| Home | Fire
Safety | Skyscrapers Home Emergencies | Glossary |
|
. Conductor Materials Copper and aluminum are the most common conductor materials used in building electrical wiring, although other materials can be used. As a general rule, solid copper conductors are used in small conductor sizes (up to about 8 AWG) because safety issues associated with aluminum are avoided and weight and cost are not significantly affected. Medium- and large-gauge stranded aluminum cable (above No. 8 AWG) is safely used on circuits as long as the terminals or connectors on the circuit are rated CU-AL (copper-aluminum) and an antioxidant paste is used on properly tightened connections. Stranded aluminum conductors are widely used on larger (above 30A) circuits serving large motors, equipment, and appliances such as clothes dryers, kitchen ranges, or central air conditioners. They are also used as service entrance and feeder conductors that carry electrical energy from the transformer to the building service equipment (the switchboard or panel board). Use of UL-listed 15 A and 20 A switches, outlets, and other devices marked CO/ALR is presently accepted. Tbl.4 shows the normal range of applications for commonly used conductor sizes. Conductor Insulation Conductors are covered with insulation to provide electrical isolation and physical protection of the conductor material. The type of insulation material determines the environment in which a wire or cable can be used safely. Wires used in doors are subjected to less exposure to the elements than those designed for outdoor use. Outdoor wiring is exposed to water and sunlight, so the insulation is designed to withstand these elements. Insulation on wires buried in the ground must also be able to withstand the damp, corrosive environment of the soil. Insulation used to cover electrical conductors are designated with a set of letters that describes the properties of the insulation. These designations were described in Section 18. ====== Tbl. **4 COMMON USES OF CONDUCTORS IN BUILDINGS BY CONDUCTOR SIZE. Conductor Gauge; Common Uses No. 20 AWG and smaller Electronic circuits and phone extensions No. 16 to 18 AWG Light-gauge extension cords, lamp cords, door chime wiring, small appliance cords No. 12 to 14 AWG Normal 15 A and 20 A branch circuits serving small appliances, convenience (receptacle) outlets, and luminaires No. 4 to 10 AWG Larger branch circuits at 30 A and above serving electrical appliances such as electric water heaters, clothes dryers, air conditioning equipment, and water pumps No. 2 to 4/0 AWG Residential and light commercial service entrance conductors and feeders to panelboards 250 kcmil and larger Heavy commercial and industrial service entrance conductors, large feeders to closet transformers, and panelboards ======= Conductor Ampacity Requirements Tables 14 through 16 and 18 through 24 contain ampacities for various conductors, conductor insulations, and sheathings. Ampacities provided in these tables are values based on a normal operating temperature of 86°F (30°C). Ampacity values for each conductor size are for different equipment terminal (where connections of wiring are made) temperatures. Heat generated at the equipment terminals can damage the conductors if it’s not properly dissipated. Unless equipment terminals are marked otherwise, circuit conductors are to be sized according to 140°F (60°C) for equipment rated 100 A and less, and equipment rated over 100 A must be sized to 167°F (75°C). Correction factors typically applied are addressed in the following sections. Temperature Correction Factor Ambient temperature is the temperature of a surrounding medium (e.g., air, soil). In the case of electrical wiring, it’s the temperature of the medium surrounding the conductor. Ambient temperature can affect allowable current-carrying capacity of a conductor. As ambient temperature rises, less current generated heat is needed to reach the temperature rating of the insulation. Therefore, ampacity is governed by contribution of ambient heat. The ambient temperature rating of a conductor refers to the normal temperature range in the environment in which that conductor is to be used (e.g., the temperature of the surrounding air, water, or earth). Conductor ampacity is adjusted for changes in ambient temperature, including temperatures below 78°F (26°C) and above 86°F (30°C). Most electrical and mechanical rooms and attic spaces where electrical equipment is installed will exceed 86°F (30°C) under typical operating conditions. A temperature correction factor (Ft ) for conductors is applied based on the ambient temperature of the conductor. For example, the temperature correction factor for THHN, THWN, and XHHW (75°C, 167°F) insulation in an ambient temperature range of 123° to 131°F (51° to 55°C) is 0.67. This means a conductor operating in this ambient temperature range will have an ampacity (allowable current capacity) that is 67% of one in the normal operating temperature range- that is, a larger conductor is required at higher temperatures. Conversely, the temperature correction factor in an ambient temperature range of 70° to 77°F (21° to _25°C) is 1.05. Refer to the local code for other correction factors and temperature ranges. Bundling Correction Factor When several current-carrying conductors are contained in a raceway or cable, the temperature of the conductors will in crease under normal loading conditions. The additional current-generated heat causes a conductor surrounded by