Why distributed audio systems use a 100 V line
In a low-impedance system the amplifier drives loudspeakers directly at 4–8 Ω. Current is high, so cable resistance matters over even short runs, and every loudspeaker has to be wired back to the same amplifier terminals. In a constant-voltage system the amplifier output is stepped up to a nominal 100 V (70 V on some systems) and each loudspeaker carries its own step-down transformer with a power tap. At 100 V a 100 W load draws about 1 A, so one cable can feed a long line of low-power loudspeakers without the losses that would make an equivalent low-impedance run unusable.
That is why the 100 V line is the normal format for background music, paging and voice-alarm coverage in hotels, retail, offices, schools, transport buildings and places of worship — many loudspeakers spread across a large floor area, each drawing only a few watts. The cable decision changes with it. On a low-impedance system you size the cable against a loudspeaker’s impedance; on a 100 V line you size it against the total load on the line and how far the last loudspeaker sits from the amplifier.
What determines the cable section on a 100 V line
Three numbers decide the conductor cross-section:
- Total load power on the line, in watts — the sum of every transformer tap fed from that amplifier output.
- Line voltage — 100 V nominal, or 70 V on systems built to that standard. It sets the current for a given load, because I = P / V.
- Run length — the distance from the amplifier to the furthest loudspeaker, counted as a loop (out and back).
The engineering target is to keep the cable’s loop resistance small relative to the load impedance. As a working rule, keep the loop resistance at or below roughly 10 % of the load impedance. That holds the power lost in the cable to about a tenth of the amplifier output — inaudible in a distributed system, and it leaves the amplifier headroom to drive the line properly.
Estimating the maximum run
For a copper pair the loop resistance is R = 2 × L × 0.0175 / A, where L is the one-way length in metres, A is the conductor cross-section in mm², and 0.0175 Ω·mm²/m is the resistivity of copper. The load impedance of a 100 V line is Z = 100² / P = 10000 / P. Applying the 10 % rule gives an approximate maximum one-way run of L ≈ 28570 × A / P metres.
The table below applies that estimate to common loads and cross-sections. It assumes a single amplifier output feeding one line, all of the load at the far end (the worst case), and copper conductors at about 20 °C.
| Total load on the line | 1.5 mm² | 2.5 mm² | 4.0 mm² |
|---|---|---|---|
| 50 W | ~850 m | ~1430 m | ~2290 m |
| 100 W | ~430 m | ~715 m | ~1145 m |
| 200 W | ~215 m | ~355 m | ~570 m |
| 400 W | ~105 m | ~180 m | ~285 m |
Two things follow from the table. First, on a 100 V line the section is driven by the total load on the line, not by the size of any single loudspeaker — a line of twenty 5 W ceiling speakers is still a 100 W line. Second, doubling the cross-section roughly doubles the reachable distance, while doubling the load halves it. Because a 70 V line draws more current for the same wattage, the distances above roughly halve on a 70 V system.
These are planning estimates, not a substitute for the equipment data. Before fixing the section, confirm the amplifier’s rated line voltage, add up the actual transformer tap settings rather than the loudspeakers’ nominal ratings, and check the result against the wiring regulations that apply where the building is.
Choosing the cable construction for the installation
A distributed line is almost always a permanent installation — in conduit, on tray, or clipped to structure — so the construction that suits a fixed install is the starting point. LBS manufactures 2-core speaker cable from 0.5 mm² up to 6.0 mm², in constructions an installer can choose between:
- Jacketed multi-core (LBSRVS) — multi-core OFC conductors with PVC conductor insulation under an overall PVC jacket, from 2 × 1.5 mm² up to 8 × 4.0 mm². On a 100 V line the 2-core sizes are the ones that apply: one pair carries the line through the building, protected by the jacket.
- Tour-grade reinforced (LBSRVH) — a reinforced construction with cotton thread and paper filling that holds the round profile under repeated handling, from 2 × 1.5 mm² to 2 × 6.0 mm². It matters more where part of the cable has to move than on a purely fixed run.
- Two-colour parallel (LBSRVB-P) — a flat parallel construction in which the two conductors are different colours, so polarity is obvious at the terminal, from 2 × 0.5 mm² to 2 × 2.5 mm².
Polarity matters more on a distributed line than on a short low-impedance run. A reversed pair in one leg of a long line puts that section out of phase with the rest of the system, and in a distributed installation that usually shows up as a zone that sounds thin or dead rather than as an obvious fault. A colour-coded parallel construction, or a jacket with printed polarity marking, makes the check quick on a building-wide line.
The fixed-installation view of cable selection is set out on the audio cable for installers page, and the conductor-selection logic behind the sections above is in the speaker cable gauge guide. For installations that combine a distributed line with network audio in the same project, the hotel and commercial AV installation guide covers how the cable types fit together.
Whether the line needs a shielded construction is a separate question from section. The constructions described here are unshielded, and on a distributed line the answer depends on the route the cable takes rather than on the signal level — that decision is set out in do speaker cables need shielding?
What to confirm before ordering
A distributed-line enquiry reaches a factory as a short list of facts. The ones that change what gets supplied are:
- Total transformer tap load on the line, in watts, and the amplifier’s line voltage (100 V or 70 V).
- The one-way run to the furthest loudspeaker, and whether the line splits into more than one leg.
- The conductor cross-section the estimate above supports, and whether the installation calls for a jacketed, reinforced or parallel construction.
- The installation environment — conduit, tray, surface — and any project-specific requirement the cable has to meet, which should be stated explicitly because it can change the jacket material.
- Jacket colour and any printing text, so the line can be identified at both ends of a building-wide run.
Keeping the specification in these terms — load, run, section, construction, environment — makes the line repeatable from one order to the next, which is what matters once a distributor is reordering for a recurring project type.
Frequently asked questions
What size speaker cable do I need for a 100 V line system?
Sizing follows the total load on the line, not the individual loudspeakers. Add up the transformer tap settings in watts, read the approximate maximum run for a given cross-section from the table above, and pick the section that covers your run with margin. A heavily loaded line or a long run needs a larger conductor; a lightly loaded line covers a long distance even in a modest section.
Can ordinary low-impedance speaker cable be used on a 100 V line?
The copper is the same. What changes is how the cable is sized and where it is installed. On a 100 V line the current is low for the power carried, so the section is set by the total load and run rather than by a loudspeaker impedance, and because the cable is normally a permanent installation, a jacket that suits the routing is the practical requirement.
How far can a 100 V line run?
Several hundred metres is normal for a light load in a 1.5 mm² section, and the reachable distance grows with the cross-section and shrinks with the total load, as the table shows. The limits are set by the acceptable power loss in the cable and by the amplifier, so the run, the load and the section have to be checked together rather than in isolation.
Next step
If you are specifying speaker cable for a 100 V or 70 V distributed installation, send the load, the run length, the required construction and the installation environment through the enquiry form, and LBS will confirm the section and the construction against the range it manufactures. The full speaker cable line is listed on the professional speaker cable category page.