I have a 24vAC 20VA transformer and several cameras that consume 4 watts, how many cameras can a feed?

Question

I don't really know how to relate VA (volt-ampers) capacity to watt consuption.

Answer 1

20VA = 20 watts

4 watt camera X 5 = 20 watts

Answer = 5 cameras

Answer 2

A volt-amp applied to a resistive load is basically a watt, if the voltage is measured as RMS.  If you have a power factor other than 1, due to a reactive load instead of a pure resistance, you'll need to factor that in.

If the cameras behave as resistive loads, expect to power 5 cameras at 4 watts from a 20 VA transformer.

Answer 3

Transformer are usually rated in watts, not VA.

also note that cameras usually need DC, and a transformer produces only AC, so that may be a problem.

Is this really a transformer or is it a DC-DC supply? or an inverter? they are all different, and using the wrong one could burn up your cameras.

.

Answer 4

I've never really figured out the VA rating thing, since the definition of a watt is volts times amps, but the VA rating is always higher than the watt rating. It has something to do with the power factor of the load, but it's mostly a rating manufacturers use to make it look like their stuff has more capacity than the other guys.

A better thing to look at is the amp rating for the power supply. Each camera uses 1/3 of an amp.

You should be able to run three or four cameras off that thing.

You can get better transformers for $5 on ebay. Just enter 24vac and supply into the search field.

Answer 5

Since VA = Volts x Amps and your transformer operates at 24 volts then the maximum current you can expect the transformer to furnish is Amps = VA / Volts = 20VA / 24V = .833 Amps.

Without knowing the power factor of the cameras the simplest and most fool proof way to solve your problem is to borrow a clamp on amp meter and measure the output current of the transformer with one camera connected to the circuit and operating. You can expect the current to increase approximately by the same amount each time you add an operating camera. Divide .833 amps by the number of amps that one camera draws and this will tell you the maximum number of cameras you can connect to the transformer.  Just to be sure measure the current again with the calculated maximum number of cameras connected and operating. The total current must not exceed .833 amps.

If you use the above method to determine how many cameras you can connect to the transformer then you don`t have to be concerned about the relationship between Volt Amps and watts in this particular application.

However for your general information: Volts x Amps x power factor = watts

Answer 6

It depends on how many of the cameras are operating at once.  Is the power mostly for the motor drive? 20VA could be 10 watts or 20 watts, depending on if it is motor, lights, or electronics.  I would guess that 2 cameras would be fine, perhaps even 5 cameras if only 2 motors are operating at once.

Answer 7

You'll  should easily get three on that. possibly 4.

Right okay, 20VA means the transformer is capable of Running a load that draws 20W  providing that load has a Power factor of 1 however. Your camaras arn't going to be a resistive load, and since they are only 4W they are not required by law to be designed to have a good power factor. I have never seen anything less than 0.7 though i work on products with higher power...(i might check a few things when i go back to work thinking about it)

If you know the power factor of the load. then you simply multiply this by the VA rating of the supply.

Therfore a supply of 20VA and a load with a power factor of 0.7 means you can run 14 Watts of load on the supply. Hope that make sense.

It's highly likely that your camaras won't have their Power factor specified. and you can only guess... or spend serious money on a power meter.