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How, When, Where: Wire

Jul 1, 2002 12:00 PM, Glen Ballou


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Wire is not the only means of getting a signal from one device to another; fiber optics, high-frequency radio, microwaves and satellite transmission are also available. Wire has one big advantage over the other methods, however, in that it does not require electronic devices on each end to convert it from one medium to another. Wire is still the most used method to connect one circuit or component to another.

When installing wire, contractors must follow the National Electrical Code, developed to ensure the safety of people and property against fires and electrical hazards. The NEC is widely accepted as the set of regulations governing the proper installation of wire and cable in the United States, though each state, county, city and municipality has the option to adopt all of the code, part of the code, or to develop one of its own. The local inspectors have final authority; so though the NEC is a good reference when questions arise about the proper techniques for a particular installation, local authorities must be contacted for verification.

Once you have met the codes, the next step is to determine the size of the wire you need to use. Resistance is an important factor in determining wire size. For instance, connecting an 8½ loudspeaker to an amplifier 500 feet away through a No. 19 wire causes 50 percent of the power to drop in the wire in the form of heat. A handy rule of thumb is that 1000 feet of No. 16 wire has a resistance of about 4½. Each time the wire size changes by three (from No. 16 to No. 19 or No. 13), the resistance doubles or halves. When determining wire resistance, the length of wire in a pair is twice the length of the total cable run.

In the United States, wire is sized by the American Wire Gauge method. Wire sizes from No. 4 to No. 30 are used most often. Wire with a number less than four is very heavy and cumbersome, and wire greater than No. 24 is fragile and has high resistance. Table 1 gives the resistance and current carrying capabilities of wire.

The current carrying capacity of wire is controlled by NEC Tables 310-16, 310-15(b)(2)(a) and Section 240-3. Table 310-16 shows the maximum current carrying capacity for insulated conductors rated from zero to 2000 V, including copper and aluminum conductors. Each conductor amperage is given at 60, 75 and 90 degrees Celsius. Because the melting point of copper is much higher than that of the insulation, the current limit on a copper wire is the melting point of the insulation and is listed by most manufacturers. The materials won't melt at the specified temperature but may begin to fail certain tests. The maximum continuous current rating for an electronic cable is limited by conductor size, number of conductors contained within the cable, maximum temperature rating of the cable and environmental conditions such as ambient temperature and air flow.

Table 1. Wire Sizes Between AWG No. 4 and No. 30.
AWG
No.
½/1000' Amperes @
1000 CM/A
Amperes @
1500 CM/A (B&S)
4
0.2536
41.70
27.70
5
0.3192
33.10
22.00
6
0.4028
26.30
17.50
7
0.5080
20.80
13.80
8
0.6045
16.50
11.00
9
0.8077
13.10
8.70
10
1.0180
10.40
6.90
11
1.2840
8.20
5.50
12
1.6190
6.50
4.40
13
2.0420
5.20
3.50
14
2.5750
4.10
2.70
15
3.2470
3.30
2.20
16
4.0940
2.60
1.70
17
5.1630
2.00
1.30
18
6.5100
1.60
1.10
19
8.2100
1.30
0.86
20
10.3500
1.00
0.68
21
13.0500
0.81
0.54
22
16.4600
0.64
0.43
23
20.7600
0.51
0.34
24
26.1700
0.41
0.27
25
33.0000
0.32
0.21
26
41.6200
0.25
0.17
27
52.4800
0.20
0.13
28
66.1700
0.16
0.11
29
83.4400
0.13
0.08
30
105.2000
0.10
0.07
Source: Radio Electronics

NEC Table 310-15(b)(2)(a) contains amperage adjustment factors for whenever more than three current carrying conductors are in a conduit or raceway. Figure 2 simplifies the number of cables in a conduit and the current carrying capacity of each conductor. Referring to the current capacity chart, determine conductor gauge, insulation and jacket temperature rating and number of conductors from the applicable product description for the cable. Next, find the current value on the chart for the proper temperature rating and conductor size. The maximum current rating/conductor is found by multiplying the chart value by the appropriate conductor factor. The chart assumes an ambient temperature of 25 degrees Celsius and no air movement. Current values are valid for copper conductors only.

The current ratings in Figure 2 are intended as general guidelines for low-power electronic communications and control applications. Current ratings for power applications are generally set by regulatory agencies such as Underwriters Laboratories, Canadian Standards Association, NEC and others and should be used for final installation.

Section 240-3 of the NEC gives the requirements for overload protection for conductors other than flexible cords and fixture wires. Section 240-3(d) Small Conductors, states that No. 14 to No. 10 conductors require a maximum protective overcurrent device with a rating no higher than the current rating listed in the 60 degrees Celsius column. These currents are 15 A for No. 14 copper wire, 20 A for No. 12 copper wire, and 30 A for No. 10 copper wire. Another important limitation comes from temperature rise. When connecting wire to a terminal strip or connector, the temperature rise in the connections must be considered. Often the circuit is not limited by the current carrying capacity of the wire but by the termination point.

TO SHIELD OR NOT TO SHIELD

When in doubt, shield. With all of the noise in the environment, you face challenges from electromagnetic and radio-frequency interference. Shielded cable and twisted pairs ensure signal integrity, prevent downtime and maintain sound and picture quality. Braid shields have good structural integrity, flexibility and flex life. They are ideal for minimizing low-frequency interference and have a lower DC resistance than foil shields. Braid shields are also effective at RF ranges. We can assume a maximum coverage of 95 percent for a single braid shield.

Mechanically induced noise is a frequent concern in the use of guitar cords and microphone cables that are constantly moving. Braided shield cable usually has fillers in it to reduce triboelectric noise, generated by the mechanical motion of a cable that changes the relative position of one wire to the other. When wires inside the shield rub against each other, small electrical discharges are created. Fillers help keep the conductor spacing constant, and semiconductive materials help dissipate charge buildup. Wire without fillers, such as shielded loudspeaker cable, does not have a filler, therefore it has high triboelectric noise so it should not be used for microphone cable.

Foil shields are aluminum foil laminated to a polyester or polypropylene film for mechanical strength. Foil shields provide 100 percent cable coverage. They are much smaller in diameter than braid shields, so they are often used to shield individual pairs of multipair cable. They have less weight and bulk and cost less than braid shields and are generally more effective in RF ranges. Foil shields can be more flexible than braid but have a shorter flex life. Drain wires are used with foil shields to make termination easier and to ground electrostatic discharges. Foil shields should never be used for microphone cable or any cable with movement.

Combination shields consist of more than one layer of shielding. They provide maximum shield efficiency across the frequency spectrum. The combination foil-braid shield combines the advantages of 100 percent foil coverage and the strength and low DC resistance of a braid. Other combination shields available include various foil-braid-foil, braid-braid and foil-spiral designs.

Spiral shields can be either single or double spirals and are more flexible and easier to terminate than braided shields. Spiral shields can be considered coils of wire; therefore, they can exhibit inductive effects that make them ineffective at higher frequencies. Spiral shields are only used for analog audio applications. The French braid shield by Belden is an ultraflexible double spiral shield consisting of two spirals of bare or tinned copper conductors tied together with one weave. The shield provides long flex life and high flexibility. It also has about 50 percent less triboelectric noise.

  • Jackets and Coverings. Each type of jacket or covering has its own characteristics and advantages and disadvantages. Vinyl is sometimes referred to as polyvinyl chloride (PVC). Several varieties have temperature properties ranging from -67 to 221 degrees Fahrenheit (-55 to 105 degrees Celsius). Vinyl also varies in pliability and electrical properties so that at least one type can fit almost any application. The price range also varies accordingly. Typical dielectric constant values can range from 3.5 at 1000 Hz to 6.5 at 60 Hz.

    Polyethylene is a good electrical insulator with a low dielectric constant value over all frequencies and very high insulation resistance. Its flexibility can be rated stiff to very hard. Moisture resistance is rated excellent, and dark formulations have excellent sunlight resistance. The dielectric constant is 2.3 for solid insulation and as low as 1.35 for gas-injected foam cellular designs.

