Section 3 - Conductor Selection

In conductor specification, copper should be evaluated first. It is a more efficient conductor than any other common conductor options, in terms of both conductivity and cost.

But above 200 C, copper rapidly oxidizes unless coated with a more heat-resistant metal such as nickel, as indicated in the following table.

And, use of other conductor metals can aid in managing other performance variables beyond heat resistance.

For example, nickel offers superior heat stability, good corrosion resistance and good welding characteristics.

When superior corrosion resistance is required, alloys of nickel and copper (Monel* alloys) are sometimes specified.

Temperature Ratings For Conductor Materials

Current-Carrying Capacity

Current-carrying capacity (ampacity) is the current a conductor can carry before temperature - of conductor AND insulation - rises beyond a permitted limit. The following are key factors in determining ampacity:

Conductor Size and Material: Conductivity of conductor materials varies widely. These variances affect current-carrying capacity. Also, as the conductor is reduced in diameter and mass, ampacity decreases.

Amperage: As applied current rises, greater conductor heat is generated. A single AWG 16 copper conductor at 30 C ambient rises to 80 C with approximately 19 amperes of current; at 22 amps, the temperature at AWG 16 copper wire rises to about 90 C.

Ambient Temperature: Electrical current is just one heat contributor. As the ambient temperature - the temperature of air surrounding the wire - rises, less current-generated heat is required to reach the temperature rating of the insulation. Thus, ampacity is governed also by contribution of ambient heat.

Insulation Type: Heat dissipation through insulation varies with insulation type. The rate of dissipation affects total heat and, therefore, the ampacity. The dissipation problem becomes even more complicated when wire is enclosed in a tightly confined space.

For these reasons, assigning a current-carrying capacity to a conductor is an inexact process. Consequently, design engineers responsible for making such decisions may evaluate wire constructions empirically, using guidelines established by various standards such as the National Electrical Code. They also may deliberately derate a wire ampacity-rating estimate to achieve a greater margin of safety and extended service life of a product.

Temperature Conversion Table

Designating Wire Size

In the well-established American Wire Gauge (AWG) system, increasingly larger numbers indicate progressively smaller wire diameters. To determine correct conductor size, the following rough measures can be used in conjunction with standard calculations of current induced heat rise:

• An increase of 6 gauge numbers (e.g.: 8 to 2) roughly doubles the wire diameter.
• An increase of 3 gauge numbers (e.g.: 11 to 8) roughly doubles the cross-sectional area and cuts electrical resistance in half, thus greatly increasing ampacity.
• An increase of 10 gauge numbers (e.g.: 12 to 2) multiplies the cross-sectional area by 10 and reduces electrical resistance by roughly a factor of 10.

American Wire Gauge Numbers and Diameters

Why Stranding?

Stranded conductors are many smaller solid conductors twisted together to form a larger AWG size. Three principal stranding arrangements are shown schematically at left. These are based on geometric strand arrangements of 7, 19, 37 and so on.

Stranded constructions are used in wire construction for improved conductor flexibility, flex life and ease in handling. Stranded conductors also show greater resistance to vibration and bending during assembly and service. In addition, scratching or nicking during assembly is usually less serious than such damage to a solid conductor. To reach equivalence with a solid 22 AWG conductor, for example, a common choice is a 7/30 arrangement - 7 strands of 30 AWG conductors.

True Concentric Equailay Stranding	Unidirectional Concentric and Unilay Stranding	Rope Stranding

7 Wire	19 Wire	37 Wire	133 Wire (19 x 7)
Concentric Lay Strands			Concentric Rope-Lay Strands

NOTE: * Monel is a registered trademark of Huntington Alloys, Inc.