HOW TO DETERMINE POWER WIRE GAGE

HOW TO DETERMINE POWER WIRE GAGE

By Frank McClatchie

This article can be used to estimate the minimum wire gauge that must be used to at various cable pair lengths to power a camera and any required illumination. To begin with there are three major factors to consider when selecting the cable wire gauge that will be used to supply power to the CCTV cameras.

The first thing to consider is the DC output voltage source of the power supply you intend to use to power the Camera and any illumination. It is powered by a 60 Hertz power source so find the ripple factor specification and subtract it from the specified source voltage at maximum load (taken from power supply equipment specification). This voltage is the lowest instantaneous voltage under the heaviest load conditions which we will call “Vsource” The ripple factor can be particularly important in the less expensive power supplies because the size of the filtering capacitor determines the magnitude of the ripple factor which in turn determines the lowest power supply voltage you can use to drive the camera. If you what the ripple factor is then subtract it from the supply voltage to get the minimum Vsource voltage. Ripple factor and line voltage sag can be ignored if you select a wire size twice the required gage for added current handling capability.

The voltage used in Vsource must be the lowest voltage that will exist at the power supply source output during heavy power industry power load conditions, such as hot summer days with all the air conditioners turned on or other similar local conditions. Inexpensive power supplies for cameras are often not regulated and so the voltage will vary directly with the power line voltage and also the number of cameras connected to the system and the illumination load at night. Even regulated power supplies will loose regulation under some power industry load conditions. These conditions must be taken into consideration when choosing a voltage for Vsource otherwise you may loose some cameras during power industry brown-outs.

The second bit of information we need is “Iload” the maximum current draw in amperes of the camera and associated illumination. You can find this information in the published camera specifications.

The third bit of information we will call the “Vload” or minimum voltage required to operate the camera under all load conditions, you will find that in the published camera specifications.

The forth bit of information we will call “Vvd” it is the voltage drop produced by the resistance in the Cable Pair used to deliver the required current to the Load or camera.

When you have collected both the Vsource voltage and the Vload voltage numbers you can use the following formula to determine the Vvd, To get the Vvd subtract the Vload from the Vsource.

FORMULA: Vsource – Vload = Vvd

Now using the Iload current draw in amperes and the maximum voltage drop Vvd the maximum resistance value for the power wires can be determined. By dividing the maximum camera current draw in amperes Iload into the maximum permissible Voltage drop in Volts Vvd the result will be the maximum resistance that can exist in the wires between the Camera and the power supply and still have the system work properly.

FORMULA: Vvd / Iload = MAXIMUM RESISTANCE

Once the maximum resistance has been determined you can use the lookup table to determine the minimum wire gage needed for the distance that must be traversed to connect the Camera to the Power supply. Determine the distance the wires will run and multiply the Resistance per foot or meters times the distance in feet or meters. In this way you can select a wire gage that has less resistance than the calculated maximum resistance for the distance traversed.
WIRE GAGERESISTANCE PER FOOTRESISTANCE PER METER

16 .008032 .02636

18 .012770 .04190

20 .02030 .06660

22 .03228 .10590

24 .05134 .16850

26 .08162 .26780

28 .12980 .42590

Of course, if you find that you cannot reach that Camera with any wire gauge that you have in stock, it is possible to parallel two or more pairs of existing wires until a lower loop resistance in achieved to be able to deliver sufficient current to any particular camera load. To parallel wires of the same gage just divide the resistance per foot or meter in half.

This calculated Voltage Drop represents the absolute maxim voltage drop that will fit within the equipment minimum specification however ordinary engineering considerations would suggest a margin and that this Voltage drop should be increased by a least 10 % so that everything need not be operating at 100% of maximum permissible load. An even greater voltage drop margin of 50% would probably result in extended equipment life and can be useful if a different camera that requires more power is installed in the future.

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