Guide to Becoming a DIY Electrician

Overloaded extension cords cause a large percentage of residential fires. Overloaded extension cords can become hot enough to ignite the insulation surrounding the conductors and the burning insulation will then ignite any combustible material near them. I will explain why this happens in the next section of this lesson. For now, look at this extension cord selection table. This table will enable to select an extension cord that is safe to use without doing the math.

EXTENSION CORD SELECTION TABLE
CORD LENGTH AWG# MAX. LOAD AMPERES
25 FEET AWG18 10 AMPS.
25 FEET AWG16 13 AMPS.
25 FEET AWG14 15 AMPS.
50 FEET AWG18 5 AMPS.
50 FEET AWG16 10 AMPS.
50 FEET AWG14 15 AMPS.
75 FEET AWG18 5 AMPS.
75 FEET AWG16 10 AMPS.
75 FEET AWG14 15 AMPS.
100 FEET AWG16 5 AMPS.
100 FEET AWG12 15 AMPS.
125 FEET AWG16 5 AMPS.
125 FEET AWG12 15 AMPS.
150 FEET AWG16 5 AMPS.
150 FEET AWG12 15 AMPS.

EXTENSION CORD RATINGS

All conductors offer resistance to the flow of electric current through them. The amount of resistance offered to the flow of current is directly proportional to the length of the conductor (the longer the conductor, the greater its resistance) and inversely proportional to its area (the greater the area, the lower the conductor's resistance). Conductor size is indicated using the American Wire Gage (AWG) and, as a rule, the smaller the number, the larger the conductors cross-section area and the lower its resistance. When current flows through a resistance, heat is generated-that is how the burners on your electric range work. Unlike the burners on your range, an extension cord needs to be able to dissipate that heat without becoming hot in the process. A properly rated extension cord is able to do that continuously

Using the above table to size an extension cord is simple and straight forward if the amperage rating of the load is known as is the case with loads such as power tools, pumps, and other motor loads where its marked right on the tool or motor. In some cases, like with an electric space heater, all you may have to work with is its voltage and wattage rating. In this case, we will have to use a simple formula to calculate the number of amperes that it draws. To calculate amperes when voltage and watts are known we use this formula"

I (current in Amperes) equals P (power in Watts) divided by V (Volts). To illustrate, let us calculate the current drawn by an 1800-Watt space heater.
A=P/E=1800/120=9A

Now, referring to our table, we can select an extension cord that can safely supply this load. If the distance between the heater and the receptacle is 25-feet or less, we could use an AWG18 cord. On the other hand, if the distance between the receptacle and the heater were between 75 feet and 100 feet we would have to use an extension cord consisting of AWG 12 conductors.

Now that you know how to select a cord based on the load served, here are a few more safety tips to follow when using extension cords.

NEVER use an extension cord that is rated for indoors use outdoors. Underwriters Lab approved cords for indoors use are not meant to be exposed to rain, snow, intense sunlight, etc and cannot be safely used outdoors.

NEVER run an extension cord under rugs or cover them with any other material, to do so is a direct violation of the NEC. Covering an extension cord keeps them from dissipating heat build up and even a properly rated cord can overheat and cause a fire.

NEVER use nails or staples to hold an extension cord in place. If you need to secure an extension cord in place, use Duck Tape or some other appropriate tape.

ALWAYS keep in mind that extension cords are meant for temporary wiring use only.

VOLTAGE, AMPERES, WATTS, AND OHMS
Before moving on, I want to spend a moment talking about four common electrical units of measurement that you will be using in future lessons.

Most homes today are wired for 120/240-volts. 120-volt circuits supply lights and small appliances, while electric ranges, ovens, clothes driers are supplied by 220-volt circuits. Voltage may be thought of as electrical pressure and the amount of current that will flow through a given circuit is directly proportional to the electrical pressure applied across the circuit.

Current is measured in Amperes with one ampere being the current that will flow through a one-ohm resistor when one volt is applied across it. The Ohm is the basic unit of electrical resistance. The relationship between volts, Amps, and Ohms is expressed in the basic Ohms Law which states that current (I) is directly proportional to voltage (E) and inversely proportional to resistance ( R ) expressed in Ohms, or I = E/R.

Watts (W) is the basic unit of electrical power (P). The basic formula for calculating power is P=IXE. This is a very handy formula to commit to memory. For example, let us say that we wanted to know what the largest floodlight load we could safely connect using a 25-foot, AWG 18 extension cord; we would multiply 120-volts by 10-Amps to get 1200-Watts.

As a homework assignment, I would like you to read Articles 90 and 100 of the NEC. You have purchased a copy of the 2008 NEC, right?

If an electrical fire started in your home would you know which circuit breaker to turn off to isolate the involved circuit(s)? In the next lesson I will introduce you to a tool that will allow you to map out the circuits in your home, tracing them back to their individual circuit breakers (Cbs) without pulling a single cover.