Electrical Wiring Basics

Order Reprints
Electrical Wiring Basics


This article provides a basic primer regarding wire types, gauges and more, for a better understanding of the wires we deal with on a daily basis. Granted, this information may not apply directly to all OEM wire, but it will provide a better understanding of wire basics.

Types of automotive electrical wiring

There are two basic types of insulated wire used in automotive applications: PVC and cross-linked polyethylene. PVC wire features an extruded insulation that is applied while running it through a die onto the wire strand package. Once exposed to operating heat, this insulation is more easily melted. Cross linked is designed to handle higher temperatures. Cross-linked insulation is created by extruding the insulation material through a tube under both heat and pressure, which “cross-links” the molecules of the insulation material, making it more stable under higher heat applications, and more suitable for automotive use.

There are three basic types of PVC wire: GPT (used for general circuit wiring, and rated to 176 degrees Fahrenheit), TWP (lead-free, thinwall wire rated to 221 degrees F) and HDT (heavy-wall wire rated to 176 degrees F).

Cross-linked wire is rated at 257 degrees F. The four most commonly used types of cross-linked wire include:

  • TXL, which features the thinnest (and lightest) insulation. Commonly used by auto manufacturers. Commonly available in 18 to 12 gauge.
  • GXL, which is slightly thicker insulation than TXL. Commonly available in 18 to 14 gauge.
  • SXL features a slightly thicker insulation than GXL. This provides added abrasion protection for heavier-gauge wires (commonly 12 to 8 gauge).
  • SGX features the thickest insulation in the cross-linked wire family, especially suited for battery cables.

For battery cable applications, the three most commonly used types include SGT, SGX and STX. SGT features PVC insulation, while SGX and STX feature cross-linked insulation. For battery cables, STX offers the thinnest insulation, useful for routing in tight spaces.

Wire gauge

Regarding wire gauge size, the smaller the gauge number, the heavier the wire; and the larger the gauge number, the lighter the wire. 22 gauge is super-light wire, while 2 gauge is much heavier (and 0, 00 and 000 is progressively even heavier). The most common sizes we see in automotive applications include 18, 16, 14, 12, 10 and 8 gauge (with battery cables typically heavier in the 4, 2, 0 and 00 gauges).

Wire is typically labeled or referred to with a gauge number, followed by “AWG.” These letters stand for “American Wire Gauge.” The gauge number indexing system was initially established to indicate the number of times a solid copper wire was passed through a drawing die. With each pass, the wire diameter decreases. For example, a 1-gauge wire passed through the drawing die one time. An 18-gauge wire passed through the drawing dies 18 times. Even though automotive wires are typically stranded as opposed to solid single strand, and regardless of how wires are made today, this old standard was used to create the gauge numbering system we use today. The thinner the wire, the higher the gauge identification number.

Rather than simply taking some chart at face value, what do these gauge numbers actually represent? Interestingly, the numbers really do translate into a value that is based on measurable factors. These include the diameter of a single strand of wire (not the strand package, but the individual wire strands that comprise the package), and the number of individual strands.

Listed here are a few examples of combinations of strand diameter and number of strands that equate to various wire gauge sizes.

There is an actual formula for calculating this, but it’s the type of formula that takes a math whiz to understand, so I won’t attempt to bore you with that (I looked at the formula and my eyes glazed over).

            Wire gauge       Strand diameter     Number of strands

            22                       0.030 inch               7

            20                       0.028 inch               7

            18                       0.030 inch               16

            16                       0.029 inch               19

            14                       0.027 inch               19

            12                       0.025 inch               19

            10                       0.023 inch               19

            8                         0.021inch                19

            6                         0.021 inch               37

Here’s a simplified way to judge wire gauge. Wire gauge is determined by the cross-section area. American replacement wire is based on the American Wire Gauge System (the larger the number, the smaller the wire diameter). By stripping off a bit of insulation, you can measure the wire package diameter with a caliper or a micrometer. Granted, most wire that you purchase (especially from any of the aftermarket wire/harness suppliers) likely will already be labeled for gauge size, and there won’t be any need to measure the wire. But if you want to measure it yourself, or if you’re dealing with unlabeled wire, following are examples.

American Wire Gauge (AWG) sizes

            Gauge size      Conductor strand package diameter

            20                      0.032 inch

            18                      0.040 inch

            16                      0.051 inch

            14                      0.064 inch

            12                      0.080 inch

            10                      0.102 inch

            8                        0.128 inch

            6                        0.162 inch

            4                        0.204 inch

            2                        0.258 inch

            1                        0.289 inch

            0                        0.325 inch

            00                      0.365 inch

(NOTE: Measure the diameter of the conductor (strand package) only, not including the insulation layer. ALSO NOTE: AWG sizing generally refers to a single, solid, round conductor. The AWG of a stranded wire is determined by the total cross-sectional area of the conductor, which determines the current-carrying capacity and electrical resistance. Because small gaps may exist between the strands, a stranded wire will usually measure at a slightly larger overall diameter than a solid wire of the same AWG size).

