This is a common question we receive from customers. The answer may vary depending on the situation, but we have compiled some information to help make your decision.
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Introduction
In order to provide power to your home, business, or other building, you’ll need to properly size the wire that runs from your main electrical panel to your meter. The main electrical panel is where the electricity comes into your property, and the meter is what measures how much power you’re using.
getting the right size wire is important for two reasons. First, if the wire is too small, it can overheat and start a fire. Second, if the wire is too large, you’ll be wasting money on material and labor costs.
The equation for sizing wire is fairly simple: Just divide the total amperage of all the devices on your circuit by the voltage of your system. This will give you the number of amps that will flow through the circuit. To determine the size of conductor required in AWG (American Wire Gauge), use this equation:
AWG = (log10(I/0.005) – 2)/0.39
Where I = Current in amps and 0.005 = derating factor
For a 240 volt system:AWG = (log10(I/0.005) – 2)/0.48
The Basics of Electrical Wiring
Generally, your local electric code will dictate the size of the wire you need to use from your meter to your panel. The amperage of your breaker will also play a role in what size wire you need. Most houses will use a 200-amp breaker, which will require a 4/0-gauge wire. This is the thickest wire available at most hardware stores.
Wire Size
The size of the wire coming from the utility company to your house is called the “service entrance line”. This line is composed of two main types of wire: a conductor and an insulator. The conductor is the part of the wire that actually carries the current, and the insulator is the part of the wire that surrounds and protects the conductor.
The service entrance line comes into your house at the meter, and then goes to your main electrical panel. From there, it branches off to various circuits throughout your house. The size of the wire that is used for each circuit depends on the amount of current that will be flowing through it.
The most common sizes of household wiring are 14-, 12-, 10-, and 8-gauge. The larger the gauge number, the smaller the diameter of the wire. So, 14-gauge wire is smaller in diameter than 12-gauge wire, which is smaller in diameter than 10-gauge wire, and so on.
The amperage rating of a circuit dictates the size of wire that should be used for that circuit. For example, a 20 amp circuit should have 12-gauge wire, whereas a 30 amp circuit should have 10-gauge wire.
In general, 14-gauge wire is suitable for most 15 amp circuits in your home, 12-gauge wire is suitable for most 20 amp circuits, and 10-gauge wire is suitable for most 30 amp circuits.
Wire Gauge
In the home, wire gauge generally refers to the physical size of the wire, not the electrical current it is rated to carry. The electrical current is measured in amperes or “amps.” The physical size or gauge of the wire is indicated by a number; the larger the number, the smaller the physical size of the wire. In North America, households are supplied with electricity at 120 volts (V).
The U.S. National Electrical Code (NEC) recommends a maximum voltage drop of 3% for individual branch circuits supplying 120-volt lighting outlets located more than 20 feet (6 meters) from the main service panel. This NEC recommendation translates to a maximum voltage drop of 0.36 volts for an entire 20-foot circuit length when using 14-gauge copper wire or a maximum voltage drop of 0.60 volts for an entire 20-foot circuit length when using 12-gauge copper wire. To minimize voltage drop in longer runs or where higher amperage loads will be used, use larger diameter (lower gauge number) wiring such as 12 AWG for long runs or for higher loads such as air conditioners and electric dryers.
Wire Types
There are three main types of residential electrical wiring: copper, aluminum and galvanized steel. Most homes built before 1960 have aluminum wiring, while homes built after 1960 usually have copper wiring. Galvanized steel is a third type of residential wiring that is no longer used but can still be found in some older homes. Aluminum wiring is not as common as copper wiring and can pose some safety risks if not installed or maintained properly. In general, copper wire is the best type of residential electrical wiring because it is less expensive than aluminum and less likely to cause fires.
Determining the Correct Size Wire
The size of the wire running from your meter to your panel is very important. If the wire is too small, it can overheat and cause a fire. If the wire is too large, it can be costly and difficult to work with. In this article, we will help you determine the correct size wire for your needs.
Step One: Determine the Total Amperage of the Circuit
The first step in correctly sizing any circuit is to determine the total amperage of the circuit. This is done by adding together the amperage ratings of all devices on the circuit. Make sure to use the maximum amperage rating of each device; do not use the “nameplate” or running amperage rating as this will result in an undersized wire and breaker.
For example, let’s say you have the following devices on a single circuit:
-A 60 watt light bulb (0.6 amps x 120 volts = 72 watts)
-A 1,000 watt clothes iron (8.3 amps x 120 volts = 1,000 watts)
-A 500 watt hair dryer (4.2 amps x 120 volts = 500 watts)
The total amperage of this circuit would be: 0.6 + 8.3 + 4.2 = 13.1 amps
Step Two: Calculate the Total Length of the Wire Run
Now that you have the total number of watts that your appliances will be using, you need to calculate the total length of the wire run. This is the distance from the main electrical panel to the furthest outlet plus the distance between each outlet. Use the following formula to find the total length of wire needed in feet:
Total length of wire run = (Distance from main panel to furthest outlet + (Number of outlets – 1) x Distance between outlets) x Number of circuits
For example, if you are running one circuit to 10 outlets spaced 6 feet apart, your total length of wire run would be:
(10 + (10-1) x 6) x 1 = 160 feet
Step Three: Select the Appropriate Wire Gauge
The National Electrical Code specifies that you must use the correct size (gauge) of wire for the amperage of the circuit. The main panel is where you get your electricity; the wires coming IN to the main panel are called DEEnergized (ungrounded), and the wires leaving out of the main panel are called LEGround (ground). Selecting the proper gauge of wire is critical to ensuring that your electrical system is safe and codes compliant.
The NEC table 310.15(B)(16) provides a guide for selecting the correct gauge of conductor, based on the length of the conductor and the amperage of the circuit. This table is reprinted below:
Ampacity of Insulated Conductors -not more than three current-carrying conductors in raceway, cable or earth (directly buried), based on ambient temperature not to exceed 30°C-
Size (AWG or kcmil) Copper Aluminum or copper-clad aluminum
14 15 17
12 20 21
10 25 26
8 30 34
Conclusion
After looking at the National Electrical Code, it is determined that 4 AWG copper wire is the proper size to use for a 100 amp service drop. This is based on using copper THWN conductors in EMT conduit, with three current carrying conductors and a grounding conductor. The distance from the meter to the panel determines the correct size wire to use. For example, if the run was 200 feet, then you would need to use 6 AWG copper wire instead of 4 AWG.
