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Article 310 of the National Electrical Code lists minimum conductor requirements for electrical systems. Tables in the NEC show installation standards that will apply in most communities where the NEC is followed. But be sure to use the version of the code that the inspectors are using. Even though the NEC is revised every three years, some cities and counties don't get around to adopting the current version until months or years after each revision.

In this book we're concerned with building construction exclusively. The scope of the NEC is much broader. We'll ignore NEC requirements that relate to ship building, aircraft manufacturing and the automobile industry.

In the construction industry, the most common wire types are identified as THW, THHN, THWN, XHHW, MTW, TF, TFF, TFFN, AF, and USE-RHH-RHW. These code letters identify the type of insulation on the wire.

Wire sizes used in building electrical systems range from #18 (called 18 gauge under the American Wire Gauge system) to 1000 KCMIL. KCMIL stands for one thousand circular mils.

The smaller the gauge number, the larger the wire. #14 gauge is smaller than #12 gauge, for example. Gauges larger than 1 follow the "ought" scale. One ought (usually written as 1/0) is larger than #1 gauge. Two ought (2/0) is larger still. Above four ought, wire size is identified in thousands of circular mils. A circular mil is the area of a circle that's one mil (1/1000th of an inch) in diameter. Wire that has a cross section area of 250,000 circular mils (250 KCMIL), for example, weighs about 12 ounces per linear foot. 1000 KCMIL wire weighs more than 3 pounds per linear foot.

Conductors in wire are either solid (one piece surrounded by insulation) or stranded. Stranded copper wire can be tinned as part of the manufacturing process and might be so specified.

Type THW

Type THW wire is Moisture- and Heat-Resistant Thermoplastic. It has a maximum operating temperature of 75 degrees C or 167 degrees F for dry or wet locations and 90 degrees C or 194 degrees F when used in electric discharge lighting equipment of 1000 open-circuit volts or less. The insulation is flame-retardant, moisture- and heat-resistant thermoplastic. It has no outer jacket or covering and is sized from #14 gauge with either solid or stranded conductors.

Type THHN

Type THHN wire is Heat-Resistant Thermoplastic and has a maximum operating temperature of 90 degrees C or 194 degrees F for dry locations. The insulation is flame-retardant, heat-resistant thermoplastic. It has a nylon jacket or the equivalent and is sized from #14 gauge with either solid or stranded conductors.

Type XHHW

Type XHHW wire is Moisture- and Heat-Resistant Thermoplastic and has a maximum operating temperature of 90 degrees C or 194 degrees F for dry locations, and 75 degrees C or 167 degrees F for wet locations. The insulation is flame-retardant cross-linked synthetic polymer. It has no outer jacket or covering and is sized from #14 gauge in either solid or stranded conductors.

Type USE

Type USE is Underground Service-Entrance Cable, Single Conductor, and has a maximum operating temperature of 75 degrees C or 167 degrees F for applications as listed in Article 338, Service-Entrance Cable in the NEC. The insulation is heat- and moisture-resistant. It has a jacket or outer covering rated as moisture-resistant nonmetallic and is sized from #12 gauge in either solid or stranded conductors.

Type MTW

Type MTW is Moisture-, Heat-, and Oil-Resistant Thermoplastic and has a maximum operating temperature of 60 degrees C or 140 degrees F. It's used for machine tool wiring in wet locations and is covered by NEC Article 670, Metal-Working Machine Tools. The insulation is flame-retardant, moisture-, heat- and oil-resistant thermoplastic. It's sized from #14 gauge with stranded conductors. There are two thicknesses of insulation. One is a little thinner than the other. The thicker insulation, Type A, has no outer jacket or covering. The thinner insulation, Type B, has a nylon jacket or equivalent covering.

Type TF

Type TF is Thermoplastic-Covered Fixture Wire and has a maximum operating temperature of 60 degrees C or 140 degrees F. It's used for fixture wiring as permitted by the NEC. The insulation is thermoplastic. Sizes are #18 or #16 gauge with either solid or stranded conductors. It has no outer jacket or covering.

