BMS Cable Selection Guide: Shielded vs Unshielded, and Getting the AWG Right

BMS Cable Selection Guide: Shielded vs Unshielded, and Getting the AWG Right 

This is a question I have pondered while staring at a list of presentations at the BMS panel meeting trying to choose whether a shielded or unshielded cable is sufficient and whether 18 AWG is truly enough for that particular sensor. This is a situation where it seems quite straightforward on paper but becomes much trickier when it comes time to pull the cable through the ceiling next to VFDs. 

This guide walks through the two decisions that matter most when specifying BMS cable shielding and wire gauge and why getting them wrong doesn't usually show up on day one. It shows up six months later, as a "ghost" fault that only happens when the chiller plant cycles. 

Why BMS cable isn't just "any cable" 

The BMS is actually the nervous system of the building, as there are temperature sensors, VAV controllers, dampers, valves, flow meters, and so on, all communicating with the controller over low-voltage wires. The signals carried via such wires are most likely analog (4-20mA, 0-10V) or low-voltage digital communication (RS-485, BACnet MSTP, KNX). They do not carry any significant amount of power, and they do not resist noise. 

This is basically the essence of the problem. BMS cable fails not because it cannot conduct electricity, but because it collects all sorts of noise from other systems operating in the same building, including fluorescent and LED drivers, VFDs, elevator motors, large transformers, and other cables laying nearby. Selecting the right type of shielding and cable gauge is mostly about dealing with noise. 

Shielded vs unshielded: what the shield is actually doing 

An unshielded cable, as you might have guessed, consists of only twisted pairs (or sometimes straight pairs) encased in an outer layer of PVC or LSF material. Shielded cables come with an additional metal layer, usually either foil, braided, or both, surrounding the conductor material. 

The shield doesn't stop noise from existing near the cable. What it does is give that noise a path to ground before it can induce a current on the actual signal conductors. Think of it less like a wall and more like a lightning rod, it intercepts the interference and drains it away instead of letting it couple onto your sensor wire. 

Here's the practical way to think about when each type belongs in your project: 

Use unshielded cable when: 

• The run is short (generally under 30 metres) and stays well clear of power cabling 

• You're wiring simple on/off contacts, relays, or basic digital I/O with no analog signal involved 

• The cable path doesn't cross near VFDs, contactors, ballasts, or high-current busbars 

• You're working in a low-noise environment; a small retail fit-out is a very different animal from a mechanical plant room 

Use shielded cable when: 

• You're carrying analog signals (4-20mA, 0-10V) over any meaningful distance 

• The route runs parallel to, or crosses, power cables especially anything feeding motors or VFDs 

• You're wiring RS-485 or BACnet MSTP trunks, where noise doesn't just cause a bad reading, it can corrupt the whole communication bus 

• The installation is in a plant room, riser, or anywhere near switchgear, generators, or large electrical loads 

A rule of thumb that holds up well in the field: if you're unsure, shield it. The cost difference between shielded and unshielded cable is usually small relative to the labour cost of re-pulling a cable after a fault shows up during commissioning. 

One detail that trips people up constantly 

A shield can only function properly if it is correctly grounded but at only one point. If both ends of a shielded cable are grounded, you have a ground loop and are producing the very noise that the shield should have kept out. This is one of the most common on-site mistakes with BMS wiring, and it's almost always invisible until commissioning, when a controller starts reporting erratic values for no obvious reason. 

Getting the AWG right 

Wire gauges (American Wire Gauge - AWG) will determine how resistant each metre of your cable will be, and resistance is what causes the majority of the issues with BMS wiring when it isn’t related to noise. Note that the lower the AWG number, the heavier the wire. 

For BMS applications, gauge selection comes down to two things: signal type and cable length. 

For simple digital contacts and short runs (under 50 metres), 22 AWG is usually fine ; these signals just need continuity, not much current. 

In analog wiring such as 4-20mA loops, 18 AWG is usually standard, but for long cables, you would be advised to move to 16 AWG. For 4-20mA loops, voltage drops in the wiring would give an inaccurate measurement at the end of a long cable, especially one that is more than 100 meters. 

For RS-485/BACnet MSTP trunk cable, most manufacturers specify 22 AWG or 24 AWG twisted pair as standard, since these networks are impedance-matched and gauge consistency across the whole trunk matters more than raw thickness ; mixing gauges on the same bus can create reflection issues at the joints. 

A simple gauge selection reference: