I'm looking for a little clarification here. I'm in the process of replacing the Knob and Tube wiring in an older 1930 home. Although I trying to preserve the original look of the house the K&T prevents me from installing insulation in the attic.
Originally there was only one circuit running into the attic that serviced all the lights in the house. I would like to run two circuits up there instead of one and split the load. That way if I happen to trip a breaker I’m not left completely in the dark.

From my service panel my electrical lines run through conduit into my basement and from there it runs up to the attic. I know I can run two lines of 12-2 Romex from my panel up to the attic to service the lights but I’m also trying to conserve space through the conduit.
So here is my question: Can I run a single 12-3 line up to my attic to service all my lights? If I run the black wire on one circuit and the red wire on another circuit (out of phase with the first circuit) can I share the common wire? From what I’ve read, and please correct me if I’m wrong, the common will carry the difference between the loads. I’m using a 20A breaker which should allow me a 1900W load safely. I’ve estimated a maximum of 1400W per circuit if all the lights were on and I used 100W bulbs.
Also, if I’m correct and I can run my line in this manner, is there any reason why I shouldn’t.

Any help would be appreciated,

Thanks

Yes, you can use a 3 wire cable for two circuits that share the neutral. Be sure the two circuits are on each on opposite legs at the panel.

Thanks TK, I was pretty sure it wasn't a problem but it's always better to get a second opinion.
Thanks again,

You can save even more room in the conduit as far as it goes, by running THHN or other singles.