Two lengths of 12/2 romex to substitute for 10/2

Question

Can I run 2 lengths of 12/2 romex 100 feet to substitute for a single run of 10/2? I'm not needing to meet code, just trying to get 120 v at 20 amps to my shop with as little light dimming as possible. Thanks for any help you can give!

Answer 1

You only need one run of 12/2, so why waste money?

12/2 is already legal for 20A so you're all set.

At 100' you will have 0.2% voltage drop per amp, or 4% drop if you redline the circuit at 20A, which you should not do, as a rule; you should only plan to run 16A (80%). You do not need a wire size bump for voltage drop at 100', and your run is actually 65' so you are all set. Figure on 0.13% voltage drop per amp. So at 20A 2.6V drop, no big deal.

For anti-dimming, use LEDs or fluorescents with modern ballasts

Specifically look for LEDs or fluorescent ballasts made for multi-voltage 100-240V (world voltages). Those will automatically adjust to voltage "on the fly" so dimming will be minimal/nonexistent; the device will simply treat a startup sag as "oh, we're in Japan now ok".

Common 15A receptacles can only be on 15A or 20A circuits.

As soon as you put a 25A or 30A breaker on it, you can't have receptacles anymore. Unless you feed that circuit to a subpanel and distribute it to one or more 15/20A circuits.

Every breaker is dual thermal/magnetic.

They all have a permissive curve allowing some seconds for a 200% load, some minutes for a 1.25x load, and instant trip for a 1000% load.

If you want that feature local to the shed, feel free to install a subpanel. This can be fed with 120v (mine is). You can then use a 30A breaker to get full use out of the #10 wire, and local breakers of appropriate sizes for the circuit. Subpanels are as cheap as $20 and breakers are typically $5 if you're not subject to AFCI/GFCI requirements.

There can be only one

One NEC rule is that you can only have one circuit of the same type and usage. So sticking two 120V circuits out there isn't allowed, unless, say, one of them is controlled by a light switch from the house.

However you can have a 120V circuit and a 240V circuit, 120/240V split-phase circuit, and an MWBC (all at once even).

So have one of your two 120V circuits switched from the house. Or run a larger 120V circuit and have a subpanel. Or run a 120V/240V circuit and have a subpanel. Or run a MWBC.

Wait, what's a MWBC?

It's 2 hots sharing a neutral. It must be punched down into a 240V common-trip breaker, you can't use a duplex/twin or you'll overload the neutral.

Say you run 12/3 cable and punch it down onto a 240V 20A breaker. You now have 2 full 20A circuits worth of power available to you. YOu split one 20A/120V circuit off black and white, and the other 20A/120V circuit off red and white. Great for saw on phase 1 and dust collector/lights on phase 2 for instance.

Answer 2

The answer is NO you can not parallel conductors smaller than 1/0 . This is specifically addressed in the NEC 310.10.H.1.

Answer 3

There are parts of code that are for convenience and some that are for extreme edge cases, but much of code is for common issues that have dangerous consequences. So, to your question, yes, you can run 2 12/2 cables and use that for a higher amperage, but I really feel like this is one of the actually dangerous things that code happens to forbid.

If any connection is weaker than another, more power will flow over the other wire. In the worst case, a connection can completely break and there will be no symptoms other than an overheating wire. It's also going to be very confusing for anyone else that needs to work on the circuit.

Your particular use case seems to be to mitigate voltage drop because a single 12/2 cable can carry 20A safely so the downsides (fire) might not be as big of a deal, but it seems like a lot of downsides for saving $100 on a roll of 10/2.

There might be other reasons to not do this, but it's hard to say because most people would just use the right cable.