70V vs 100V Lines: What's the Difference and How to Choose

When you need to distribute sound across an entire building — from the rack room out along corridors, floors and halls — the first decision is the line itself. Distributed voice alarm and background music systems almost always run on a constant-voltage (high-impedance) 70V or 100V line, and within that there's a second fork: 70 or 100 volts. That choice drives how far your cable runs can reach, how many loudspeakers you can hang on a single pair of conductors, and what the amplifier has to deliver. Let's break down what separates the two standards, how loudspeaker power taps work, how to avoid overloading the line, and when a low-impedance setup makes more sense.

How does a 70/100V line differ from a low-impedance one?

A constant-voltage line runs at 70 or 100V through matching transformers, letting you connect dozens of loudspeakers to a single pair of conductors over long cable runs. A low-impedance line (4–16 ohms) is built for short runs and a small number of loudspeakers close to the amplifier, but delivers higher audio quality.

70 or 100 volts — which is better?

For new, long-distance systems, 100V is the way to go: at the same current it carries more power with lower losses. A 70V line tends to survive in smaller or legacy installations. VPA equipment works with both.

How many loudspeakers can one line carry?

As many as the sum of their tapped power ratings allows, provided that total stays below the amplifier's output with a 20–30% headroom margin. The limit isn't a head count — it's the total connected load, and it depends on the taps you select and the amplifier model.

Can you mix 70V and 100V on the same line?

No. The line voltage is set by the amplifier, and every loudspeaker connects to it through a tap rated for that voltage. Stick to one standard within a single line. That said, many amplifiers provide separate 70V and 100V outputs — you simply wire the line to the output you need.