top of page

🛠️ Sensor Signals for BMS Installers: 4–20 mA, 0–10 V, 2–10 V, and Thermistors 10k/20k (with Pro Installation Tips)

  • Writer: Mikhail Strashnov
    Mikhail Strashnov
  • 6 days ago
  • 3 min read

If you're a BMS installer, you deal with sensors every day—temperature, pressure, light—and they usually send information using analog signals. The most common types are 4–20 mA, 0–10 V, 2–10 V and Thermistors. Here’s a clear, easy guide (grade‑school simple!) with smart installation tips.


1. ✅ 4–20 mA Current Loop

How it works: Sensors send current between 4 mA (zero) and 20 mA (full-scale) .

Why it's great:

  • Resists noise & voltage drops, even over long cable runs

  • Fault alert: reading 0 mA means wire is broken

  • Industry standard in factories and process plants

Cons:

  • Needs 24 V power and proper loop wiring

  • Slightly pricier

Pro Tips for Installers:

  • Always turn off power before wiring 4–20 mA loops—power is present on terminals, risking short circuits

  • Use shielded cable, grounded on one side only, to reduce noise

  • For 2‑wire transmitters, confirm whether the power is sourced or sunk—don’t mix types


2. 🔌 0–10 V Voltage Signal

How it works: Voltage varies from 0 V (zero) to 10 V (max)

Why it's easy:

  • Simple and cheap

  • Easy to test with a voltmeter

Cons:

  • Voltage drops over cables reduce accuracy

  • No failure alert—0 V could mean no signal

Installer Tips:

  • Use twisted-pair shielded cable to reduce noise

  • Keep voltage drop under 0.3 V for reliable performance

  • Separate signal cables from power lines to avoid interference 


3. ⚠️ 2–10 V Voltage Signal

How it works: Voltage ranges 2–10 V where 2 V is zero, giving a “live-zero” .

Why it’s useful:

  • Adds simple fault detection

  • Better noise immunity than 0–10 V

Cons:

  • Less common—might not fit all devices

Installer Know-How:

  • Treat wiring like 0–10 V—use shielded twisted-pair and ground the shielding 

  • Confirm your controller accepts a 2 V minimum reading


  1. 🌡️ Thermistor Sensors: 10k & 20k

What is a thermistor? A thermistor is a temperature sensor whose resistance changes with temperature.


🧊 10k Thermistor

  • Most common type in BMS

  • Resistance at 25°C = 10,000 ohms (10k)

  • Typically NTC (Negative Temperature Coefficient) — resistance drops as temperature rises

  • Used in room sensors, duct sensors, pipe sensors

Pros:

  • Very accurate at measuring temperature

  • Simple 2-wire connection

  • No need for external power — sensor is passive

Cons:

  • Resistance signals can't go long distances — limited to ~100–150 ft (30–45 m)

  • Controller must match sensor curve (type II, III, etc.)

Installer Tips:

  • Use twisted-pair wire

  • Don’t run near AC power cables — noise affects resistance readings

  • Always verify thermistor type (10k Type II is not the same as Type III!)


🔥 20k Thermistor

  • Works just like the 10k but starts at 20,000 ohms at 25°C

  • Less common, but sometimes used for longer runs

  • Slightly less sensitive to noise and voltage drop

Installer Tips:

  • Use 20k thermistors if spec sheet recommends it for longer cable runs

  • Always match the controller’s expected input curve


🧰 Quick Summary Table for BMS Installers

Sensor Type

Output

Power Required

Distance

Fault Detection

Use Case

4–20 mA

Current

Yes

✅ Long

✅ Live zero

Industrial, long cable runs

0–10 V

Voltage

Yes

⚠️ Short

HVAC, lighting

2–10 V

Voltage

Yes

⚠️ Short

✅ (2V = zero)

HVAC with fault monitoring

10k Thermistor

Resistance

❌ No

⚠️ Medium

⚠️ (depends on controller)

Room & duct temp sensors

20k Thermistor

Resistance

❌ No

⚠️ Medium+

⚠️

Long-run temp sensing

🔧 Real Installer Insight

I once replaced a failed 0–10 V temp sensor in a long duct run with a 10k thermistor. But the controller showed the wrong reading. Turned out the system was expecting a 10k Type III, and we had used Type II. That 5-minute fix turned into an hour! Moral of the story: know your thermistor curve.


✅ Final Advice for BMS Installers

  • For noisy sites or long cables: go with 4–20 mA

  • For simple HVAC installs: use 0–10 V or 2–10 V

  • For precise temp readings in short runs: use 10k thermistors

  • For longer thermistor runs: choose 20k, if supported


Choosing the right sensor signal helps reduce callbacks, improves system uptime, and keeps your clients happy.


LightCom - pro BMS Installers team

Comments


bottom of page