Type TFF

Type TFF is Thermoplastic-Covered Fixture Wire, Flexible Stranded. It has a maximum operating temperature of 60 degrees C or 140 degrees F and is used for fixture wiring, with some NEC restrictions. The insulation is thermoplastic. Sizes are either #18 or #16 gauge with stranded conductors. It has no outer jacket or covering.

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Type TFFN

Type TFFN is Heat-Resistant Thermoplastic-Covered Fixture Wire, Flexible Stranded. It has a maximum operating temperature of 90 degrees C or 194 degrees F and is used for fixture wiring as permitted by the NEC. The insulation is thermoplastic and is sized at #18 or #16 gauge with solid or stranded conductors. It has a nylon or equivalent outer jacket.

Type AF

Type AF wire is Asbestos Covered Heat-Resistant Fixture Wire. It has a maximum operating temperature of 150 degrees C or 302 degrees F for fixture wiring. It's limited to 300 volts and indoor dry locations. The insulation is impregnated asbestos. Gauges run from #18 to #14 with stranded conductors. Moisture-resistant and impregnated asbestos insulation is sized from #12 to #10 gauge with stranded conductors.

Many other types of insulation are listed in the tables of the NEC. The types just listed are the ones most commonly used in building construction. Many other types of insulated wire were used in the past and have been discontinued. The NEC still lists some types that are seldom used and are no longer stocked by most dealers. For example, the smaller sizes of THW are no longer popular. Type THHN or THHN-THWN has become more popular as the industry standard changed to newer types of insulation. The insulation jacket on THHN-THWN is much thinner. Yet the wire has a higher ampacity than older THW. The NEC tables "Maximum Number of Conductors in Trade Sizes of Conduit or Tubing" permit more current-carrying capacity in conduit of a given size when using THHN wire than when using THW wire.

Flexible Cords

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Flexible Cords

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Many types of flexible power cords are listed in the NEC. Some of these apply to the construction industry and are important to the electrical estimator.

Type SJ

The trade name for Type SJ is Junior Hard Service Cord. It's made in sizes #18 or #10 with two, three or four conductors. The insulation is thermoset plastic with an outer jacket or covering of thermoset plastic. It's used for portable or pendant fixtures in damp locations where hard usage is expected. The insulation is rated at 300 volts for 60 degrees C.

Type S

The trade name for Type S is Hard Service Cord. It's made in gauges from #18 to #2 with two or more conductors. The insulation is thermoset plastic with an outer jacket or covering of thermoset plastic. It's used for portable or pendant fixtures in damp locations where extra hard use is expected. The insulation is rated at 600 volts for 60 degrees C.

Type SO

The trade name for Type SO is also Hard Service Cord. It's made in gauges from #18 to #2 with two or more conductors. The insulation is thermoset plastic with an outer jacket or covering of oil-resistant thermoset plastic. It's used for portable or pendant fixtures in damp locations where extra hard usage is expected. The insulation is rated at 600 volts for 60 degrees C.

Type STO

The trade name for Type STO is also Hard Service Cord. It's made in gauges from #18 to #2 with two or more conductors. The insulation is thermoplastic or thermoset with an outer jacket or covering of oil-resistant thermoplastic. It's used for portable or pendant fixtures in damp locations where extra hard usage is expected. The insulation is rated at 600 volts for 60 degrees C.

Type NM

The trade name for Type NM is Non-Metallic-Sheathed Cable. It's made in gauges from #14 to #2 with from two to four conductors and either with or without a ground wire. The ground wire may be either insulated or bare. The insulation is TW or as listed in NEC Table 310-13. The cable has an outer jacket or covering of a flame-retardant and moisture-resistant plastic. Type NM is commonly used in house wiring. The insulation is rated at 600 volts for 60 degrees C.

Type UF

The trade name for Type UF Cable is Underground Feeder and Branch Circuit Cable. It's made in gauges from #14 to #4/0 with one, two, three or four conductors. Gauges from #14 to #10 have 60 mil insulation. Gauges from #8 to #2 have 80 mil insulation. Gauges from #1 to #4/0 have 95 mil insulation. It comes with or without a ground wire. Single conductor is rated for 600 volt, 60 degrees C or 140 degrees F. The insulation is moisture-resistant and is integral with the jacket or outer covering. Multiple conductor cable is rated at 600 volts and 75 degrees C or 167 degrees F. The insulation is moisture- and heat-resistant.

