System design
How to Wire a 70 Volt System
Quick answer
Add up every speaker tap, then size the amplifier at that total divided by 0.8. Sixteen ceiling speakers tapped at 8 watts is 128 watts of load, which needs a 200 watt amplifier. The 80 percent rule covers tap tolerance and leaves room to add speakers later.
A 70 volt system, properly called a constant voltage distributed system, is how you put many small loudspeakers on one amplifier over long cable runs. It solves a specific problem: covering a large area evenly with speech and background music using dozens of ceiling speakers, where wiring each one as a low impedance load would require impossibly thick cable and impossible impedance arithmetic.
It is also frequently the wrong answer for a sanctuary, and that is worth saying before the wiring detail. A distributed ceiling system is excellent for a fellowship hall, a narthex, corridors, classrooms, a nursery and an overflow space. It is poor for a worship band, because ceiling speakers have limited low frequency output and fire down at people rather than covering a room from a point source. If your main room has a band, you want point source loudspeakers and this page is about your other spaces.
On this page
How constant voltage actually works
An ordinary loudspeaker is a low impedance load, typically 8 ohms, and the amplifier delivers power into it at a relatively low voltage and high current. High current over a long cable run means voltage drop and wasted power, which is why low impedance runs are kept short and thick.
A constant voltage system steps the amplifier output up to a nominal 70.7 volts, which lowers the current for the same power. Each loudspeaker has a small transformer that steps it back down and, crucially, lets you select how much power that speaker draws by choosing a tap. Standard taps are 0.5, 1, 2, 4, 8, 16 and 32 watts. So each speaker takes exactly the power you assign it, and the amplifier simply has to supply the sum.
That is the whole advantage. You can run dozens of speakers on one pair of wires over hundreds of feet, set the level of each individually at its tap, and add another speaker later without recalculating an impedance network. The direct comparison against ordinary low impedance wiring is at 70 volt versus 8 ohm.
Standard taps and what each one is for
Tap selection sets how loud each speaker is. These are the standard values and where each one belongs in a church.
Most church ceiling speakers sit at the 4 or 8 watt tap, and sixteen speakers at 8 watts produces a 128 watt load needing a 200 watt amplifier.
| Tap, watts | Typical application | Relative level |
|---|---|---|
| 0.5 | Very quiet zones, a nursery or a quiet corridor | Lowest |
| 1 | Background music in a small quiet room | Minus 9 dB from 8 watt |
| 2 | Offices, small classrooms and restrooms | Minus 6 dB from 8 watt |
| 4 | General corridors, narthex and fellowship space | Minus 3 dB from 8 watt |
| 8 | The usual church ceiling tap for speech in an occupied room | Reference |
| 16 | Noisy areas, a kitchen or a room with a high ceiling | Plus 3 dB from 8 watt |
| 32 | Outdoor horns and very high ceilings, rarely needed indoors | Plus 6 dB from 8 watt |
Each doubling of tap power adds about 3 dB. Taps are selected at the speaker, usually by moving a wire or turning a rotary selector, and can be changed after installation without rewiring, which is the main practical advantage of the system.
Sizing the amplifier: sum the taps, divide by 0.8
The arithmetic is deliberately simple. Add the tap wattage of every loudspeaker on the line to get the total load. Then divide by 0.8 and round up to an amplifier size that is actually sold. A 16 speaker system tapped at 8 watts each is 128 watts of load, which divided by 0.8 is 160, which rounds up to a 200 watt amplifier.
The 80 percent margin is not padding. Transformer taps have real tolerance, the load is never exactly what the labels say, and an amplifier run at its absolute limit into a transformer load runs hot and distorts. Just as importantly, the margin is what lets you add two more speakers in three years without buying a new amplifier, and churches always add speakers. Run your own list through the 70 volt system calculator.
Amplifiers must be the constant voltage type, with a 70 volt output. The Crown CDi 1000 at $1,487.52 and the CDi 2000 at $1,125.00 both provide 70 volt outputs directly. An ordinary low impedance amplifier cannot drive a 70 volt line without a separate step up transformer, and connecting one directly to a distributed line is a way to destroy the amplifier. The field is at 70 volt amplifiers.
How to wire a 70 volt system, step by step
- Lay out the speakers before choosing anything else. Coverage diameter at ear height is twice the distance from the ceiling to the ear plane, multiplied by the tangent of half the coverage angle. Ear height is 4 feet seated and 5 feet standing, and using the floor instead of the ear plane overstates coverage by about a third. The ceiling speaker spacing calculator does this properly.
- Assign a tap to every speaker. Start at 8 watts for a general occupied room, 4 watts for a corridor or narthex, 2 watts for offices and 16 watts where the ceiling is high or the room is noisy. Taps can be changed later without rewiring, so a sensible first guess is fine and adjustment during commissioning is expected.
- Total the taps and size the amplifier. Add every tap on the line, divide the total by 0.8, and round up to an amplifier size that is sold. Sixteen speakers at 8 watts is 128 watts, which needs a 200 watt amplifier. Never size an amplifier at exactly the load total.
