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Had a weird voltage drop on a long run for a keypad last week | Alarm System Installers - Rigidtalk

Had a weird voltage drop on a long run for a keypad last week

I was putting in a new system at a big house in the hills, and the main panel was in the garage. The owner wanted a keypad by the front door, which was a good 150 feet away through finished walls. I figured I'd just use a standard 4-conductor cable, no big deal. When I got it all hooked up, the keypad screen was dim and the buttons were slow to respond. My meter showed the voltage at the keypad was only 8 volts, down from the 12 at the panel. I learned the hard way that on long runs like that, the wire gauge matters a lot more than I thought. I had to go back and pull a heavier 18/4 cable to fix the voltage drop. Has anyone else run into this on a big property, and what's your go-to wire for these long keypad pulls?

5 comments

5 Comments

samward5mo ago

Honestly I think everyone is missing the real issue here. It's not just about voltage drop from the wire itself. Those thin 22 gauge cables have a lot more resistance and that resistance creates heat when power is trying to flow through them over 150 feet. On a hot day that wire heats up and the resistance goes even higher making the voltage drop worse. I had the same problem on a gate keypad in Arizona where the thing would work fine at night but crap out in the afternoon sun. Swapping to 18/4 fixed it but also I started running the cable in a separate conduit away from any hot water pipes or attic spaces that bake all day. If the house you were working on has the cable running through an unconditioned attic or crawlspace that gets hot the wire temperature is absolutely making that voltage drop worse than what you'd calculate at room temperature. Nobody checks for that and it bites you in the ass every time.

jenny_jackson5mo ago

Oh man, that exact thing got me on a gate intercom last year-eight volts at the end of a 200-foot run and I was baffled for an hour before I checked the gauge. Now I keep a spool of 18/4 in the truck for anything over 75 feet and just treat it like standard practice.

grace_perry446mo ago

That's a huge drop over 150 feet! What gauge was the original 4-conductor cable you used before swapping it out? I've seen 22-gauge wire cause similar issues on long runs for door stations, where the voltage just disappears. Makes me wonder if there's a rule of thumb for when to jump up to 16-gauge instead of 18.

wendyc536mo ago

Yeah, that "huge drop" happens all the time with skinny wire. I always use 18/4 as a minimum now, but for anything over 100 feet I just grab 16/4. It costs a bit more but saves you a callback when the keypad acts up. The math for voltage drop is a pain, so my rule is just to go a size bigger on any long run.

victorross5mo ago

You're way overthinking this, @grace_perry44. Honestly, a 4 volt drop on a 150 foot run with 22 gauge is fine for most keypads. Those things are designed to work anywhere from 6 to 14 volts. The dim screen and slow buttons weren't from the voltage drop, they were from a loose connection at the panel or a bad splice somewhere in the wall. I've run 100 foot runs with 22 gauge speaker wire for keypads and never had a problem. People get too hung up on gauge numbers instead of just checking their terminations first. That 18/4 you swapped in probably has the same drop if the wire was the real issue, you just got lucky.