How Can we calculate the rating of a cable?Show the formula?
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Answer Posted / prof. nazar m. karyar
VD = 2 x K x Q x I x D/CM - Single Phase
VD = 1.732 x K x Q x I x D/CM - Three Phase
“VD” = Volts Dropped: The voltage drop of the circuit
conductors as expressed in volts.
“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 ohms and 21.2
ohms is used for aluminum conductors. The “K” constant is
suitable for alternating current circuits, where the
conductors do not exceed No. 1/0.
“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 2. Feeders – This FPN
recommends that feeder conductors be sized to prevent a
maximum voltage drop of 3%. The maximum total voltage drop
for a combination of both branch circuit and feeder should
not exceed 5%. [215-2(d) FPN No. 2],
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