RS-485 Maximum Cable Length by Baud Rate
How far an RS-485 segment reaches depends on how fast you drive it and what you drive it through. This table gives the planning figures Bit2Bus uses for BACnet MSTP and Modbus RTU networks.
Reference table
| Baud rate | Base length | Cat5e | Cat6 | Shielded Twisted Pair | RS-485 Cable | Shielded 1mm² | Belden 3105A |
|---|---|---|---|---|---|---|---|
| 9 600 baud | 1 200 m | 1 080 m | 1 140 m | 960 m | 1 200 m | 1 200 m | 1 200 m |
| 19 200 baud | 1 200 m | 1 080 m | 1 140 m | 960 m | 1 200 m | 1 200 m | 1 200 m |
| 38 400 baud | 1 000 m | 900 m | 950 m | 800 m | 1 000 m | 1 100 m | 1 000 m |
| 76 800 baud | 500 m | 450 m | 475 m | 400 m | 500 m | 550 m | 500 m |
| 115 200 baud | 350 m | 315 m | 332 m | 280 m | 350 m | 385 m | 350 m |
All values in metres, for a single segment without repeaters.
Open the cable length calculatorWhy 1200 m keeps showing up
1200 m (4000 ft) is the figure the RS-485 standard is usually quoted with, and it is the practical ceiling for a segment. A better cable can beat a poorer one at the same speed, which is what the cable factors in the table express - but no cable quality pushes a segment past that ceiling, so the numbers above are capped there rather than extrapolated.
What the cable types mean
- RS-485 cable - purpose-made low-capacitance twisted pair, the reference case.
- Belden 3105A - a common 22 AWG shielded pair for industrial field bus, comparable to the reference.
- Shielded 1 mm² - thicker conductors, lower loop resistance, slightly better at a given speed.
- Cat5e / Cat6 - works, and is often already installed, but the impedance is 100 Ω rather than the 120 Ω RS-485 expects.
- Shielded twisted pair - generic STP with no controlled impedance; the most conservative entry.
Things that shorten a segment in practice
- Missing termination. A segment wants 120 Ω at both physical ends, and nowhere else.
- Stubs. Spurs off the trunk should be as short as you can make them; a daisy chain beats a star.
- Device count. Every transceiver loads the line - see the max devices calculator for the 32 unit load budget.
- Electrical noise. Running alongside power cabling costs you margin that this table assumes you still have.
Related
These are planning figures for a healthy installation, not a substitute for the device datasheets on your bus. Always check what your specific transceivers state.