KNX Voltage Drop Calculation – Complete Guide for KNX TP Bus Design

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Table of Contents

1. Why KNX Voltage Drop Matters

KNX TP devices receive their operating power through the same twisted-pair bus used for communication. As the distance between the KNX power supply and a device increases, the resistance of the cable causes the bus voltage to decrease.

This is known as voltage drop.

In a small installation, voltage drop may not be noticeable. However, in larger KNX projects with long cable runs, many devices, or an uneven distribution of loads, it becomes an important design consideration.

A KNX line should therefore be checked for:

  • Bus voltage at the power supply
  • Cable length
  • Cable resistance
  • Device current
  • Voltage at remote devices
  • Distribution of devices along the line
  • Position of the power supply
  • Additional voltage-drop points

A useful design principle is:

Do not check only the KNX power supply rating. Check the voltage available at the most demanding point of the bus.

2. What Causes Voltage Drop on a KNX Bus?

Every electrical cable has resistance.

When current flows through the cable, the voltage loss can be calculated using:

Vdrop = I × R

Where:

  • Vdrop = voltage drop in volts
  • I = current flowing through the cable in amperes
  • R = resistance in ohms

For a two-conductor circuit:

R = ρ × L / A

Where:

  • ρ = resistivity of the conductor
  • L = conductor length
  • A = conductor cross-sectional area

In a KNX TP installation, however, the practical calculation should use the actual resistance specified for the selected KNX cable, rather than assuming a generic copper-cable value.

3. KNX Bus Voltage

A KNX TP installation is normally supplied by a KNX power supply producing approximately 30 V DC.

The voltage available at a device will be lower than the supply voltage because of voltage drop along the cable.

Simplified:

KNX PSU → Cable → KNX Device

For example:

30 V → voltage drop → 28 V at device

The exact acceptable operating range should always be checked against the KNX requirements and the device manufacturer’s specifications.

The important engineering question is therefore not simply:

“Is my KNX power supply 30 V?”

but:

“What voltage is actually available at the furthest and most heavily loaded device?”

4. Basic KNX Voltage Drop Formula

The simplest calculation is:

Vdrop = I × R

Suppose:

  • Bus current = 400 mA
  • Cable resistance = 0.08 Ω

Then:

I = 0.4 A

Therefore:

Vdrop = 0.4 × 0.08

Vdrop = 0.032 V

This illustrates an important point:

The resistance value must correspond to the actual electrical path being considered.

For KNX design, you should not blindly apply a generic cable resistance figure to the entire 300–350 m bus length.

The current is not necessarily the same throughout the complete line because KNX devices are distributed along the cable.

5. Why KNX Voltage Drop Is More Complicated Than a Simple Cable Calculation

Consider a KNX line with the power supply at one end:

KNX PSU
  │
  │
  ├──── Device
  │
  ├──── Device
  │
  ├──── Device
  │
  ├──── Device
  │
  └────────────── Device
                 ↑
            Furthest device

The current flowing through the first section of cable is the current required by all downstream devices.

After the first group of devices, some current is consumed.

Therefore the current flowing through the next section is lower.

This means a realistic calculation needs to consider:

Cable section + downstream current

rather than simply multiplying:

Total line current × total line length

This is particularly important in large installations.

6. A Practical Section-by-Section Calculation

Consider the following simplified line:

PSU
 │
 ├── 100 mA load
 │
 ├──────── 50 mA load
 │
 ├──────── 100 mA load
 │
 └──────── 100 mA load

Total load:

100 + 50 + 100 + 100 = 350 mA

But the current in the cable changes as we move along the line.

Cable SectionDownstream Current
PSU → Load 1350 mA
Load 1 → Load 2250 mA
Load 2 → Load 3200 mA
Load 3 → Load 4100 mA

Therefore, the voltage-drop calculation should consider each section separately.

Total voltage drop:

Vdrop(total) = Vdrop₁ + Vdrop₂ + Vdrop₃ + Vdrop₄

This approach gives a much more meaningful result than assuming the full 350 mA flows through every metre of cable.

7. KNX Cable Resistance

KNX TP installations normally use a dedicated KNX twisted-pair cable.

A common KNX cable specification is:

2 × 2 × 0.8 mm

The actual electrical resistance depends on the cable construction and manufacturer.

