What the calculator is for
A two-wire transmitter is powered by the same current it uses to signal. Everything in series with it takes a voltage of I × R: the receiver’s input resistor, a barrier or isolator, and both cable conductors. Whatever is left must stay above the transmitter’s minimum (lift-off) voltage at the highest current the loop will carry. If it does not, the reading clips or a fault signal never reaches the control system.
- Vtransmitter = Vsupply − fixed drops − Imax × Rloop
- Rmax = (Vsupply − fixed drops − Vmin) / Imax
- Cable resistance counts both conductors: 2 × length × Ω/m.
Worked example
A 24 V supply feeds a transmitter needing 10.5 V, a 250 Ω input and 500 m of 1.0 mm² copper (17.24 Ω/km per conductor, so 17.24 Ω for the pair). At a 21.5 mA upscale alarm the loop drops 0.0215 × 267.2 Ω = 5.75 V, leaving 18.25 V at the transmitter, a margin of 7.75 V. The loop can take up to 627.9 Ω in total, so up to 11,000 m of this cable.
Add a zener barrier of 340 Ω and the same loop has only 10.94 V left. Barriers are where most loop budgets fail.
Conductor resistance
| Size | Ω/km | Ω per 100 ft |
|---|---|---|
| 0.5 mm² | 34.5 | 1.05 |
| 0.75 mm² | 23 | 0.701 |
| 1.0 mm² | 17.2 | 0.526 |
| 1.5 mm² | 11.5 | 0.35 |
| 2.5 mm² | 6.9 | 0.21 |
| AWG 22 | 53 | 1.61 |
| AWG 20 | 33.3 | 1.02 |
| AWG 18 | 20.9 | 0.638 |
| AWG 16 | 13.2 | 0.402 |
Common mistakes
- Checking at 20 mA only. Size for the upscale alarm current (at least 21 mA under NAMUR NE43, commonly set at 21.5 to 22.5 mA), so a failed transmitter can still drive its alarm.
- Counting the cable once. Current flows out and back: both conductors count.
- Nominal supply voltage. Use the lowest voltage the supply guarantees, after redundancy diodes and distribution.
- HART without its resistor. HART communication needs at least 230 Ω in the loop, usually the 250 Ω input, and HART transmitters often need a higher minimum voltage.
Basis
Ohm’s law around a series loop. Copper resistance uses 0.017241 Ω·mm²/m at 20 °C, the annealed copper standard behind IEC 60228 conductor tables, with a temperature coefficient of 0.393 % per °C. AWG sizes follow their defining diameter formula. Real cables are specified by maximum resistance; use the datasheet figure when you have it.