Copper, yes/aluminum, no

Copper wire is favored as opposed to aluminum wire. In fact, you should avoid aluminum wire altogether. Why? First of all, copper is a better conductor than aluminum. In addition, the type of soft aluminum required to make wire tends to work-harden quickly (bend, bend, bend, break). And, the dissimilar metals will tend to react and will oxidize, which increases resistance, which can lead to poor conductivity and/or overheating of the wire. In a nutshell, using aluminum wire isn’t worth the aggravation or the risk.

Typical (12V) wire gauge sizes per automotive applications

(NOTE: AWG gauges listed here represent minimum size — you can go heavier, but never go lighter. Example specs here do not represent all circuits currently found on production vehicles.)

A/C circuit         10 gauge

Alternator          8 gauge

Ammeter           10 gauge

Cigarette lighter/accessory port  14 gauge

Clock    18 gauge

Coil wire            16 gauge

Dome light        16 gauge

Auxiliary driving lights    14 gauge

Gauges 14 gauge

Headlights        14 gauge

Headlight switch to fuse block         12 or 10 gauge

Heater leads      16 gauge

Heater switch    10 gauge

Horn     14 gauge feed and 16 gauge ground          

Horn to relay     14 gauge

Ignition switch   12 or 10 gauge

Parking lights    16 gauge

Radio/CD to fuse block       14 gauge

Starter to relay        12 gauge (to solenoid)

Stop lights        16 gauge

Tail lights          16 gauge

Turn signals      16 gauge

Windshield wiper/washer        14 gauge

 Amps            Length of wire

@ 12V  Numbers in chart indicate AWG wire gauge

                        3’          5’          7’          10’        15’        20’

0 to 1                18         18         18         18         18         18

1.5                    18         18         18         18         18         18

2                      18         18         18         18         18         18

3                      18         18         18         18         18         18

4                      18         18         18         18         18         18

5                      18         18         18         18         18         18

6                      18         18         18         18         18         18

7                      18         18         18         18         18         18

8                      18         18         18         18         18         16

10                     18         18         18         18         16         16

11                     18         18         18         18         16         16

12                     18         18         18         18         16         16

15                     18         18         18         18         14         14

18                     18         18         16         16         14         14

20                     18         18         16         16         14         12

22                     18         18         16         14         12         12

24                     18         18         16         14         12         12

30                     18         16         14         12         10         10

36                     16         14         14         12         10         10

40                     16         14         12         12         10         10

50                     16         14         12         10         10         10

100                   12         12         10         10         6          6

The chart indicates the appropriate (minimum) wire gauge to be used, based on the maximum length of wire (feet) that will safely handle a specific amperage draw (at 12 volts). For example, a circuit that draws 20 amps that needs a wire that runs 10 feet long requires a minimum of a 16 gauge wire (or heavier).


Relays feature two basic circuits: one circuit turns the relay on and off, and the other circuit passes current through the relay once the relay is turned on. A relay acts like a switch. It turns power on and off on demand and serves as an isolator, preventing the high power demands of certain accessories from damaging other circuits that aren’t designed to handle heavy loads. Placing a relay in the circuit allows the actual control switch to experience only a small level of the power running through the circuit. Basically, a relay is a heavy-duty switch that’s activated by the primary control switch. Relays in the 30 amp range are required for accessories such as auxiliary driving lights, audio system amplifiers, electric fans, electric fuel pumps, electric water pumps, etc.

Always install a dedicated relay with each applicable circuit (don’t assume that a single relay can handle multiple circuits). The relay handles the heavy amperage in the circuit, allowing the control switch to simply turn the relay on and off. A relay also can help to boost a signal in a long wire run. For example, when a wire runs from the battery to the switch and from the switch to the accessory, a lengthy wire can reduce the power available at the accessory. A relay can be positioned to shorten the length of the power circuit, maximizing power available to the accessory motor.


Always select an alternator based on total amperage requirements. NOTE: Since one-wire alternators don’t charge well at engine idle speed, avoid one-wire alternators if you have EFI, electric fans, big-draw audio systems, etc. While some wiring shops I spoke with said that they generally don’t have a problem with “older” EFI systems (GM LT1/LT4 generation, etc.), a one-wire alternator does pose problems with the later generation LS engine control systems.

If you do run a one-wire alternator, be aware that the one-wire circuit eliminates the wiring normally used to control other functions such as warning lights (idiot lights) on the dash. In addition to gauges, warning lights provide a quick indication that something has reached the danger point (a fuel injected engine is about to misfire, a cooling fan is about to overheat, etc.). A low-voltage sensor can be used with any alternator to provide power to an indicator light when system voltage drops below 11 volts. An example is the Ron Francis LS-11 lo voltage sensor. You can use this sensor to power any LED or factory type “idiot” light.

