Caught a new guy running alarm wire parallel to Romex for 20 feet the other day

I keep seeing people forget that line voltage creates interference on low voltage signal wires, even in 2025 with all the fancy shielded cable out there. Did your mentor drill that into you or did you learn it the hard way like me?

2 comments

2 Comments

riverknight19d ago

I used to think a big enough gap and modern shielding would take care of it... then i had a call back where some cheap security system was picking up the 60hz hum every time the kitchen lights were on. Had to go back and re-pull the whole run with twisted pair and keep it separated from the power cables. Yeah, that mistake only happens once.

harper_wells18d ago

I used to think a big enough gap and modern shielding would take care of it" yeah, and I bet @riverknight still thinks that after that one kitchen light incident. Honestly, I've run alarm wire right next to 120v for way longer than 20 feet in old houses where there's just no other path, and it's worked fine every time. The 60hz hum thing only seems to happen with cheap gear or when the shield isn't grounded properly, not from just being parallel. If your wire is twisted pair and the shield is tied to ground at one end only, you can practically tape it to the Romex and still get clean signal.