“What cable do I pull for KNX?” is one of the questions we hear most from US integrators and electrical contractors. The short answer: the KNX bus is forgiving, the National Electrical Code is not. This guide covers what the bus needs electrically, which cable markings an inspector will look for, how far you can run it, and how DALI wiring differs.
The 30-second answer
- Electrically, KNX TP needs one twisted pair of 0.8 mm (≈20 AWG) solid copper conductors, ideally shielded.
- In the US, the jacket must carry an NEC listing marking — CL2, CL3, CM or their riser/plenum versions — that fits the space it runs through.
- Distances: per line segment, max 1,000 m (3,280 ft) of cable, 350 m (1,150 ft) from the power supply to any device, and 700 m (2,300 ft) between any two devices.
- DALI is different: two conductors, no shield, sized for a 2 V drop — and not SELV, so how you classify the circuit decides the cable.
What the KNX bus actually needs
KNX TP carries data (9,600 bit/s) and device power on the same pair, at a nominal 30 V DC SELV supplied by a KNX power supply with an integrated choke. One pair is all the bus needs. Look for these characteristics:
| Characteristic | What to look for |
|---|---|
| Conductors | One pair, 0.8 mm (≈20 AWG) solid copper |
| Polarity | Red (+) / black (−) |
| Screen | Overall foil shield with drain wire |
| Conductor resistance | ≈10 Ω per 1,000 ft per conductor (20 AWG) |
Can I use Cat5e or Cat6 instead?
On a short run it will often work, and it's the most common shortcut we see. We don't recommend it, for three reasons:
- Thinner conductors. Cat5e/Cat6 conductors are 24–23 AWG, with roughly two to two-and-a-half times the resistance of 20 AWG. The 350 m and 700 m limits assume 0.8 mm conductors; with thinner wire, voltage at the far devices drops faster.
- Terminals. KNX bus connectors are designed for 0.6–0.8 mm solid wire. Undersized conductors are more prone to loose contacts — the classic cause of intermittent bus faults that eat commissioning days.
- Identification. A gray Cat6 run looks like any network drop, and sooner or later someone patches it into a switch. Use cable that is unmistakably the KNX bus, and label both ends.
What US inspectors look for
The NEC doesn't care whether a cable says “KNX” on it; it looks for a listing marking printed on the jacket. For a low-voltage control circuit like the KNX bus, that means a Class 2/Class 3 cable (CL2, CL3), or a communications cable (CM) permitted to substitute for it, in the fire rating required for the space. Since the 2023 NEC, the listing requirements for these cables sit in Article 722; the circuit rules remain in Article 725.
| Where the cable runs | Minimum marking | Also acceptable |
|---|---|---|
| Plenum (return-air ceiling or space) | CL2P | CL3P, CMP |
| Riser (vertical run between floors) | CL2R | CL3R, CL2P, CL3P, CMR, CMP |
| General purpose (walls, trays, non-plenum ceilings) | CL2 | CL3, CM, any riser or plenum type above |
| Limited use (e.g., dwellings) | CL2X | Any of the above |
Rule of thumb: a plenum-rated cable (CMP or CL3P) can go anywhere a Class 2 bus can go. Two more points decide whether the job passes:
- Class 2 depends on the source, not the wire. A circuit is Class 2 because it's powered by a listed Class 2 power source. Before you call the KNX bus Class 2 on the drawings, check the listing of the bus power supply you're using. Many KNX devices are CE certified but not UL/NRTL listed — our product pages show the certification status of each device.
- Separation inside the panel. KNX actuators put the bus terminal a few inches from 120/277 V load terminals. NEC 725.136 requires Class 2 conductors to be kept separate from power conductors in enclosures, raceways and boxes — by a barrier or by the spacing the code specifies. Plan wireways in the enclosure before it's built, not during inspection.
Code adoption and interpretation vary by jurisdiction. Confirm the approach with your AHJ before rough-in.
The cables we stock
- KNX Cable, UL Listed Type CMP — 20 AWG solid, shielded pair, FEP insulation, low-smoke jacket in orange with green stripe (UL file E179333). Plenum-rated, so it covers plenum, riser and general-purpose runs. Sold per 100 ft or as a 1,000 ft spool.
- KNX Cable 2×2×0.8 mm (20 AWG), halogen-free — standard 2-pair KNX bus cable, CPR class Dca. Not UL listed (CE certified; not UL/NRTL listed — verify local code requirements before specifying in US installations).
- DALI Bus Cable, UL Listed plenum — unshielded 2-conductor, 16 AWG (CMP) or 14 AWG (CL3P), for DALI buses installed as Class 2.
- NM-B PCS — power conductors plus a 16 AWG Class 2/Class 3 control pair in one jacket, for residential luminaire drops with DALI or 0-10V control.
Browse the full cable collection →
How far you can run it: KNX limits
| Limit (per line segment) | Metric | Imperial |
|---|---|---|
| Bus devices | 64 | |
| Power supply to any device | 350 m | 1,150 ft |
| Between any two devices | 700 m | 2,300 ft |
| Total cable length | 1,000 m | 3,280 ft |
Topology is free — line, star or tree — but never a closed ring, and no termination resistors are needed. For more than 64 devices, add line repeaters (up to three per line, for 255 devices) or split the installation into lines with couplers. Where devices are concentrated far from the supply, a line can take a second power supply; follow the manufacturer's minimum spacing between supplies.
Sizing the supply: add up the bus current of every device from its datasheet. A 640 mA supply covers 64 devices at about 10 mA each; a 160 mA supply, about 16. Options include the Siemens 640 mA, the Phoenix Contact STEP3 640 mA and the decentralized Siemens JB 125C33 160 mA (110–277 V AC input).
DALI wiring is a different animal
DALI shares KNX's “two wires, almost any topology” convenience, but the electrical rules differ:
- Two conductors, polarity-free. No shield and no twisted pair required.
- Up to 300 m (984 ft) of total cable, with no more than 2 V of drop between the bus power supply and the farthest device at up to 250 mA.
- Up to 64 control gear per line. No closed rings.
- Not SELV. In Europe, DALI routinely shares a cable with mains. In the US, the classification decides the cable: with a UL Listed Class 2 bus power supply and drivers whose DALI terminals are rated for Class 2, you can use listed low-voltage cable run separately from power. If any driver or supply puts its DALI connection in Class 1, the whole bus becomes Class 1 and needs 600 V conductors — THHN/THWN in raceway, or MC cable with integral control conductors.
| Longest DALI run | IEC cross-section | US equivalent* |
|---|---|---|
| Up to 100 m (328 ft) | 0.5 mm² | 20 AWG |
| Up to 150 m (492 ft) | 0.75 mm² | 18 AWG |
| Up to 300 m (984 ft) | 1.5 mm² | 16 AWG (14 AWG for margin) |
*Our calculation for the worst case: 250 mA and 2 V of drop. Real bus loads are usually well below 250 mA.
For bus power, see the LTECH DALI-2 DIN-rail bus power supply and the Lunatone DALI PS2 — and check each one's listing before you put Class 2 on the drawings.
Pre-pull checklist
- Agree with the AHJ how the KNX bus and any DALI bus will be classified (Class 2 or Class 1).
- Check the listing of every bus power supply.
- Choose the cable rating by space: plenum, riser or general purpose.
- Measure the longest runs against 350 / 700 / 1,000 m for KNX, and 300 m with 2 V of drop for DALI.
- Lay out panel separation between bus and power conductors.
- Give each bus its own jacket color, and label both ends.
- Record the bus topology on the as-builts — it saves hours in ETS commissioning.
FAQ
What gauge is KNX cable?
KNX bus conductors are 0.8 mm solid copper, roughly equivalent to 20 AWG.
Does KNX cable need a UL listing in the US?
The cable needs an NEC listing marking on the jacket (CL2, CL3, CM or their riser/plenum types) that suits the space it runs through. Our KNX Cable Type CMP is UL listed for plenum, riser and general-purpose runs; our 2-pair halogen-free KNX cable is not UL listed.
How many devices can one KNX power supply feed?
It depends on device current. As a rule of thumb, 640 mA covers about 64 devices and 160 mA about 16, but always add up the datasheet values. The limit of 64 devices per line segment applies either way.
Does DALI cable need to be shielded?
No. The DALI standard requires neither a shield nor a twisted pair. What matters is conductor size for the 2 V drop limit and a cable rating that matches the circuit classification.
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This guide summarizes general NEC concepts for planning purposes. It is not a substitute for the code edition adopted in your jurisdiction; the AHJ has the final word.