Electrical – Counting Smoke Detector Wires In Junction Box

That looks like a doorbell transformer. Those smaller wires should lead to your doorbell chime unit (assuming you have a doorbell). The junction box is there simply to provide power to the doorbell transformer.

From the picture it looks like you have 12/3 and 14/2 in that box, but it is difficult to tell. How confident are you the larger wires are larger than #12?

Figure out what breaker is supplying power to that circuit. As long as it is sized appropriately for the smallest conductors on the circuit, it will be fine. For example, if that is indeed a mix of #12 and #10 copper, your breaker must be 20A or smaller.

Also as a side note, NM cables entering that metal junction box need to be secured with a cable clamp. Any unused knockout holes need to be plugged. It looks like you've got a couple of code violations there at the top of your picture.

You should find an installers manual for the main panel and read it thoroughly. But I will give you my thoughts below.

First off, your black wire is connected to the positive terminal not the negative terminal according to your picture and wiring diagram.

Secondly, I am suspicious that this detector ever worked since there is no second wire connected to the power supply terminals. According to the label it takes 12 volts DC to power it and you can't do that with just one wire.

Without seeing any installers manual, I would say the black and white are 12 volts DC power and the red and blue are the common and signal wires.

So test for voltage between the black and white wires. If you get 12 volts move the white wire to the other power supply terminal. Usually the auxiliary contacts would be for another system to be notified that the detector is in alarm. The red and blue should probably go to the ground and signal terminals and the auxiliary terminals are just that, auxiliary. Then see if the detector works.

Notice on the backs of either detector they don't show any wires connected to the auxiliary contacts.