I install two 120v outlets on each post along with a single 240v outlet.
And then a small note says, "Either one L2 or two L1's but not both".
Each post has properly rated #10 conductors for the 16 amps on Line1 and
Line2 and Ground and a #12 for Neutral since the Neutral carries either
zero or only the 12 amps of a single L1.

Is this legal under the NEC?

It depends entirely upon your breaker size. As long as you don't have a breaker larger than 20 amps on either phase you should be good to go.

I'm worried about you sizing the neutral smaller than the hots, as somebody may see the 10 AWG hot line and put a 30 amp breaker on it, not realizing the neutral wire still needs to be limited to 20 amps.

I'm also worried about the ability for somebody to overload one of your hots by plugging into both the L2 and an L1 at the same time.

A polite sign isn't the same as an interlock. You must assume some bozo will plug into the L2 and both L1's at the same time.

(I assume the two 120v L1 outlets are on opposite phases of a split phase 240 setup for the 240v outlet.)

For example, if somebody sets up a space heater that draws 15 amps on one of the L1 outlets, and then somebody else starts to charge their car at 16 amps on the 240 outlet, the total number of amps on one of your hots would be 31 amps, which is more than should be going over a 10 AWG wire. If you had a 20 amp breaker no problem. If you are using a 30 amp breaker, no problem for the 240 volt circuit, but somebody could successfully draw 25 amps over your 10 AWG neutral if they (for example) had a 30 amp 120v RV circuit going through a 15 amp plug adapter.

So either the breaker must trip because it's a 20 amp, or all the wires including the neutral must be sized to accommodate the full possible load, 10AWG for a 30 amp breaker.


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