Residential Electrical Wiring: Key Safety Considerations

Residential Electrical Wiring Overview

A residential electric power service typically consists of a three wire ac system supplied by the local power utility company. A distribution transformer with a primary and two secondaries formed with a center tap on the secondary side is shown below:

The distribution transformer is located on the utility pole, from which the three wires originate. Two of the wires are ‘hot’ wires, while the third is called a neutral wire, which is connected to earth ground. As per the standard insulation color codes, ‘hot’ wires are represented by either B (black) or R (red), neutral by W (white), and ground  by G (green).

Domestic loads comprising of 120 V appliances and lighting, divided nearly equally between the two 120 V (rms) secondaries, are connected from hot wires to neutral as shown in Figure 1.0 above. Appliances such water heaters are supplied with 240 V (rms) power from the series connected secondaries as illustrated in the figure above.

The reduction of power loss in the lines also referred to as I²R is imperative from the perspective of efficiency and minimizing the amount of heat generated in the wiring for safety considerations. Because the power loss in the lines is directly related to the current needed by the load, a lower line loss will be incurred with the 240 V wiring in delivering the required power to a load. However, for the lower voltage case, the size of the wires is increased, thereby reducing the wire resistance, in an effort to minimize losses.

Normally the three-line cable coming out of the secondaries of the distribution transformer on the power utility pole passes through the electric meter that measures energy consumption in kilowatt-hours and terminates at the main panel. This is illustrated in the figure below:

At the main panel, circuit breakers serve the joint role of disconnecting switches and overcurrent protection; the neutral is connected to a busbar (bus) and in turn to the local earth ground; the hot lines are connected to individual circuits for lighting and appliances.

Safety Considerations for Residential Electrical Wiring

The circuit breaker labelled GFCI (ground fault circuit interruption), is used for safety primarily with outdoor circuits and in bathrooms. It is important to keep in mind that every outgoing ‘hot’ wire must be connected to a circuit breaker, whilst every neutral wire and ground wire must be tied directly to earth ground at the neutral busbar.

Typically in most homes you will find three-wire connections to their outlets, as illustrated below:

From safety perspective the ground wire is used to connect the metallic chassis of the appliance to earth ground. Without the ground conductor connected to the metal case of the appliance, the appliance chassis could be at any potential with respect to ground, possibly even at the ‘hot’ wire’s potential if a part of the ‘hot’ wire were to lose some insulation and come in contact with the inside of the appliance’s chassis.

Poorly grounded appliances can therefore be a significant hazard by providing a path to ground through the body of a person touching the chassis with a hand. An undersized ground loop current limited by the body resistance may flow directly through a person’s body to ground and could be very dangerous. Ordinarily, the circuit breaker would not operate under such circumstances.

Nevertheless, if the ground conductor is present and properly connected to the chassis of the appliance, the metal case will remain at ground potential or at worst a few volts from the ground if a fault results in current through the ground wire. The ‘hot’ conductor may be shorted to ground under fault conditions, in which case the circuit breaker would operate.

The body resistance of a normal person typically ranges from 500 k𝝮 down to 1 k𝝮, depending upon whether the skin is dry or wet. Thus a person with wet skin risks electrocution from ac voltages as low as 100 V. The amount of current is the key factor in electric shock. The 100 – 300 mA current range is the most dangerous.

The ground fault circuit interrupter (GFCI) provides the best possible shock protection. A sensing coil located around the ‘hot’ and neutral ‘wires’ in the GFCI detects the imbalance of currents between the neutral and the live conductor under fault conditions and opens the circuit in response when |I_B – I_W > | 5 mA. The GFCI may be located either at an outlet or at the main panel. Ground fault interrupters are now required in branch circuits that serve outlets in areas such as basements, bathrooms, garages, and outdoor sites.

Bottom Line

Safety is a very important factor to consider when working with electrical power. Various electrical standards or codes have been set up to provide protection to personnel and property, while specifying the requirements for the installation and maintenance of electrical systems. Only qualified and properly certified persons should undertake installation, modification or repair of electrical systems according to the standards set by the relevant authorities.

Related articles:

• How Ground Fault Interrupter (GFI) is used as a Safety Device
• Main Types of Earthing Systems
• Electric Wiring Testing & Inspection Instruments Requirements
• Fuses & Circuit Breakers vs. Overload Heaters for Industrial Motor Protection