Slides Notes 2
CM
"CM" = Circular-Mils: The circular mils of the circuit conductor as listed in Chapter 9, Table 8.
I
"I" = Amperes: The load in amperes at 100 percent, not 125 percent for motors or continuous loads.
K
"K" = Direct Current Constant: This is a constant that represents the direct current resistance for a one thousand circular mils conductor that is one thousand feet long, at an operating temperature of 75º C. The direct current constant value to be used for copper is 12.9 and 21.2 is used for aluminum conductors. The Units of K=Cmil Ohms per foot. The "K" constant is suitable for alternating current circuits, where the conductors do not exceed No. 1/0. The equation for adjust K for temperature change: R 2 = R 1 [1 + α (T 2 - 75°C)], where α cu = 0.00323, αAL = 0.00330 at 75°C. NOTE: Field expedient method will use unadjusted K unless ambient temp is above 75°C.
L
"L" = Length: The distance the load is located from the power supply, not the total length of the circuit conductors.
Q
"Q" = Alternating Current Adjustment Factor: Alternating current circuits No. 2/0 and larger must be adjusted for the effects of self-induction (skin effect). The "Q" adjustment factor is determined by dividing alternating current resistance as listed in NEC Chapter 9, Table 9, by the direct current resistance as listed in Chapter 9, Table 8. NOTE: Metal coatings are often applied to the surface of individual wire strands in electrical conductors to improve solderability, reduce oxidation (corrosion) or improve electrical properties at high frequencies. Three of the most common coating (plating) materials used with copper conductors are tin, silver and nickel. A coated conductor with seven individual strands is illustrated above (coating thickness exaggerated for clarity). Decades ago metal coatings were also used to provide easy stripping of the insulation, i.e., to keep the insulation from sticking to the conductor. Improved insulation materials have made that practice unnecessary. NOTE: Assume uncoated unless otherwise specified.
310.15 (B)(3) Adjustment Factors.
(a) More than Three Current-Carrying Conductors. Where the number of current-carrying conductors in a raceway or cable exceeds three, or where single conductors or multi-conductor cables are installed without maintaining spacing for a continuous length longer than 600 mm (24 in.) and are not installed in raceways, the allowable ampacity of each conductor shall be reduced as shown in Table 310.15(B)(3)(a). Each current carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor. (UGLY'S p. 78)
An ideal insulating material should have the following properties:
- Long life - Long-time high dielectric strength - Resistance to corona and ionization - Resistance to high temperature - Mechanical flexibility - Resistance to moisture - Low dielectric loss
Low Voltage Cable Components
--- Conductor Carries the current from source to load Solid, or strands of copper or aluminum Used as ungrounded, neutral/grounded, and grounding --- Insulation First layer over conductor Contains the voltage Mainly made of plastic, rubber Contains the voltage within --- Jacket Layered over the insulation. Protects the insulation from abrasion Made of nylon, plastic, etc.
Control Wiring
-Wire designed to carry an electrical signal from a pilot device or controller -May be grouped in cables with J-Plugs. -May be shielded to prevent static build up or prevent stray currents
(3) Supervised Installations
. For supervised installations, feeder conductor sizing shall be permitted to be determined by qualified persons under engineering supervision. Supervised installations are defined as those portions of a facility where all of the following conditions are met: (1) Conditions of design and installation are provided under engineering supervision. (2) Qualified persons with documented training and experience in over 600-volt systems provide maintenance, monitoring, and servicing of the system.
Start with the equation:
AmpAllowable (from table) = Amp(load) /(CF x AF) Find correction factor for 97oF of 75oC insulation: (0.88). Find adjustment factor for 4 current carrying conductors: (0.80). Find minimum AmpA required for load: 152A/(0.88 x 0.80)=215A On Ugly's Pg 73, go to the Copper, 75oC, column. Move down and identify the first AmpA that is greater than 215A is 230A. Go to the furthest left column to find the required conductor size: 4/0.
310.15 (B) (2) Ambient Temperature Correction Factors.
