Hot-Ground Reverse on Three-Light Tester

by Mike Morgan | Apr 18, 2020

A hot-ground reverse on an electrical outlet occurs when the connection of the hot and ground wires have been crossed or reversed. In other words, the hot wire is connected where the ground should be connected, and the ground wire is connected where the hot wire should be. 

Hot-ground reverse outlets would be a shock hazard, but they almost never happen in real life.

Why then does the three-light tester indicate a hot/ground reverse? It is because of the way three-light testers work.

A three-light tester can give the indication of a hot/ground reverse with a completely different from a hot/ground reverse.

When a three-light tester shows a hot/ground reverse, the actual problem is almost always a disconnected neutral somewhere upstream of the receptacle being tested.

Let me explain how this can happen.

Hot-Ground Reverse on Three-Light Outlet Tester

Let’s talk about the hot-ground reverse indication.

One of the few common light configurations on a three-light tester is one that is labeled as a “Hot-Ground Reverse.” As mentioned above, this is a very rare situation that would be very dangerous.

If you connect the hot wire to the ground screw on an outlet, then that hot wire on the ground will electrify the body/frame of whatever is plugged into that outlet.

For example, if you have a washing machine plugged into an outlet with a hot/ground reverse, the washing machine body will be energized and could electrocute anyone who touches it.

No one who has even a minimal knowledge of electricity would wire an outlet with a hot/ground reverse. That is why this wiring setup would be so rare.

Let’s look at the diagram below.

The diagram shows where the voltage will be on an outlet with a hot-ground reverse.

Red indicates 120 volts on a particular slot. Green indicates zero volts.

A particular light on a three-light tester will only illuminate if there are 120 volts between the two slots corresponding to that light. Since there is green (zero volts) on both sides of the Center light in the diagram, the center light will not illuminate.

Since there is green and a red slot on the two sides of the other two lights, both the right-hand light and the left-hand light will be illuminated.

This is shown the photo below.

While the hot ground reverse is very rare, it is not too difficult to achieve a hot-ground reverse reading with a 3-light tester plugged into an outlet that is wired very differently from the example shown above.

The situation that can give this indication starts with a properly-wired outlet, but there is there must also be a disconnected neutral wire upstream of the outlet being tested.

The only other thing needed to get the 3-light tester to show a hot-ground reverse is to have a load plugged into the receptacle being tested or into an outlet downstream of this outlet, and that load must be turned on.

The load that is plugged in will not work because the disconnected neutral upstream prevents current from flowing, but the plugged-in load will cause the 3-light tester to indicate a hot-ground reverse.

This happens because the plugged-in and turned-on load will essentially interconnect the hot and neutral slots of the receptacle, causing them to both be at 120 volts.

In other words, the load will act just like a conductor supplying the 120 volts to the neutral side of the load.

The 120 volts will travel through the neutral side of the cord, back to the outlet, and will apply the 120 volts to the neutral side of the outlet.

The diagram below shows what the voltage will be at each slot on this outlet.

Both the hot and neutral slots are at 120 volts since these two slots are interconnected via the load. The ground slot is at zero volts just like it should be in a properly wired outlet. 

Again, because of the voltage differences between the various slots, the right-hand light and the left-hand light will be illuminated while the center light will be off as shown below. This is the same indication as shown in the example above.

The voltage on this outlet is exactly the opposite the voltage on the outlet shown above, but you get the exact same indication on the tester.

Why is the Neutral Slot at 120 Volts When a Neutral Wire is Disconnected?

If you don’t understand why you have 120 volts on both hot and neutral slots, here is another way of looking at it.

Let’s look at an example of a lamp that is plugged into the outlet.

When there is a correct neutral and the current is flowing, the voltage drops from 120 volts to 0 volts as it the current flows through the bulb.

As a result, the only part of the circuit that is energized to 120 volts is the part shown in red in the diagram below. The rest of the circuit (shown in black) is at zero volts.

Now, let’s look at the situation when the neutral wire is disconnected somewhere upstream of the receptacle shown.

When the neutral wire returning back to the panel is not connected, an open circuit is created, and current cannot flow.

With NO current flowing, everything in the circuit will be energized to 120 volts because the light bulb is no longer a load (or voltage drop). The light bulb is simply a conductor just like a piece of wire.

This means that everything in the open circuit (including what should be the neutral wire) is at 120 volts. This is shown in the diagram below.