    Teflon has excellent electrical properties, temperature range and chemical resistance. It is not suitable against nuclear radiation, and it does not have good high voltage characteristics. Teflon cables are rated 500 and 392 degrees Fahrenheit maximum (260 and 200 degrees Celsius). The cost of Teflon is approximately eight to ten times more per pound than vinyl insulations. The dielectric constant for solid Teflon is 2.1. Foam Teflon (FEP) has a dielectric constant of 1.42.

    Polypropylene is similar in electrical properties to polyethylene and is primarily used as an insulation material. Typically, it is harder than polyethylene, which makes it suitable for thin wall insulations. UL maximum temperature rating may be 140 to 176 degrees Fahrenheit (60 to 80 degrees Celsius). The dielectric constant is 2.25 for solid and 1.55 for cellular designs.

    Silicone is a very soft insulation that has a temperature range from -112 to 392 degrees Fahrenheit (-80 to 200 degrees Celsius). It has excellent electrical properties plus low moisture absorption and resistance to weather, ozone and radiation. Unfortunately, it has low mechanical strength, poor scuff resistance and a high cost.

    Neoprene has a maximum temperature range from -67 to 194 degrees Fahrenheit (-55 to 90 degrees Celsius). The most stable colors are black, dark brown and gray. Neoprene is oil and sunlight resistant, making it ideal for outdoor applications. The electrical properties are not as good as other insulation material, so thicker insulation is required.

    Rubber normally has natural rubber and styrene-butadiene rubber (SBR) compounds. Some formulations are suitable for -67 degrees Fahrenheit (-55 degrees Celsius) minimum while others are suitable for 167 degrees Fahrenheit (75 degrees Celsius) maximum. Rubber jacketing compounds are exceptionally durable, withstand high-impact and abrasive conditions better than PVC and are resistant to degradation or penetration by water, alkali or acid. The compounds also have excellent heat resistance and provide greater cable flexibility in cold temperatures.

  • Insulation Color Codes. Wire insulation colors help trace conductors or conductor pairs. The most common codes are given in Tables 2 and 3.

  • Snakes. Snakes are an easy way to connect stage microphones and such to a remote mixing board. Snakes have individually jacketed and shielded pairs for optimal protection against signal loss in a single jacket. Snakes can be specified as to the type of connectors or termination and number of pairs. Snakes offer the following advantages:

    1. Low-capacitance insulation materials

    2. Spiral, braid and other foil tape shields for special interference problems

    3. Low profile appearance, high temperature resistance and cold temperature pliability

    4. Jacket and insulation materials that meet NEC requirements

    5. Easy termination and identification

    6. No overall shield required to reduce crosstalk

    7. Long cable runs are cheaper than running multiple single channel cables.

    Color codes for snakes are given in Table 4.

  • Plenum Cable. Plenum cable must be used in ceilings where the air handling system uses the plenum as the delivery or the return air duct. In a typical modern commercial building with hung ceilings, cables are installed in the enclosed space between ceilings and the floor above. That area may also be used as a return air plenum for a building's heating and cooling system — an invitation to disaster if a fire breaks out. If the fire is able to feed on combustible materials (such as cable insulations) in the plenum, the fire can spread rapidly. That's the reason NEC requires that conventional cables always be installed in metal conduit when used in plenums.

    In 1981 the jacket and insulation compound used in plenum cables was tested and found acceptable under the terms of the NEC and was classified by Underwriters Laboratories Inc. for use without conduit in air return ducts and plenums. Originally, plenum cable was Teflon jacketed. Today most plenum cables have a special PVC jacket that meets the fire rating. Although plenum cable costs more than conventional cable, the overall installed cost is dramatically lower because it eliminates the added cost of conduit and saves time and labor required to install it.

  • Multiconductor Cables. When many lines carrying different programs or signals are run in the same conduit, they induce crosstalk currents into each other. Crosstalk is induced by two methods: electromagnetically due to unbalanced coupling between one circuit and others or electrostatically due to unbalanced capacitance to other circuits, which develops a voltage difference between one circuit and the others (or to its own or other shields carrying current). The main ways to eliminate crosstalk include using shielded cables, using transformers on both ends and grounding the circuit. Be sure that the two wires of each pair are twisted, as this ensures close spacing and aids in canceling pickup by transposition. The measurements in Figure 3 were made many years ago but are still valid today. All pickup was capacitive because the leads were twisted, eliminating inductive coupling. The test was made in a 250-foot conduit with two twisted pairs run through it. One carried 70.7 V while the second cable was measured for crosstalk. Measurements made for half this length produced half the voltages; therefore, the results at 500 and 1000 feet were interpolated. The crosstalk figures are for 1 kHz. The voltages at 100 Hz and 10 kHz are one tenth and ten times those figures, respectively.