Using your DMM to locate poor battery/starter connections

Using your DMM (digital multimeter), you can perform a voltage drop test. Naturally, resistance in the cables or the connections to either the battery or the starter will reduce power to the starter and may be the source of poor cranking performance. In order to check your connections, check battery negative to body ground, body ground to engine ground and the positive cable to the starter solenoid. Most commonly, connection problems occur at the ends of the cable where they connect to either the battery or to ground or solenoid. In most cases, a connection is not solid/tight or it’s corroded and unable to carry the required high-amperage current.

With the DMM in its voltmeter mode and set on either a low DC volt scale or a millivolt scale, connect the voltmeter across each of the connections. Place the probes with one on the negative battery terminal and the other on the cable end. If your DMM features a min-max function, press it before starting the engine. Crank the engine, and then shut it off. Push the min-max button again to read the maximum voltage drop that occurred across that connection. Perform this same test for all suspect connections. Each test must be performed when the starter motor is cranking, because you need the heavy current of the starter draw to see a voltage drop. Acceptable voltage drop should be less than 0.5V. If you find a higher voltage drop, re-do or replace the connection(s).

Test the battery during cranking

You also can use your DMM to perform a quickie battery test, using the meter’s min-max feature. In this test, you’ll look for battery voltage drop under the load (during cranking). This is similar to a resistance load test, using the starter as the load.

Using alligator clip leads, connect the DMM (set to DC volts) to the positive and negative battery terminals. Press the button to use the min-max feature. Crank the engine, let it run for a second or two and then shut it off. Battery voltage will initially read around 12.6 volts, and as the starter is engaged, the battery voltage will drop. When the engine fires, battery voltage will climb back to a normal level as the alternator charges the system.

Push the min-max button to observe the voltages. This will provide useful information, including the nominal voltage (be sure to have the headlights on so that you are not fooled by a surface charge); that the battery is not discharged; and the drop-in voltage during cranking (this informs you of the battery’s ability to supply current).

Stand your ground

Whenever we talk about the electrical system, we always need to address the issue of proper grounding, since poor grounds can create a host of common problems.

Insufficient grounds are responsible for headaches such as dim headlights, wire overheating and instrument malfunctions, to mention only a few.

For those who are dealing with fiberglass/composite bodies, grounding becomes even more of a concern.

Instead of relying on a single engine ground strap (block to frame), consider adding multiple ground cables such as battery negative post to the transmission tailshaft housing, a ground between body and frame (on a steel body), and a ground from the battery negative to the frame (especially when remote-mounting a battery to the trunk area).

If you’re running a metal fuel tank, also consider adding a ground strap from the fuel tank filler neck to the frame. And naturally, make sure that every ground connection (at the body, frame, engine block, etc.) provides a clean, solid reliable ground (even if that means shaving a small bit of paint or powdercoat).

As we’ve mentioned in past articles, poor grounds can also promote engine coolant electrolysis and can actually degrade an aluminum radiator over a period of time. And, according to leading engine bearing experts I’ve spoken with over the years, poor grounding can even lead to crankshaft main bearing damage (cavitation and pitting — yet another reason to make sure that your engine block receives a solid ground connection).

Installing a negative power distribution block can help, especially when you’re dealing with a fiberglass body. This allows you to run a ground from the frame to the distribution block, then obtain grounds for various components at the multi-connection distribution block.

Insulated wiring consists of only half the circuit in the electrical system.

The ground circuit (which includes the vehicle body, frame and engine) must carry the same amount of circuit back to the battery negative post as leaves the positive post.

Loose or corroded ground connections will add too much resistance for proper circuit operation.   ‚óŹ


American Autowire Inc., 150 Heller Place #17W, Bellmawr, NJ 08031

(800) 482-9473 /

ApogeeKits, 1131 S. Airport Circle, Suite 140, Euless, TX 76040

(800) 980-7966 /, PO 636, Galt, CA 95632

(209) 481-6496 /

Current Performance, (EFI harnesses). 6330 Pine Hill Rd. #16, Port Richey, FL 34668

(727) 844-7570 /

Hot Rod Wires Inc., 937 Meadowdale Circle, Garland, TX 75043

(972) 240-6851 /

Kwik Wire, N4936 Hwy. V, Fond Du Lac, WI 54937

(920) 921-2637 /

Painless Performance, 2501 Ludelle St., Ft. Worth, TX 76105

(817) 244-6212 /

Panduit, 18900 Panduit Dr., Tinley Park, IL 60487

(800) 777-3300 /

Rebel Wire, 1701 Fritschle St., Olney, IL 62450

(618) 395-8216 /

Ron Francis Wiring, 200 Keystone Rd., Suite 1, Chester, PA 19013

Related Articles

The Dreaded Engine Surge

Brake pad and rotor technology

Wheel Alignment: Driver-Assist Systems Are Directly Affected by Wheel Alignments

You must login or register in order to post a comment.