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Grounding and Neutral in Power Wiring.

Partial diagram of typical wiring for a residence.

Partial diagram of typical wiring for a residence.

Recently I changed a setting on this blog to put no time limit for comments and I got a very interesting comment/question in the post: Ground, Common, Neutral, Reference, and Buses.   I am not going to repeat the question and my answer here, but that question and my reply made me realize how important this subject is to everyone. There really is a lot of confusion about grounding and shielding for both electrical noise reduction as well as the safety aspects. There is also a lot of bad advice floating around out there. For example, the recommendation from an electrician to Bryan about connecting the ground terminal is the neutral terminal is VERY VERY BAD advice. My goal is to arm you with general knowledge about the subject so you can make decisions on specific question you have with your own wiring.

The first picture is an overview of typical modern residential wiring showing one branch circuit with three receptacles. Because the drawing is hard to read I will also provide pictures of each area and explain the important ideas and concepts. Also, remember you can click on each picture and it will enlarge to full size and then you can use your browser return arrow to return back to this page.  Please note, this will be specifically about North American standards and specifically USA standards and our National Electric Code  (NEC or simply in this post “the code”).

The transformer in the upper left of the diagram is owned by the utility company and is either pole mounted or mounted on a pad to provide power for the residence. The main purpose of the transformer is to convert the high voltage transmission line voltage to a voltage safe to be available in your house.   There are a lot of rules about the wires running from the transformer to the first circuit breaker  These wires are called the Service Conductors and there are a lot of special rules applied to them because these are not protected by a circuit breaker or fuse.  The main thing i want to point out here is there are three wires.  One wire which will become our neutral wire is connected to the center tap of the transformer and the other two wires we will refer to as “hot wires”.  This will make sense in just a moment. The voltage between either of the “hot wires” to the neutral wire is 120 Vac rms.  However, the rms is understood so this is the last time I will use it in these posts.   (For a technical discussion on what rms means refer to my post “Sinusoidal waveforms, AC, and DC signals“.   The voltage between the two “hot wires” is 240 Vac so this system is called a 240/120 V system.

Power Panel and the Ground Rod

Power Panel and the Ground Rod

The next drawing shows what happens when these wires are connected to the power panel. (This is called a Panel Board in Code speak). Please note I chose to not draw the meter box before the power panel. The grounding functions I am about to talk about often occurs in the meter box but for the sake of simplicity I omitted it from this drawing. The two hot wires are connected to the main circuit breakers and after passing through the main circuit breakers these wires are connected to buses for the branch circuit breakers.   The important wire for our discussion is the neutral conductor which not fused but is connected to a set of terminals in a metal bar called the neutral bus.

At this point we also create a new wire for our power system, the ground wire and the ground bus.  The ground system is actually connected to earth.  Usually this is done on house wiring by driving a copper or copper coated steel rod into the ground.  The code word for this is the grounding electrode.  There are other options for creating a grounding electrode including connecting to a metallic cold water pipe that is also connected to the earth for at least 10 ft.   On newer houses such as mine, that is not an option because the water service to my house is a plastic pipe.   This grounding electrode is connected to the grounding bus by a conductor called the grounding electrode conductor.   The metal enclosure of the panel board is also connected to the ground bus, usually this is done by a screw that actually connects the two parts together.

Now we are at the most important conductor for this discussion is the jumper between the neutral bus and the ground bus.  This is called the “Main Bonding Jumper” in the NEC.  This is the one and only place where these two conductor systems are connected together.  In the next post I will show exactly why this is a very important rule.  Why is the Neutral connected to ground?   Think about the transformer sitting on a power pole. The high voltage side of the transformer is connected to a voltage of several thousand volts and although the low voltage side is not directly connected to that voltage the two conductors are capacitively coupled to each other.   This means that without a ground connection the low voltage side could drift upward and could be at a very high voltage compared to ground.   You, however, have you feet firmly placed on the ground so you will be at the potential of ground.   If the neutral was not connected to earth the voltages could be high enough to arc over the insulation.  By connecting the neural to ground we know exactly what the potential of the neutral and hot wires will be compared to the ground potential.

A branch circuit consisteing of three recepticles

A branch circuit consisting of three receptacles

Now that we have the ground circuit created and the neutral bonded to ground we are ready to wire up a branch circuit. The branch circuit could be a 240 V circuit and this could have two hot wires and a ground wire (for an example a water heater) or it could have two hot wires, a neutral wire, and a ground wire, for example a clothes dryer. Notice that in all modern wiring a ground wire is always included. Now it is time to learn the terms used in the NEC.
The “hot” conductors for a branch circuit are called “ungrounded conductors” and these must have some form of overcurrent protection, either a fuse or circuit breaker.  This means that on a 240 Volt circuit two circuit breakers have a handle that ties the two breaker handles together.   The undgrounded conductors may have any color wire except white, grey, or green.  Usually on most house wiring this wire is black, but it may be red or some other color.  The official code name for the neutral wire is the “Grounded Conductor” and the official name for the ground wire is the “Grounding Conductor”.  Those names are very close to each other, the significance is the ed suffix on the neutral wire is it is current carrying conductor that happens to be grounded.   The grounding conductor is not intended to carry current in normal situations and has the primary purpose to connect metal enclosures to ground so these have no potential that can cause a shock to a person.   The neutral or grounded conductor by code is required to be either a white or grey conductor.  (However, never assume that because a wire has white insulation that it is connected to ground.)  The grounding conductor has either green insulation or is a bare conductor.  Usually in house wiring this is a bare wire.  Again, just because a wire is green don’t assume it is a ground wire.   There is a lot of bad wiring practices out there.

A normal 120 V receptacle

A normal 120 V receptacle

In the diagram I show three receptacles connected to together in a 120 V branch circuit.  The hot wire connects to the small slot on the receptacle and usually the receptacle has brass colored screws to help you remember which side.  Code requires that only one conductor be placed under a screw but the receptacle has two of these screws making it possible to extend the circuit.  The large slot is to be connected to the neutral conductor and there is normally two silver colored screws for these wires.   (White wire goes to sliver or lighter colored screws and the black wire goes to the brass.or darker colored screws).  The ground wire is connected to a green screw and this is internally connected to the semi-round slots.   However, as shown in the diagram there is a very important special rule here.   The ground wire to extend the circuit must be connected to the incoming ground wire and a short wire is used to connect this junction to the one and only receptacle ground screw.   The reason this is done this way is to keep the ground wire intact even if a receptacle is removed for replacement.

For example, lets say receptacle No 2 is faulty and you decide to replace it.   Working safely, you turn off the breaker, check the power to the circuit and make sure it is dead and remove the receptacle.   Then you find out you do not have one in your spare parts like you thought you had so you temporarily protect the disconnected wiring until you can get to a store to buy a new receptacle.   Even though the circuit breaker is in the off position and receptacle No. 2 is removed, the device plugged into receptacle No. 3 is still connected to ground.

In the next post I will describe two bad situations and why they are bad and some special things about plugs to these receptacles.  After that I will show how a Ground Fault Circuit Interrupter. GFCI. operates and why it is a very important device.  Later we will talk about some special situations and maybe a few “war stories” I have ran into over the years.


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