Table 2: Color codes for nonpaired cables per ICEA No. 2 and No. 2R.
Cond# Color Cond# Color Cond# Color
1st Black 18th Orange/Red 35th Wht/Red/Org
2nd White 19th Blue/Red 36th Org/Wht/Blue
3rd Red 20th Red/Green 37th Wht/Red/Blue
4th Green 21st Orange/Green 38th Blk/Wht/Grn
5th Orange 22nd Blk/Wht/Red 39th Wht/Blk/Grn
6th Blue 23rd Wht/Blk/Red 40th Red/Wht/Grn
7th White/Black 24th Red/Blk/Wht 41st Grn/Wht/Blue
8th Red/Black 25th Grn/Blk/Wht 42nd Org/Red/Grn
9th Green/Black 26th Org/Blk/Wht 43rd Blue/Red/Grn
10th Orange/Black 27th Blue/Blk/Wht 44th Blk/Wht/Blue
11th Blue/Black 28th Blk/Red/Grn 45th Wht/Blk/Blue
12th Black/White 29th Wht/Red/Grn 46th Red/Wht/Blue
13th Red/White 30th Red/Blk/Grn 47th Grn/Org/Red
14th Green/White 31st Grn/Blk/Org 48th Org/Red/Blue
15th Blue/White 32nd Org/Blk/Grn 49th Blue/Red/Org
16th Black/Red 33rd Blue/Wht/Org 50th Blk/Org/Red
17th White/Red 34th Blk/Wht/Org
NOTE: No. 2 cables feature a spiral strip; No. 2R cables feature a ring-band strip.
Courtesy Belden Wire and Cable Company.


Table 3: Color codes for paired cables (Belden Standard).
Pair No. Color
Comb.
Pair No.

Color
Comb.

Pair No. Color
Comb.
Pair No. Color
Comb.
1 Black/Red 11 Red/Yellow 21 White/Brown 31 Purple/White
2 Black/White 12 Red/Brown 22 White/Orange 32 Purple/Dark Green
3 Black Green 13 Red/Orange 23 Blue/Yellow 33 Purple/Light Blue
4 Black/Blue 14 Green/White 24 Blue/Brown 34 Purple/Yellow
5 Black/Yellow 15 Green/Blue 25 Blue/Orange 35 Purple/Brown
6 Black/Brown 16 Green/Yellow 26 Brown/Yellow 36 Purple/Black
7 Black/Orange 17 Green/Brown 27 Brown/Orange 37 Gray/White
8 Red/White 18 Green/Orange 28 Orange/Yellow
9 Red/Green 19 White/Blue 29 Purple/Orange
10 Red/Blue 20 White/Yellow 30 Purple/Red
Courtesy Belden Wire and Cable Company.


