Calculation reviewed: 4 October 2026 · Engineering Desk Tools
How to use the 4–20 mA loop voltage budget calculator
Check whether a loop supply can power a transmitter at maximum current with the connected loop resistance. Start with measurements or documented values for the equipment and reporting period you actually want to assess. The calculator does not fill in a typical installation for you. Its blank fields help keep a demonstration value from being mistaken for your own operating data.
Choose the required options, enter the values in the labelled units and select Calculate. If a field is rejected, check its range and unit before changing it. The result is accompanied by the equation and assumptions so you can decide whether it answers your engineering question.
Inputs and measurement basis
- Loop supply voltage (V). The voltage at the power supply or barrier output, for example 24 V DC. The value must be greater than zero.
- Transmitter minimum operating voltage (V). Minimum operating voltage from the transmitter data sheet. Some devices need more when HART communication is used.
- Maximum loop current to check (mA). Highest loop current to check: 20 mA for full scale, or the high alarm current, commonly 21 to 22 mA. Allowed range: 4 to 30.
- Receiver or input resistance (Ω). Resistance of the receiver, input card or sense resistor. A 250 Ω resistor converts 4 to 20 mA into 1 to 5 V.
- Total cable loop resistance, out and back (Ω). Resistance of both conductors, out and back: one-way length times resistance per metre times 2.
- Other series resistance (Ω, optional). Barrier, isolator, indicator or other series devices. Leave blank if there are none.
- Fixed voltage drops such as diodes (V, optional). Fixed drops such as a protection diode. Leave blank if there are none.
Record the measurement source and operating condition. Retain unrounded values for verification.
Method and interpretation
A loop-powered transmitter needs a minimum voltage across its own terminals to operate. Every other element in series, such as the receiver resistor, cable, barrier and diode, drops voltage as the loop current flows. The check is whether the supply, minus those drops at the highest current, is still above the transmitter minimum. The same numbers give the largest total loop resistance the supply can drive.
Total resistance = receiver + cable + other. Loop drop = I(max) × total resistance + fixed drops. Margin = supply − transmitter minimum voltage − loop drop. Maximum loop resistance = (supply − minimum − fixed drops) / I(max).
Check at the highest current the loop can carry: 20 mA for full scale, or 21 to 22 mA if a high alarm current is configured. Use the transmitter minimum voltage from its datasheet, including any HART communication requirement. Add barrier or isolator resistance as series resistance. A negative margin means the transmitter may not reach full output or alarm current.
Worked example
This example does not populate the form. Enter your own installation data.
Calculated result: 7.56 V margin. The loop has positive voltage margin at the checked current. Confirm the figure against the transmitter datasheet and any communication requirement.
Check the arithmetic independently using the displayed equation. Retain intermediate precision and compare the final value with your equipment documentation.
Quick-reference table
These illustrative rows retain the worked example assumptions except for the varied input. They are not certified ratings or manufacturer data. Use the form for intermediate values.
Common mistakes
Checking at 4 mA or 12 mA instead of the highest current; forgetting the cable resistance of both conductors; leaving out barrier or isolator resistance; using the typical rather than the minimum transmitter voltage; ignoring the extra requirement for HART communication. Before relying on the output, check the decimal format, unit scale and source of each value. Recheck unusual results against the formula.
Frequently asked questions
Are the example values used automatically?
No. Inputs remain blank when the tool opens or when you clear them. The worked example and table are reading material below the calculator. Enter the actual values you want to check each time, and label saved reports so their context is not lost.
What does the result mean?
A positive margin means the supply can drive the transmitter at the highest current you checked. A negative margin means the transmitter may fall short at high output, so reduce the loop resistance or raise the supply voltage.
Can I use a zero or a negative value?
The allowed range follows the meaning of each input. A divisor needs a positive value; other fields may allow zero. Check the form labels and error message rather than forcing an invalid value. A rejected input must be corrected before a result can be shown.
Is this a final design or equipment approval?
No. Check at the highest current the loop can carry: 20 mA for full scale, or 21 to 22 mA if a high alarm current is configured. Use the transmitter minimum voltage from its datasheet, including any HART communication requirement. Add barrier or isolator resistance as series resistance. A negative margin means the transmitter may not reach full output or alarm current. Use the calculation as support for a checked decision, and review equipment documentation and applicable requirements before changing an installation or operating setting.
How can I save or share a calculation?
Calculate first, then use Copy result or Print report. Some maintenance tools also provide a CSV summary. Reports contain your own entered data, so review them before sharing. You can share the public page URL without including plant records or confidential measurements.
How do I report an unexpected result?
Use the Contact page and include the tool URL, input values, units, expected output and the reference you used. Explain the measurement basis. Exclude confidential information.
Related tools and records
Use related calculations with consistent units, periods and equipment scope.
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