Instrument Loop Calculator

A 4-20 mA loop is a series circuit: the power supply, the cable resistance, and the transmitter's own burden all share the same current and each drop some voltage. This calculator adds up the cable and burden voltage drops at worst-case 20 mA and checks whether enough voltage is still left over (headroom) for the transmitter to operate correctly.

Copper's resistivity rises with temperature (about 0.393% per °C above 20°C). A cable run through a hot area or outdoors in summer has measurably higher resistance than its rated value at 20°C, which increases the voltage drop and reduces headroom at the transmitter. Enter the expected cable temperature to get a realistic worst-case number rather than a 20°C best case.

Most transmitter manufacturers specify a minimum operating voltage; anything above that is headroom. Industry practice is to keep at least 2 V of headroom at 20 mA and to size the supply at 1.3 × the transmitter's minimum voltage plus loop drop for a comfortable safety margin — tight headroom leaves no room for supply sag, connector resistance, or added devices later.

Smart (HART) transmitters can briefly output up to 22-24 mA during diagnostics or alarm conditions, above the nominal 20 mA full-scale signal. A loop that just barely passes at 20 mA can collapse when the transmitter tries to signal a diagnostic fault at 22 mA — this calculator checks both currents so a diagnostic event doesn't take the loop down.

This calculator works from physical cable properties — length, cross-sectional area, and temperature — to derive cable resistance and voltage drop. The Loop Power Supply Calculator instead works from a component budget — barrier, isolator, and other fixed resistances you already know — to check the same headroom question without needing cable geometry.