Type SEU

The trade name for Type SEU is Service-Entrance Cable. It's made in gauges from #12 to #4/0 with one or more conductors. The cable is rated 600 volts. The temperature rating of the wire is the same as the rating of the conductor itself. The cable has a flame-retardant, moisture-resistant jacket or covering.

Bare Copper Wire

Bare copper wire is made in gauges from #14 to #4/0 in either soft drawn, medium hard drawn or hard drawn temper. It's usually sold by the hundredweight, which is 100 pounds. It comes in solid strands in sizes up to #4 and stranded up to #4/0 gauge. The most common use for bare copper wire is in grounding electrical systems.

The NEC has a table that gives weights for 1000 linear feet of soft drawn bare copper wire. This simplifies the conversion of lengths to weights so you can convert the hundredweight price to a cost for the length you need.

Metal-Clad Cable

Type MC cable is a factory assembled cable of one or more insulated conductors enclosed in a flexible interlocking metallic sheath, or a smooth or corrugated tube. The conductors have a moisture-resistant and flame resistant fibrous cover. An internal bonding strip of copper or aluminum is in contact with the metallic covering for the entire length of the cable. It's rated for 600 volts and comes in gauges from #14 to #4 in single or multiple conductors. An approved insulating bushing or the equivalent must be installed at terminations in the cable between the conductor insulation and the outer metallic covering.

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Flexible Cords

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There are some restrictions on the use of armored cable. It's prohibited in theaters, places of assembly, motion picture studios, hazardous locations, where exposed to corrosive fumes or vapors is expected, on cranes or hoists, in storage battery rooms, in hoistways or elevators and in commercial garages. Check for exceptions in the NEC when working with MC armored cable.

Other types of insulated conductors and cables are available and used occasionally in construction. But be sure you refer to the code and discuss the material with the inspection authority before including them in your bid. Some cities and counties prohibit certain conductors and cables.

High Voltage Wire and Cable

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High Voltage Wire and Cable

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Medium and high voltage conductors and cables require special consideration. In most projects they're specified for a particular use or service. Many types of high voltage conductors are produced by some manufacturers. Some are stocked only by the factory. Others are not stocked at all and have to be made to order. These will be very expensive unless the factory run is quite large. In any case, be prepared to wait many weeks for delivery.

Terminations for high voltage conductors are also a specialty item. Installation should be done only by an experienced craftsman, usually called a cable splicer. Shielded conductors will require a stress cone for termination at the point of connection to switching equipment or some other device. Stress cones can be made as a kit that's adapted to the end of the cable to relieve stress from the shield.

Splicing cable is also a specialty that should be done by a qualified cable splicer. The splicer prepares the conductor and either uses a splice kit or tapes a splice. The process may take many hours to complete. Once a stress cone or a splice has been started, it should be completed as quickly as possible to keep moisture out of the splice.

Most professional electrical estimators have to figure the cost of extending an existing underground electrical system occasionally. For example, you may have to figure the cost of splicing new XLP cable to three conductor lead covered cable. An experienced cable splicer will be needed to make this splice properly. In an underground system that's subject to moisture or even continuous submersion in brackish water, this splice will have to be made correctly.

High voltage splicing requires experience and precision. If the cable is damaged or if the splice isn't made correctly, replacing the cable and the splice can be very expensive. If you can't locate an experienced cable splicer, consider subcontracting the work to a firm that specializes in this work. Having it done by a qualified expert can preserve many nights of sound sleep.

Most specs require a high-potential test on the cable after it's been installed. The test is done with a special portable test unit. An experienced operator is needed to run the test. Follow the test procedure recommended by the cable manufacturer.

