DPDT switching branch circuit from one breaker to another?

rmully · Oct 6, 2023

I’m planning a 240V welder circuit in a detached garage with a 100A sub panel. For the 160A or 180A output welders I’m looking at, the conductor can be #10, and a 30A breaker is probably OK, but a 40 would help the duty cycle. Conductor must handle welder's 1eff rating, breaker can be up to double 1max rating.

I have a dedicated 240V, #10 conductor, 30A breaker circuit in place. I don’t want to modify that by upping the breaker size and making it unusable for other things, and I’d rather not run a long, entirely new circuit, wanting to save $ and not run a lot of new wire, conduit or wire mold through the space and on to an exterior wall.

I have on hand a few feet of #10 wire, and a 30 amp 120/277 volt 3 position toggle DPDT wall switch.

Is this OK or stupid: install a new circuit, 240V, #10 wire, 40 or 50A breaker, running to a new junction box about a foot or so from the panel. Sever the cable of the existing aforementioned circuit and run it into that box. Now there are two circuits running into the junction box, both with #10 wire, one with 30A breaker for conventional uses, and one with the upsized breaker for welder only. Then install the 30 amp 120/277 volt toggle DPDT wall switch in the box, connecting it to the existing branch circuit that continues on through the building. I could mark the switch plate something like WELDER and OTHER, and switch between the two as needed.

rmully · Oct 8, 2023

Here's a diagram of what I'm trying to describe.

Unless I'm mistaken, depending on a welder's 1eff and 1max rating, code for a dedicated, single welder branch circuit may use #10 wire with a 50A breaker.

I have a 240V, #10 conductor, 30A breaker branch circuit in place. I don’t want to change that by upping the breaker size and making it unusable for other things.

So I'm wondering if I can make a new circuit, same wire size but with a 50A breaker and switch between the two with a DPDT switch.

I have spare wire, a DPDT switch and 50A breaker so it's an inexpensive job.

[ElectriciansForums.net] DPDT switching branch circuit from one breaker to another?

rmully · Oct 8, 2023

Making the diagram less stupid looking...

pc1966 · Oct 10, 2023

It sounds bonkers.

If the socket is only rated for 30A then that should be the maximum breaker size. If you have very high surge peaks then you can get breakers with a higher magnetic to thermal ratio (for example D-curve MCB instead of typical B-curve or C-curve in most EU/UK boards), but in UK regulations you have to ascertain that it will disconnect under fault conditions fast enough so a low enough earth loop fault impedance. I'm not familiar with USA code to advise on these aspects for you, but someone like @Megawatt will know far more.

If you need a 50A socket for the welder just put one in. It is likely not much more expensive than the switch and box, etc, needed above and you might find a use for both sockets at once.

rmully · Oct 16, 2023

I think the root of my question is easily obscured by the uncommon aspect of code for welders.

Many welders have two ratings - I 1eff, and I 1max.
I'm looking at one unit that is rated at 25A I 1eff, and 35A I 1max.
By code, a single dedicated circuit for it can be #10 with a 40 or 50A breaker.
Given that, I have two questions:
1) If for welders, #10 wire is OK with a 40A breaker, wouldn't a 30A switch be OK within that circuit? It's the same capacity as the wire on either side of it.
2) is the overall idea to switch between two circuits OK?
It seems doable and safe to me.

rmully · Oct 19, 2023

I appreciate the input on this. I decided it is not a good idea, and will spend the money and time to install a new dedicated circuit with conventional receptacle, conductor and breaker arrangement. At least, that way the new circuit is sound for non-welder use.