Neutral will carry the same current as the hot on a 120v circuit so IT MUST be upsized.
Not really. For a 120V circuit, the neutral and the ungrounded conductor just need to be protected by the breaker. So for a 30A circuit, they both need to be #10 copper or bigger. If you were pulling individual wires in conduit, and you ran out of #10, you could substitute a #8 or a #6 for one of the circuit conductors. You'd get half the reduction in voltage drop as if you upsized both wires. If you upsize the ungrounded conductor, then you are required to upsize the EGC proportionately, 250.122(B).
I only ran 10/2 wire and the voltage drops to 105 volts. The run is 135 feet from my panel box so I was going to run 6 awg copper thhn wire.
10/2 sounds like a cable type wiring method such as NM (Romex) cable. Individual conductors like THHN can not be run unprotected, they are only for use in a conduit wiring method. So unless you have conduit installed, you can't just add one more THHN wire.
Also, you have a technical violation if you replace your 10/2 Romex with 8/2 or 6/2 Romex, as those both come with a #10 EGC. But when you upsize for voltage drop, you are supposed to upsize the EGC as well. So you could use something like Al 6-6 (or 4-4) SEU with #6 (or #4) ground.
The downside there is the receptacle and breaker lugs may not take that large a wire. In which case you'd need to pigtail short sections of copper wire to the aluminium wire, e.g. using Polaris style connectors. In fact, if you have space to set a junction box at each end, the easiest way to do it would just be to leave short pieces of the existing 10/2 NM at each end, and use the junction boxes to splice over to the larger wire size.
Cheers, Wayne