- Choose the cable gauge for the run length. Constant voltage lines carry low current, so cable is less critical than in a low impedance run, but long runs still lose power. Keep total line loss under about half a decibel, use 16 or 14 gauge for typical church runs and step up to 12 gauge for very long runs or heavy loads.
- Wire the line in parallel, observing polarity throughout. Every speaker connects across the same pair, in parallel, never in series. Keep polarity consistent at every single speaker, because one reversed speaker cancels against its neighbours in the overlap region and creates a dead zone that is very hard to find afterwards.
- Have a licensed electrician handle anything in the building fabric. Speaker cable in a plenum ceiling must be plenum rated, runs through fire rated assemblies must be properly sealed, and anything in conduit alongside mains wiring has code implications. This is where a licensed electrician working to local code is not optional.
- Verify the load before connecting the amplifier. Measure the line impedance with a meter designed for constant voltage systems and confirm it matches the calculated load. This catches a wiring error, a shorted transformer or a tap set wrong before the amplifier finds it the hard way. Do this with the amplifier disconnected.
- Commission by ear, room by room. Play speech material and walk every space, adjusting taps where a room is too loud or too quiet. Speech intelligibility, not music, is the test that matters for a distributed system, because that is what it exists to deliver.
Spacing the ceiling speakers correctly
Speaker layout matters more than anything electrical here. The coverage diameter of one ceiling speaker at the listening plane is twice the distance from the ceiling to the ear, multiplied by the tangent of half the published coverage angle. Most 70 volt ceiling speakers are 90 degrees, so in a 10 foot ceiling with seated listeners at 4 feet, the coverage circle is about 12 feet across.
Then choose a spacing pattern. Edge to edge spacing, where the circles just touch, is the cheapest and leaves gaps at the diagonals, which is fine for background music. Minimum overlap at about 0.71 of the diameter is the normal choice for speech reinforcement. Tight overlap at half the diameter gives even coverage everywhere and is used where intelligibility is critical or the ceiling is high. The ceiling speaker spacing calculator produces the grid, and the chart is at ceiling speaker coverage chart.
The most common mistake is calculating coverage to the floor rather than to the ear plane, which overstates coverage by roughly a third in a 10 foot room and leaves dead spots between seats. The second most common is quoting the coverage angle at 500 Hz rather than 2 or 4 kHz, which also overstates it badly.
Sources
- Constant voltage distributed system design practice and standard transformer tap values
- Loudspeaker manufacturer application guidance on ceiling speaker coverage angles and spacing patterns
- Crown published specifications for the CDi series 70 volt and 140 volt amplifier outputs
- National Electrical Code continuous load rule limiting a circuit to 80 percent of breaker rating
Frequently asked questions
How do I size a 70 volt amplifier?
Add up the tap wattage of every loudspeaker on the line, then divide by 0.8 and round up to an amplifier that is sold. Sixteen speakers at 8 watts is 128 watts of load and needs a 200 watt amplifier. The 80 percent margin covers tap tolerance, keeps the amplifier out of distortion and leaves room to add speakers later.
Can I use a normal amplifier on a 70 volt line?
Not directly. A constant voltage line needs an amplifier with a 70 volt output or a separate step up transformer between a low impedance amplifier and the line. Connecting an ordinary amplifier straight to a distributed line presents it with a load it was never designed for and is a reliable way to destroy it.
What tap setting should church ceiling speakers use?
Start at 8 watts for a general occupied room such as a fellowship hall, 4 watts for corridors and a narthex, 2 watts for offices and small rooms, and 16 watts where ceilings are high or the space is noisy. Each doubling of tap adds about 3 dB, and taps can be changed during commissioning without rewiring anything.
Are ceiling speakers suitable for a worship band?
No, and this is worth being direct about. Ceiling speakers have limited low frequency output and fire downward at individual listeners rather than covering a room from a point source, so a band sounds thin and localised directly overhead. Use point source loudspeakers for the main room and reserve distributed ceiling systems for speech and background music elsewhere.
How far apart should ceiling speakers be?
It depends on ceiling height and coverage angle, calculated to the ear plane rather than the floor. In a 10 foot ceiling with seated listeners, a 90 degree speaker covers about 12 feet across, and minimum overlap spacing for speech puts them about 8.5 feet apart. Calculating to the floor overstates coverage by roughly a third and leaves dead spots.
Can I add speakers to an existing 70 volt system?
Yes, provided the amplifier has capacity. Add the new taps to your existing total and check the sum is still under 80 percent of the amplifier rating. This is the main practical advantage of constant voltage wiring, and it is why sizing with the 80 percent margin at installation matters so much: it is what makes the expansion possible without a new amplifier.
Researched, not professional advice. This page is compiled from published manufacturer specifications, operator manuals, FCC rules, published standards and owner-review consensus, not hands-on testing. Sound system design, rigging loudspeakers overhead, and any electrical work are jobs for a qualified professional: have flown loudspeakers and their attachment points signed off by a structural engineer or a certified rigger, and have all wiring done by a licensed electrician to your local code. Wireless microphone rules change, so confirm the current FCC position before buying. As an Amazon Associate we earn from qualifying purchases.