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FEATURE ARTICLES

Electrical Connections: The Importance of Soldering

By Chris Wantuck

So much depends on the electrical system that during restoration or maintenance, extra care should be given to not only the wiring, but more importantly, its connections. This restoration tip examines good connection techniques and the importance of soldering.

Electric currents run through the vehicle's wiring from many different components. There are sources such as the battery or generator and there are loads, the components that require the current to operate. Ignition coils and breaker points, headlights and taillights, instrument panels, horns, and heater fans are all examples of electrical loads. All of these have one thing in common, they use wire between them and all use some sort of connector to transition the wire to a fastener. The most common connector is a wire terminal or lug. They can be open type such as a spade or hook type and the closed type is called a ring terminal. Regardless of which type is selected, electrical connections to these terminals have two properties associated with them: mechanical and electrical. A mechanical connection is a friction mating of the terminal metal to the wire itself, crimping or crushing the metal around the wire. All terminal types make this mechanical connection to the copper wire and in some cases include the wire's insulation for greater strength. The mechanical connection is intended to provide an electrical connection while also providing strength if the wire is pulled or strained. Electrical connections are those that use solder around the wire to improve the electric current flow and should not be confused with a mechanical connection. While strong most times, a solder connection is not a substitute for a good mechanical connection. The best electrical connections include both mechanical crimping of the terminal around the wire followed by soldering.

Wire is rated in sizes usually called American Wire Gauge (AWG) which is essentially the wire's diameter, expressed in either inches or millimeters. A 10 AWG gauge wire is approximately 1/8 inch in diameter and 36 AWG gauge, one of the smallest gauge wires, is a mere 0.005 inches or about the size of a human hair. The higher the AWG number, the smaller the wire. Wire gauges can be a single piece of wire called solid core or made up of multiple strands of smaller gauge wires called stranded core. The benefit of stranded gauge wire is that it is more flexible than solid core. Stranded wire can be made up of a few medium size wires, a couple dozen smaller size wires or in certain cases over fifty very small (human hair) size wires. Stranded wire examples of a 14 gauge wire include 7/22 (seven 22 gauge wires) and 19/27 (nineteen 27 gauge wires)

Cross section example of two types of 14 AWG stranded wires, before and after crimping. Left a 7/22 stranded wire and on the right a 19/27 stranded wire. The 7/22 would be more common for an automotive application.
Cross section example of two types of 14 AWG stranded wires, before and after crimping. Left a 7/22 stranded wire and on the right a 19/27 stranded wire. The 7/22 would be more common for an automotive application.

Overall, the total diameter of the wire doesn't change, just its composition. Stranded wires are tightly twisted together while the insulation is applied during manufacture. The significance of a wire's gauge is the amount of current it can safely pass through it while the number of strands indicates its flexibility. The following table shows this for some common size wires.

AWG Strands
(Flexible)
Strands
(More Flexible)
Diameter
(inches)
Maximum
Ampere Rating
10 37/26 49/27 0.115 30
12 7/20 19/25 0.093 20
14 7/22 19/27 0.073 15
16 7/24 19/29 0.060 10
18 7/26 16/30 0.048 6

The flow of current through the wire occurs evenly among the strands and when meeting the terminal, predominantly flows through the outmost strands of wire where they make contact with the terminal's metal collar. The contact of the single strands—their round edges against the terminal's collar—doesn't provide much contact area to carry the current.

Illustration of current flowing in individual wire strands. Note each strand's minimal contact to the lugs metal collar.
Illustration of current flowing in individual wire strands. Note each strand's minimal contact to the lug's metal collar.

When crimped, the wires flatten out to conform with the collar, but the outer strands are still the connection point for current flow.

Crimped number 10 terminal, but notice how much space remains between the strands and the collar. Corrosion will enter the air gaps and deteriorate the connection.
Crimped #10 terminal, but notice how much space remains between the strands and the collar. The gaps between the collar and strands let air reach the surfaces of the wire and consequently, corrosion will occur and deteriorate the connection.

Even if the crimp is perfect and all strands are making contact, there is limited contact between the strands and the terminal. Furthermore, a long term looming problem began the minute the wire's insulation was stripped: Oxidation.

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