Ampacities for ambient temperatures other than those shown in the ampacity tables shall be corrected in accordance with Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b), or shall be permitted to be calculated using the following equation: I′=I[(Tc-Ta′)/(Tc-Ta)]1/2 where: I′=ampacity corrected for ambient temperature I=ampacity shown in the tables Tc=temperature rating of conductor (°C) Ta′=new ambient temperature (°C) Ta=ambient temperature used in the table (°C)
1
Ampacity - current, in amperes, that a conductor can carry continuously without exceeding its temperature rating Cable - a stranded conductor or a combination of conductors insulated from one another Conductor - a wire or combination of wires not insulated from each other, suitable for carrying a single current Wire - a slender rod or filament of metal
From NEC:
Conductor, Bare. A conductor having no covering or electrical insulation whatsoever. (CMP-6) Conductor, Covered. A conductor encased within material of composition or thickness that is not recognized by this Code As electrical insulation. (CMP-6) Conductor, Insulated. A conductor encased within material of composition and thickness that is recognized by this Code As electrical insulation. (CMP-6)
215.2 Minimum Rating and Size. Feeders Not More Than 600 Volts. (1) General.
Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. Conductors shall be sized to carry not less than the larger of 215.2(A)(1)(a) or (b). (a) Where a feeder supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum feeder conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load. (b) The minimum feeder conductor size shall have an allowable ampacity not less than the maximum load to be served after the application of any adjustment or correction factors.
(3) Supervised Installations.
For supervised installations, feeder conductor sizing shall be permitted to be determined by qualified persons under engineering supervision. Supervised installations are defined as those portions of a facility where all of the following conditions are met: (1) Conditions of design and installation are provided under engineering supervision. (2) Qualified persons with documented training and experience in over 600-volt systems provide maintenance, monitoring, and servicing of the system. Allowable ampacity (AmpA) -Table 310.15(B)(16) through Table 310.60(C)(86) (UGLY'S pp. 73-76)
210.19 Conductors — Minimum Ampacity and Size. (A) Branch Circuits Not More Than 600 Volts.
Informational Note No. 1: See 310.15 for ampacity ratings of conductors. Informational Note No. 2: See Part II of Article 430 for minimum rating of motor branch-circuit conductors. Informational Note No. 3: See 310.15(A)(3) for temperature limitation of conductors. Informational Note No. 4: Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation. See Informational Note No. 2 of 215.2(A)(1) for voltage drop on feeder conductors.
Insulation
It is impossible to find any one material that is best when all these essential characteristics are considered. For instance, impregnated paper has the highest electrical breakdown strength coupled with the longest life of all the materials employed for the insulation of conductors. On the other hand, it is not moisture resistant, is not so flexible as other materials, and will not withstand higher temperatures as others. Several different types of insulation are therefore employed. The insulation whose overall characteristics best meet the conditions of service for the particular application should be selected. (from AEH, 2-25)
Power Cables
Low voltage power cables Carry power from low voltage generator Carry power from secondary of service transformers to residential customers. Nominal 600 volts or less.
Check on learning
Q. Using the previous example: Calculate the voltage drop and voltage drop percentage. Calculate the voltage drop and voltage drop percentage using the field expedient method. Is the cable sufficient for the load? Why? Answer: Use the 3 phase formula: Vd = 1.732 x K x Q x I x D/CM K=12.9 (Copper) Kadjust = 12.9 [1 + 0.00323 (38°C - 75°C)] = 11.4 Q = 0.0630/0.0608 = 1.0362 (required for AC cable larger than 2/0, Ohm Ch9T9/Ohm Ch9T8 for 4/0 uncoated copper Ohm/kFt) I = 152A D = 500ft CM = 211600 Vd = 1.73 x 11.4 x 1.0362 x 152A x 500Ft / 211600cmil = 7.34V %Vd = (7.31V/480V) x 100 = 1.52% (Less than 3% for feeder) Vd(FE) = 1.73 x 12.9 x 1.0362 x 152A x 500Ft / 211600cmil = 8.3V %Vd(FE) = (7.31V/480V) x 100 = 1.73% (Less than 3% for feeder)
Check on learning 2
Q: What are the 3 components of low voltage power cable? A: Conductor, Insulation, Jacket Q: On which NEC Table(s) would you find the following information? A: Ampacity of low voltage cables in open air: Table 310.15 (B) (17) Ampacity of single conductor, insulated, aluminum (5kV) cables in open air: Table 310.60(c)(70) Adjustment Factors for more than three current carrying conductors: Table 310.15(B)(3)(a) Correction Factors for 30C/40C rated conductors: Tables 310.15(B)(2)(a/b)
Q/A
Q: Why does AWG 8 have two rows of data? A: 8 AWG can be made of one or seven stands. While the conductor area does not change, the overall dimensions will. Q: How is the AWG gage written 3/0 or 000 pronounced? A: "triple-ought" or "Three-ought" Q: why do 250 AWG and above not have a number in the Circular mils column? A: 250 AWG, also referred to as 250MCM and above are using their circular mils instead of a gage size. For example 250 AWG = 250 MCM = 250,000 circular mils. Q: What does MCM stand for? A: MCM stands for Thousands of circular mils or kCmil.
