Cable and power
70 Volt Tap Chart
Quick answer
Twenty-four ceiling speakers tapped at 4 watts draw 96 watts, so the amplifier must be rated 150 watts once the 80 percent installer rule is applied. Each 4 watt tap presents 1,225 ohms at 70 volts and delivers about 90 dBA at the ear plane six feet below.
A 70 volt system solves a problem low impedance wiring cannot: feeding twenty or fifty loudspeakers from one amplifier, over long runs, with the level of each one set independently. It does it by running the line at high voltage and low current through a transformer at every speaker, and the tap on that transformer is the volume control for that seat.
Choosing taps is the whole design job. Too low and the fellowship hall is inaudible under conversation; too high and you have spent the amplifier on the first eight speakers. This page gives the standard taps, what each one presents to the line, what each one produces at ear height, and how much amplifier a given layout needs. Size a full layout with the 70 volt system calculator and space the speakers with the ceiling speaker spacing calculator.
On this page
What does each 70 volt tap do?
The seven standard taps. Impedance is simply 4,900 divided by the tap wattage, because 70 volts squared is 4,900. The level column assumes a typical 89 dB sensitivity ceiling loudspeaker and a listener whose ears are six feet below the baffle, which is a nine or ten foot ceiling with a seated congregation.
A 4 watt tap presents 1,225 ohms and produces about 90 dBA at the ear plane six feet below an 89 dB sensitivity ceiling loudspeaker, which is the normal setting for speech reinforcement.
| Tap | Impedance at 70 V | Level at 1 m | Level 6 ft below | Where this tap belongs |
|---|---|---|---|---|
| 0.5 W | 9,800 ohms | 86.0 dB | 80.8 dBA | Quiet corridors, nurseries, restrooms. Barely above the air handling. |
| 1 W | 4,900 ohms | 89.0 dB | 83.8 dBA | Background music in a foyer or an office corridor. |
| 2 W | 2,450 ohms | 92.0 dB | 86.8 dBA | Low ceiling classrooms and small meeting rooms. |
| 4 W | 1,225 ohms | 95.0 dB | 89.8 dBA | The default for speech reinforcement in a fellowship hall. |
| 8 W | 612.5 ohms | 98.0 dB | 92.8 dBA | Noisy rooms, kitchens, gymnasiums, overflow seating. |
| 16 W | 306.3 ohms | 101.0 dB | 95.8 dBA | High ceilings above about 16 feet, or music that has to carry. |
| 32 W | 153.1 ohms | 104.0 dB | 98.8 dBA | Rarely right in a church. Usually a sign the speaker is in the wrong place. |
Impedance is 4,900 divided by the tap wattage. Levels assume an 89 dB at 1 watt at 1 metre ceiling loudspeaker and free field propagation; a real ceiling adds a few decibels of boundary gain in the low end. Not every transformer offers every tap, and the 0.5 W tap in particular is often missing on inexpensive units.
How big an amplifier does my ceiling system need?
Add the taps, then divide by 0.8. The 80 percent rule is the installer convention: it absorbs transformer tolerance, insertion loss and the fact that somebody will add two more speakers next year without telling you. The result is then rounded up to an amplifier size that is actually sold.
Thirty-two ceiling speakers at the 4 watt tap total 128 watts and need a 200 watt amplifier after the 80 percent rule, while the same thirty-two speakers at 8 watts need 400 watts.
| Speakers | Tap | Total tap load | With 80% rule | Amplifier to buy |
|---|---|---|---|---|
| 8 | 4 W | 32 W | 40 W | 100 W |
| 12 | 2 W | 24 W | 30 W | 100 W |
| 12 | 4 W | 48 W | 60 W | 100 W |
| 16 | 4 W | 64 W | 80 W | 100 W |
| 16 | 8 W | 128 W | 160 W | 200 W |
| 20 | 4 W | 80 W | 100 W | 100 W |
| 24 | 2 W | 48 W | 60 W | 100 W |
| 24 | 4 W | 96 W | 120 W | 150 W |
| 24 | 8 W | 192 W | 240 W | 300 W |
| 32 | 4 W | 128 W | 160 W | 200 W |
| 32 | 8 W | 256 W | 320 W | 400 W |
| 32 | 16 W | 512 W | 640 W | 800 W |
| 40 | 4 W | 160 W | 200 W | 200 W |
| 40 | 8 W | 320 W | 400 W | 400 W |
| 48 | 8 W | 384 W | 480 W | 500 W |
| 60 | 2 W | 120 W | 150 W | 150 W |
| 60 | 4 W | 240 W | 300 W | 300 W |
| 80 | 2 W | 160 W | 200 W | 200 W |
| 80 | 4 W | 320 W | 400 W | 400 W |
| 100 | 4 W | 400 W | 500 W | 500 W |
Total tap load is the sum of every tap on the line. The 80 percent figure is the load divided by 0.8, and the last column is the next amplifier size that exists as a product. Mixed taps on one line are normal: add the actual settings rather than assuming every speaker is the same.
