4–20 mA Loop Power Budget Calculator

4–20 mA Loop Power Calculator

Check whether a two-wire (loop-powered) transmitter still has enough voltage to operate at full current once the cable, receiver and any other loop devices take their share.

Loop supplyV DCTransmitter minimumV DCFrom the datasheet (lift-off voltage).

Receiver and other resistanceΩInput card or HART resistor (often 250 Ω), barriers, indicators in the loop.

Cable run (one way)mCopper gauge

Maximum loop currentmAUse the transmitter's high-alarm current (often 21–23 mA), not just 20 mA.

Result

Cable resistance (both conductors)12.57 Ω

Total loop resistance262.6 Ω

Voltage at transmitter @ 22 mA18.22 V

Margin over minimum7.72 V

Maximum allowable loop resistance614 Ω

OK: the transmitter keeps enough voltage at full current.

How the loop budget works

In a two-wire loop, everything is in series: the power supply, the receiver (input card or HART resistor), the cable, and the transmitter. Every resistance drops voltage in proportion to the loop current (V = I × R). Whatever is left over has to be at least the transmitter's minimum operating voltage from its datasheet.

V_transmitter = V_supply − I_max × R_loop
R_{loop max} = (V_supply − V_{transmitter min}) ÷ I_max

Worked example

24 V supply, 250 Ω HART resistor, transmitter minimum 12 V, checked at 20 mA:

Maximum loop resistance = (24 − 12) ÷ 0.020 = 600 Ω
Voltage at the transmitter with only the 250 Ω resistor = 24 − 0.020 × 250 = 19 V
That leaves 350 Ω for cable, barriers and indicators.

Why check at the alarm current, not 20 mA

If the transmitter is configured to fail upscale, it has to drive about 21–23 mA to signal a fault (NAMUR NE 43 uses ≥ 21 mA). A loop that only just works at 20 mA can starve the transmitter at the alarm current. The high-alarm signal then never reaches the control system, which is exactly when you need it.

Things that eat voltage

The input card's sense resistor, or a 250 Ω resistor added for HART communication
Intrinsic safety barriers (a zener barrier can add a few hundred ohms) and isolators
Loop-powered indicators and any other series devices
Long cable runs, especially in smaller wire gauges (counted out and back)

Converting the loop current to a process value? Try the 4–20 mA scaling calculator.