70V/100V Line Speaker Load Calculator
Find out how many ceiling or wall speakers a 70V or 100V constant-voltage amplifier can drive. Enter the amplifier power, the wattage tap each speaker is set to, and a headroom margin, and the tool returns the maximum speaker count plus the load that leaves on the amp.
Enter Values
Minimum 0
Minimum 0
Minimum 0
How to use this calculator
- Enter the amplifier's rated power in watts.
- Enter the tap setting (watts) each speaker's transformer is set to — commonly 5 W, 10 W or 15 W.
- Optionally set a headroom percentage (20% by default) so the amp is not run at full power, then read the maximum speaker count and the remaining power.
How it works
On a constant-voltage line every speaker draws its tap wattage regardless of position, so the total load is simply the number of speakers times the tap wattage. Usable power is the amp power reduced by the headroom margin: usable = amp power × (1 − headroom ÷ 100). The maximum count is floor(usable ÷ tap), and the remaining amp power is the rated power minus the total tap load at that count.
Worked example
Worked example. A 240 W amplifier with speakers tapped at 10 W and 20% headroom: usable = 240 × 0.8 = 192 W, so max speakers = floor(192 ÷ 10) = 19. Nineteen speakers draw 190 W, leaving 50 W of the amp's 240 W spare.
Common mistakes
- Loading the amp to 100% of its rating with no headroom, which leaves nothing for peaks and can overheat the amplifier.
- Adding up speaker impedances as if it were a low-impedance system — on a 70V/100V line you add tap wattages, not ohms.
- Forgetting transformer insertion loss, which means a little extra power is consumed beyond the nominal tap figure.
Frequently asked questions
What is the difference between 70V and 100V lines?
Both are constant-voltage distribution systems; the number is the line voltage at full amplifier power. The speaker-count maths is the same — you add tap wattages — but 100V lines allow slightly thinner cable or longer runs for a given loss.
Why leave headroom instead of using the full amp power?
Headroom keeps the amplifier out of clipping on musical peaks, reduces heat, and leaves room to add a speaker later. A 20% margin is a common rule of thumb for background music systems.
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Cite this tool
Zerdly. (2026). 70V/100V Line Speaker Load Calculator. Retrieved from https://www.zerdly.com/tools/constant-voltage-speaker-load-calculator/
Tip: Enter any known values to calculate the remaining results.
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