Why is the use of relays in home wiring frowned upon?

Question

It seems that some folks on the main site frown upon (this is the only direct indication I can find, but I've seen a few comments and non-upvoted answers that indicate this as well) the use of relays in house wiring, but why? The linked relay:

• is UL listed (not just UL component recognized!) for mains service
• can switch a typical branch-circuit lighting or single device load (most relay jobs are switching relatively fixed loads, vs a receptacle outlet)
• can be mounted to any junction box that has a 1/2" NPT conduit knockout via its builtin nipple
• and is not terribly expensive or scarce (about $30, and Grainger + several major electrical supply houses nationwide carry the RIB line)

Is there some weird Code-related wackiness related to relays in dwelling units? Or is it simply unconventional enough to make people squirm?

Answer 1

They are slightly unusual, but not in violation of code or "frowned upon" by anyone that knows what they are doing. Few home loads are such that they benefit from the use of a relay rather than a direct switch, so direct switching is often simpler/cheaper, and that's all. As an example where they are common, even in houses - electric heating circuits, run by a 24V thermostat; Or pretty much any heating/cooling system generally has relays associated with it.

Answer 2

What the post you cite is describing looks just fine as long as you are using everything according to its listing, labeling and manufacturer's instructions.

That is the only thing the NEC requires of field engineered devices and circuits.

The only other thing is you may void warranties by hacking your home equipment.

Otherwise I say hacking is great since that is the edge of discovery.

Answer 3

I don't have an axe to grind on the subject, but having evaluated relay options for a few projects, I can see where there might be concerns.

• it's odd and unexpected, and therefore more difficult/expensive to troubleshoot especially if Mr. Wizard has left the scene.

• it's all too easy to grab relays which are not listed, especially since many who explore the idea come from automotive or Arduino DC electronics, and have the habit of sourcing from non-Code vendors, apocryphally, Ali Express.

• Finding relays of appropriate spec is a lot harder than you think. There are a sea of products which seem perfect until you read the data sheet in close detail. E.g. "20A 250V" relays that have much lower ratings for inductive, tungsten and ballast loads - the latter refers to magnetic ballasts; electronic ballasts have very different issues.

• Packaging it in a code-legal way is most of the work, and it's really tempting to skip that and leave it in zip-ties and duct tape.

I have been going crazy trying to find relays rated for switching 20A of "ballast load" (clunky old sodium lights with magnetic ballast) at any feasible price.

Answer 4

If you have to ask the internet how to wire this stuff up; DON'T.

This an area common only to industrial electrical engineering. Knowing how to run a branch circuit and how to program a raspberry pie DOES NOT qualify you to install these type of things.

I gave up on electrical questions. I either don't know the answer (because it's professionals talking shop) or I'd have to begin my answer with a warning (just like that one does) so, no thanks, I'll pass.

I had posted a rather lengthy and somewhat embarrassing post that explains my position on this further.