100 V line explained: why the amplifier's power ends up in the loudspeaker, not the cable
100 V line is a way of wiring loudspeakers so that almost all of the amplifier's power reaches the loudspeakers instead of heating the cable. A small transformer on each loudspeaker raises its impedance from a few ohms to hundreds, and because an amplifier shares its power in proportion to impedance, the cable's share shrinks to almost nothing. The "100 V" is simply the agreed reference voltage that makes the sums easy.
Most explanations stop at "it works like the National Grid". That is true in spirit and unhelpful in practice, because 100 V is not a high enough voltage for that argument to carry much weight on its own. The real reason is simpler.
Everything on the circuit takes a share of the power
Whatever sits in the path of the current puts some of that current to work. The loudspeaker turns its share into sound. The cable turns its share into heat.
The amplifier divides its power between them in proportion to their impedance (or, for plain copper, its resistance). So the question is never "how much power does the amplifier have?" but "what share of it does the cable take?"
Worked example 1: a low-impedance loudspeaker on 20 m of thin cable
Imagine a loudspeaker connected over one pair of a Cat 5e cable about 20 m long. Out and back, that pair measures about 4 ohms.
| Item | Measured |
|---|---|
| Cable pair, looped | 4 ohms |
| Loudspeaker | 6 ohms |
| Circuit total | 10 ohms |
The cable is 4 of the 10 ohms, so it takes 40 % of the amplifier's power. From a 100 W amplifier, the loudspeaker gets 60 W and the cable gets the other 40 W as heat.
Worked example 2: the same cable on 100 V line
Fit a 100 V line transformer to the loudspeaker. Its impedance rises from 6 ohms to around 996 ohms. The cable stays at 4 ohms.
| Item | Measured |
|---|---|
| Cable pair, looped | 4 ohms |
| Loudspeaker with transformer | 996 ohms |
| Circuit total | 1 000 ohms |
The cable is now 4 of 1 000 ohms: 0.4 % of the power. Same cable, same loudspeaker, and the loss has fallen a hundredfold. That is the whole trick.
A useful side effect: because every loudspeaker has a high impedance, they can all be connected in parallel along one circuit, which is what makes 100 V line cabling so straightforward on a large building.
Why it is called "100 V" (and "constant voltage")
Power, voltage and impedance are tied together by one formula:
Power (W) = Voltage² ÷ Impedance (ohms)
Amplifier sizes vary, loudspeaker sizes vary and every circuit's impedance is different. Fixing the voltage is the only way to make the other two quick to work out, so the industry standardised it. Europe settled on 100 V; North America uses 70 V. "Constant voltage" means constant in the maths, not on your meter: with speech and music the actual voltage on the line changes all the time.
With the voltage fixed, measuring a circuit tells you its load directly. A circuit measuring 50 ohms on 100 V line is a load of 100² ÷ 50 = 200 W, so it needs a 200 W or larger 100 V amplifier output.
Because everything is referenced to the same voltage, the design shortcut follows: add up the tap settings of every loudspeaker on the circuit, and the amplifier output must be at least that figure.
What the shortcut leaves out
Two cautions a maintainer will recognise:
- Measure impedance, not resistance. A multimeter reads the DC resistance of a transformer winding, which is not what the amplifier sees. Loudspeaker circuits are measured with an impedance meter at audio frequency. The cable itself is mostly resistance, so a multimeter is close enough for the cable alone.
- The cable share is small, not zero. On very long runs or with many loudspeakers, the total load impedance falls and the cable's share rises again. That is one reason voltage-drop and loss checks still belong in a voice alarm design, and why a circuit's measured impedance is worth recording at every maintenance visit: a change in it is often the first sign of a fault.
For sizing the amplifiers and zones on your own building, the ZOE estimator does these sums for you with the working shown.