For engineering calculations, use the resistance stated in the cable manufacturer’s datasheet.

This is important because:

  • Different cable constructions can have different resistance.
  • Temperature affects conductor resistance.
  • The resistance value may be specified per conductor length or as a loop resistance.
  • Some calculations use Ω/km while others use Ω/m.

Always check the units.

For example:

Ω/km

must be converted to:

Ω/m

before using it in a metre-based calculation.

8. Example: KNX Voltage Drop Calculation

Consider a KNX line with:

  • KNX supply = 30 V
  • Total bus load = 400 mA
  • Cable length = 100 m
  • Cable resistance = manufacturer-specified value
  • Devices distributed along the cable

Instead of treating the entire 100 m as one resistor carrying 400 mA, divide the installation into sections.

For example:

SectionLengthDownstream Current
PSU → A25 m400 mA
A → B25 m300 mA
B → C25 m200 mA
C → D25 m100 mA

For each section:

Vdrop₁ = I₁ × R₁

Vdrop₂ = I₂ × R₂

Vdrop₃ = I₃ × R₃

Vdrop₄ = I₄ × R₄

Then:

Vdrop total = Vdrop₁ + Vdrop₂ + Vdrop₃ + Vdrop₄

Finally:

Vdevice = VPSU − Vdrop total

This gives the estimated voltage at the remote device.

9. The Furthest Device Is Not Always the Only Device to Check

It is tempting to assume:

The physically furthest KNX device automatically has the lowest voltage.

That is not necessarily true.

The voltage at a device depends on:

  • Distance from the power supply
  • Current flowing through the preceding cable sections
  • Distribution of other KNX devices
  • Cable resistance
  • Branch arrangement
  • Location of the power supply

A device that is not physically furthest can potentially experience significant voltage drop if it lies behind a heavily loaded cable section.

Therefore, professional design should consider the electrically worst-case location, not just the longest cable run.

10. KNX Power Supply Location Matters

The location of the KNX power supply can affect voltage distribution.

Consider:

Configuration A

PSU ───────────────────────── Devices

Most of the line is downstream of the PSU.

Configuration B

Devices ───────── PSU ───────── Devices

The supply is positioned between groups of devices.

The second arrangement can distribute current more effectively in some designs, although the permitted KNX topology and installation rules must always be respected.

The power supply location should therefore be considered during the design stage rather than treated as an afterthought.

11. Measuring KNX Bus Voltage in the Field

Calculation is only one part of commissioning.

Actual bus voltage should also be measured during troubleshooting and commissioning.

A multimeter can be used to measure the DC voltage between the KNX bus conductors, provided the measurement is performed correctly and safely.

Typical procedure:

  1. Identify the KNX TP pair.
  2. Set the meter to an appropriate DC voltage range.
  3. Measure the voltage near the KNX power supply.
  4. Measure the voltage at selected remote devices.
  5. Compare the readings.
  6. Pay particular attention to the electrically remote or heavily loaded sections.

For example:

Measurement PointMeasured Voltage
At PSU30.1 V
Mid-line29.3 V
Remote device28.5 V

The difference between the measurements provides useful diagnostic information.

12. Voltage Drop vs Low Power Supply Voltage

These two problems can look similar but have different causes.

Problem A — Low PSU output

If the power supply itself is producing significantly less voltage than expected, investigate:

  • Power supply fault
  • AC input issue
  • Overload
  • Protection state
  • Incorrect measurement
  • Device-specific power-supply behaviour

Problem B — Excessive cable voltage drop

If:

PSU voltage = normal

but:

Remote device voltage = significantly lower

then investigate:

  • Cable resistance
  • Cable length
  • Excessive load
  • Poor connection
  • Loose terminal
  • Damaged conductor
  • Incorrect wiring
  • High-resistance connection

This distinction is extremely useful during troubleshooting.

13. Loose Connections Can Create Local Voltage Drop

Not every voltage-drop problem is caused by cable length.

A poor connection can introduce additional resistance.

For example:

KNX Cable ────X──── KNX Device
              ↑
        High-resistance
          connection

Possible causes include:

  • Poor terminal connection
  • Damaged conductor
  • Incorrect stripping
  • Loose connector
  • Corroded contact
  • Mechanical damage

The result can be:

Normal voltage near PSU + unexpectedly low voltage downstream

If the voltage changes significantly when the load changes, investigate the connection and cable path carefully.

