch-current mode uses the following junction balance. For the voltage-reading mode, R = V_rated / I_rated and I = V_measured / R. For sizing, R = V / I and dissipation P = I ร— V.

Is = It - Ir
  • Where Is is the Shunt Current (amps)
  • It is the total current (amps) 
  • Ir is the current through the other branch (amps) 

To calculate the shunt current, subtract the current through the other branch from the total current.

How to Calculate Shunt Current?

The following two example problems outline how to calculate the Shunt Current.

Example Problem #1:

  1. First, determine the total current (amps). In this example, the total current (amps) is measured to be 50.
  2. Next, determine the current through the other branch (amps). For this problem, the current through the other branch (amps) is calculated to be 30.
  3. Finally, calculate the Shunt Current using the formula above: 

Is = It – Ir

Inserting the values from above and solving the equation with the input values gives: 

Is = 50- 30= 20 (amps)

Example Problem #2: 

Using the same process as example problem 1, we first define the variables outlined by the formula. In this case, the values are:

total current (amps) = 40

current through the other branch (amps) = 12

Entering these values into the formula or calculator above gives us: Is = 40 – 12 = 28 (amps)