Currently, I have a client running an inappropriate cat 5E cable through a buried conduit, underground from their house to their pool control unit. This conduit also facilitates two lines of 120 V. The amps it’s pulling is probably around 50 in total.

If I run a shielded ethernet cable through this conduit to resist EMF, would a smaller AWG (cat5e 24 AWG) or thicker (cat 6 23 AWG) be more resistant to the neighboring EMF?

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    \$\begingroup\$ Not enough to make a difference. \$\endgroup\$ Jul 14, 2023 at 21:22
  • \$\begingroup\$ it depends on the location, but it may be illegal to run power and low voltage circuits in the same conduit \$\endgroup\$
    – jsotola
    Jul 15, 2023 at 1:08

1 Answer 1


No. Run fiber.

You have two problems and a non-problem running ethernet through power lines. Let's go through them.

Low voltage not allowed to mix with AC power

This is for the simplest reason: so that if the conduits or wiring takes a hit, it cannot put 120V on stuff that is not insulated for 120V. I.E. when it leaves the power conduit and presents at an Ethernet faceplate, or runs on a random cat6 wire, or goes into a router or PC. Sticking ethernet in a power conduit is an amateur move and don't repeat it or presume it was approved by the inspector, because it wasn't.

Now in rare cases where AC mains electrical equipment are talking to each other, where the entire low voltage network stays inside Class I wiring methods, that is allowed. But that is limited to cases like CANbus between a solar charge controller and an energy management module in the main panel, or a "disconnected" or "actuate" signal between a Tesla PowerWall and the utility isolation switch.

Fiber which does not have a conductive component is allowed in AC mains conduits. Yes, I know it's not what you'd prefer, but you have to do it or route your ethernet elsewhere.

Indoor rated cable in outdoor conduits

Inside a conduit is not indoors. Outdoor conduits are presumed to be 100% full of water 100% of the time.

Signal interference

This is a moot point, since ethernet has no business in the same pipe with power.

The Navy uses light signals between ships. What keeps this signal from being jammed by the sun, which pulsates between day and night? Well, the signals are about 2 Hz, and the sun is at 0.00001 Hz, too far apart to interfere. Same deal with 100 MHz+ ethernet vs 0.000060 MHz electricity.

Yes, there are surges and spikes, but you can tamp them with surge suppression, and they too are simply too glacial to jam Ethernet. The bigger problem is phantom voltage being induced onto the adjacent wires due to capacitive coupling. A 40 VAC bias on the ethernet wires may not be appreciated too much by the transceivers.

  • \$\begingroup\$ Just on that last point, the ethernet spec requires device isolation to withstand at least 2kV, so you should be fine in terms of any reasonable induced voltage \$\endgroup\$
    – BeB00
    Jul 14, 2023 at 22:22
  • \$\begingroup\$ 40VAC on a pair is nothing. Ethernet data pairs are galvanically isolated with transformers. And the shield will (hopefully) be grounded and prevents capacitive coupling. \$\endgroup\$
    – Justme
    Jul 14, 2023 at 22:36
  • \$\begingroup\$ @justme 120VAC will appear as DC to a ~100 MHz transfomer. And DC can saturate a transformer. In fact I once worked on a 1945 locomotive that had an AC alternator for no other purpose than to run through transformers which could then be saturated by DC, with the surviving current controlling something else etc. etc. An absolutely bonkers analog computer using AC in a DC world. \$\endgroup\$ Jul 15, 2023 at 1:47
  • \$\begingroup\$ @Harper-ReinstateMonica 40VAC won't be between wires of a pair, that much differential voltage of pair would melt the transformer windings. But because Ethernet is a transformer isolated, it can handle a pair having much more than 40VAC of common mode bias voltage. \$\endgroup\$
    – Justme
    Jul 15, 2023 at 7:39

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