The Difference Between Wires, Cables and Conductors
A wire is a single rod of metal with a small ratio of diameter to length.
A conductor is a wire suitable for carrying an electric current.
A stranded conductor is a conductor made up of a group of wires. These wires are usually twisted together. For example, cables may be referred to as 7/36. This means it is constructed from 7 strands of 36 gauge wire. (And from the Stranded Wire chart below, we can see that a 7/36 wire is 28 AWG.
A cable is either a single stranded conductor or a combination of conductors insulated from one another (a multi-core cable). Cables in the oil and gas and petrochem industries are generally always insulated and often protected with an armoured sheath, and are referred to as armoured cables. In general, stranded conductors are more flexible and less susceptible to fatigue-failure than solid wires.
The Importance of Using the Correct Cable Size
Wires can carry only a limited amount of current safely. If the current flowing through a wire exceeds the current-carrying capacity of the wire, excess heat is generated. This heat may be great enough to burn off the insulation around the wire and start a fire. Therefore each conductor or cable will have a specified current carrying capacity, also sometimes referred to as its ampacity.
An increase in the diameter, or cross section, of a wire conductor decreases its resistance and increases its capacity to carry current.
Other reasons for choosing an increased cross sectional area of wire is to limit volt drop along its length - this is of particular concern in long cable runs, and in Intrinsically Safe (IS) circuits.
Limitations to Cable Size Selection
Wires and cables are made in standard diameters. When selecting cables it is common to select the next standard size up from that calculated.
The terminals (e.g. Weidmuller, Phoenix etc) into which the cable or wire will terminate are made to accommodate a range of sizes. Be aware of any limitations this may place on your selection.
Standard Instrument, Electrical and Power Cable Sizes
Wire diameters are often specified in American Wire Gauge (AWG) rather than in square mm (sq mm) or inches. The cable size charts below give dimensions of common wire diameters and the corresponding AWG.
How to Convert AWG to mm
When calclating AWG from diameter or cross sectional area, the diameter and cross sectional area are rounded to the nearest AWG equivalent values.
Wire Diameter Calculation
The n gauge wire diameter dn in millimeters (mm) is equal to 0.127mm times 92 raised to the power of 36 minus gauge number n, divided by 39:
Wire Cross Sectional Area Calculation
The n gauge wire's cross sectional area An in square millimeters (mm2) is equal to π (Pi) divided by 4 times the square of the wire diameter d in millimeters (mm):
The following pages on Control and Instrumentation.com give more resources and look up tables that will be of use to the instrument design team:
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For those who want to delve further into the world of wiring and electrical installations in hazardous areas, then the following books from Amazon will be of interest: