A single KNX TP line supports 64 devices and 1000m of cable. For a typical apartment, that's more than enough. But a large villa, a commercial floor, or a multi-floor office needs more — and that means adding lines with line couplers.

When you need multiple lines

You'll need a second TP line when: device count exceeds 64 (including power supply), total cable length exceeds 1000m, you want galvanic separation between areas (fault isolation — a short on line 1.2 doesn't affect 1.1), or you're designing a large project where different contractors work on different building sections.

Line coupler vs backbone coupler

A **line coupler** (Linienkoppler) connects an area line to the backbone. A **backbone coupler** connects an area to the main backbone in very large projects. For most projects up to 8 lines, line couplers on a backbone line are the right architecture. The backbone itself needs its own power supply and carries only coupler traffic — it should have minimal devices on it.

Individual addresses and the area/line structure

KNX uses a 3-part address: Area.Line.Device. The coupler's individual address defines the area and line it serves. A coupler at address 1.2.0 creates line 1.2. Devices on that line get addresses 1.2.1 through 1.2.255 (in practice, 1–63 for TP). The area number (1) groups lines 1.1 through 1.15 — this forms an "area".

Telegram filtering at the coupler

The line coupler acts as a filter. By default, it only passes telegrams that are addressed to devices on the other side. This reduces bus load on each line. In ETS, the coupler's filter table is automatically calculated from your group address assignments — but you must download the filter table to the coupler after any group address change. Forgetting to download coupler filter tables is a common cause of "my blind stopped responding from the other room" bugs.

Power supply per line

Each TP line needs its own 640mA or 320mA power supply. Don't share a PSU across a line coupler. The coupler electrically separates the lines. If you share a PSU and the coupler fails, you lose bus power on both sides.

Practical example: 300 m² villa

Area 1, Line 1.1: Ground floor — 45 devices. Line 1.2: Upper floor — 35 devices. Line 1.3: Technical (meters, HVAC) — 20 devices. Backbone 1.0: 3 line couplers + KNX IP router. Total: 100 devices, 4 power supplies, 1 IP router, 3 line couplers. Clean, fault-isolated, expandable.