Okay, let's break down the code requirements, electrical theory, and your calculations.

Code Standpoint (NEC) 210.19(A)(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. In addition, conductors of multioutlet branch circuits supplying receptacles for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit. 210.19(A)(3) 15-Ampere Branch Circuit. Conductors shall be rated not less than 15 amperes.The specific application of 210.19(A)(1) and 210.19(A)(3) to your question means: You cannot reduce the wire size mid-circuit if you intend to maintain a 15-ampere branch circuit. The entire branch circuit from the breaker to the last receptacle must be at least 12 AWG. 14 AWG can only be used on 15-ampere circuits when it feeds lighting outlets (210.21(B)(1)).

Therefore, from a code perspective, your initial scenario of running 12/2 for the homerun and then transitioning to 14/2 for downstream receptacles is a violation of the NEC.Electrical Theory Standpoint Ampacity

12 AWG wire is rated for 20 amps (NEC Table 310.16, ampacities are typically based on 90°C insulation, derated for terminations as required). 14 AWG is rated for 15 amps. The weakest link in the chain determines the circuit's overall capacity. You are essentially creating a situation where a portion of the circuit is only rated for 15 amps even though it's being fed by a conductor capable of handling more. This is dangerous, because it could easily overload the 14-gauge section before the breaker trips, causing the conductor to overheat.

Voltage Drop

You are correct that voltage drop occurs across the entire circuit length. Reducing the wire size increases the resistance of that section, leading to a larger voltage drop in that area and affecting the voltage at all receptacles further down the line. Even if most of the load is near the beginning of the circuit, a user could still theoretically draw a substantial load from the furthest receptacle, overloading the 14 AWG section.

Addressing Your Questions/Calculations Voltage Drop Calculation and 8 AWG

Your voltage drop calculation is generally correct in principle. The formula you're using is a simplified approximation for voltage drop, and assumes 2-wire circuit and a K of 12.9 for copper. VD = (2 K I D) / CM, where: VD = Voltage Drop (in volts) K = Constant (approximately 12.9 for copper at standard temperatures) I = Current (in amperes) D = One-way distance (in feet) CM = Circular Mils (wire size) Your Calculation: (2 x 12.9 x 130' x 12A) / 3.6V (your allowable drop) = CM = 11180 The CM value corresponds to #10 wire

"Are people doing this (running larger gauge wires)?"

Yes! Upsizing wire to mitigate voltage drop is a common practice, especially in long runs, for sensitive equipment (like electronics), or in commercial/industrial settings where even small voltage drops can significantly impact performance. However, 8 AWG is likely overkill for a 15A branch circuit for typical residential use.

What are you missing/not understanding?



NEC Mandates a continuous wire size for the full load (210.19(A)(1)), from beginning to end on the full branch circuit. You are proposing a reduction of the wire size mid-run. This is a major issue. Overthinking

You're over-emphasizing the voltage drop calculation to the point where you're considering dramatically upsizing wire gauge for a relatively small current draw and distance.

Revised Recommendations1. Stick with 12/2 NM for the entire run. If voltage drop is truly a concern, increase to #10 AWG or even #8 AWG for the entire run.2. Consider splitting the circuit. If the load is consistently high, consider running a separate 15A or 20A circuit to dedicated loads.3. Evaluate the actual load

How much current are you actually planning to draw on this circuit? Are you just trying to be very conservative? If the load is relatively light, voltage drop may not be as significant as you think.

Important Considerations



Cost

Larger gauge wire is more expensive.

Termination

Make sure you can properly terminate the larger gauge wire in the electrical boxes and devices. Older receptacles and switches may not be designed for larger wire sizes. You might have to change to specific 'spec grade' devices to allow for this larger conductor.

Derating

If you're running multiple conductors through conduit or in close proximity, you may need to derate the ampacity of the wires, which might necessitate even larger sizes. NM-B Cable (Romex) at 12AWG is only good for 25amps, you must derate it in order to achieve 20ampacity (20amps / 25amps = 80% 20amps/.8 = 25 amps) - This is still sufficient for the 20ampacity rating*

Consult a Licensed Electrician

If you are unsure about any of this, please consult a qualified and licensed electrician. They can properly assess your situation, perform accurate voltage drop calculations, and ensure the installation meets all applicable codes and safety requirements.

In Summary: You cannot reduce the wire size on the branch circuit. Continue with the 12/2 run. If Voltage Drop is a concern, then either run 10/2 or 8/2 romex the entire branch circuit. Flag for review