several other current-carrying conductors to exceed the temperature rating of the insulation more easily. As a result, ampacity of a conductor grouped with several other conductors is influenced by contribution of ambient heat. Simply, when several (more than three) current-carrying conductors are added to a raceway or bundled in a cable, ampacity of a current-carrying conductor is decreased to compensate for the extra heat. A bundling correction factor (FN) must be applied for four or more conductors in a raceway or cable installed in the same raceway or conduit or any bundled cables that are more than 24 in (0.63 m) long. For raceways and cables with 4 through 6 current-carrying conductors, the bundling correction factor is 0.80; for 7 through 9 conductors, it’s 0.70; and for 10 through 20, it’s 0.50. Refer to the local code for correction factors for additional conductors. The number of current-carrying conductors includes any ungrounded conductor or grounded conductor. A shared neutral (see definition in the Neutral Conductor section above: a single 120 V circuit is not served by a neutral conductor) that is not current carrying, and is not counted. However, on four-wire, three-phase wye-connected systems the shared neutral must be counted as a current-carrying conductor. Equipment-grounding conductors are not current-carrying conductors and are not counted. To account for ambient temperatures outside of this nor mal range, ampacity for a conductor in the normal operating temperature range (the base ampacity) is adjusted with reduction factors discussed in Ex. **3. These factors are applied such that the ampacity (I_ampacity) of a conductor at a specific operating temperature is the ampacity (I_normal ) at normal operating temperatures multiplied by any applicable reduction factors for ambient temperature (Ft ) and conductor bundling (FN): Ex. **2 Determine the ampacity of a No. 8 AWG copper conductor with THHN insulation that will be used in a 120 V, two-wire circuit in an environment with an average ambient air temperature of no greater than 86°F (30°C). No corrections must be made for temperature or more than three conductors in a raceway, so I_ampacity _ I_normal Circuit conductors are sized according to 140°F (60°C) for equipment rated 100 A and less. From Tbl. 11 in Section 18, the ampacity of this conductor is 40 A. Ex. **3 Determine the ampacity of a No. 8 AWG copper conductor with THHN insulation that will be used in a 120 V, two-wire circuit in an industrial environment with an average ambient air temperature of no greater than 125°F (51.7°C). Three similar circuits will be carried in a conduit. Assume that a shared neutral is not used. From the main body of the text, the temperature correction factor for THHN insulation in an ambient temperature range of 123° to 131°F (51° to 55°C) is 0.67. From the main body of the text, the temperature correction factor for more than 6 conductors in a raceway is 0.80. As in Ex. **2, circuit conductors are sized according to 140°F (60°C) for equipment rated 100 A and less. From Tbl. 11 in Section 18, the ampacity of this conductor is 40 A. The ampacity of this conductor under these conditions is 21.4 A. Ex. **4 Determine the ampacity of a No. 8 AWG copper conductor with THHN insulation that will be used in a 120 V, two-wire circuit in an industrial environment with an average ambient air temperature of no greater than 125°F (51.7°C). Three similar circuits will be carried in a conduit. Assume that a shared neutral is not used. Equipment terminal connections are rated at 167°F (75°C). From the main body of the text, the temperature correction factor for THHN insulation in an ambient temperature I_ampacity _ I_normal (Ft)(FN) _ 40 A (0.67)(0.80) _ 21.4 A I_ampacity _ I_normal (Ft)(FN) range of 123° to 131°F (51° to 55°C) is 0.67. From the main body of the text, the temperature correction factor for more than 6 conductors in a raceway is 0.80. Equipment terminal connections are rated at 167°F (75°C), so from Tbl. 11 in Section 18, the ampacity of this conductor is 50 A. The ampacity of this conductor under these conditions is 26.8 A. Conductor Voltage Drop Requirements In addition to ampacity requirements, branch circuits and feeders should be analyzed for voltage drop because of the adverse effect it can have on performance and operating life of appliances and equipment. Although not required, branch circuits are typically designed to ensure that voltage drop at full load does not exceed 3% from the panelboard to the farthest outlet. Feeders are typically designed to ensure that voltage drop at full load does not exceed 2%. Total voltage drop in the feeders and branch circuits should not exceed 5%. For circuits feeding critical and sensitive equipment (e.g., medical, testing, and other high-fidelity electronic equipment), tighter limits are recommended: no more than a 1% voltage drop on branch circuits and a total voltage drop of 2% for feeders and branch circuits. When the feeder and branch circuit conductors for long circuits are sized on this basis, conductor sizes are increased beyond that required for ampacity. The basic formula for determining voltage drop (E_drop) in a two-wire AC circuit or three-wire AC single-phase circuit with a balanced load at 100% power factor (neglecting reactance) is based on the one-way circuit length (L), in feet or meters; conductor resistance (R), in O/1000 ft or O/1000 m; and the circuit load (I) in amperes: The percentage of voltage drop is