I was looking for space to add a circuit to an existing sub panel at my mother's house and ran into a problem that I need some advice on. Upon opening the sub panel, I discovered that it is being fed with two 12-3 Romex wires originating from two 20A circuits at the main. Now, I think that I should change those two breakers to one 20A two-pole bonded breaker. Here is the issue, the neutrals are both terminated on the neutral bar in the sub panel. I figure that it probably doesn't hurt anything, but I am not sure. A standard installation would have two hots and a neutral all the same size, so perhaps it is no issue to have two neutrals. I thought that I would ask the community's advice, though. Thanks! (by the way - it currently feeds three 15A general purpose circuits)

Let me see if I understand you correctly, main panel has two single pole 20 amp breakers, each has two #12 wires that feed the subpanel?

The neutral is not the problem that you think, the neutral from the main panel connects to the neutral bar in the sub, and any branch circuits from that sub then connect to the sub neutral bar.

But I am having a problem understanding the feeder to this panel.

Simply put, do you have two 20 amp breakers feeding a sub, that then feeds three 15 amp breakers?

Can you elaborate and explain further?

Thanks

Thanks for the reply. Yes, that's it, I don't know the history behind the sub panel as it came free with the house ;) . There are two 20A circuit breakers in the main. Two runs of 12 AWG between the main and the sub. Three 15A circuits from the sub to two adjacent rooms. The 12 AWG is landed as 1 black on 1 hot bar, 1 black on the other hot bar, 2 whites on the neutral bus, and 2 grounds on the ground bus.

It seems as though the larger of the two rooms was used as a shop or hobby room of some sort before she bought the house.

Before you add any more circuits, please consider changing this arrangement to something more proper. Install a two pole breaker, say 40 amps, and #8 wire to feed the subpanel, or larger depending on the rating of the subpanel.

I don't get what they were trying to do, seems a bit Disney to me. The could have used tandam breakers, two in one slot, in the main panel for what they achieved.