How to Select 4‑20 mA Signal Cables

In automation‑control engineering projects, the selection of 4‑20 mA signal cables directly determines system anti‑interference performance, signal transmission accuracy and long‑term operational reliability. Combining industry standards and field engineering practices, precise cable selection can be performed from the following five core dimensions.

I. Core Construction and Anti‑Interference Design

The 4‑20 mA signal is a weak analog signal highly susceptible to electromagnetic interference from on‑site sources such as frequency converters, large electric motors and welding machines. Therefore, cable construction must satisfy the following requirements.

1. Twisted‑Pair Structure

Twisted‑pair cables are mandatory. The symmetry of twisted pairs offsets interference induced by external electromagnetic fields. Shorter twist pitches deliver stronger anti‑interference capability.

2. Group‑Based Shielding

Adopt a layered‑shield S/FTP structure: individual aluminium‑foil shielding for each pair plus overall copper‑braid shielding. This effectively mitigates crosstalk between pairs and external common‑mode interference.

3. Pure‑Copper Conductors

High‑purity copper conductors (copper content ≥ 99.9 %) shall be used. Aluminium‑core or copper‑clad‑aluminium conductors are strictly prohibited, to guarantee sound conductivity and prevent oxidation and overheating at joints.

4. Insulation‑Layer Materials

Insulation shall be polyethylene (PE) or polyvinyl chloride (PVC), with insulation resistance ≥ 500 MΩ·km. PE insulation offers a lower dielectric constant and superior high‑frequency performance, suited for high‑speed data‑transmission scenarios.

II. Conductor Cross‑Section vs. Transmission Distance

Loop voltage drop shall be calculated during selection to guarantee that the voltage at the transmitter terminal remains above its minimum operating voltage (typically ≥ 16 V) at maximum transmission distance. Below are reference selections for standard 24 V power‑supply systems.

Voltage‑drop calculation formula:

ΔU = I × R × L × 2

Where: ΔU = loop voltage drop (V); I = loop current (mA); R = resistance per unit length (Ω/km); L = one‑way transmission distance (km).

III. Jacket Material and Environmental Compatibility

Cable outer jackets shall be selected according to actual installation environments.

IV. Key Installation and Grounding Specifications

Proper cable selection is only the first step; correct installation also determines final performance.

1. Single‑Point Shield Grounding

The cable shield must be reliably grounded only at the control‑cabinet (PLC/DCS card‑module) end. The shield at the field‑instrument end shall be insulated and floating. Grounding both ends is forbidden, as ground‑potential differences create ground loops that introduce severe interference and signal drift.

2. Segregation of Low‑Signal and Power Cables

4‑20 mA signal cables shall never share conduits or cable trays with power cables (e.g., 380 V supply cables, frequency‑converter output cables). For parallel routing, maintain a minimum clear separation of 30 cm. Where crossing is unavoidable, route cables at a 90‑degree perpendicular angle.

3. Minimum Bending‑Radius Control

Avoid sharp bends or kinks during installation. The minimum cable bending radius shall be no less than 6‑7.5 × the outer cable diameter, to prevent internal‑insulation damage or broken shield braid wires.

4. Conduit Protection

Route cables inside galvanised‑metal conduits or cable trays; both ends of metal conduits shall be reliably grounded. Conduit fill factor shall not exceed 40 % to ensure heat dissipation and maintenance space.

V. Common Selection Pitfalls and Mitigations

VI. Quick‑Reference Selection Table

Closing Remarks

Selecting 4‑20 mA signal cables is a systematic engineering task. Signal integrity, electromagnetic compatibility, environmental durability and installation specifications must all be considered comprehensively. Engineers shall fully investigate site operating conditions before selection, strictly follow relevant national standards, and enforce quality control over critical steps such as shield grounding during installation, to guarantee stable long‑term operation of automation‑control systems.

Reference Standards

‑ GB/T 19666‑2019 General Rules for Flame‑Retardant and Fire‑Resistant Electric Cables and Optical Cables

‑ GB/T 5023‑2008 Polyvinyl‑Chloride‑Insulated Cables of Rated Voltages up to and Including 450/750 V

‑ HG/T 2006‑2022 Thermoplastic Polyurethane Elastomers

‑ IEC 61034‑2 Measurement of Smoke Density of Cables Burning under Specified Conditions

‑ GB 50057‑2010 Code for Design of Building Lightning Protection

‑ SH/T 3019‑2016 Specification for Design of Instrument Piping and Cabling in Petrochemical Engineering