Loop Voltage Headroom Calculator
Instrument and controls engineers use this when specifying or troubleshooting a 4-20 mA current loop. It answers the practical question: after the current drops voltage across the wiring, the sense resistor, and any intrinsic-safety barrier, is there enough voltage left for the transmitter to operate at full-scale current?
A two-wire transmitter needs a minimum terminal voltage to run. The supply has to cover that minimum plus every series voltage drop in the loop at the highest expected current. This calculator adds the series resistances, computes the voltage left at the device, and reports the headroom against the minimum you enter.
The Calculator
Result
Positive headroom means the loop has enough voltage at this current.
The Formula
Series resistances add, then Ohm's law gives the drop at the loop current:
Headroom is the device voltage minus the transmitter's minimum:
Worked example. With V_supply = 24 V, I = 20.8 mA, R_wire = 20 ohm, R_sense = 250 ohm, and no barrier, R_total = 270 ohm. The drop is (20.8 / 1000) × 270 = 5.616 V, so V_device = 24 - 5.616 = 18.38 V. Against a 10.5 V minimum the headroom is 18.38 - 10.5 = 7.88 V, comfortably positive.
Assumptions and Limits
- All drops are treated as ideal resistances at the single worst-case current. Real barriers and isolators may specify a voltage drop rather than a resistance; use that figure if the manufacturer gives one.
- The default 20.8 mA covers typical over-range; if your device's failure current is higher (for example 21 or 22 mA), use that so the loop still works when the transmitter is driven high.
- Wire resistance is round-trip. If you only have one-way resistance, double it before entering.
- Results are engineering estimates. Confirm the transmitter's minimum terminal voltage and any barrier drop against the datasheets, and measure the loop where you can.
FAQ
Why size at 20.8 mA and not 20 mA?
The loop must still have enough voltage when the transmitter drives over-range or a high-fault current. Sizing at the highest current the device can output leaves margin so it does not brown out exactly when it is trying to signal a problem.
Do I need a 250 ohm sense resistor?
250 ohm is common because it turns 4-20 mA into a 1-5 V signal and sits in the range HART communication needs. If your input card senses the current directly, set R_sense to that card's input resistance instead.
What if headroom is negative?
The transmitter can drop below its minimum voltage at high current and misbehave. Raise the supply, shorten or thicken the wire, reduce the sense resistance, or remove an unnecessary series device until headroom is positive with margin.
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