Utilize a conductor properties chart to determine the resistance of the cable at the end of the circuit. An 18 AWG conductor, for example, has a resistance of 7.77 Ohms per 1,000 feet, so divide 600 by 1,000 and multiply that by 7.77 to find the actual resistance: 600/1,000 = 0.6 X 7.77 = 4.662 Ohms. A 16 AWG conductor has a listed resistance of 5.08 Ohms at 1,000 feet. Again, do the math by multiplying 5.08 by 0.6 = 3.048 Ohms.
Step 5: Determine the voltage at the end of the NAC. To do this, we use simple Ohms Law: I X R = E. We know the total current in our circuit is 0.968A (see Step 2) and we know that the actual resistance of our circuit to be 4.662 Ohms using 18 AWG and 3.048 Ohms using 16 AWG wire. Doing the math: 0.968A X 4.662 Ohms = 4.513 Volts using 18 AWG, and 0.968 X 3.048 Ohms = 2.95 volts for 16 AWG.
Step 6: Determine the actual voltage drop at the end of the NAC. To do this, begin by subtracting the voltages in Step 5 from the operating voltage at the head of the NAC: 24V - 4.513V = 19.487V for 18 AWG, and 24V - 2.95V = 21.05V for 16 AWG.
Step 7: Determine which cable size to use based on actual voltage at the end of the circuit. Use the manufacturer’s specification sheet to find out the lowest and highest voltage allowable for proper operation. In this case, our GE3-24 visible-audible signaling appliances have an operating voltage range of 16VDC to 33VDC. As you can see, using the voltages derived in Step 6, either gauge size will work — but clearly 16 AWG would be the better choice.
To simplify the process of determining wire size, check out the “NAC Circuit Calculator” on Automatic Fire Alarm Association’s (AFAA) Web site in the resources section. It will provide an automated go/no-go method of determining wire gauge using a common spreadsheet program.
Al Colombo is an award-winning writer who has covered electronic security and life safety since 1986. Visit his Web site at www.alcolombo.info, and check out his Security Sense blog.
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