Which tap should each room get?
Tap selection is a level decision, not a power decision. Start from the noise floor of the room and the ceiling height, and remember that every step up the tap ladder is 3 dB and doubles that speaker’s share of the amplifier.
A fellowship hall with a ten foot ceiling used for speech wants the 4 watt tap, which lands about 90 dBA at seated ear height, while a nursery corridor wants 0.5 or 1 watt.
| Space | Typical ceiling | Background noise | Tap | Why |
|---|---|---|---|---|
| Nursery, restroom, quiet corridor | 8 to 9 ft | 35 to 40 dBA | 0.5 W | Presence without intrusion. Paging must be audible, nothing else. |
| Foyer or narthex, background music | 10 to 14 ft | 45 to 55 dBA | 1 W | Conversation continues over it. Higher taps make people shout. |
| Office corridor | 8 to 9 ft | 40 to 45 dBA | 1 W | Paging and emergency announcements only. |
| Classroom or small meeting room | 8 to 10 ft | 40 to 45 dBA | 2 W | Speech at conversational level from a ceiling three feet above a head. |
| Fellowship hall, speech | 10 to 12 ft | 50 to 60 dBA | 4 W | The default. About 90 dBA at the ear, 30 dB above the noise floor. |
| Fellowship hall, meals in progress | 10 to 12 ft | 65 to 70 dBA | 8 W | Announcements have to beat a hundred conversations and cutlery. |
| Overflow or cry room | 9 to 11 ft | 50 to 60 dBA | 4 W | A feed of the service, mixed for intelligibility rather than impact. |
| Gymnasium or multi-purpose room | 18 to 24 ft | 55 to 65 dBA | 8 to 16 W | The throw is double, which costs 6 dB before anything else happens. |
| Kitchen | 9 to 10 ft | 65 to 75 dBA | 8 W | Extraction fans set the floor. Expect to need more than feels right. |
| Covered outdoor walkway | Varies | 50 to 60 dBA | 8 W | No room gain at all, and weather-rated pendants are less sensitive. |
| Sanctuary, traditional, speech only | 16 to 24 ft | 40 to 50 dBA | 8 to 16 W | Possible, but a distributed ceiling system in a sanctuary is a compromise. |
| Sanctuary with a worship band | Any | Any | None | Do not do this. Use point-source loudspeakers. |
Background noise figures are typical occupied levels. The working target for intelligible speech is about 25 to 30 dB above the room’s noise floor at the listener. Where two adjacent spaces need very different taps, split them onto separate amplifier zones rather than fighting it with taps alone.
How to plan a 70 volt line without getting caught
- Lay out the speakers before choosing taps. Coverage comes first. Work the grid from the ceiling height and the speaker’s dispersion angle, measured to the ear plane at four feet seated, not to the floor. Using the floor over-states coverage by about a third in a ten foot room.
- Assign a tap to each zone, not to each speaker. Every speaker in one room should sit on the same tap. Mixed taps inside a single space create a level that changes as you walk, which is exactly the fault a distributed system exists to avoid.
- Add the taps and divide by 0.8. Sum the actual settings across the whole line. Divide by 0.8 for the installer margin, then round up to an amplifier size that exists. Never specify an amplifier whose rating equals the tap total.
- Check the line impedance before you power up. With the amplifier disconnected, measure the line. It should read close to 4,900 divided by the total tap wattage. A reading far below that means a speaker is tapped higher than the drawing says, or a transformer is wired straight through.