310.15(B)(3)(c)
Raceways and Cables Exposed to Sunlight on Rooftops. Where raceways or cables are exposed to direct sunlight on or above rooftops, raceways or cables shall be installed a minimum distance above the roof to the bottom of the raceway or cable of 23 mm (7∕8 in.). Where the distance above the roof to the bottom of the raceway is less than 23 mm (7∕8 in.), a temperature adder of 33°C (60°F) shall be added to the outdoor temperature to determine the applicable ambient temperature for application of the correction factors in Table 310.15(B)(2)(a) or Table 310.15(B)(2)(b). Exception: Type XHHW-2 insulated conductors shall not be subject to this ampacity adjustment.
Voltage drop
Single Phase - VD = 2 x K x Q x I x L/CM Three Phase - VD = 1.732 x K x Q x I x L/CM "VD" = Volts Dropped: The voltage drop of the circuit conductors as expressed in volts.
(B) Feeders over 600 Volts.
The ampacity of conductors shall be in accordance with 310.15 (0-2000V) and 310.60 (>2001V) as applicable. Where installed, the size of the feeder-circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel. Feeder conductors over 600 volts shall be sized in accordance with 215.2(B)(1), (B)(2), or (B)(3).
215.2 Minimum Rating and Size. (3) Ampacity Relative to Service Conductors. (B) Feeders over 600 Volts.
The ampacity of conductors shall be in accordance with 310.15 (0-2000V) and 310.60 (>2001V) as applicable. Where installed, the size of the feeder-circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel. Feeder conductors over 600 volts shall be sized in accordance with 215.2(B)(1), (B)(2), or (B)(3).
(1) Feeders Supplying Transformers.
The ampacity of feeder conductors shall not be less than the sum of the nameplate ratings of the transformers supplied when only transformers are supplied.
(2) Feeders Supplying Transformers and Utilization Equipment.
The ampacity of feeders supplying a combination of transformers and utilization equipment shall not be less than the sum of the nameplate ratings of the transformers and 125 percent of the designed potential load of the utilization equipment that will be operated simultaneously.
Ampacity-
The current, in amperes, that a conductor can carry continuously without exceeding it's temperature rating. I = amperes = Intensity of current Ampacity (Amp) - depends on conductor's cross-sectional area, if conductor is copper or aluminum, and type of insulation around the conductor
Ampacity Relative to Service Conductors.
The feeder conductor ampacity shall not be less than that of the service conductors where the feeder conductors carry the total load supplied by service conductors with an ampacity of 55 amperes or less.
Cables
The size of a wire is usually expressed in wire gage, and the different sizes are referred to by gage numbers. Unfortunately several systems of gages have been originated by different manufactures for their products. However, it has become standard practice in the United States to employ the American Wire Gage (AWG), also known as the Brown and Sharpe (B&S), to designate copper and aluminum wire and cable used in the electrical industry. In most cases the larger the gage number, the smaller the size of the wire. (Americans Electricians Handbook, AEH, 2-3) Dimensions for the wire are given in ASTM B 258 and NFPA 70 Table 8. (ASTM American Section of the International Association for Testing Materials) NOTE: Have students find the "Conductor Properties" in their Ugly's (Pg. 71). Use the following question or your own to assess students comprehension of the table and the footnotes.
