Your Home's Electrical Panel—Knowledge is Safety

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  • Intermediate

With light coming on at the flip of a switch, electricity is often taken for granted without giving it a second thought. This is an attempt at a brief summary of understanding the 6 steps of what’s happening beyond an electrical outlet.

1. The Transmission Grid

The electricity as we know it is produced at a power plant and processed through its Transmission Substation that uses transformers to convert the thousands of Volts of generated electricity to extremely higher levels ranging between 150,000 and 765,000 Volts to reduce line losses on long-distance transmission grids which run typically on a distance of about 300 miles (almost 500 Km).

2. The Distributing Grid

TheTransmission Grid then feeds into a Power Substation that reduces that high voltage down to a distribution voltage usually under 10,000 Volts, which in turn is the stepped down to usually around 7200 Volts by the Distribution Station which will be the line voltage running through the neighborhood on three wires (3 phases).

3. The Transformer Drum

Finally, the distribution grid brings the three or at least one of the wires to a pole near the house, or else, like in some suburban neighborhoods where large green boxes are placed at every house or two, the distribution grid lies underground. A large transformer drum attached to the pole taps from the 7200 Volts from the wire (or one of the three wires). This transformer steps the voltage down from 7200 Volts to 240 Volts (Fig.1). Every utility pole in the world also has a grounding wire running down the pole with 6 to 10 feet (1.8 to 3 meters) buried in the ground.

diagram of transformer wiring

There are two insulated wires running from the transformer to the house and one bare wire which is the ground wire. As shown by the Sine Wave of figure 2, one of the insulated wire carries +120 Volts in relation to the ground while the other is at -120 Volts of the sinewave, also in relation to the ground, creating a difference of 240 Volts. This configuration also permits the use of both 120V and 240V appliances around the house.

4. The Watt-Hour Meter

Both of the black insulated hot wires are then mechanically hooked up to black insulated wires coming out of the electric mast (Fig. 3), with the bare ground wire connecting with the mast’s white wire. The wires are then fed into a watt-hour meter like the one in figure 4. A ground wire is also added clamped to the galvanized mast (Fig.5) at one end and clamped to the ground rod deeply driven in the ground at the other end.

home power meter wiring

The purpose of the Watt-Hour meter is to record the amount of current passing through it to operate every appliance and other electrical equipment running in the house. The Kilowatt/hour readings are recorded on the dials or an electronic meter.

home power wiring and panel

5. The Circuit Breaker Panel

From the watt-hour meter, the wires go down the conduit coming out inside the circuit breaker panel (Fig. 6) where you can see the two black (hot) wires connected at the top terminals of the main circuit breaker with the other side of the double breaker connected to the two Hot bus bars, while the white wire is connected to the panel’s neutral bus bar. The main breaker in figure 7 lets you cut power to the entire panel when necessary.

The two bus bars connected to the main breaker are shaped as a series of fingers crisscrossing alternatively between both bus bars and each a terminal to every breaker feeding a circuit within the house.

circuit breaker main panel

6 Fuses and Circuit Breakers

Fuses and Circuit Breakers are safety devices. Let’s say there weren’t any fuses or circuit breakers between the main breaker and the outlet in the wall where:

  • there’s a power tool plugged in and the motor burns out shorting the power directly to ground,
  • or there’s an extension cord getting severed using an electric power tool causing a direct connection between power and ground,

The only objective of that 120-volt power line connected directly to the ground is to draw as much electricity through that connection as it possibly can, causing either the device or the wire in the wall to burst into flames. That wire would get hotter than a range’s heating element. This is why a special current limiting device with a power rating no larger than what the circuit’s wiring can handle is placed across the line.

The fuse is the most basic safety device, disposable after one surge, it uses a small wire or foil that’s designed to evaporate should the power rating be exceeded.

The more common and popular safety device now is the circuit breaker that basically uses that very heat generated with the overload to trip a switch that can be reset after it cools down.

With either device, the reaction immediately kills the power on that line, protecting it from overheating. The main breaker at the top determines how much current will be distributed throughout the electric panel at any given time. Anything more will simply trip the breaker killing the power for the entire house. So when installing or upgrading a breaker panel, special considerations must be given to all the electrical appliances that will be in that house even as future additions, making sure that the supply will always meet the demand.

For the Do-It-Yourselfer’s Safety

The point where the power reaches the main breaker when it enters the electric panel is where the everyday handyman must draw the line. There must never be any tampering or working on wires or components before that point which has practically unlimited current and is impossible to shut down except by the service provider. On this note, it is very important to respect the following guidelines:

The main circuit breaker at the top with the very high rating (100 amps plus) is not to be exposed by removing its cover plate, should not be removed, replaced, or otherwise tampered with in any way.

If work needs to be done where the front cover with the access door has to be removed, the main circuit breaker must be turned OFF to prevent any power from getting access to any of the house’s circuits.

When the main breaker is turned OFF a battery-powered light or other alternatives will be required to work inside the panel.

It’s important and common practice for electricians that one hand be always kept in the pocket while only working with the other hand. This is to prevent a lethal electric shock from going through the chest cavity and vital organs when one hand touches a live wire while the other hand is holding on or touching a grounded metal part. This must become second nature for anyone working in electrical circuits.

When reaching inside the panel for any reason such as passing wires through, it should only be done using insulated screwdrivers, cutters, pliers, or any other tools. The hands should never come in contact with any metal parts of the tools used.

Anyone noticing repeatedly slipping on the last two pieces of advice should stay away from electrical DIY projects. It has to become a habit or instinct.