70 V and 100 V distribution are the same idea wearing different jackets: send audio down the cable at a high voltage and a low current, let each speaker take the small slice it needs through a transformer. Nearly everything written about choosing between them is regional habit dressed up as engineering. There is exactly one technical difference that matters, it is a factor of two, and once you can see where that two comes from, every practical rule — tap impedance, cable length, what burns when you mix systems — follows in a line.
The short answer
Use whichever voltage your amplifier is built around, and match every speaker on that line to it. In North America that usually means 70 V (70.7 V, to be exact); in Europe, the Middle East, most of Asia and Latin America it means 100 V. Neither sounds better. A 100 V line carries the same power with less current, so it tolerates longer cable runs and thinner conductors — that is the whole advantage, and it only becomes visible on long lines.
| 70 V line | 100 V line | |
|---|---|---|
| Exact line voltage | 70.7 V | 100 V |
| Current at 100 W | 1.41 A | 1.00 A |
| Line impedance at 100 W | 50 Ω | 100 Ω |
| Power lost in a given cable | 2× that of 100 V | reference |
| Max run for 0.5 dB loss, 2 × 1.5 mm² | ≈ 230 m | ≈ 465 m |
| Same speaker, same tap setting | half the power, 3 dB quieter | reference |
| Where it dominates | North America | Europe, Asia, Middle East, Latin America |
Why constant voltage exists at all
Connect speakers the way you would in a living room — 4 or 8 Ω, straight to the amplifier — and two problems appear the moment the job grows. Adding speakers in parallel drops the load impedance until the amplifier is running into a short. And at 8 Ω, delivering 100 W means about 3.5 A flowing down the cable, so every metre of copper eats a measurable slice of the signal.
Constant-voltage distribution solves both at once. The amplifier is designed to swing a fixed voltage at full output — 70.7 V or 100 V — and every speaker hangs on the line through a step-down transformer that draws only the wattage its tap is set to. Speakers connect in parallel, in any number, until the sum of the taps reaches the amplifier's rating. Nothing is critical about the order or the position on the line.
- Design becomes addition. Twenty speakers on 6 W taps is 120 W of load. That is the entire load calculation.
- Cable gets cheap. Ten times less current than a low-impedance line means a hundred times less power lost in the same copper.
- Runs get long. Hundreds of metres are routine, which is what makes a building-wide system possible at all.
- Levels are set per speaker. The tap on each transformer decides how loud that speaker is relative to the others — no attenuators, no separate amplifier channels.
Where the two voltages came from
70.7 V is a regulatory artefact. In the United States, wiring that stays below 100 V RMS falls under the lighter Class 2 rules of the National Electrical Code — thinner insulation, simpler conduit requirements, no electrician's licence for parts of the job. 70.7 V sits comfortably under that ceiling, and it is not an arbitrary number: it is the voltage that delivers exactly half the power of 100 V into the same impedance.
100 V systems grew up in Europe under different wiring rules, where the extra headroom was allowed and the longer reach was welcome. Both were standardised decades ago and both are now everywhere; equipment sold internationally usually supports one primarily and mentions the other in the specification. There is no ongoing technical argument between them — only installed base and local code.
The one number that differs: a factor of two
A speaker tap is not really a wattage. It is an impedance chosen so that the tap draws the rated power at the rated voltage: Z = V² / P. A 6 W tap on a 100 V line is a 1667 Ω load. Put that same transformer on a 70.7 V line and the impedance has not changed — but the voltage across it has, so the power drawn is (70.7 / 100)² = half.
| Tap rating | Impedance | Power on a 100 V line | Power on a 70.7 V line | Level difference |
|---|---|---|---|---|
| 1.5 W | 6667 Ω | 1.5 W | 0.75 W | −3 dB |
| 3 W | 3333 Ω | 3 W | 1.5 W | −3 dB |
| 6 W | 1667 Ω | 6 W | 3 W | −3 dB |
| 10 W | 1000 Ω | 10 W | 5 W | −3 dB |
| 20 W | 500 Ω | 20 W | 10 W | −3 dB |
| 30 W | 333 Ω | 30 W | 15 W | −3 dB |
Three decibels is audible but modest — roughly the difference between a system that is comfortably loud and one that is merely adequate. That is why a 70 V installation generally uses taps one step higher than the same room would on 100 V, and why simply moving a speaker from one system to the other rarely destroys anything: it just gets quieter.
What happens when you mix them
The two failure directions are not symmetrical, and only one of them is expensive.
- 100 V speaker on a 70 V amplifier — safe, quiet. Each speaker draws half its tap power, the whole system sits 3 dB down, and the amplifier is loafing at half its expected load. Nothing is damaged. Raise the taps a step and you are back where you wanted to be.
- 70 V speaker on a 100 V amplifier — this is the one that burns. Each transformer now sees twice the voltage it was designed for, so it draws twice its rated power. Twenty speakers you counted as 120 W present 240 W to the amplifier. The transformers saturate and run hot, the amplifier goes into protection or clips, and after long enough at that level something gives — usually a voice coil or the amplifier's output stage.