- Leave one tap step of headroom in every room. Set the system one tap below the maximum the room could take. A church that taps everything at 8 W has no way to make the fellowship hall louder for a funeral lunch without rewiring.
- Label the tap at every speaker. Write the setting on the back can and on the drawing. The next volunteer will have no other way to find out, and chasing an untagged tap means a ladder and a ceiling tile for every speaker in the building.
Where this chart does not apply
It is not a design for a worship band. A distributed ceiling system is built for even, moderate level speech and background music. It has no low frequency extension worth the name, it cannot produce the level a contemporary service runs at, and firing down from above a congregation’s heads gives no sense of the sound coming from the platform. If your sanctuary has a band, you want point-source boxes. Say so even when a ceiling system is the cheaper quote.
It assumes 70 volt, not 100 volt or 25 volt. A 100 volt line, common outside the United States, presents 10,000 divided by the tap wattage instead, so every impedance in the first table roughly doubles. A 25 volt line presents 625 divided by the tap wattage. Transformers are usually marked for one standard only.
Insertion loss is real on cheap transformers. A budget line transformer can eat 1 to 3 dB, so a 4 watt tap on a bargain speaker does not reach the level a 4 watt tap on a quality one does. That difference is why two identical-looking installations sound a grade apart.
Low frequency is the first casualty. Small line transformers saturate in the bass, and manufacturers roll the low end off deliberately to protect them. A 70 volt ceiling speaker that measures flat to 80 Hz on its 8 ohm bypass tap may only reach 150 Hz through its transformer.
The 8 ohm bypass tap defeats the point. Most 70 volt speakers include a direct 8 ohm connection. Using it on a distributed line puts a dead short across a constant voltage amplifier. It exists for the case where one or two speakers run from an ordinary amplifier instead, and wiring one into a 70 volt run is the single most common installation fault on these systems.
Sources
- 70 volt constant voltage line arithmetic and the 80 percent amplifier sizing convention, as implemented in this site’s 70 volt calculator
- NFPA 70 National Electrical Code Article 640 for the installation of distributed audio wiring, plenum rating and support requirements
- Ceiling loudspeaker sensitivity and dispersion figures from installed loudspeaker manufacturer application guides
Frequently asked questions
How many speakers can one 70 volt amplifier drive?
As many as the amplifier’s rating divided by 0.8 will cover. A 200 watt amplifier supports 160 watts of tap load, which is forty speakers at the 4 watt tap, twenty at 8 watts or eighty at 2 watts. The count is irrelevant on its own; what matters is the sum of the taps. Always leave the 20 percent margin so you can add speakers later.
What tap setting should I use for a fellowship hall?
The 4 watt tap is the normal answer for a ten to twelve foot ceiling used for speech, landing about 90 dBA at seated ear height, roughly 30 dB above a typical occupied noise floor. Move to 8 watts if the hall is used for meals while announcements are made, because a hundred conversations and cutlery raise the floor by ten decibels or more.
What impedance does a 4 watt tap present?
1,225 ohms, because impedance on a 70 volt line is 4,900 divided by the tap wattage. A 1 watt tap is 4,900 ohms, an 8 watt tap is 612.5 ohms and a 32 watt tap is 153 ohms. Measuring the finished line with the amplifier disconnected and comparing against 4,900 divided by your total tap load is the fastest way to find a miswired transformer.
Can I mix tap settings on the same 70 volt line?
Yes, and you usually should. The line does not care, because the taps simply add. What matters is keeping one setting consistent within a single room so the level does not change as somebody walks across it. Mix taps between rooms on one line, never within a room, and write every setting on the drawing.
Is a 70 volt system good enough for a sanctuary?
For a spoken word service in a room with a low flat ceiling, it can work. For anything with a band it is the wrong tool: there is no usable low frequency through small line transformers, the level a contemporary service needs is out of reach, and sound arriving from directly overhead gives no sense of the platform. Use point-source loudspeakers instead.
What happens if I overload a 70 volt amplifier?
It clips, then protects, then in the worst case fails, and a clipping amplifier on a distributed line can cook transformers as well. Overload usually comes from someone adding speakers to a finished line without re-adding the taps. That is precisely what the 80 percent rule buys you, so if the tap total has crept past the amplifier rating, drop a zone onto a second amplifier rather than tapping everything lower.
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.