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Electronic Parts

Discrete Components

Wires

Basics

A metal conductor contains electrons that can move through the material when an electric field is applied. Wire does not create current; it provides a path. The amount of current is determined by the complete circuit.

Resistance of wire

Every ordinary conductor has resistance. A longer wire has more resistance, while a larger cross-sectional area has less resistance.

R = ρL / A
Why wire gets warm

Current flowing through resistance converts electrical energy into heat:

P = I2R

Doubling current produces four times the resistive heating in the same wire. This square-law relationship is one reason conductor sizing and overcurrent protection are so important.

Voltage drop

A wire carrying current develops a voltage drop:

Vdrop = I R

The load therefore receives less voltage than the source supplies. Long runs, large current, small conductors, high temperature, and poor connections all increase voltage drop.

If the complete out-and-back conductor path has 0.20 Ω resistance and the load draws 10 A:
Vdrop = 10 × 0.20 = 2 V
and the wire dissipates 20 W.
AWG: the number runs backward

For ordinary AWG sizes, decreasing the gauge number means increasing conductor diameter. Approximately every three AWG sizes changes cross-sectional area by a factor of two, and approximately every six sizes changes diameter by a factor of two.

Do not select a building conductor solely from an informal “amps per gauge” chart. Ampacity depends on the wiring method, conductor insulation, temperature, terminal limitations, conductor grouping, and the code article governing the particular circuit.

Insulation

Insulation keeps conductors from touching each other, grounded metal, people, or other unwanted paths. Different compounds are designed for different temperatures, voltages, wet or dry locations, sunlight, oil, flame exposure, flexing, and chemical conditions.

Color and identification

Color is useful, but meaning depends on the type of wiring and the applicable standard. In U.S. premises wiring, white or gray is generally associated with the grounded conductor and green or bare conductors are used for equipment grounding. Ungrounded circuit conductors use other permitted identification. There are special cases and marking rules, so electricians should consult the adopted NEC rather than relying on color memory alone.

Never assume that an existing conductor is de-energized because of its color. Older work, alterations, improper installations, switched conductors, and control wiring may not match expectations. Test for absence of voltage using an appropriate safe procedure.
Splices

A splice must provide both a sound electrical connection and mechanical security. It must be made with a connector or method suitable for the conductor material, size, number of conductors, temperature, environment, and enclosure. Twisting wires together and covering them with tape is not a general-purpose substitute for a listed connector.

Terminations

A poor termination can have more resistance than many feet of wire. Loose connections, damaged strands, corrosion, incorrect strip length, wrong connector size, and improper torque can produce overheating and arcing. Terminals marked for copper, aluminum, or both must be used only with the materials for which they are identified.

Electrical terminals increasingly have manufacturer-specified torque values. Proper torque is part of making a reliable connection; “tight enough” by feel is not a universal standard.
Mechanical protection

A conductor can be electrically large enough and still be unsafe if its insulation is cut, crushed, pinched, exposed to sunlight it is not rated for, or placed where nails and screws can reach it. Wiring methods therefore include rules for support, bending, protection, enclosures, raceways, bushings, and physical damage.