- Mixed speakers on one line — unpredictable. The line is fine electrically, but half your speakers are 3 dB louder than the other half and the tap arithmetic no longer describes the load. Coverage design goes out of the window.
Cable: where 100 V earns its keep
Power lost in the cable is I²R, and the 70 V line carries 1.41 times the current for the same delivered power. Square that and the loss in identical copper is exactly double. In a small shop this is invisible. Across a warehouse or a hotel it decides your conductor size.
The usual design target is to keep line loss under 0.5 dB — about 11 % of the power. At that budget, with a 100 W load, the runs work out like this:
| Conductor | ≈ AWG | 100 V line | 70.7 V line |
|---|---|---|---|
| 0.75 mm² | 18 AWG | 220 m | 110 m |
| 1.0 mm² | 17 AWG | 305 m | 150 m |
| 1.5 mm² | 16 AWG | 465 m | 230 m |
| 2.5 mm² | 14 AWG | 775 m | 390 m |
Two things about that table catch people out. The limit scales with the load, not just the distance: halve the connected wattage on the line and the permitted run doubles, because the current halves. And the loss is set by the whole loop, so a single long spur to one remote speaker can quietly eat the budget for the entire line. Our online calculator does this arithmetic per line for the actual cable you plan to pull, alongside the speaker layout.
Sizing the amplifier — same on both systems
Whichever voltage you are on, the amplifier is sized from the taps you actually set, not from the speakers' maximum ratings, plus headroom for the transformers' magnetising current and for the speakers somebody will add later.
- Add up the tap settings across the line. Twenty-four ceiling speakers on 6 W taps: 144 W.
- Add 20–30 %. That is 173–187 W.
- Choose the next amplifier up — here a 240 W model such as the VPA-MK 240Z6, which also splits the job into six switchable zones. Smaller rooms are served by the 120 W model; the full range is on the mixer amplifiers page.
- On a 70 V system, remember that the same taps deliver half the power — so either raise the taps and keep the amplifier, or accept 3 dB less and enjoy the headroom.
Our ceiling range is specified for 100 V lines with taps from 3 W (VPA-PP3, VPA-P3) through 3/6 W (VPA-PM6) and 3–10 W (VPA-P10) up to 10/20 W (VPA-P20); wall and horn models follow the same logic with their own tap sets. If you are working on a 70 V system, check that the transformer in front of you is rated for the voltage you are feeding it — that single check prevents the expensive failure direction above.
So which should you use?
- Follow the local standard. It decides what your wholesaler stocks, what the next contractor expects to find, and which code applies to the cable in the wall. That is worth more than any 3 dB.
- Extending an existing system? Match it exactly. Voltage, tap conventions, cable type. Do not create a building with two conventions in it.
- New build, free choice, long runs? 100 V, for the cable economics.
- New build in North America? 70 V, because Class 2 wiring rules will save more on installation than you will ever spend on copper.
- Speakers spread over more than a few hundred metres? Neither voltage saves you on its own — split the load across more lines, or move the amplifier closer to the speakers.
And the honest summary: for the overwhelming majority of shops, offices, hotels, schools and factories, this choice changes nothing about how the finished system sounds. What changes it is the number of speakers, where they are placed, and whether the amplifier has honest headroom — which is what our ceiling speaker layout guide and the calculator are for.
Frequently asked questions
Is 100 V better than 70 V?
Only in cable economics. A 100 V line carries the same power at 70 % of the current, so it loses half as much in identical copper and reaches roughly twice as far before the loss matters. Sound quality, reliability and speaker choice are unaffected.
Can I connect a 100 V speaker to a 70 V amplifier?
Yes, safely. It draws half its tap power and plays about 3 dB quieter. Move each speaker one tap step up to recover the level, as long as the total stays within the amplifier's rating.
Can I connect a 70 V speaker to a 100 V amplifier?
Don't. Each transformer sees twice its design voltage and draws twice its rated power, so the amplifier is overloaded by a factor of two and the transformers run hot. This is the direction that damages equipment.
Why 70.7 V and not a round 70?
Because it is 100 divided by the square root of two — the voltage that delivers exactly half the power into the same impedance. It also keeps the line under the 100 V RMS ceiling for Class 2 wiring in the US National Electrical Code, which was the point of it.
How do I tell which system an existing installation uses?
Read the amplifier's output terminals first — they are labelled with the voltage. Then confirm from the speakers: their transformer tap labels are printed for a specific line voltage. If the two disagree, measure the line impedance with the amplifier disconnected before powering anything up.
Does the cable itself need to be different?
No special cable is required for either voltage — ordinary two-conductor copper of an adequate cross-section is what both use. Size it from the load and the run length, and keep it away from mains and dimmer circuits.
Check your line before you pull the cable
Tap totals, line impedance, cable loss on the actual conductor size, amplifier headroom — all of it is arithmetic, and all of it is easier to get right before the cable is in the wall than after. Enter the room, the speakers and the cable you intend to use, and the calculator returns the line loading, the loss per line and the amplifier that fits.
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