0

Backstory: I am running power (temporarily) to a workshop through a 25' and a 50' extension cords. I was worried about the voltage drop over this length so I was measuring voltage to see if it dropped below what the tools needed. As it turned out, there was almost no voltage drop, which really surprised me, so I connected another 50' cable to the end of that. Still no very little voltage drop.

The resulting daisy-chain looks like this:

Outlet -> [25' 10AWG] -> [50' 12AWG] -> [50' 14AWG]

Then I decided to measure neutral to ground to see if it was zero. I tested at each of the four locations, and had various readings from 8V to 0V. It seemed that the farther I got from the original outlet the higher the voltage. What can explain this?

3
  • what neutral to earth voltage do you measure at the outlet where the extension cords are plugged in?
    – Jasen
    Commented Oct 2, 2020 at 23:49
  • Are you measuring to the dirt underfoot or to the ground conductor in the extension lead?
    – Jasen
    Commented Oct 2, 2020 at 23:50
  • ground wire not literal dirt Commented Oct 3, 2020 at 12:03

3 Answers 3

4

Keep in mind that voltage drop in the wire is totally load dependent; when you measure voltage without load at the end of long wires, you won't see much if any voltage drop. If you use a splitter on the end of your chain of extension cords, and plug in your shop vac and turn it on, and test voltage on another receptacle on the splitter, you'll see voltage drop.

The small voltages you're measuring from neutral to ground could be induced voltages, and will disappear if any load is placed in the test circuit, or if you use a low impedance volt meter, which is in itself enough of a load to make the phantom voltage vanish.

3

Voltage drop is a function of the current actually flowing on the wires at that moment. You can measure that with a clamp meter or a plug-in meter like a Kill-a-Watt. You can compute expected voltage drop using a voltage drop calculator, though you will need to compute each segment separately and add them up.

Voltage drop is measured between hot and neutral, and affects both hot and neutral. half the voltage drop happens on hot, and half happens on neutral. So if you have 10 volts of voltage drop, the hot wire drops 5 volts... and guess what happens on neutral: it rises 5 volts!! Hot drop and neutral rise should be equal.

Ground is not affected by voltage drop, so it is a reference point from which you can see how hot falls and neutral rises.

Remember to also measure voltage at the origin, so you know what you are comparing to.

If the numbers you are seeing do not correllate to the above, you may have a poor connection at a socket or plug.

2

This is perfectly normal.

The neutral is carrying the return current of your workshop.

The ground should be carrying no current other than faults.

The neutral and ground, if done properly, are bonded only at the service entrance panel.

So, you have current and resistance making voltage on the neutral, and no current on the ground, so no voltage.

Just as expected.

Or, if measuring with nothing plugged in, you'd see "no voltage drop" (on the hot and neutral) because no current - and you are probably seeing "phantom" or induced voltage from neutral to ground from the long extension cords acting as a low-grade transformer if measured with a modern meter that has a very high input resistance.

1
  • 1
    Phantom voltages are generally capacitive, not inductive Commented Oct 2, 2020 at 23:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.