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I exchanged my double pole 40 ampere breaker that supplied my sub panel for my garage with a single pole 40 ampere and now out of my six breakers in the sub panel only 1,3, and 5 have power. How do I resolve the issue to get power to all my breakers?

The reason I exchanged breakers is my sub panel has three lugs, and I had to completely disconnect it to do some work. When I wired it back up, even with the breakers in the sub panel in the off position, I had power to all my outlets and switches. I do not know what I did wrong as it was fine before.

To Harper, yes, there are only three wires supplying the sub panel.

You have a 240 volt sub panel which was fed from a 240 volt breaker. Now you’re feeding only half of the sub panel with a 120 volt breaker. Sorry, what did you think would happen?

Maybe you don’t understand how dual-leg 240 volt systems work. The incoming 240 volts comes in as two 120 volt legs which are 180 degrees out of phase. You get 120 volt between either hot and neutral and 240 volts between the two hots. Most breaker panels are designed so that the even numbered slots connect to one leg and the odd to the other. Therefore, a full-sized dual breaker connects to both legs to give 240 volts. Combine this with neutral to also give 120 volts.

Assuming you have no 240 loads (dual breakers) in the sub, you could use a pigtail wire to connect both hot wires (that were previously connected to the 240 you removed) to the new 120 volt breaker. However, this would give you half the total amperage in the sub (40 amps instead of 80).

Otherwise, you really need a dual (240 volt) breaker there.

You need a double pole breaker. More specifically, you need a breaker that supplies power from both phases on the main panel.

Half your breakers in the sub are now not powered because they were connected to the second phase. It’s not clear what they are tied to now.

Oh dear. This is a foogly mess.

First, you did the right thing by punching that main panel breaker down onto a single. The problem is with the subpanel; it is very badly misconfigured by a guy who cut a lot of shortcuts.

First, it is illegal to double-tap neutral bar screws like that, unless the panel's labeling or instructions say they are intended for that, and these aren't. A few panels will allow 3 grounds on a screw, but always one neutral.

Second, this sub panel has neutrals and grounds on the same bar. That is 100% improper in a subpanel. Those neutrals and grounds need to be separated, and the grounds need to be attached to the subpanel chassis/frame. The ideal way to do this is to install an accessory ground bar for the panel - however the last guy didn't leave you much length to reach it. So I would maybe convert the neutral bar to a ground bar, add a jumper from this ground bar to the panel chassis... and use wire nuts and a pigtail chain to attach supply neutral to the other neutrals. Inelegant, but effective.

On the hot wire, you need to get more #8 copper cable and make 2 pigtails off the solitary hot wire. One goes to where the black wire now is. The other goes to where the ground wire now is.

This converts the panel to a 120V panel. But it's not legal to power this subpanel in 120/240V mode, with only 3 wires.

The panel will only able to supply 40A for all circuits added together, somewhat less than the 2 poles of 40 A powerable before. But chances are this will be alright.

If the route of the wire between subpanel and main panel is all conduit, a red wire of the same size should be added to the pipe. Then it can be hooked up like a normal panel.