14. Symptoms of Excessive Voltage Drop

KNX voltage-drop problems can produce intermittent symptoms such as:

  • Devices going offline
  • Programming failures
  • Communication errors
  • Devices restarting
  • Unstable sensors
  • Actuators becoming unavailable
  • Intermittent ETS communication
  • Problems occurring only when the system is heavily loaded

This is why voltage-drop problems can sometimes be incorrectly diagnosed as:

“KNX communication problems”

or:

“Faulty device.”

The physical bus should be checked before replacing multiple devices.

15. Voltage Drop and KNX Bus Current Are Connected

The previous KNXHUB article on KNX power supply sizing established that total bus current must be calculated.

Voltage drop is the next step.

The relationship is:

Higher current → higher voltage drop

because:

Vdrop = I × R

Therefore:

  • More devices can increase current.
  • Higher-current devices can increase current.
  • Longer cable paths increase resistance.
  • Higher resistance increases voltage drop.

This creates a direct relationship between:

Power Supply → Bus Current → Cable Resistance → Voltage Drop

16. Common KNX Voltage Drop Design Mistakes

Mistake 1: Using only device count

“100 devices means I need a 640 mA supply.”

Device count alone does not determine voltage drop.

Mistake 2: Assuming 30 V at the PSU means 30 V everywhere

The voltage at the remote device can be lower.

Mistake 3: Ignoring current distribution

The entire line does not necessarily carry the same current.

Mistake 4: Using generic cable resistance

Use the actual KNX cable manufacturer’s electrical specification.

Mistake 5: Checking only the longest cable

The electrically worst-case point depends on both distance and downstream load.

Mistake 6: Ignoring connections

A poor terminal connection can introduce significant local resistance.

Mistake 7: Replacing devices before measuring the bus

A simple voltage measurement can sometimes identify a supply or wiring problem much faster than replacing KNX devices.

17. KNX Voltage Drop Troubleshooting Workflow

When a KNX installation shows intermittent device failures:

Step 1 — Check the power supply

Measure the KNX bus voltage near the PSU.

Step 2 — Check total bus load

Compare the actual installation against the PSU rating.

Step 3 — Measure remote voltage

Check voltage at the affected device.

Step 4 — Compare measurements

Look for a significant voltage difference between the PSU and remote device.

Step 5 — Follow the cable path

Inspect:

  • Junctions
  • Terminals
  • Connectors
  • Cable damage
  • Distribution points

Step 6 — Check the topology

Confirm that the physical installation follows the intended KNX topology.

Step 7 — Test under operating conditions

A problem may become visible only when the system is operating with its normal device load.

18. Designing a Reliable KNX TP Line

A reliable KNX line should be designed as a complete electrical system.

The design process should include:

1. Device schedule

↓

2. Device current calculation

↓

3. Power supply selection

↓

4. Cable and topology design

↓

5. Voltage-drop calculation

↓

6. Worst-case voltage check

↓

7. ETS commissioning

↓

8. Physical bus measurements

This approach reduces the risk of discovering electrical problems after the installation has been completed.

19. KNX Voltage Drop Calculation Checklist

Before approving a KNX TP design:

  • Record actual current consumption of every device.
  • Calculate total bus current.
  • Select an appropriate KNX power supply.
  • Obtain cable resistance from the cable datasheet.
  • Divide long runs into relevant cable sections.
  • Consider downstream current for each section.
  • Calculate cumulative voltage drop.
  • Check the electrically worst-case device.
  • Consider future expansion.
  • Verify the installed bus voltage during commissioning.
  • Inspect terminals and connections if unexpected voltage drop is found.

Conclusion

KNX voltage-drop calculation is an important part of professional KNX TP system design.

The basic electrical relationship is simple:

Vdrop = I × R

But a real KNX installation requires more than applying this formula to the total cable length.

The designer should consider:

Bus current + cable resistance + current distribution + topology + device location

A properly sized KNX power supply is only the starting point. The final question is whether sufficient bus voltage remains available at the devices under realistic operating conditions.

For integrators and consultants, the most effective approach is to combine calculation during design with actual voltage measurements during commissioning.

Design the KNX bus electrically on paper, then verify it physically on site.

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