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I'm curious how to make this work in a pinch as I'm having to update my business wiring in 3 months for renovation.It'll eventually be a larger amp panel or two seperate panels

Current I have a small panel running 20amp fed by the main.

I need additional circuits for some equipment coming any day.

I have an unused MC wire I could run right now to that subpanel.

Is it possible to use the subpanel and split one side for the current machines, and use the other sides for the new machine? I have an extra neutral bus I can install to isolate from the first feed and use the current ground bar for it all.

This is only temporary and can it be done?

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It mostly doesn't work that way. A panel typically has two hot buses, one down each side, with interlocking fingers so that the end result is alternating rows connected to each bus. In theory (but almost certainly not to code) if you only had 120V loads and you installed an extra neutral bar then you could run one hot to one source and the other hot to the other source. But it would be a mess and the moment you need a single 240V load (a.k.a. a double-breaker) you have a big dangerous mess.

The one exception, though almost certainly not to code, is an old Rule of 6 panel. This typically has one main power source coming in, connected to lugs, not to a breaker. The top section typically has 6 double-breakers, 5 for separate loads (e.g., water heater, dryer, oven) and 1 supplies power to the bottom section. In theory, you could power that bottom section with a backfed breaker (which would technically work but be another code violation because the panel would not be labeled for that use) and have it power several circuits independent of the top. But that is theory ONLY. In practice, if your existing panel is not a Rule of 6 (and I doubt that it is), you would not have that flexibility, and you would definitely not install a new Rule of 6 panel, which brings us to...

Install a new panel now. Panels are not, in the grand scheme of things, that expensive. For example, semi-randomly searched, here is a Square D Homeline 30 space panel with some breakers for under $100:

panel 30 space

Includes 3 single 20A and 2 double 30A breakers.

Is that the best panel around? No. But it is decent and new and not much in the grand scheme of things.

Alternatively, try and find out what kind of panel you will install in 3 months and either get that panel now, or at least get one of the same brand/type so that you can reuse the breakers.

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    Despite the fact that pricing for everything is high right now due to "supply chain issues", may as well invest in the final panel now. Prices probably won't drop all that much in the 3 months between now and the renovation. Consult with the contractor doing the renovation work to determine what panel to get now to support the future work.
    – FreeMan
    Commented Feb 4, 2022 at 14:17

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