4 to 20 mA Loop Power Budget Calculator
Check that a 4 to 20 mA loop has enough voltage for the transmitter at full alarm current, after the drops across the receiver, barrier and cable. The workbook also gives the maximum cable length and checks HART resistance.
Download
- Calculator sheet with a worked example
- How to Use sheet with steps
- 9 built in formulas, no macros
- Yellow input cells, grey results
- Calculation sign off block
- Excel, Google Sheets and LibreOffice
Worked Example
These are the results of the example already filled in the workbook. Replace the yellow cells with your own data.
About this Calculator
A transmitter that works at 12 mA but resets when the process goes high is almost always a loop power problem: at 20 mA and above, the voltage drops across the receiver, barrier and cable leave too little for the transmitter. A two minute loop budget catches it at the design stage.
Basis: Ohm's law; copper conductor resistance from IEC 60228 class 2 at 20 °C with temperature correction.
How to use
- Enter the supply voltage and the transmitter minimum operating voltage from its data sheet.
- Enter the maximum loop current (use the upscale alarm current).
- Add the receiver input resistance, any barrier or isolator resistance and other series items.
- Enter cable length and conductor size; the cable resistance is calculated for both cores.
- Check the margin, maximum cable length and HART resistance.
Formulas used
- Cable resistance = 2 × length × R/km × (1 + 0.00393 (T − 20)).
- Vtx = Vs − Imax × Rtotal; margin = Vtx − Vmin.
- Rmax = (Vs − Vmin) ÷ Imax.
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Frequently Asked Questions
How do I calculate the maximum load of a 4 to 20 mA loop?
Rmax = (supply voltage − transmitter minimum voltage) ÷ maximum current.
Why use 22 mA instead of 20 mA?
The transmitter may drive up to its upscale alarm or saturation current, so the loop must work there too.
What resistance does HART need?
At least 230 Ω (250 Ω is typical) between the power supply and the transmitter.
Learn more on the blog
Analog input stability: cable length problems
A practical guide on instrumentationblog.in that explains the engineering behind this workbook.
Read Full Article