70V / 100V speaker line calculator
Add up your taps, check the amp, and see how much the wire run costs you before you pull cable.
How it works
A constant-voltage line looks to the amp like a single impedance set by the voltage and the total of the transformer taps:
Ten speakers tapped at 15 W on a 70.7 V line add up to 150 W, so the amp sees 70.7² ÷ 150 ≈ 33.3 Ω.
The wire adds its out-and-back resistance in series with that load. The calculator treats every speaker as if it were at the far end of the run, which is the worst case. Real daisy-chained lines lose less, because the near speakers don't pull current through the whole run.
loss (dB) = 20 × log10((Z + R) ÷ Z)
500 ft of 16 AWG is about 4.0 Ω out and back. Against 33.3 Ω that's a loss of about 1.0 dB, so roughly 80% of the amp's power reaches the speakers.
Field notes
- Leave the amp headroom. A common rule is to keep total taps at or under about 80% of the amp's rating at the line voltage. That covers transformer insertion loss, the tap tolerance and future speakers someone adds without telling you.
- Count the wattage you actually tapped, not the speaker's maximum tap. A speaker with taps of 1, 2, 4 and 8 W that's set to 2 W counts as 2 W.
- Common targets for line loss are 0.5 dB (about 11% of the power) for paging and background music, and tighter for anything where level matters.
- Fixing a long run: going up two gauges halves the loop resistance. Moving the amp closer, splitting the zone into two home runs or switching to 100V all help too. On 100V the same wattage puts a higher impedance on the line, so the same wire loses less.
- Check the amp's rating at your line voltage. Many amps list one wattage at 4 Ω and a different one at 70V or 100V, and some need a bridged or switched mode for constant-voltage output.
Wire resistance used
| AWG | Ω per 1000 ft (one conductor) | Ω per km |
|---|---|---|
| 10 | 0.999 | 3.28 |
| 12 | 1.588 | 5.21 |
| 14 | 2.525 | 8.28 |
| 16 | 4.016 | 13.18 |
| 18 | 6.385 | 20.95 |
| 20 | 10.15 | 33.30 |
| 22 | 16.14 | 52.95 |
Solid copper at 20 °C. Stranded cable and warm plenum spaces run a few percent higher.
FAQ
Is 70V really 70 volts?
The nominal figure is 70.7 V RMS, which is 100 V peak. In practice it's the voltage the amp puts out at full rated power, and music and paging sit well below that most of the time.
Can I mix 70V speakers on a 100V line?
Only if the transformers have 100V taps. A 70V transformer on a 100V line draws about twice its marked wattage: (100 ÷ 70.7)² ≈ 2, so a 10 W tap pulls about 20 W. The transformer can saturate, and the amp may clip trying to feed the extra load. Either way the speakers get a flattened, almost square wave instead of a clean sine. That dumps far more heat into the voice coils and can overheat the transformer and blow drivers, tweeters first.
Why is this worst case?
Speakers spread along a run each pull their current through only part of the wire. Lumping them all at the end gives the highest loss, so if this number is fine, the real line is fine.