About electrical calculations
Instrumentation work sits on an electrical foundation, and the calculations here span both worlds: the low-power domain of signal loops, and the power domain of motors, cables and protection that those loops ultimately control.
Cable sizing is the recurring theme, and it has three independent checks that must all pass. Current-carrying capacity is a thermal limit, and the tabulated value is never the answer on its own — ambient temperature and grouping corrections routinely reduce it by 40% or more. Voltage drop often governs on long runs regardless of thermal capacity. Fault-loop impedance governs whether protection disconnects in time.
For instrument circuits the constraints invert. A 4-20 mA loop carries milliamps, so ampacity is irrelevant; what matters is loop voltage budget, and for RTD circuits, the lead resistance that the instrument cannot distinguish from the sensor.
How these tools fit together
- Establish the design current. For motor circuits, calculate full load current from rating, voltage, power factor and efficiency — and remember nameplate kW is shaft output, not electrical input.
- Check thermal capacity. Take the tabulated ampacity for the conductor size and installation method, then apply ambient and grouping derating factors.
- Check voltage drop. Verify the drop over the actual run length stays within limits — this frequently governs on long cables even when thermal capacity is ample.
- Coordinate the protection. Design current must not exceed the device rating, and the device rating must not exceed the derated cable capacity.
Frequently asked questions
How do I calculate motor full load current?
For three phase, current equals shaft power divided by the product of root three, voltage, power factor and efficiency. Efficiency must be included because nameplate rating is mechanical output rather than electrical input.
Why is cable derating necessary?
Published ampacity assumes reference conditions of about 30 °C ambient and a single circuit. Higher ambient temperature and grouped cables both reduce heat dissipation, so the tabulated value must be corrected downward before it means anything.
All calculators are provided for reference and education. Verify independently before use in safety-critical work — see our disclaimer.