Discussion

Do I use 12Vdc or 24Vac for my CCTV System?

Most CCTV cameras on the market offer 12Vdc or 24Vac operation (they are usually fitted with a small switch or jumper to swap between voltages).

So, AC or DC – which one and why ?

It all starts out with the simple fact that if you double the operating voltage, you halve the current required – or if you halve the operating voltage, you effectively double the operating current required.

For example, a camera that has a rated power consumption of 6W (6 Watts) will draw approx. 500mA @ 12V or 250mA @ 24V  (W / V = A) – And this is before we factor in the voltage drop caused by cable gauge / length of run / termination resistance, etc. Assuming that we stick with industry best practice which recommends that you do not connect more than 70-80% of the rated load of the power supply, in this example, in order to power 16 of these cameras, you would require a 10A /12Vdc power supply or a 5A /24Vac power supply.

There’s a difference between 12Vdc & 24Vac Supplies (apart from the obvious):

Due to manufacturing complexity, DC supplies can be up to 3 times more expensive than an equivalent AC supply for any given current rating i.e. a 12Vdc 5A supply can be up to 3 x more costly than a 24Vac 5A. In the example above, if we select 12Vdc over 24Vac, we will need a supply that is up to 6 x more expensive (twice the current rating + 3 x more expensive to manufacture).

Now, if it was as simple as that, everyone would only install 24Vac power supplies and leave the 12Vdc ones in the van – you knew it wasn’t going to be that straight forward, didn’t you ? 🙂

12Vdc Regulated Supplies:

There are a couple of other things that need consideration – first is that good quality 12Vdc power supplies are designed to deliver a constant (regulated) 12Vdc. You should be able to measure this from no load (nothing connected) through to full load (the power supply rating – eg 2.5A, 5A, 10A) So, cable run induced voltage drop aside, the connected devices should always have a constant (12V) DC voltage supplied to them, regardless of input mains voltage (240V) – note that the mains supply voltage can vary both up and down depending upon location, cabling infrastructure, local load and time of day.

24Vac Supplies:

24Vac supplies on the other hand have no internal voltage regulation, in fact the only time that you will be able to measure approximately 24Vac on the output of these supplies is if two factors are met:

1) The incoming mains is 240V (not 215V or 260V, etc. – as it can sometimes be in certain areas)

AND

2) The connected load is close to (or at) the maximum rating of the transformer (eg 2A load on a 2A power supply, 9.9A load on a 10A supply, etc.)

If  output load remains stable but the incoming mains voltage varies up or down, the output voltage will track up and down proportionately:

eg: 230Vac In > 23Vac Out   /   255Vac In > 25.5Vac Out

Likewise, (assuming that incoming mains is 240Vac) – if a 24Vac 10A power supply has a connected load of 1.5A (as can sometimes be the case where an installer specifies a 10A supply to cater for future system expansion), then the output voltage can be anywhere between (say) 26V and 28V – depending on transformer quality and design. Now, if the incoming mains rises to (say) 255V from 240V, then output voltage could be over 30Vac – potentially leading to longterm connected equipment damage.

Depending upon the installation requirements, 24Vac power is generally more flexible when it comes to medium to long cable runs – Ohm’s Law tells us that for a given equipment current draw and cable length, the voltage drop over the distance will be similar for 12Vdc or 24Vac – however, if the calculated voltage drop is (say) 2.5V, then the camera connected will probably work satisfactorily on 24Vac, but may not start at all on 12Vdc (24V-2.5V =  21.5V  ~ usually within the operating parameters of most cameras,  while 12V – 2.5V = 9.5V ~ probably below the minimum operating voltage of most cameras)

The Bottom Line:

While there are a couple of exceptions, if mains voltage is reasonably stable and your cameras can accept either 12Vdc or 24Vac, then 24Vac power is generally the way to go – for both performance and cost effectiveness.

This is an extract from the Tactical Technologies website

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