CORRECTION RE-ISSUE: Anatomy of a Conductor Part 3: Voltage Levels and Base Current Value
This issue continues discussion on the anatomy of a conductor. It focuses on voltage levels and base current value.
Base Current Value
In Part 1, we discussed the different types of conductors in relation to their location in the building's power distribution system and the service provided. Anatomy of a Conductor, Part 1 The types are service conductors, service entrance conductors, feeder conductors, branch circuit conductors, ground electrode conductor and neutral conductor. The factors to consider in meeting the NEC's detailed but confusing requirements were also listed. Then in Part 2, we discussed sizing the conductor. Anatomy of a Conductor Part 2
Back to factors, those for consideration by the electrical professional can be organized into three categories: (1) Primary Factors, (2) Secondary Factors and (3) Ancillary Factors.
Primary Factors are those used to determine load as "seen" by the conductor: (1) system voltage, (2) system phase, (3) equipment # of poles, (4) equipment utilization voltage and (5) total electrical loads.
Secondary Factors are those that impact the size and type of conductor as referenced in the NEC. These include: (1) temperature limitations, (2) temperature correction factor, (3) adjustment factor, (4) equipment grounding conductor, (5) load type (receptacle, lighting, motor and welding equipment), (6) conductor insulation (TW, THW, THHN and others), (7) ground electrode conductor and (8) material (CU or AL).
Last, there are the Ancillary Factors that support the Secondary Factors. These are design driven and considered in relation to installation. They include: (1) voltage drop, (2) conductors in parallel, (3) tap rule, (4) current carrying conductor, (5) demand load calculation and (6) amperage interrupting capacity (AIC).
The outcome of applying the Primary Factors is the base current value in AMPS. The designer based on design requirements and location, then applies the Secondary Factors to obtain conductor size and the associated overcurrent protection device (OCPD). Once these 2 basic values are determined, then the designer finally applies the Ancillary Factors to complete the design and specify the correct wire size and insulation rating.
Voltage Levels
The Primary Factors are applied as part of the following basic engineering equation for power to determine the base current value (l).
P = l x V x √3 for a 3 Φ distribution system
P + l x V for a single Φ - 1 pole and single Φ - 2 pole distribution system
Where P stands for Power (VA); V stands for Volt (V); l stands for Current (A)
Any electrical circuit, in its simplest form, consists of two wires which carry electrical current at a voltage level (electrical potential). The motion of the free electrons (charges) in a given electrical potential in a solid conductor constitutes an electric current (l). Said electric current (l) travels at the speed of light (186,000 miles/second). The unit of electric current is Ampere (Amps). One Ampere of current = 6.251 x 10 to the power of 18 electrons pass given cross section in 1 sec.
The higher the voltage, the higher the current flow for a given resistance. The flow of current (l) in an electric circuit is impeded by the resistance (R) [R = V / l] and produces heat (wasted energy). This wasted energy can be calculated by the following equation: P = l² R.
It is because of this equation, one should properly size the conductor to minimize wasted energy.
Distribution systems in a building are classified according to the voltage level used to transfer the power needed to operate its equipment. The most common distribution system voltage levels that are widely used in the United States are:
480Y / 277 V 3 Φ; 208Y / 120 V - 3 Φ; 230 V closed Δ, 3 Φ; 230 V open Δ; 230 V - 2 poles, 1 Φ; 120 V - 1 pole
In summary, the voltage, the phase, equipment # of poles, equipment utilization voltage and total electrical loads determine the base current value.
This discussion about the complex requirements of the conductor will continue in future newsletters.
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