70V / 100V Line
Sum the taps (W) of the speakers on a constant-voltage line (70.7 V or 100 V), compute the total load, the equivalent impedance the amplifier sees, amplifier usage and headroom per the 80% rule. 100% in your browser.
Amplifier usage
Per-row breakdown
| Speaker | Tap | Qty | Load | % of total |
|---|
How it is calculated · 70V / 100V lines
1. On a constant-voltage line each speaker has a
transformer with taps in watts. The system load is the sum of the taps used:
load = Σ(tap × quantity).
2. The equivalent impedance the amplifier sees depends only on the
line voltage and the load: Z = V² / load. At 70.7 V,
V² = 5000; at 100 V, V² = 10000.
3. Amplifier usage: usage % = load / power × 100.
4. The 80 % rule: the safe limit is
power × margin. The headroom = limit − load leaves room for peaks,
cable loss and future speakers.
5. Verdict: OK if the load ≤ safe limit ·
tight if it exceeds the limit but fits the amp · overload
if the load exceeds the amplifier power.
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How it works
The calculator sizes a constant-voltage speaker line (70.7 V, the North American standard, or 100 V, the European one), the usual scheme for paging and distributed audio. Each speaker has a transformer with taps in watts; you add rows with the tap and quantity of each model and the tool sums the total load: load = Σ(tap × quantity).
From that load it computes the equivalent impedance the amplifier sees (Z = V² ÷ load: at 70.7 V, V² = 5,000; at 100 V, V² = 10,000), the amplifier utilization percentage and the headroom according to the 80 % rule: the load should not exceed the amplifier power times the margin, leaving room for peaks, line loss and future expansion. The verdict is OK if the load fits within the safe limit, tight if it exceeds it but still fits the amplifier, and overload if it exceeds the total power.
Example: office paging with a 240 W amplifier at 70.7 V
- Rows: 12 ceiling speakers at 8 W + 6 at 4 W + 2 horns at 15 W → load
= 96 + 24 + 30 = 150 W. - Equivalent impedance:
5,000 ÷ 150 ≈ 33.3 Ωas seen by the amplifier. - Amplifier utilization:
150 ÷ 240 = 62.5 %; the safe limit at 80 % is240 × 0.8 = 192 W. - Verdict: OK, with
192 − 150 = 42 Wof headroom for future speakers.