Before we learn how to drop voltage calculations, we must understand what voltage drop is.
As current flows through a conductor or wire, its flow is inherently resisted by the wire itself, this is called
impedance. The impedance caused by the wire is determined by the physical properties of the wire; these physical properties include material type, cross-sectional area, and length. The increase in impedance causes or voltage drop.
How did we jump from impedance to voltage dropping?
How are these two things related? Recall that impedance is measured in ohms
(a measurement of resistance) and also recall that voltage, resistance, and current are related (ohm's law).
Let's think about the formula V = I x R. If I increase R and have to keep I the same, what must happen to V in
order for the equation to remain true (left side of the equal sign the same as the right side)?
V must decrease.
Now that voltage drop makes a list more theoretical sense, let's examine one of its practical implications.
Looking at the diagram alone, assuming it is to scale, what can we assume about the voltage drop to be
calculated at A and B? The voltage drop is bigger at B, why? Because it is further away from the 120V source.
Recall that the NEC (National Electrical Code) limits the amount of voltage drop allowed at the furthest point of a circuit.
Before attempting to solve the problem, make sure to have the following information:
Notice that if you are asked to find the maximum length of wire for a given voltage drop you just need to solve for length on the equations alone.
If asked to find the maximum current, solve for it in the equations alone.
You were given a resistance factor (per 1000 ft of wire) of 21.2 for aluminum and 12.9 for copper
We did so by
See table 8, Chapter 9 in the NEC