I want to replace a 30A breaker with a 20A breaker at the panel.
At the moment there is a small bit of 10/2 wire (12 inches long) that goes into a relay box and from that relay box comes 12/2 wire powering 4250W baseboards at 240v = 17.7A.
This is probably safe but not necessarily to code.

I am replacing the circuit with 3500W of baseboards and keeping the existing 12/2 wire but I am going to add a 240v thermostat instead and make all connection in a new junction box near the panel instead of the relay box.

To make things easier can I just plug the 10/2 wire in the current panel into the new 20A 2 pole breaker and then connect up the 10/2 to 12/2 in the junction box?
Or is it better/safer to replace that with all 12/2 wire?

 

The way I understand it, you can always use a heavier gauge wire than what the breaker requires. As far as connecting two different gauge wires together, I'm not sure.

 

It won't break code, but can become confusing to others. As long as your 10-2 is at the breaker and the 12-2 is downline. You can use the 12-2 for your 3500 watt heater. Since it is such a small piece of cable, replace it with a continuous 12-2 from breaker to thermostat.

 

(+1) In this case, with the 10 gauge piece so short, replace it with 12 gauge if convenient.

(For those eavesdropping) No problem keeping the 10 gauge wire for your to-be 20 amp circuit. Ten gauge wire is sometimes used for 15 and 20 amp circuits where the distance is long such as from the house to a boat dock.

Suggestion: Rather than connecting the black (and/or red) 10 gauge wire end(s) entering the panel directly to the breaker, splice on a short length (pigtail) of 12 gauge wire and connect that to the breaker. This will let a future electrician know that the circuit is 20 amps. No need to pigtail the neutral unless it it too short to reach the bus bar.

 

There are only 3 heaters connected to the end of this circuit totalling 3250W (13.5A).
As I understand it a baseboard heater can never pull more thatn it's maximum wattage.
So, how safe is it to leave this on a 30A breaker until someone more experienced with panels can come and change it out for a 20A?

 

An unexpected unpredictable unforeseeable spontaneous malfunction could possibly result in an increased current draw.

No one is going to knock on your door to inspect the breaker box but do realize that it is not safe to leave this on a 30A breaker flipped on for any length of time.

 

Changing out a breaker is easy.

 

For instances like this, I'll label the cable something like "oversized 12/2 down the line".