Current Shunt Resistor Formulas
Current shunt formulas: the resistance from the full-scale voltage and current, the power at full current, the current from a measured millivolt and the drop.
The formulas
| To find | Formula |
|---|---|
| Shunt resistance | R = Vfs / Ifs |
| Power at a current | P = I² × R = V × I |
| Current from the voltage | I = Vm / R |
| Resistance for an allowed drop | R = Vdrop / I |
Vfs is the voltage across the shunt at its rated current Ifs, Vm the voltage that you measure and I the current. A shunt is a low resistance in series with the load, and the meter reads the small voltage across it. Four-wire (Kelvin) connection takes the sense leads from the shunt itself, so the lead resistance does not add an error. The shunt heats at full current and its resistance shifts with temperature, and the maker gives the power rating, the temperature coefficient and the accuracy. A shunt in the live conductor needs the meter or the amplifier to take the common-mode voltage. The values here are examples.
Worked example: a 75 mV, 100 A shunt
- R = 0.075 / 100 = 0.75 mΩ
- At full current, P = 100² × 0.00075 = 7.5 W
- A reading of 42 mV is I = 0.042 / 0.00075 = 56 A
Worked example: a shunt for 20 A with a drop of 100 mV allowed
- R = 0.1 / 20 = 5 mΩ
- At 20 A, P = 20² × 0.005 = 2 W
75 mV shunts by rated current
| Rated current | Resistance | Power at full current |
|---|---|---|
| 10 A | 7.5 mΩ | 0.75 W |
| 50 A | 1.5 mΩ | 3.75 W |
| 100 A | 0.75 mΩ | 7.5 W |
| 200 A | 0.375 mΩ | 15 W |
Frequently asked questions
Why use a low resistance?
A low value drops little voltage and wastes little power in the circuit. The trade-off is a small signal, which needs an amplifier or a sensitive meter.
Why does a shunt have four terminals?
Two carry the load current and two sense the voltage. The sense leads carry almost no current, so their resistance does not add to the reading.
Why is the power rating important?
The power turns into heat in the shunt. If it runs hot, the resistance drifts and the reading with it, and a hot shunt can fail.
What about an AC current?
A shunt also works for AC, but its inductance adds to the voltage at a high frequency. For a high-frequency or isolated measurement, use a current transformer or a Hall sensor.
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