As I'm investigating my bathroom's wiring in anticipation of an update, I've found a violation of NEC 300.3(B) (that conductors need to run in the same raceway, cable, etc.). I understand that problem, and it won't be hard to fix (see background below). I understand that code is focused on prevented heating and other unwanted physical effects, and not primarily about the EMF interference.

What I'm curious about is how much EMF inference this short loop of wire is actually generating in practice and how much effect it would have in the surrounding area. The loop is probably not more than 20 feet (roughly 6 meters), and the grounded and ungrounded conductors are probably 3–6 feet (roughly 1–2 meters) apart. It's a 15 amp branch circuit. Would this cause noticeable interference with wifi or cell reception in the home (and if so, any more than heavy plaster walls and metal lath and cement backer behind bathroom tile)?


This all happened before my time in this house, but from the different cables types (old NM, newer NM, etc.) I can tell that the bathroom evolved over time. It appears that it probably used to be to power to the switch first, and then later when a fan and light unit was added, it became power to the fan and light first, with two switch loops via a run of 12/3 to the switch. Then, for reasons not entirely clear to me, tied in to one conductor of an existing cable running to the older lamp, and then one conductor of another cable running from the older lamp. That is, the current reality is the first image (a), but what should have been done is (leaving the "bottommost" cable in place, but unused) is the second image (b):

(a) current reality, violation

(b) what should have been done

  • 2
    rf interference to what, exactly? (Although one could well argue that 50/60 Hz is not radio frequency.
    – Jon Custer
    Commented May 6 at 15:00
  • What are the blue & black lines between the bottom two boxes? Is that just a "spare" cable that's unused, or do those actually complete a loop? If it's actually a loop, A) it's unnecessary band B) it's a code violation.
    – FreeMan
    Commented May 6 at 15:16
  • @FreeMan In the first image, which is how things are right now, the bottom cable and the left hand cable are both cables in which one conductor is used (which is the problem). In the second image, which is how things probably ought to be, it's a completely unused cable that's capped off at both ends. What I think happened, based on the left and bottom cables being old black NM with rubber/cloth covered conductors and the 12/3 on the right being newer Romex, is that when the fan was added, they added the switch loop and (wrongly) figured they should use the bottom cable for something. Commented May 6 at 18:45
  • @JonCuster Good point -- I should have said EMF, as RF is a specific subset. Updated. Commented May 6 at 18:48

2 Answers 2


This will not cause any interference with cell phone signals, Wi-Fi, Bluetooth, Ethernet, etc. Those all operate on much higher frequencies and will have filters that cut off frequencies below a limit that is far above 60 Hz. It may induce some hum in audio amplifiers nearby. It’s an audible frequency so it can’t just be filtered out by audio amplifiers without possibly throwing away part of the legitimate input.

It’s a code violation because it can lead to heating and vibration that can cause connections to fail, leading to arcing and fires.

  • Good point mentioning why it's a violation; thank you! I do and did understand that, especially with regard to passing through metal, etc.; I'm just focusing on the other aspect here. But thank you for pointing it out--I wouldn't want someone to come away with the impression that "oh, it won't mess with my wifi, so it must be OK". Maybe I can bring some audio equipment up to observe the before and after. :) Commented May 6 at 13:58

Here's a mystery for you.

As you know, the sky and everything under it flashes between very bright and very dark. This is the day-night cycle.

So with all that flashing, how is it possible for ships at sea to communicate with light signals? Doesn't the flashing of the sun interfere with the light signals?

LOL of course not, they are on very different frequencies, separated by a 200,000:1 ratio in baud rate. And the sun being a sinusoidal pattern, is too gradual to affect a square wave.

Same deal with power lines vs Ethernet.

  • Slightly cheeky, but I appreciate the "mystery". :) I should have said "EMF" rather than "RF". As nobody's answer mentioned, this could produce unwanted interference, but in things like audio hum, rather than in the RF range. That said, I do like the analogy as a very clear way to explain how vastly different frequencies don't necessarily interfere and/or can be disambiguated. It could even be extended to phase, if ships used 12 hour long signals that started and stopped at midnight and noon, right? Commented May 6 at 18:53
  • @Joshua even line noise is far too low frequency to bother ethernet. More like "clouds going over" lol. Good point about phase but then you need to know your timing and be in sync. It could be messy if you were off sync enough to have a beat frequency. Commented May 9 at 5:10

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