Wow, that's some batch of questions!

Q. Reactance across supply, etc.

In the case of the long feeder, it just means that the 0.5uF capacitance is in parallel with the supply.

Have you studied the construction of a basic capacitor? You can make a very crude one by taking a couple of strips of aluminum kitchen foil, sandwiching a slightly wider strip of thin insulating material between them and on top, then rolling the whole lot into a cylinder with one wire connected to each foil. In earlier times capacitors were often just that, with waxed paper as the insulation.

The point is that it is nothing more than two strips of metal held a short distance apart by an insulating dielectric. If you took a 0.5uF capacitor and connected it between a hot breaker terminal and neutral busbar at a panel, you would get 22mA of current flowing through that capacitor.

A cable is also two long strips of metal with insulating material between them, so if you make it long enough you can get the same 0.5uF (or more). The capacitor just packs it into a much smaller space by deliberately minimizing the gap between the conductors and getting the most surface area in the smallest possible space.

So, the 0.5uF capacitor above has one foil connected to the hot and the other to the neutral. The two wires in the long feeder cable are also connected one to hot, one to neutral, so it's a parallel capacitance.

You'll get such a capacitance between any two such conductors which are close enough. If you took a length of, say, 3-wire armored cable, there would be a measurable capacitance line A to line B, B to C, and A to C. There would also be capacitance between each of those lines and the grounded armor.

The longer the cable, the higher the capacitance will be. The greater the spacing between conductors, the lower the capacitance.

Regarding your two effects: In the very broadest of terms, just placing capacitance in parallel with the supply will cause additional current to flow, while adding capacitance in series with some other device will increase the overall impedance and reduce the current flow, so you're on the right track.

HOWEVER, this is not necessarily the case where the rest of the circuit contains inductance (e.g. a motor, fluorescent lamp ballast etc.). The explanation for that had better wait until later.

Q. Difference between reactive/resistive ohms, why we can't add them directly.

Basically because there is a phase difference between the resistive and the capacitive portion of the circuit. In this case of the meter, it is a series circuit. The voltage across the resistive section (the meter) will be in phase with the current, but the voltage across the capacitive section will lag the current by 90 degrees.

Q. Why 10 meg input on a DMM.

That's the way the circuitry is designed. High-impedance meters are needed for some sensitive electronics to avoid loading effects, and that's the field in which the DMM first appeared. 10M has just become a common design value, although there are others.

Q. Why did we use 120V on the I=E/R (or more precisely, I=E/Z).

At first look you might think that the meter started on an open circuit, but we've already seen that that "open" conductor was really capacitively coupled to the energized hot wire.

The impedance (Z) calculated was that of the meter resistance (R) and the cable reactance (X) in series, so we need to use the voltage appearing across the whole series circuit. That's from the energized hot wire to neutral, i.e. 120V.

Q. Optimum value of reactance.

I'd better not get into too long an explanation, but the power-factor correction capacitor would be a good example.

Inductance (e.g. a fluoro. ballast) causes a phase shift between current and voltage on the supply which is undesirable (i.e. the power factor is less than 1).

Fitting a p.f. capacitor of the correct value can bring I and E back in phase (or very nearly so). Too small and the phase shift will not be enough to do that; too big and you'll "overshoot" and take it out of phase again in the other direction.

Notes on conduit later... Must eat! [Linked Image]