7.1.6 CALCULATION OF VOLTAGE DROP

7.1.6.1 DESIGN CONDITIONCurrentThe full load currents based on the loads upon application of relevant demand factor and those that are to be used in conductor sizing.

PFThe voltage drop verification of conductors will be based on an 85% system power factor the final values will be verified by the results of a load flow study.

Conductor Sizes and ImpedanceThe preliminary sizing will be based on conductor ampacity refer to section 7.1.4The voltage drop of conductor will be based on NEC conductor impedances which will be compared with IEEE Tables and manufaucturing catalogs for verification pruposes

DistancesThe location of utilization equipments will be selected according their proper application and from architecturally designated areas.

Power system components will located as near as architecturally allowable to the load center and at those approved by the utility company.

Proposed alternative locations that will minimize the excessive voltage drop will be provided evaluations. Voltage SpreadThe difference between the maximum and minimum voltages which appear at any location in asystem under any operating conditions.

Steady state Voltage Conditions:Percent of Nominal VoltageLow Voltage SystemsLigthing Outlets :91.7% -104.2% Other Utilization Equipment : 90% - 104.2%At Distribution Equipment Busses (panelboards,switchgears):95%-105%

High Voltage SystemsAt Distribution Equipment Busses (panelboards,switchgears):97.5%-105%At Utilization Equipment : 90% - 104.2%

Motor Starting Voltage ConditionsAt Utilization Equipment : 85% of Nominal VoltageAt Terminals of motor being started: Less than 85% Motor rated Voltage

Voltage Regulation MethodsRegulating Equipment:Tap settings or Ratio Selection At Transformer for primary distribution voltage drop for constant full load.Auto Tap ChangersVoltage Regulators Equipment

Impedance Reduction:Load Grouping and/or isolationReduction of feeder loads having long distance routesConductor Paralleling and Close Spacing

Locating High voltage or Primary Distribution near bulk loads.Load Centers

Power Factor Improvement

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IMPEDANCESCABLES:NEC : Table 8 & 9IEEE : Table 10,11 Std 242Cable Manufacturers Catalog

BUSWAYSIEEE : Table 12, Std 242, T59,pp290,std 241

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FORMULAApproximate :V = I*R*cos q + I*X*sin q

VL-L = 2*V for single phase volts

VL-L = 1.732*V for three phase volts

V = Voltage drop in circuit, line-to-neutral.I = Current flowing in conductor.R = Line resistance for one conductor, Ohms.X = Line reactance for one conductor, Ohms.q = Angle whose cosine is the load power factor.cos q = Load power factor, in decimals.sin q = Load reactive factor, in decimals.

Cable Units : Assuming 85% Power Factor, Negligible Inductance

V = K1 * K2 * I * L / CMIL

Where:I = Current in conductor, AmpsL = Length of one run, FtV = Line voltage drop

K1 = 0.866 Three phase = 2 Single phase = 1 3phase 4wire L-N voltage drop

K2 : Resistivity of conductor at 30degC, Cmil*Ohms / FtK2 = 12 For circuits loaded more than 50% of cable ampacity = 11 For circuits loaded less than 50% of cable ampacity = 18 For aluminum conductors

V = I * L * K3 * 1000

Where:I = Current in conductor, AmpsL = Length of one run, mV = Line voltage drop, voltsK3 = Catalog Constant for three phase 3 wire circuit,mV/A/m

%VD = V * 100/ Vrated

General Formulas:In terms Of Ur , Line to Neutral voltage drop :

Ud = sqrt( (Ur*cosq + I*R)^2 + (Ur*sinq + I*X)^2 ) – Ur

In terms of Us

Ud = Us + I*R*cosq + I*X*sinq - sqrt(Us^2-(I*Xcosq-I*R*sinq)^2 )

Ur = Us*(ZL/Zs)

Ud = Us - Ur

Ud * 2 = UdL-L single phase

Ud * 1.732 = UdL-L three phase

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Aproximate for Concentrated Load at end of conductor

Ud = I*R*cos q + I*X*sin q

Upr = S*( R*cos q + X*sin q )/ (10 * Uk^2)

Aproximate for distributed load on a line:

Upr = [S*( R*cos q + X*sin q )/ (10 * Uk^2)]* L*(1- La/(2*L))

where

Ud = Voltage drop, in volts.Upr = Voltage drop, in percent of voltage at sending end.Us = Line-to-neutral voltage at sending end, in volts.UR = Line-to-neutral voltage at receiving end, in volts.q = Angle whose cosine is the load power factor.R = Resistance of circuit, in ohms per phase.X = Reactance of circuit, in ohms per phase.I = Load current, in amperes.ZL = Load impedance, in ohms.ZS = Circuit impedance, in ohms, plus load impedance, in ohms, added vectorially S = Three-phase apparent power for three-phase circuits or single-phase apparent power for single-phasecircuits, in kilovoltamperes.Uk = Line-to-line voltage, in kilovolts.La = Distance from source to desired point, in feet.L = Total length of line, in feet.

SELECTION OF FEEDERS BY VOLTAGE DROP CRITERIAALLOWABLE PERCENTAGE VOLTAGE DROPNEC 210.19 FPN No.4:and 215.2 FPN no.2Conductors for branch circuits and feeders as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation.

SELECTION OF VOLTAGES

LOCATION OF LOADCENTERS

7.1.6.2 CALCULATION RESULTS

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