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AN ENGINEERING SELECTION FOR SIZE OF LONG CONDUCTORS CONSIDERING FACTORS IN ADDITION TO NEC

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. Abstract. Conductor Sizing Long conductor Voltage Drop Diameter, Length, # Phases

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AN ENGINEERING SELECTION FOR SIZE OF LONG CONDUCTORS CONSIDERING FACTORS IN ADDITION TO NEC

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

  2. Abstract • Conductor Sizing • Long conductor • Voltage Drop • Diameter, Length, # Phases • Current, Temperature

  3. Conductor SizingCommon Part of Electrical Design • NEC = insulation temperature • I => I2R losses => heat • For runs < 100 ft • For runs > 100 ft, voltage drop begins dominate

  4. Conductor SizingImpedance Zallowed = Vdropallowed / Irequired Its the basics

  5. 2 p d 2 2 A ( mil ) = = 0 . 7854 d 4 2 A ( cmil ) = d 2 mil = 0 . 7854 cmil ConductorSmall Size d = 10 mils A = 100 cmils = 78.54 mils2

  6. æ ö o 234 . 5 C + T ç ÷ r = 10 . 371 * 1 . 02 ç ÷ o 254 . 5 C è ø r = 10 . 786 W cmil / ft r = 17 . 35 W cmil / ft Conductor Resistance • Correct resistivity for temperature, mat’l, config. • Base = annealed copper @ 20o C • CU @ 25o C • AL @ 25o C

  7. Wire DimensionsFirst Pass - Resistance Only

  8. Wire DimensionsResistance - Phase # - Distance • 1 phase: ph factor = 1 # cond / ph = 2 • 3 phase: ph factor = # cond / ph = 1 ( ) ( ) r D * # cond phase factor * I A = V D

  9. Voltage DropEffect of Wire Dimensions & Distance Manipulate area calculation ( ) ( ) r D * # cond phase factor * I V = D A

  10. Wire DimensionsAC -vs- DC • Calculations are DC • Sizes < 4/0 DC AC • Larger conductors ZAC 1.3 * RDC • Total Z has inductive & capacitive components • Z = R + jX

  11. X = 2 p f L = 377 L L æ ö d æ ö - 7 L = 4 * 10 l 1 + 4 ln ç ÷ ç ÷ è ø r è ø AC ConsiderationsInductive • Wire dimensions reactance • Skin effect negligible @ power frequency • Divide by 2 for coax and transmission

  12. 2 p e l C = ( ) ln d / r AC Considerations Capacitive • Parallel, round conductors • Divide by 2 for 2 wires of single phase • Xc negligible < 2400 V • Yc very large for lines < 10 miles • Ignore for cable

  13. V D = Z = R + j X I 2 2 2 V r l d æ ö æ ö æ ö æ ö D - 7 = + 2 p f l * 13 . 816 * 10 ln ç ÷ ç ÷ ç ÷ ç ÷ è ø è ø è ø 2 è ø I 4 r r Voltage DropBased on Impedance • Parallel Conductors • Neglect Capacitance • Calculate VD straightforward

  14. LengthFrom Wire Dimensions - Voltage Drop • Contains ‘r’ and ln (r) • Can’t solve directly • Make assumptions • ‘d’ between conductors 3 * radius • Equations for VD and A are radicals

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

  16. SummaryWire Size - Conductor Properties • Considerresistance and inductance • Correctresistivity for mat’l, config, T • Calculate area from resistivity, length, • phase, current, V drop • Manipulate for voltage drop and distance • It’s Radical

  17. Questions ?

  18. Questions ?

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