Table 4: Color codes for snake cables.
Pair No. Color Comb. Pair No. Color Comb. Pair No. Color Comb. Pair No. Color Comb. Pair No. Color Comb.
1 Brown 13 Lt. Gray /Brown stripe 25 Lt. Blue/ Brown stripe 37 Lime/
Brown stripe
49 Aqua/
Brown stripe
2 Red 14 Lt. Gray/ Red stripe 26 Lt. Blue/ Red stripe 38 Lime/
Red stripe
50 Aqua/
Red stripe
3 Orange 15 Lt. Gray/ Orange stripe 27 Lt. Blue/ Orange stripe 39 Lime/
Orange stripe
51 Aqua/
Orange stripe
4 Yellow 16 Lt. Gray/ Yellow stripe 28 Lt. Blue/ Yellow stripe 40 Lime/
Yellow stripe
52 Aqua/
Yellow stripe
5 Green 17 Lt. Gray/ Green stripe 29 Lt. Blue/ Green stripe 41 Lime/
Green stripe
53 Aqua/
Green stripe
6 Blue 18 Lt. Gray/ Blue stripe 30 Lt. Blue/ Blue stripe 42 Lime/
Blue stripe
54 Aqua/
Blue stripe
7 Violet 19 Lt. Gray/ Violet stripe 31 Lt. Blue/ Violet stripe 43 Lime/
Violet stripe
55 Aqua/
Violet stripe
8 Gray 20 Lt. Gray/ Gray stripe 32 Lt. Blue /Gray stripe 44 Lime/
Gray stripe
56 Aqua/
Gray stripe
9 White 21 Lt. Gray/ White stripe 33 Lt. Blue/ White stripe 45 Lime/
White stripe
57 Aqua/
White stripe
10 Black 22 Lt. Gray/ Black stripe 34 Lt. Blue/ Black stripe 46 Lime/
Black stripe
58 Aqua/
Black stripe
11 Tan 23 Lt. Gray/ Tan stripe 35 Lt. Blue/ Tan stripe 47 Lime/
Tan stripe
59 Aqua/
Tan stripe
12 Pink 24 Lt. Gray/ Pink stripe 36 Lt. Blue/ Pink stripe 48 Lime/
Pink stripe
60 Aqua/
Pink stripe
Courtesy Belden Wire and Cable Company.

In a future article I will discuss digital audio cable, coaxial cable, CCTV/CATV cable and installation techniques.


Glen Ballou owns Innovative Communications and is the author and editor of the first, second and third editions of the Handbook for Sound Engineers (Focal Press).


The National Electrical Code

NEC's code book has nine chapters that are divided into articles pertaining to specific subjects. Five articles pertain to communication and power-limited cable.

Article 725 covers Class 1, Class 2 and Class 3 remote control and signaling cables as well as power-limited tray cable. Power-limited tray cable can be used as a Class 3 or Class 2 cable. Cable listed as multipurpose, communications or power-limited fire protective can be used for Class 2 and Class 3 applications. A Class 3 cable can be used as a Class 2 cable.

Article 760 covers power-limited fire protective cable. Cable listed as power-limited fire protective cable can also be used as Class 2 and Class 3 cable. Cable listed as communications and Class 3 can be used as power-limited fire protective cable with restrictions to conductor material and type gauge size and number of conductors.

Article 770 discusses three general types of fiber optic cable: nonconductive, conductive and composite.

Article 800 covers multipurpose and communication cable. Multipurpose cable is the highest listing for a cable and can be used for communication, Class 2, Class 3 and power-limited fire protective cable. Communication cable can be used for Class 2 and Class 3 cable and also as a power-limited fire protective cable with restrictions.

Article 820 covers community antenna television and RF cable. CATV cable may be substituted with multipurpose or communication coaxial cable.

The NEC has designated four categories of cable for various environments, and these are listed from the highest to the lowest. A higher-listed cable can always be used as a substitute for a lower one.

Plenum cable is suitable for use in air ducts, plenums and other spaces used for environmental air without conduit because it provides adequate fire-resistant and low smoke-producing characteristics.

Riser cable is suitable for use in a vertical run, in a shaft or from floor to floor, and it has fire-resistant characteristics capable of preventing the spread of fire from floor to floor.

General Purpose cable is resistant to the spread of fire. It is suitable for general-purpose use, with the exception of risers, ducts, plenums and other space used for environmental air.

Restricted Applications cable has limited use, is suitable for use in dwellings and in raceways and is flame retardant. Restricted use is limited to nonconcealed spaces of 10 feet or less, fully enclosed in conduit or raceway, or cable with diameters less than 0.25 inches for a residential dwelling.

NEC cable hierarchy, Figure 1, defines which cables can replace other cables. The chart starts with the highest listed cable on the top and descends to the lowest listed cable on the bottom. Following the arrows defines which cable can be substituted for others.

When choosing cable for an installation, check the application and environment to determine which type of cable to use and what rating it should have. Because the NEC code is a general guideline that can be adopted in whole or in part, contact local authorities to verify local codes.

Any substitutions must be done with a higher-rated cable than what the code calls for. The local inspector or fire marshal has the final authority to approve or disapprove any installation of cable based on the NEC or on the local code, so make sure you check with those authorities.



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