There are good reasons for running this test. Of course, every reel of electrical cable gets a high potential test at the factory. But cable can be damaged between the factory and the job site. Your cable may have been cut from the original reel and re-coiled on a smaller reel for shipment to the job site. It probably sat in storage for at least several months and may have been exposed to adverse conditions for the entire time. When your electricians finally install the cable, it may be defective, even though it looks fine. But if moisture has entered from a cut end or if the cable was damaged in transit, it's much better to find out before the cable is energized. A fault in a high voltage cable can cause damage to other cable in the conduit or to other parts of the electrical system.

The high potential test is not a destructive test. If the cable is sound before the test begins, it will be unchanged by the test. The test proves that the insulation is in good condition and that the conductor will safely carry the rated voltage.

Here's the usual test procedure. First, identify the conductors by circuit and by phase (A, B, C, etc.). Voltage is applied for a few minutes and is increased to a certain value over a set amount of time. The operator records the voltage, time and amount of leakage up to a predetermined voltage. The maximum test voltage will be more than the rated voltage of the cable. The technician performing the test enters results on a special test form and submits it to the designing engineer for review. The form should then be filed with permanent project records.

If a fault shows up during the high potential test, the operator should have equipment available to locate the fault. Usually the problem will be at a stress cone or in a splice. One method of locating a fault in the cable is to install a thumper, a large capacitor-type unit, in the line. The thumper gives off an electrical discharge every few seconds. The charge will create a pop or snap when it hits the fault, making location easy.

Repair the problem by resplicing. If the fault is in the cable, pull out the cable and examine it carefully. It was probably damaged during installation. If the cable jacket is good and doesn't have signs of damage, the cable itself may be at fault. The manufacturer may want to test the cable to determine the cause of failure. Most manufacturers will replace at no charge cable that fails a high potential test before being put into service. Once the cable is put into use, it's hard to get a free replacement, even if there is no apparent cable damage.

Aluminum Wire

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Aluminum Wire

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The types of insulation listed for copper wire are the same as used on aluminum wire. There are two major differences between copper and aluminum wire: First, aluminum usually costs less for a similar gauge. Second, the ampacity of aluminum wire is less than that of the copper wire. That means that aluminum wire has to be heavier gauge to carry the same current. Tables in the NEC show the allowable current rating for aluminum wire.

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Aluminum Wire

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Aluminum wire is much lighter than copper wire and can be installed in less time. Compare the difference in the labor units in this book for copper and aluminum wire. Many electricians prefer to install aluminum wire on some types of applications. But aluminum wire comes with a major problem: it oxidizes when not covered with insulation. When the wire is stripped and left bare, it develops a thin white oxide coating that resists the flow of current. Eventually the wire begins to heat up at the point of connection. Heat expands the conductor. Cooling causes it to contract. Constant expansion and contraction makes the connection looser and accelerates oxidation. Eventually the conductor will fail. Sometimes overheating will cause a fire.

There are ways to reduce the problem of oxidation. An anti-oxidation material can be applied immediately after the cable is stripped and before it's terminated in the connector. The material can also be applied to the connector. Then the aluminum wire is inserted into the connector and tightened. A good practice is to retighten the connector after a few days and again after the circuit has been in operation for about a year. Some wire and cable connectors are made with an oxidation inhibitor material inside the area where the wire will be inserted. Only connectors rated for aluminum wire should be used on aluminum circuits.

Today, aluminum wire is seldom used on branch circuits to convenience receptacles or lighting switches. But it's common on feeder circuits such as runs from distribution switchboards to lighting or power panels.

When you decide to change a feeder from copper wire to aluminum wire, be sure to compare the carrying capacity of the two materials. You'll find that it takes larger aluminum wire to carry the same amperage as a copper wire. This may mean that you'll have to use larger conduit size to accommodate the larger wire diameter. But using aluminum on feeder circuits will probably still reduce the labor and material cost.

Wire Connectors

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Wire Connectors

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Many types of wire connectors and lugs are available. Some are insulated, made for multiple conductors, or intended to be watertight. Be sure the connections you price will comply with the code and job specs. Not all types of connections are listed in this manual. But you'll find a representative selection here that gives you a good idea of how to price most of the common wire connectors. It would take a 200-page book just to list labor and material costs for all the connectors and lugs that are available.

Pulling Wire in Conduit