Quick Answer / Summary: Two distribution families: low-impedance (4-16 ohm) for short runs and single rooms, and 100V/70V line for multi-speaker buildings where long runs at low impedance would waste power. Fixed installs also require the correct jacket rating for the route, consistent polarity across every tap, and gauge sized to the circuit’s current rather than to habit.
Loudspeaker cable in a building is a different discipline from speaker wire in a living room: the runs are long, the circuits are many, and half the world’s installed systems distribute audio at 100 volts rather than 8 ohms. The rules that follow are the ones inspectors and commissioning engineers actually apply.
Low Impedance Versus 100V Line
Low-impedance distribution delivers power at amplifier voltage into a few ohms; cable resistance is therefore a large fraction of the load and long runs lose both level and damping. Constant-voltage line systems step the amplifier up to 100V (or 70V), each loudspeaker carries a step-down transformer, and the same power travels at far lower current – which is why a 100V line can run hundreds of metres on modest gauge. Choosing between them is a function of run length and speaker count, not of taste.
Tap Settings and Power Budgets
Each 100V loudspeaker’s transformer carries tap settings in watts. Summing the taps across the circuit yields the load the amplifier must supply, and the sum must stay below the amplifier’s rated line output with headroom. A mis-set tap is the most common commissioning fault: one speaker at double its intended setting can consume the headroom for several others.
Gauge in Line Systems
Because the current is low, 100V line circuits tolerate 16 or 18 AWG over long distances where low-impedance circuits would demand 12 AWG. The limit is line loss – commonly kept under ten percent of the nominal voltage – which is a voltage-drop calculation rather than a damping one.
Jackets, Routes, and the Polarity Discipline
In-wall and plenum routes require CL2, CL3, or the equivalent local fire rating, printed on the jacket and checked at inspection. Plenum spaces demand the higher grade because their air is recirculated. Polarity, meanwhile, scales: with dozens of loudspeakers on one circuit, a single reversed tap cancels that speaker against its neighbours and is inaudible until commissioning – which is why every tap is marked during rough-in, not after.
Labelling Schemes That Survive
A workable scheme names the circuit, the tap number, and the polarity at both ends – for example C3-T07+ and C3-T07-. Printed labels survive where marker ink does not, and the commissioning sheet mirrors the same identifiers so the test record and the building agree.
Testing Before Handover
Commissioning checks three things per circuit: continuity and polarity at every tap, line loss against the design figure, and tap settings against the schedule. The tests are cheap; the rework after a failed inspection is not, especially where ceilings have closed.
Commissioning Checklist for Installed Systems
Before handover, verify three things per circuit: continuity and polarity at every tap, line loss within the design figure, and tap settings against the schedule. Record the results with the circuit identifiers used on the drawings, so the next technician inherits evidence rather than assumption.
Distribution Comparison
| Line | Low Impedance | 100V Line |
|---|---|---|
| Typical load | 4-16 ohms | Transformer taps in watts |
| Run length | Short to moderate | Hundreds of metres |
| Gauge pressure | High – damping budget | Low – voltage drop only |
| Speaker count | One or two per channel | Many per circuit |
| Failure mode | Loss and soft bass | Mis-set tap, reversed polarity |
Practical Applications
Specifying a retail or school paging system, wiring a multi-room distributed audio installation, replacing lamp-cord runs in a venue, and preparing cable schedules that pass inspection the first time.
Frequently Asked Questions
What is a 100V line loudspeaker system?
A distribution method where the amplifier output is stepped up to 100V and each loudspeaker steps it back down through a transformer. It lets long runs and many speakers share one circuit with low current and modest cable gauge.
Which cable rating do I need in a ceiling void?
Follow the route: CL2 or CL3 for in-wall and riser routes, and the plenum-rated grade where the void recirculates air. The rating is printed on the jacket and verified at inspection.
How do I size cable for a 100V circuit?
By voltage drop, not damping: keep line loss within the design limit, commonly under ten percent, and check the summed transformer taps against the amplifier’s line output.