4–20 mA Loop Resistance Calculator (Maximum Loop Load)

How to use the 4–20 mA loop resistance calculator

Enter the loop supply voltage, the transmitter’s minimum operating (lift-off) voltage from its datasheet, the maximum loop current, the receiver’s input resistance, any barrier or isolator resistance, and the cable length and conductor resistance. The calculator shows the maximum resistance the loop can tolerate, the actual resistance and the margin.

For more background, read 2-Wire, 3-Wire and 4-Wire Transmitters.

4–20 mA loop resistance formulas

R_max = (V_supply − V_{transmitter,min}) ÷ I_max

R_loop = R_input + R_barrier + 2 × length × r_cable

Example

24 V supply, transmitter needs 10.5 V, 22 mA maximum, 250 Ω PLC input, 500 m of 1.5 mm² cable:

• R_max = 13.5 ÷ 0.022 = 614 Ω
• Cable = 2 × 0.5 × 12.1 = 12.1 Ω; total loop = 262 Ω
• Margin = 352 Ω — the loop works, with room for a barrier

Typical voltage drops in a loop

Component	Typical drop at 20 mA
250 Ω input resistor	5 V
Zener barrier	6–9 V (about 300–450 Ω)
Galvanic isolator (loop-powered input)	2–4 V
Loop-powered indicator	1–3 V
1 km of 1.5 mm² cable (2 cores)	0.5 V

Conductor resistance (copper, 20 °C)

Cross-section	Resistance per conductor
0.5 mm²	36.0 Ω/km
0.75 mm²	24.5 Ω/km
1.0 mm²	18.1 Ω/km
1.5 mm²	12.1 Ω/km
2.5 mm²	7.41 Ω/km

Troubleshooting a low-voltage loop

If the reading clips before 100%, measure the voltage at the transmitter terminals while forcing 20–22 mA. If it is below the lift-off voltage, raise the supply, remove a series device, or use a lower-resistance input.

Frequently asked questions

How do I calculate maximum loop resistance?

R_max = (supply voltage − transmitter minimum voltage) ÷ maximum loop current. With 24 V, a 10.5 V transmitter and 22 mA, R_max ≈ 614 Ω.

Why use 22 mA instead of 20 mA?

Transmitters signal faults and over-range above 20 mA (NAMUR NE 43 uses ≥ 21 mA for failure). The loop must still deliver enough voltage at those currents or the alarm signal is lost.

What happens if the loop resistance is too high?

The transmitter runs out of voltage at high currents, so readings clip near the top of the range — often seen as a reading that never reaches 100%.

Does HART need a minimum resistance?

Yes. HART communication needs at least 230–250 Ω of loop resistance between the power supply and the transmitter.

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