AN ENGINEERING SELECTION FOR SIZE OF LONG CONDUCTORS CONSIDERING FACTORS IN ADDITION TO NEC Marcus O. Durham THEWAY Corp / U of Tulsa Robert A. Durham Radco Consulting Marcus O. Durham THEWAY Corp / U of Tulsa Robert A. Durham Radco Consulting

Abstract u Conductor Sizing –Long conductor –Voltage Drop –Diameter, Length, # Phases –Current, Temperature

Conductor Sizing Common Part of Electrical Design u NEC = insulation temperature u I => I 2 R losses => heat u For runs < 100 ft u For runs > 100 ft, voltage drop begins dominate

Conductor Sizing Impedance Z allowed = V drop allowed / I required Its the basics

Conductor Small Size Amil d d( )   Acmild()  2 milcmil . d = 10 mils A = 100 cmils = mils 2

Conductor Resistance u Correct resistivity for temperature, mat’l, config. u Base = annealed 20 o C u 25 o C u 25 o C           *... o o CT C  cmil/ ft   cmil/ ft 

Wire Dimensions First Pass - Resistance Only

Wire Dimensions Resistance - Phase # - Distance u 1 phase: ph factor = 1# cond / ph = 2 u 3 phase: ph factor = # cond / ph = 1  A DcondphasefactorI V D   *#*

Voltage Drop Effect of Wire Dimensions & Distance  V DcondphasefactorI A D   *#* Manipulate area calculation

Wire Dimensions AC -vs- DC u Calculations are DC u Sizes < 4/0 DC AC u Larger conductors Z AC 1.3 * R DC u Total Z has inductive & capacitive components u Z = R + jX

AC Considerations Inductive u Wire dimensions reactance u Skin effect power frequency u Divide by 2 for coax and transmission XfLL L  2377  L d r               *ln l

AC Considerations Capacitive u Parallel, round conductors u Divide by 2 for 2 wires of single phase u X c negligible < 2400 V u Y c very large for lines < 10 miles u Ignore for cable   C dr  2  l ln/

Voltage Drop Based on Impedance u Parallel Conductors u Neglect Capacitance u Calculate V D straightforward V I ZRjX D  V Ir f d r D                              l l*.*ln

Length From Wire Dimensions - Voltage Drop u Contains ‘r’ and ln (r) u Can’t solve directly u Make assumptions u ‘d’ between conductors 3 * radius u Equations for V D and A are radicals

Comparison Conventional -vs- Dimensions u For single load, > 100’ u Determine size based on I & T, using NEC u Calculate size based on V & Z, using technique u Select size that is largest for I or V

Summary Wire Size - Conductor Properties u Consider resistance and inductance u Correct resistivity for mat’l, config, T u Calculate area from resistivity, length, phase, current, V drop u Manipulate for voltage drop and distance It’s Radical

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