I recently bought this ceiling speaker (http://www.3acctv.com/upload/uploads/Specification/T-103C.pdf), as shown in the PDF it has 3 wires attached to it. I am no expert in this, so I was wondering if someone could help explain the difference between the 3 wires.

I do understand the concept of +/- wires and how to connect them, the fact that this speaker has 3 wires baffles me though.


  • \$\begingroup\$ Is the third wire attached to the metalwork? \$\endgroup\$
    – user16324
    Commented Jul 9, 2015 at 16:06
  • \$\begingroup\$ @BrianDrummond Nope, all are attached to the speaker itself, do not thing it's a ground wire. \$\endgroup\$
    – SU-GDL
    Commented Jul 9, 2015 at 16:16
  • \$\begingroup\$ Do you have a 70V audio system? Ceiling speakers of this type have a built in transformer, with multiple taps for different volumes. Some have a selector for the taps, others simply have extra wires. \$\endgroup\$ Commented Jul 9, 2015 at 16:19

2 Answers 2


As it says in the PDf as well:

With power tabs for 3W and 6W.


"Black:Com Red:1.7KΩ White:3.3KΩ"

Or drawn, its equivalent resistances are:


simulate this circuit – Schematic created using CircuitLab

You connect Black & Red for 6W at 100V, Black & White for 3W at 100V. Do NOT use the other wire in either case.

  • \$\begingroup\$ Thank you! Any idea why would I want to choose 6W or 3W? \$\endgroup\$
    – SU-GDL
    Commented Jul 9, 2015 at 16:17
  • \$\begingroup\$ @SU-GDL Because: 1. Your amp has a specific rating. 2. 6W is too loud. 3. 3W is too quiet. Etc etc. Usually ceiling speaker systems use one set of wires to route all speakers on a floor and you can then choose to make some louder than others. For example. \$\endgroup\$
    – Asmyldof
    Commented Jul 9, 2015 at 16:24
  • \$\begingroup\$ In a 70/100V audio system, you don't have to worry about impedances, hence the transformers. What you do have to worry about is that you don't exceed the wattage output of your amp. If your amp is rated for 500 Watts, you should ideally not run more than 400 Watts through it (20% headroom to not stress it). You can also select the tapping to control the volume in a certain area, 6W for most but 3W for quieter areas. \$\endgroup\$ Commented Jul 9, 2015 at 16:24

These speakers are designed for a 100V loudspeaker distribution system.

There are 3 common loudspeaker distribution voltages: 25V, 70V, 100V. North America uses mostly 70V systems, with a very few systems using 25V distribution. I think that 100V distribution is used in places like Japan and Europe but I've never worked with any of those systems. Regardless, the principles are the same no matter what the target voltage is.

The concept behind constant-voltage loudspeaker distribution systems is simple: it's too much work to figure out how to match the load impedance to the amplifier. It's much easier to design the amplifier to output 70 Vac RMS when it is operating at full power. It is then a simple matter to use a transformer to drop that voltage such that the desired amount of power is fed to an individual speaker.

Most loudspeakers designed for use with constant-voltage distribution systems have multiple taps so that the amount of power being fed to that particular speaker is made appropriate for the space it is being used in. Most common taps on the small speakers that I've worked with are octave steps starting at 1/4 Watt and going up to the power rating of the speaker. That is: 1/4W, 1/2W, 1W, 2W, 4W, 8W, etc.

Conversely, larger speakers start at the full rated power of the speaker and work downwards: the Community W2-218WT speakers installed recently at our local football stadium have taps at 100W, 50W, 25W, 12.5W.

In general, speakers that are designed for use at 100V can also be used at lower voltages but you have to recalculate the power that is available at each tap. However, speakers should not be used on a distribution system that has a higher voltage than the speaker rating - the transformer may saturate and bad things can happen.


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