WIRING FOR SUCCESS
Sep 1, 2002 12:00 PM, STEVE FILIPPINI
POP QUIZ: WHAT IS THE ONE COMMON component of any security, audio, or video system? If you answered “wire” or “cable,” then you know just how important it is to use the proper wire for your applications. Pick up any wire catalog, and you will see hundreds of gauges, various kinds of conductors, and many types of cable. For now I'll focus on the correct type of wire needed for security systems.
To the layperson, wire is often identified by only two categories: gauge and the number of conductors in a jacket. The gauge, or size of a cable's diameter, is given a number that can range from 14 to 24 when dealing with low-voltage systems. The lower the number, the larger the gauge of the conductor and the more current it can handle. Telephone punchboards typically use 24-gauge wire, because it carries data and voice signals at a very low current draw. House wire used for 110V sources is usually 12 gauge, which can handle much higher current draws. The number of conductors needed depends on the application for each cable. Early home telephone systems used 22-gauge, 3-conductor wire runs. The red and green wires carried the voice signals, and the yellow wire was used for ringing the bells on the phone. As residential telephone systems improved, it was common to see the yellow wire combined with the red wire. That eventually led to the removal of the yellow wire altogether. The red telephone wire is called the ring, and the green wire is called the tip. The wire remained at 22 gauge. That's because though the wires carry voltage to ring the bells, the current required wasn't much.
Security systems are usually 12V and carry no more than two to three amps of current at any given time. In this case, 22-gauge, 4-conductor (22/4) cable is used. For infrared and glass-break devices, two conductors are used for powering the sensor and two conductors are used for monitoring the protection-sensing circuit. Security system transformers used for power sources run anywhere from 12V, 12V/amps to 24V, 50V/amps in size. For those devices, it is highly recommended to use a larger gauge wire, such as an 18 gauge, 2-conductor (18/2) cable. Don't fall into the trap of using a 22/4 cable and doubling up on two of the wires. The official reason is because your wire gauge should match the application requirements. The everyday reason is because people don't always match the same two colors at both ends of the wire run. A red/black, green/yellow combination on one side and a red/yellow, black/green combination on the other side will provide only a short circuit and permanently damage your system's step-down transformer.
Fire alarm systems that include 24V horn/strobes used in the notification appliance circuits need at least a 14-gauge, solid conductor cable with two or four conductors, depending on the type of audible devices used. Because the components are used for fire systems, the color of the jacket needs to be red and the chemical makeup of the jacket needs to be approved for fire applications. If the wire is routed through open plenum ceilings, then the wire also needs to be plenum jacketed.
But how does one know what gauge wire to use when approaching a new application? First, you must know Ohm's Law, which states that voltage is equal to current multiplied by resistance. If the source voltage is already known and the total sum of the current consumption from the peripheral devices is determined, dividing the voltage source by the total current draw will give you the maximum value of resistance allowable for a correctly operating circuit. As previously mentioned, the larger the wire gauge, the lesser the value of resistance for a particular wire run. The National Electrical Code has a chart that identifies the resistance value of a specific wire type and gauge and wire run distance (based on 1,000 feet of wire, 500 feet one way) at an environmental temperature of 167 degrees Fahrenheit. Eighteen-gauge, uncoated copper wire, for example, varies between 7.77ž (for solid wire) and 7.95ž (for stranded wire) per 1,000 feet. In a real-world application, you wouldn't subject your wires to 167-degree temperatures. Instead, the resistance for a stranded, 18-gauge, uncoated copper wire at 77 degrees Fahrenheit is 6.51ž per 1,000 feet.
That said, you should notice a few things regarding wire properties. A solid wire has less resistance than a stranded wire, and the lower the operating temperature, the lower the resistance for that particular wire. Knowing the resistance of the wire you are using will help you make sure you do not exceed the 10 percent voltage drop rule for your circuit. If the total current draw is multiplied by the known circuit resistance, then the result will be the voltage drop for that circuit. If the source voltage is 12V, then the voltage drop cannot exceed 1.2V. If you exceed the 10 percent rule, you run the risk of having the powered devices acting erratically.
You may be wondering if specifying a larger gauge wire for all of your applications might make the process easier to get through. Why stock multiple rolls of 22/4 and 18/4 gauge wire if you can order several rolls of 14/4 instead? If you are a field technician, then yes, it will make it easier. If you are a salesperson, it won't make it easier. The larger the gauge means less resistance, resulting in a greater cost. In my occupation, I am required to determine the routing and total footage of wire required to complete a parts list for calculating the customer's final cost. If I overshoot the amount needed, I run the risk of overbidding the job and losing the account. If I underbid the amount needed and have to ship wire to the job site later, then my company will eat the cost of the wire and shipping expenses, not including field installation downtime and possible overtime.
Calculating the total wire run length of a job is simple enough. Take a scaled drawing of the building to be protected and lay out the expected security devices on it. Determine the individual wire runs, allowing room for slop and distance from device to ceiling height (ranging anywhere from 5 to 20 feet, depending on the business or application) by using a wheelie tool that digitally measures a wire run from a protection device to the control panel. Add the totals up and throw in a few hundred feet just for kicks, and you should be fine.
Other types of wire you may need to know about include shielded, twisted, and stranded. Shielded wire traps and drains (among other things) RF interference and AC induction away from sensitive data circuits. Remember to ground only one side of the shield to an earth ground connection. Grounding both sides to the ground creates a giant dipole antenna and introduces damaging interference instead of draining it away. Twisted wire reduces the amount of interference, but just not as effectively as shielded.
Stranded wire is more pliable, but if you don't properly treat the ends of a stripped-back conductor, you may create more problems than you solve. I have worked on hundreds of problem-riddled security systems that were corrected by simply shortening the length of exposed, unjacketed wire leads. Also, many installers don't twist the stranded leads together before connecting them to a terminal or splice, creating “whiskers” that short out or ground out, producing false alarms and system failures. I take the stripped leads in my left hand and, in a clockwise direction, use my right hand to twist them tight. Some technicians solder the ends of the leads, and others use spades and wire connectors before tying them down to wire terminals. If you are not using wire connectors or spades, you should get into the habit of curving the end of the wire to the right before placing it under an open screw terminal. That way when the screw rotates to the right and tightens down on the wire, the leads will be less apt to roll out from under the terminal, causing loose connections and system problems.
Hopefully, this sheds some light on why some field technicians use the wire they do. If you are doing it differently and feel strongly about it, send comments my way.
Steve Filippini is a senior security technician with more than 20 years of experience in the security-installation industry. He can be reached at firstname.lastname@example.org.