I'm slowly remodeling my house one room at a time. I'm to the point where I need to wire a window for the future alarm system. This post talks about how to wire a home during new construction for a security system and partially answers my question saying to use 22/4 wire.

What about 18/4? 24/4? Solid or stranded?

Also, since I won't be able to connect the alarm system for some time yet, and I'm not sure where the main board will be yet, I need to have leeway on the cable length.

How long can a particular gauge of wire run without signal degradation or interference issues?

  • What voltage is it running?
    – Jason
    Jul 18, 2013 at 18:59
  • I'm not sure what voltage is running. Is there not a standard? Security Systems are new to me.
    – John Smith
    Jul 18, 2013 at 19:22
  • Sadly no, if you are wiring for simple magnetic/reed switches, the typically set of two plastic bar, for the windows then you'll have more leeway since they are not powered, as opposed to something like a motion detector or a lit keypad. I'm writing up formulas and such, answer to follow.
    – Jason
    Jul 18, 2013 at 19:36
  • Short answer, go bigger (lower), not smaller (higher).
    – Jacob S
    Jul 18, 2013 at 20:54

2 Answers 2


Most security system have a minimum of 9v DC, but I've seen up to 60v AC or DC, running to the sensors. The concern here is voltage drop as these are extremely low current devices. If you are using magnetic/reed switches, which it sounds like you are, they are unpowered digital switches (either on or off). Digital logic circuit typically require >50% of the feed voltage to trigger as high (on) and anything else is low (off); however, you always want more than the minimum so any interference and such doesn't affect you.

enter image description here

VD = Voltage drop (conductor temp of 75°C) in volts

L = One-way length of the circuit's feeder (in feet)

R = Resistance factor in ohm/kft

I = Load current (in amperes)

  • 28ga copper = 64.898 ohm/kft
  • 24ga copper = 25.669 ohm/kft
  • 18ga copper = 6.3851 ohm/kft

Let's assume the wire is going to a back-lit keypad and draws .5A (trying for worst case scenario)

VD(100ft, 28ga) ~= 6.5V. 9v source means 2.5v at the end which won't work

VD(100ft, 24ga) ~= 2.5V. 9V source means 6.5v, a reed switch should work fine, a keypad or motion sensor may not.

VD(100ft, 18ga) ~= .64. 9v source means 8.36v, almost anything will work.

Now that is the worst case scenario (within reason). Most modern security systems are 24v so 28ga is pushing it for keypads/motion sensor (depending on what they actually need) but 24ga should be fine for everything. It really depends on what your trying to hookup for each stretch of wire, the length, and the input voltage.

If this breaks any Low-Voltage NEC, someone please add details.

*Source: http://en.wikipedia.org/wiki/Voltage_drop, http://amasci.com/tesla/wire1.html

  • Good answer. Thank you! Any thoughts on stranded vs solid?
    – John Smith
    Jul 18, 2013 at 20:14
  • Solid for fixed installation wiring like this.
    – hookenz
    Jul 18, 2013 at 20:28
  • Please note, I used 100ft for everything including the 24v quick stats. Adjust accordingly.
    – Jason
    Jul 18, 2013 at 21:07

A rule of thumb I use, based on a class in residential electrical wiring, is to increase the wire size by one gauge for every 100 feet of wire used. So if the window will be at most 245 feet from the panel as the wire runs, use 16ga wire. Please don't ask me to show the math behind that, I wouldn't be able to remember it all. That won't necessarily address interference, just voltage loss.

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