(2) Grounded Conductor.
The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel. Additional minimum sizes shall be as specified in 215.2(A)(3) under the conditions stipulated.
250.66 Size of Alternating-Current Grounding Electrode Conductor.
The size of the grounding electrode conductor at the service, at each building or structure where supplied by a feeder(s) or branch circuit(s), or at a separately derived system of a grounded or ungrounded ac system shall not be less than given in Table 250.66, except as permitted in 250.66(A) through (C). NOTE: You will have a class on grounding later in this module.
Thermoplastic insulation.
Thermoplastic counpounds have been developed for the insulation of electrical wires. Those meeting the specifications of the Underwriters Laboratories are designated as Type TW, THW, THWN, THHN, THHW, TBS, or MTW. Type TW, THW, or THWN is moisture-resistant and can be used in wet locations. 1) Type THW or THWN is both moisture- and heat-resistant and is approved for use in both wet and dry locations at a maximum conductor temperature of 75⁰C. 2) Type THHW is moisture- and heat-resistant and may be used in wet and dry locations at a maximum conductor temperature of 75⁰C in wet locations and 90⁰C in dry locations. 3) Type TBS is a thermoplastic insulation with a flame-retardant fibrous outer braid, and it is acceptable only for switchboard wiring. 4) Type THWN is a moisture- and heat-resistant insulation approved for both wet and dry locations at a maximum conductor temperature of 75⁰C. It has an outer nylon jacket. 5) Type THHN is similar to Type THWN except it is not moisture-resistant and may only be used in dry and damp locations at a maximum conductor temperature of 90⁰C. 6) Type MTW is moisture-, heat-, and oil-resistant and is restricted to machine-tool wiring at a maximum conductor temperature of 60⁰C in wet locations and 90⁰C in dry locations.
Thermoplastic cont
Thermosetting insulation. Four high-quality wire are Types XHH, XHHW, FEP, and FEPB. 7) Type XHHW is a moisture- and heat-resistant, crosslinked, thermosetting polyethylene with and insulation rating of 75⁰C in wet locations and 90⁰C in dry locations. 8) Type XHH has a thermoset insulation that is rated at 90⁰C in dry and damp locations. 9) FEB has a heat-resistant, fluorinated ethylene propylene insulation and is suitable for use in dry and damp locations at a maximum conductor temperature of 90⁰C 10) Type FEPB is suitable for dry locations at 200⁰C for special applications. (AEH 2-27) Other insulating material such as mineral insulation, paper insulation, as well as others can also be used and will be covered in Medium Voltage Cables.
Rubber insulation.
With respect to insulation, rubber is the word used to designate insulations consisting of compounds of natural rubber or synthetic rubber, or both, combined with such other ingredients as vulcanized agents, antioxidants, fillers, softeners, and pigments. These natural-rubber and synthetic rubberlike compounds are used more than any other material for the insulation of electrical conductors. They have the desirable characteristics of moisture resistance, ease of handling and termination, and extreme flexibility. On the other hand, the will not withstand such high temperatures or voltages without deterioration as will some other types of insulation. A large variety of rubber compounds are available, with different characteristics that depend upon the particular service conditions for which they have been developed.
Moisture-resistant rubber compounds
are available for installations in which the wire will be subjected to wet conditions. These compounds are called moisture-resistant or submarine compounds.
Moisture- and heat-resistant compounds
combine the temperature- and moisture-resistance characteristics of the Type RH and the Type RHW insulations. They are recognized by the Underwriters Laboratories designation Type RHW, which is approved for use in wet or dry locations at a maximum conductor temperature of 75⁰C. (AEH, 2-26)
Vulcanization
vulcanisation is a chemical process for converting natural rubber or related polymers into more durable materials by heating them with sulfur or other equivalent curatives or accelerators.
Heat-resisting rubber compounds
which will resist considerably higher temperatures that the Code-grade rubber compound have been developed. Such compouds meeting the Underwriters Laboratories specifications are designated Type RH or Type RHH insulation.
