Wiring a RIB (relay in a box)


Senior Member
I am putting a few relays around the house for various things and want to make the installs code compliant and also look reasonable for when I might sell the home. In one location I want a low voltage outdoor motion sensor for some outside lights. For the logic I will use a microprocessor board (Arduino or similar) to provide the delay so it operates independently of an automation system. I want to keep things simple for future owners so they don't have to have an automation controller for basic operation of things. The micro can send serial messages to report its status or receive to modify the time on, force it off, etc. but the serial link is not needed for basic operation. I can sense the voltage using an optoisolater so I can flip the switch off momentarily to force the light on forever and a longer off time to return to normal operation (switch is before the micro). I design totally different things that have this sort of voltage isolation requirement and get UL approvals on them so understand the needed spacings and such but not as familiar with NEC. While I can put together something that meets UL spacings my HA device won't have UL approval. A RIB seems like the way to go to avoid any issues. May still build up the micro board although a production board would look better for that too. My question is mainly how to package up the installation. The RIB goes on a electrical box and the low voltage control wires are long enough to be routed outside the box and spliced to wires going the micro board. Having the low voltage wires spliced in the open seems ok but might raise questions for someone unfamilar with the RIB, especially since they are coming out of a line voltage box.. I need a box for the micro board anyway so I am thinking of putting the micro in its own box (would fit in a plastic outlet box). I can sense the voltage by adding an outlet and using a small wall wart with output routed to the micro box. Wondering how others do this to make it look neat and also code compliant.

In another location I want to sense current so have a similar issue with routing wires from a current transformer to a micro board.

Most of these are in the attic or unfinished space but one is outside to control a pump so needs to be weatherproof. Thinking of putting things in a sprinkler timer box as they have a high voltage compartment and low voltage section. They are set up for just an outlet so I would need to add a hole to get the RIB low voltage wires into the low voltage section.
I'd get a NEMA box with DIN rail, use DIN rail relays and then separate the cabling that way. Looking at your outward goals, I'd look at a time delay RIB before complicating things too much. Just the nature of adding all those mini boards is going to complicate things for any future owner compared to a HA system or lighting controller with a backplane to a HA system.
Wouldn't recommend using the nipple mounted RIB's on anything outdoor. I'd go with track mount and a suitable enclosure and not a sprinkler box, field modified.
In the specific case of separation of the LV and HV portions of the RIB, normally you'd either use a divided box/enclosure or you're looking at 2 boxes with a nipple between. The HV side would be it's own and the LV would be routed to it's own separate box.
While you can technically route the LV cables outside of the enclosure, it's an area that can be colorful and up for interpretation as to whether or not the LV junction is or should be allowed outside of an enclosure.


Senior Member
That's helpful. Several angles I hadn't thought of.

The NEMA box sounds like a good solution for the outdoor one. And it could house the needed current sensor. Any issue with putting a mini board in the same enclosure as the high voltage if given enough spacing? Any references to the rules covering such field wiring so I can read up on it? Any specific NEMA boxes or sources for those you might suggest?

I had thought of a box with a separator but the separator would need to be modified (small hole) for the low voltage wires. That's something I have wondered if allowed. There are relays that could be placed in the bottom of a box with low voltage on one side of the divider and line voltage on the other but it would require a cut out in the separator. In pool control panels the relays sit in the high voltage area and low voltage wires enter the relay area. The low voltage wires are not 600V but are routed to maintain significant clearance between HV and LV. But that is a UL approved unit not field wired. Here is a relay I have seen used for that.


I suppose those could also be mounted in a NEMA box. Just not quite as easy as DIN rail mount...

The nipple between boxes for the LV wires is an obvious solution for the nipple mounted RIB I hadn't considered.
The big IF is whether or not the LV cables introduced for the specific connection of the LV to HV (relay) are rated the same as the HV cables. It's usually a necessary evil that is done for the purposes of interconnection and only interconnection of LV and HV systems.
That said, I'd look at Hoffman or their contemporaries and DIN rail. It's how it's done in the building controls business. HV on one side, LV on the other, finger duct or panduit to maintain separation. Same way the elevator guys do it...and their stuff has from LV up to polyphase HV in the same enclosure.
Plenty of DIN mounted devices that separate out the voltage connections.
Rather than get too complex, if you're only looking for current, get a RIB no contact current sensor (donut) and call it a day.


Senior Member
Was talking to someone here about phone company boxes. Seems a few years ago a subcontractor put in a number of splice boxes (free standing - not sure of what they are called). They had trouble with many of them because they trapped condensation (and perhaps moisture rising from the ground). There was a vent but it was set up closed. Once they opened it problems went away. Since I have a conduit stub going into the ground with direct burial cable I am concerned about the same thing happening with a sealed NEMA box. I found some PVC ones at the big box stores but they were sealed. Then I found a metal one that is built more like an outdoor subpanel. It has small holes in the bottom corners. That should breath enough to minimize condensation.
Usually condensation in an enclosure is a function of the environment and gasketing of the enclosure....as they alluded to, sometimes 100% sealed isn't the best answer. Also has to do with the area of the country you're in.