70V Speaker System Reference

Constant-voltage audio in three tables: the tap-to-impedance ladder (all of it derives from V² ≈ 5,000), the amplifier loading rule, and wire gauge by run length. The tap math is physics; the wire table is the published 2-conductor, 0.5 dB-loss basis — a circulating table prints double these distances by counting one-way conductor length.

Tap wattage → primary impedance (Z = 5,000 / W)

COMPUTED
What each tap presents to the line. Minimum line impedance = 5,000 ÷ amplifier watts — a 100 W amp must never see less than 50 Ω.
Tap (W)Primary impedance
1 W5,000 Ω
5 W1,000 Ω
10 W500 Ω
30 W167 Ω
60 W83 Ω
100 W50 Ω

Wire gauge by run length (70.7 V line)

PUBLISHED
Maximum 2-conductor run length by gauge and total load, at 0.5 dB SPL (11% power) loss with the load at the end of the run — the published Lowell basis, independently corroborated by Belden's resistive model.
AWG30 W load60 W100 W250 W
10 AWG5,000 ft2,500 ft1,500 ft550 ft
12 AWG3,100 ft1,550 ft940 ft375 ft
14 AWG1,900 ft950 ft600 ft225 ft
16 AWG1,200 ft600 ft370 ft145 ft
18 AWG750 ft375 ft230 ft90 ft
Distributed loads (speakers spread along the run) effectively see less loss than this end-loaded worst case. A widely-copied chart prints ~2× these figures by measuring one-way conductor length — check the basis before mixing tables.

The three voltage tiers

70.7 V — the us standard — paging, bgm, mass notification (100 v peak × 0.707; born of the old conduit threshold); 25 V — school intercom, nurse call, stricter local rules (cable loss ~8× worse than 70 v — short runs only); 100 V — international standard (same math with v² = 10,000; ~2× the reach of 70 v). On the code side, amplifier output wiring lives under NEC Article 640: amplifier output wiring per 640.9(c) → article 725; class 2/3 where the amp is listed and marked for it, and audio circuits run as class 2/3 shall not share cable or raceway with other class 2/3 conductors — the practical version of which is: keep 70 V lines away from mic-level cable, in separate pathways.

Common questions

How does a 70V speaker system work?

The amplifier drives a 70.7 V line; every speaker hangs in parallel off it through a step-down transformer with selectable wattage taps. Each speaker draws its tap wattage regardless of how many share the line, so design is addition: sum the taps, stay under the amplifier's rating. High voltage means low current, which means small wire over long runs — the same reason utilities transmit at high voltage.

How do you size a 70V amplifier?

Add up every speaker's tap setting and keep the total at or below 80% of the amplifier's rated power (some designers derate to 70% where transformer insertion loss and long lines stack). Twenty ceiling speakers tapped at 5 W = 100 W of taps = at least a 125 W amplifier.

Why 70.7 volts exactly?

A code artifact that stuck: circuits under 100 volts peak historically avoided conduit requirements, and 100 V peak × 0.707 = 70.7 V RMS. The 25 V tier exists for stricter local rules (school intercoms, nurse call) at ~8× the cable loss; 100 V is the international standard with twice the reach. Same math throughout — the magic number is V².