1. EET 323 – Electrical System Design Lecture 6: Conductors and Over-Current Protection
Radian Belu, PhD

2. Lecture Objectives

3. Conductor Sizes and Types
Conductors used in electrical power distribution cables are made of copper or aluminum. Aluminum alloy is permissible only in the larger sizes, provided it is of an approved alloy.
Conductor sizes are specified in terms of American Gauge Wire (AWG) or thousand circular mils (kcmil). The AGW designation for conductors ranges from #14 up through #4/10, while the kcmil designation is used from sizes from 250 up to 2000 kcmil.
For power distribution circuits including feeders and branch circuits, wire sizes in the range of #14 AWG to 500 kcmil are the most commonly specified. Wire sizes larger than 500 kcmil are frequently avoided due to difficulty with installation
Breaker and fuse-holder terminal lugs rarely have provisions for larger than 750 kcmil conductor sizes. Solid conductors are used in sizes up to #10 AWG. Standard conductors are generally used for #8 AWG and larger to ease installation. The actual number of strands varies according to conductor size, with common values of 7, 19, 37, and 61 strands

4. Conductor Sizes and Types

6. Conductor Resistance
The resistance of a conductor has an effect ob the current-carrying capability of the conductor. Higher resistance implies higher power loss , higher conductor temperature, and larger voltage drop.

10. Cable Construction and Insulation Types
The conductors, made usually from copper or aluminum (solid or stranded, depending on the size and the amount of flexibility) are packaged in several ways to from electric power cables
The most common type of cable construction for low-voltage (600 V) power is the single-conductor cable covered with a single layer insulation (Figure 6-1 (A)) or by an outer nylon jacket (Figure 6-1 (B))
These cables are typically installed in conduit or other suitable raceway systems. The thickness of the of the insulating material is generally determined by the voltage rating of the cable
Common voltage classes for cables are 600 V, 2 kV. 5 kV, 15 kV, 25 kV, and 35 kV. Various insulation thicknesses are permitted with a give class

14. Cable Construction and Insulation Types
Type MC (metal-clad) cable (Figure 6-3(B)) consists of individual insulated conductors grouped together with one or more grounding conductors and enclosed in an outer sheath
Other variations include cable containing both power and control conductors in the same sheath and cable containing power conductors and fiber-optic cables in the same sheath. Fillers are used between the individual conductors to keep the conductors in place within the cable.
Article 330 of the NEC permits type of MC cable to be installed indoors and outdoors in wet or dry locations, as open runs of the cable supported at intervals not exceeding 6 feet, in cable try, in an approved raceway or conduit, or directly buried where listed for direct burial

19. Cable Construction and Insulation Types
Service entrance cables are typically identified as SE or USE; two variations of type SE cable include type SEU and SER, shown in Figure 6-5
The three-conductor configuration will typically consist of two insulated phase conductors and an insulated neutral. The four-conductor configuration consists of two insulated phase conductors, an insulated neutral, and a bare equipment grounding conductor
Type USE cable is designed to be installed underground, either in conduit or directly buried. Article 338 of the NEC describes type SE and USE cables. Article 230 of the NEC discusses service entrance requirements

22. Insulation Types
The application often dictates what type of cable insulation is required. Factors such as temperature, wet or dry location, exposure to sunlight, and so on all influence the type of insulation required
The environment in which a cable is applied has an effect on the life of the insulation
There are two major types of insulation used for building wire: thermoplastic (which melts at temperature higher than rated) and thermoset (not melts at temperature higher than rated, but deteriorate at higher rate). Polyethylene (PE) and polyvinylchloride (PVC) are examples of thermoplastic insulation, and cross-linked polyethylene (XPLE) and rubber insulations are examples of thermoset insulations

25. Conductor Ampacities
One of the limiting factor in determining the conductor ampacity is the current flow required to bring the conductor up to a certain temperature (higher current means higher power loss => higher temperature). Different type designations on insulation reflect the ability of the insulation to withstand various temperature, for example 60 C, 70 C, or 90 C
The heat generated internally as power loss in the conductor must be dissipated in order for the temperature to reach an equilibrium condition, and is affected by the ambient temperature in which the conductor is located and the proximity of other current-carrying conductors in the same raceway
Tables and of the NEC are the most widely used tables for determining the conductor ampacities

