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I have a question about whether a GFCI outlet leaks power to the neutral contact. I have a 12 AWG (Black/White/Bare) wire coming straight off the panel directly to a GFCI. Prior to wiring the GFCI, the hot wire shows 120 VAC to ground and the neutral wire shows 0 VAC to ground (No surprise there). When I connect ONLY the hot wire to the line terminal on the GFCI, the neutral terminal (the one that is for the white wire) goes hot at about 20 VAC and about 0.02 maAC to ground. There is nothing plugged into the GFCI, the white and ground were not connected yet. After I finished wiring, the GFCI works fine and does not trip...was able to use drills, belt sanders, heaters, you name it. I searched numerous websites but could not find anything about why the neutral side of the GFCI would show hot when nothing is plugged into it. Is it normal for a GFCI to draw/leak power to operate?

  • What is the model of GFCI ? Most trip from 3-6ma. Also what type of meter are you testing with? I don't ever remember a GFCI lighting a outlet tester the 3 neon light type. The electronics may have some small leakage when not in use to power the green led if it has that but we would need the type / model to know for sure as there are many different types of circuits used by the different types of GFCI's and mfg's – Ed Beal Feb 28 '16 at 10:48
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    I am wondering WHY you felt the need to make this test? It is really telling you nothing and served no purpose other than to confuse you and make you think there is a problem. .... Yes, you are reading the load from the circuitry in the GFI by making this test. Attach the neutral and the voltage goes away. – Speedy Petey Feb 28 '16 at 14:55
  • I wonder why somebody always asks for the make and model of everything, even when that can't possibly be useful to the answer. – Harper Feb 28 '16 at 15:08
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Without the neutral attached to the receptacle the terminal is floating. So, taking a voltage reading (comparing it to ground) you could get different readings and 20 VAC is not surprising. The current you were measuring was the leakage current through your meter.

It wasn't actually drawing any power during your test since you hadn't completed the circuit and were just taking a static voltage reading. The device may draw micro amps of current to operate the circuitry while it is operating normally. Modern test equipment is very sensitive by design. This is why a static voltage test on a car battery doesn't really show the condition of the battery. A better test would be performed with the headlights and defroster on.

The main goal of a piece of test equipment is to measure the circuit without affecting the circuit.

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Modern GFCI's have internal circuitry that will use a little power, and an LED to indicate it's properly functioning. These will result in a slight bit of power going to neutral that you're measuring on the silver terminal with the neutral disconnected. I don't believe there's any cause for concern and your GFCI is properly functioning.

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Congrats, you found the traditional, correct way to measure amps: putting the meter inline with the circuit under test. As such, you have a reasonable reading for the current flow (amps) the GFCI itself uses. In fact, if you'd plugged in a USB charger at that point, you'd have correctly measured the current drawn by charger+GFCI.

A DVM in ammeter mode has very low resistance - it's practically a dead short. So the ammeter effectively completes the circuit. When you had the voltmeter in amps mode, you were connecting the GFCI to neutral - with the ammeter in the circuit to watch!

However, measuring volts that way does not work... and is completely meaningless. A perfect voltmeter has infinity resistance, but this is not one. So you are seeing the internal resistance of the GFCI's own electronics in series with the internal resistance of the meter. Google a "resistor ladder" to see how resistors in series relate. Since the voltmeter was dropping 20 volts, the GFCI itself had to be dropping the other 100 volts. Which means the voltmeter had 1/5 of the resistance (well, impedance) of the GFCI. All this knowledge is totally useless, except to observe that at these very low current flows, the voltmeter's own impedance is interfering with true readings. That might affect the ammeter reading above.

A perfect voltmeter would have shown 120v on the neutral side of the GFCI. Because no current was flowing, the entire GFCI floated up to the 120v mark, electrons eager to get back to neutral.

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