Wire Types and Sizes

Basic Electricity

  • Question 1

    Most electrical wire is covered in a rubber or plastic coating called insulation. What is the purpose of having this “insulation” covering the metal wire?

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  • Question 2

    In the early days of electrical wiring, wires used to be insulated with cotton. This is no longer accepted practice. Explain why.

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  • Question 3

    Electrical wire is often rated according to its cross-sectional diameter by a gauge scale. Which is the larger-diameter wire size, 14 gauge or 8 gauge?

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  • Question 4

    In the Unites States of America, an organization known as the National Fire Protection Association, or NFPA, publishes an important set of standards called the National Electrical Code. What type of information is contained in this set of standards?

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  • Question 5

    Describe the difference between solid and stranded wire types.

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  • Question 6

    Suppose you pick up a piece of electrical wire and notice this label printed on the insulation:

    14 AWG MTW

    What does this label tell you about the wire?

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  • Question 7

    What is meant by the ampacity rating of a wire? What criteria establish the ampacity rating of any given wire?

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  • Question 8

    Examine the following American Wire Gauge table. Please note that most of the odd-numbered gauges have been omitted, because the even-numbered gauges tend to be more common:


    Gauge # Diameter (inches) Area (circular mils)

    4/0 0.4600 211,600

    3/0 0.4100 168,100

    2/0 0.3650 133,225

    1/0 0.3250 105,625

    1 0.2890 83,521

    2 0.2580 66,564

    4 0.2040 41,616

    6 0.1620 26,244

    8 0.1280 16,384

    10 0.1020 10,404

    12 0.0810 6,561

    14 0.0640 4,096

    16 0.0510 2,601

    18 0.0400 1,600

    20 0.0320 1,024

    22 0.0253 640.1


    How many gauge numbers must you increase to (approximately) double the diameter of any given wire gauge? What effect does the doubling of diameter have on the cross-sectional area of the wire?

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  • Question 9

    How many gauge American Wire Gauge sizes must you increase to (approximately) double the ampacity of any given wire gauge?

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  • Question 10

    Describe what electrical conduit is, and what applications it is commonly used in. Contrast “conduit” against raceways and wire trays.

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  • Question 11

    What is a busbar?

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  • Question 12

    The cross-sectional area of wires is often measured in units of circular mils rather than square inches or other common area units. Explain how the “circular mil” is defined, and how it is calculated for round wires.

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  • Question 13

    Determine the cross-sectional area of these conductors (viewed from one end), in units of “square inches” as well as “circular mils”:



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  • Question 14

    Small electrical wires are almost always sized by gauge rather than by circular mils (cmils). So, if you had to calculate the resistance of a length of 22-gauge copper wire and did not have the proper tools to measure the wire’s diameter, how could you find the equivalent cross-sectional area in circular mils so as to use the equation R = [(ρl)/A]?

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  • Question 15

    If an electric drill is plugged into a very long extension cord instead of being plugged directly into a power receptacle, what will happen to the drill’s performance? Explain your answer, with reference to Ohm’s Law.

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  • Question 16

    Find a piece of wire or electrical cable and bring it with you to class for discussion. Identify as much information as you can about your conductor prior to discussion:

    • Gauge
    • Ampacity
    • Voltage rating of insulation
    • Temperature rating of insulation
    • Insulation type (plastic, Teflon, silicone, etc.)
    • Insulation service (conduit, wireway, direct burial, etc.)
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