Wiring question -- should I provide additional protection?


Active Member
I plan on using a Function Devices RIBU2S2 relay as part of this project . The wires provided by the relay are 18ga. On the "line in" side, they will be connected to 12ga romex wires from a circuit on a 20A circuit breaker. The "load" side will go to a couple of Halo HLB6 LED wafer recessed light drivers that are about 14 watts each, so power usage will be minimal.

The relay and drivers will be in an easily accessible location in a conditioned attic that will be used for storage. I'm wondering if I should connect the romex to one side of a 5A circuit breaker (maybe a miniature breaker in a box mounted next to the relay) as an extra protection device for all of the wiring downstream from the romex. It would also provide an obvious shutoff switch for these two drivers/lights if, for example, on of the drivers needed to be replaced. Electrician says it isn't necessary.


Curious why you feel that extra circuit breaker is necessary? Are the wire connections protected in some sort of standard junction box with proper fittings? Is the wiring routed so it will not get pinched or stepped on?
I'm just trying to understand the reasoning behind wire sizes vs breakers. In this case, the relay and LED drivers will be mounted on plywood mounted on an attic brace to get it all off the attic floor. All of the connections to the relay wires will be done inside a junction box that will also be on the plywood. I'm using low voltage transformers to power the relay coils, and will use 600V wire, so that's not a concern.

I guess the idea is that if the load downstream of the relay is known to be 10A or less, and there's no way to "accidentally" increase the load, e.g., there's not a receptacle downstream that can be plugged into, and the mains voltage wires between the relay and the drivers are protected, there's no need for a breaker.

On the other hand, if something happened that caused excessive current on the 18ga wires (driver malfunction?), could the wires melt and start a fire without tripping the breaker?

To simplify the question (forget the relay)... why is it okay to have 12ga wire on a 20A circuit breaker feeding the drivers, when the driver wires are all 18ga?
It is quite hard to protect devices typically. However in a sensitive location the first trick is metal encasement for preventing fire spread.

If desired, a fuse with a lower rating than the breaker would be appropriate, sized for your wiring. However, with relay coils and motors use a time delay fuse so you still get the "slow-burn" overload protection without nuisance tripping and short circuit, fast tripping, as well for harder wire faults. (dual element fuse)
If you are worried about the ability of the 18 AWG wire to carry the rated 10A load, don't be. 14 AWG and 12 AWG are required for 120V circuits because they are often long runs of cable and wire resistance can become significant. The RIB relay has just a few inches of 18 AWG and the resistance will be insignificant - only about 6 milliohms for 12". In addition, the RIB relays are UL Listed, so they have been tested and approved to carry the full current that they are rated for. Even under a short circuit condition, the 18 AWG wire will handle the overload for an instant before the 20A breaker will trip.

The worst risk you have is that, in the future, someone who is not familiar with the contact rating of the relay might add a higher load to it, beyond the rated 10A.

If you really want to play it safe so the relay could handle a full 20A load, I would use two RIB2401B's.

I'm curious why you went with the RIBU2S2. Was it because it gives you the 2 relays in one package?

Also, the relay coil can work with low voltages (24V) or with 120V line voltage. If you use 120V, you can get rid of the transformer. Or are you using low voltage switches for some reason to control the relays?