Sub panel supplied by two different lines from main panel

Question

I looked but couldn't find this asked already. I found a sub panel in the crawl space of my home. It looks to supply an upstairs addition.
It has 2 different 14/2 cables going into it from the main panel, each on a 15 amp breaker. Inside the sub panel are a 15 and 20 amp breaker, with the 20 amp connected to 12/2 running up to the addition and 14/2 on the 15 amp. My best guess is this was done to get 20 amps up to the bathroom by "splitting" the current over the 2 lines supplying the sub panel.
Is this even legal? I could probably replace the existing 14/2 going into the sub panel with 12/2, but I'm not sure that is an acceptable solution.

Answer 1

Either it's 120V subpanel and he's plain paralleling... which is rather bad.

Or it's a 240V panel, and he's supplying hot/neutral/ground with one cable and the other hot with the other cable. That's a different kind of bad.

Both are unacceptable. You should replace both cables with a single /3 cable:

• 4/3 Aluminum or 6/3 Copper if you want 60A in this subpanel
• 8/3 Copper if you want 40A
• 10/3 if you want 30A
• 12/3 if you want 20A
• 14/3 if you want 15A, but this is fairly pointless.

And, both hot wires need to land on a 2-pole breaker which has common trip, meaning both legs trip together, and, move either handle, the other goes too.

Note that a duplex/tandem/twin/double-stuff has independent handles and must not be used, as it will create a dangerous situation if you do.