28. Derating Based on Ambient Temperature
The conductor ampacity is determined based on a 30 C. In some applications the ambient temperature may be higher, and in such cases o temperature correction must be applied to the table values. The correction factor is a function of conductor insulation and the ambient temperature

30. Derating Based on Number of Current-Carrying Conductors
A large number of current-carrying conductors in the same raceway restrict heat dissipation (see Table ). This condition applies to most 3-phase balanced circuits, with load current flowing in ungrounded conductors
If there are than three current-carrying conductors in the raceway, a filling adjustment factor must be applied, which similar to the temperature correction factor, the fill adjustment factor reduces the table listed ampacity of the conductor (table 6-4)

34. Derating Based on Number of Current-Carrying Conductors
The number of current-carrying conductors in a raceway is determined by the ungrounded (phase) conductors, plus any grounded (neutral) conductor that is considered to be current carrying current under normal conditions. The neutral conductor of a 3-phase, 4-wire, wye-connected circuit supplying linear loads will carry the unbalanced current in the circuit and need not to be considered also as a current-carrying conductor
Neutral conductors considered to be current-carrying conductors are identified in Section 310/15(B) of the NEC (Figure 6-7)

38. Temperature Limitations on Device Terminals
The conductor temperature is function on the current magnitude, the ambient temperature, and the raceway fill. The derating procedures are both needed due to restriction of heat dissipation when conductors are placed in elevated temperature or in close proximity to other current-carrying conductors (Table and of the NEC listed the allowable ampacities of various conductor sizes for the various insulation temperature ratings
Branch circuits and feeder conductors are typically terminated on the supply side by connection to a circuit breaker terminal lug or fuse-block terminal. On a load side, the conductors are typically terminated on a device terminal such as a screw terminal of a receptacle or switch
The heat produced by the conductor is transmitted to the device terminal causing the heating of it, until an equilibrium is established.

40. Temperature Limitations on Device Terminals

41. A 3-phase, nonlinear load is to be supplied in 35 C ambient
A 3-phase, nonlinear load is to be supplied in 35 C ambient. Determine the maximum allowable ampacity for the following conductors: a) #6 THW copper (75 C rating), and b) #6 THW-2 copper (90 C rating)

42. Conductor and Over-current Protection Device (OCPD) Selection
Continuous and Non-continuous Loads
To determine the minimum size and OCPD required for a branch circuit or feeder to supply a load, the loading on the circuit (in A) need to be determine. The load on a circuit is divided as continuous and non-continuous loads.
Article of 100 of the NEC defines a continuous load as a load that is energized for three or more hours. Non-continuous loads are general-purpose receptacle outlets, residential lightning outlets, and os

62. Parallel Conductors

63. Parallel Conductors
The rules for parallel conductor installation are specified in Section of the NEC, and is designed to produce even division of current in the conductors, while generally requiring the same insulation type, temperature rating, conductor material, size, length, and the same conductor termination
The raceway or conduit system must be the same for each set of conductors, and the conductors must be #1/0 or larger

65. Parallel Conductors
The parallel conductors must be grouped to minimize the effect of inductive heating (the result of an AC magnetic flux induced in the wall of the conduit). The alternating flux produces hysteresis and eddy current losses in the conduit wall (similar to transformer core), which are dissipated as heat
To minimize the inductive heating, the AC magnetic flux must be cancelled or reduced (as shown in Figure 6-9(B))
For a 3-phase circuits, the conductors must be arranged as shown in Figure 6-9(C). The conduit will contain a phase conductor from each phase and a neutral conductor, and is assumed that the currents are balanced, having the same magnitude and 120 phase displacement. Any unbalanced current is returned to the neutral, and the net result is that the magnetic flux is again canceled out, minimizing inductive heating in the conduit (Figure 6-10)

68. Parallel Conductors
The ampacity of the feeder conductors supplying a parallel circuit is summation of the individual currents. The total load current to be supplied is divided by the number of anticipated parallel conductors pr phase.
The ambient temperature correction factors and raceway fill adjustment factors are applied as required
Device terminal temperature limitations are based on the size of each conductor per pahse

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