determined by the ratio of voltage drop and system voltage. Ex. **5 A No. 8 copper conductor with THHN insulation has a one way length of 200 ft. It’s carrying a load of 16 A on a 120 V, two wire circuit. Approximate the voltage drop and percentage of voltage drop in the circuit. From Tbl. 11 in Section 18, a No. 8 AWG conductor has a resistance of 0.6410 O/1000 ft. Voltage drop is found by: _ (2 _ 200 ft _ 0.6410 O>1000 ft _ 16 A) _ 4.1 V Edrop _ (2 LRI) E_drop _ 2 LRI>1000 I_ampacity _ I_normal (Ft)(FN) _ 50 A (0.67)(0.80) _ 26.8 A The percentage of voltage drop is determined by: In Ex. **5, voltage drop is excessive (above 3%). Conductor size would need to be increased. The following equations can be used to approximate the maximum one-way distance (L) that a set of conductors can run, in feet. It’s based on voltage (E), amperage (I), conductor size in circular mils (cmil), and voltage drop (Edrop) expressed as a coefficient (i.e., 2% voltage drop _ 0.02, 3% _ 0.03, and so on). These equations are an approximation based on the basic voltage drop formula introduced earlier. Accuracy is limited to sizes up to 4>0 conductors. For single-phase circuits, the equations are: In SI (metric) computations, substitute the constant 70.8 for 21.6 and 118 for 36 to find length (L) in meters. For three-phase circuits, equations are: In SI (metric) computations, substitute the constant 61.3 for 18.7 and 102.2 for 31.2 to find length (L) in meters. Ex. **6 Approximate the maximum distance two No. 10 AWG conductors can carry a current of 20 A on a 120 V, single-phase circuit. Use a maximum voltage drop of 3%. No. 10 AWG conductor has a cross-sectional area of 10 380 cmils (from Section 18). For copper conductors: For aluminum conductors: The terms in Ex. **6 equations can be rearranged to create an equation to determine minimum conductor size in circular mils (cmil): For single-phase circuits, the equations are: For three-phase circuits, the equations are: Aluminum conductors: cmil _ 31.2 IL>E(Edrop) Copper conductors: cmil _ 18.7 IL>E(Edrop) Aluminum conductors: cmil _ 36 IL>E(Edrop) Copper conductors: cmil _ 21.6 IL>E(Edrop) _ (120 V)(0.03)(10 380 cmils)>(36)(20 A) _ 51.9 ft L _ E(Edrop)(cmil)>36 I _ (120 V)(0.03)(10 380 cmils)>(21.6)(20 A) _ 86.5 ft L _ E(Edrop)(cmil)>21.6 I Aluminum conductors: L _ E(Edrop)(cmil)>31.2 I Copper conductors: L _ E(Edrop)(cmil)>18.7 I Aluminum conductors: L _ E(Edrop)(cmil)>36 I Copper conductors: L _ E(Edrop)(cmil)>21.6 I 4.1 V>120 V _ 0.034 _ 3.4% for no more than three conductors in a raceway, cable, or earth burial for ambient temperatures in the normal operating range. Values in italics were governed by voltage drop for the one-way distance provided, based on a maximum 3% voltage drop. Insulation Color Coding and Identification Markings The insulation on small- and medium-size conductors is color coded for identification. Larger conductors requiring color identification are marked at the terminal ends with a hand-painted stripe or colored tape wrapped around the conductor insulation. Multiple ungrounded conductors in a race way or wire gutter must be indicated with a colored stripe. The insulation of grounded and neutral conductors at No. 6 AWG or less in size must be color coded with a continuous white or natural gray color. The grounding (ground) conductor insulation must be color coded green, green with one or more yellow stripes, or may be a bare conductor on small conductors in cables. Code no longer requires specific colors for color coding of ungrounded (hot) conductors, except the Phase Y on delta-connected, three-phase systems such as the 240 ?/120 AC, 3F-4W system discussed earlier. On delta-connected systems, Phase Y has a higher voltage to ground (e.g., 208 V on the 240 ?/120 AC, 3F-4W system) than the other legs (120 V on the 240 ?/120 AC, 3F-4W system). Code requires that this phase be marked by the color orange, with the intent of preventing the connection of single-phase loads to this phase and avoiding the resulting equipment damage. Other wise, ungrounded conductors may be any color, except white, gray, and green. The North American standard for color coding is black or any color, except white, gray, and green (ungrounded/hot); white (grounded/neutral); and green (ground). The commonly used but not mandatory color sequence of conductors serving single-phase circuiting is: Two wires: grounded: white ungrounded: black Three wires: grounded: white ungrounded: black and red Four wires: grounded: white ungrounded: black, red, and blue Five wires: grounded: white ungrounded: black, red, blue, and yellow On three-phase circuits, the color sequence of conductors tend to be: Four wires: grounded: white ungrounded: brown, orange, and yellow. Internationally, the standard is brown (ungrounded/hot), blue (grounded/neutral), and green with yellow strip (ground). Historically, different color codes have been used. On three phase circuits, the color sequence of conductors tends to be: first-phase conductor (brown), second-phase conductor (black), and third-phase conductor (gray). In large commercial and industrial facilities where several system voltages are available, it’s useful (and safe) to mark or label conductors and equipment for identification in addition to color coding it. Identification markings typically indicate system voltage and phase. Markings must be durable so they withstand the environment. A sample color and marking code for conductors is shown in Tbl.6. Prev: The Building
Electrical System |