EEM 105 Chapter 9Voltage Drop Across Conductors

*Voltage DropsVoltage drop is the loss of electrical pressure in a conductor due to its resistance.This can be observed when home lights dim when an appliance (washer, refrigerator, furnace) starts up.

*Voltage DropsResistance in a conductor is a factor of its length and its cross sectional area (in mils).Use resistance charts to find the rated resistance of a conductor .Resistance is generally given in an amount of resistance per foot (100 feet or 1000 feet).

*Voltage DropsThe total voltage applied to a motor can be found by using Ohms law to find the voltage drop and subtracting that from the source voltage.

*Voltage DropsIf a wire has a resistance of 0.308 ohms per thousand feet then the total resistance in a wire 584 feet long can be found:

584 feet X .000308 ohms/ft= 0.18 ohms

*Voltage AppliedThen the total voltage applied to a motor in a circuit where a generator supplies 180 volts will be:VM= EG - VD VM = 180 3.6VM = 176.4 Volts

*Voltage DropsTo determine the voltage drop:

VD = KIL

A

VD = permissible voltage drop, K = constant from chart, I = current in amps, L = length of circuit in feet, A = area in circular mils

*To Find the Wire Size.A = KIL

VD

VD = permissible voltage drop, K = constant from chart, I = current in amps, L = length of circuit in feet, A = area in circular mils

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*Voltage DropGenerally, voltage drop is not affected by the insulation on a conductor

*CalculatorThis web site will help you calculate voltage drops.http://www.stanselectric.com/vdrop.html

To find the proper wire size, one needs to know the type of conductor (copper or aluminum), the distance in feet, and the amperes that are needed for the load.

*How to Calculate Voltage DropsA primary concern when installing lengths of wire is voltage drop. The amount of voltage lost between the originating power supply and the device being powered can be significant. Improper selection of wire gauge can lead to an unacceptable voltage drop at the load end. The following chart is designed to help calculate voltage drop per 100 feet of paired wire as a function of wire gauge and load current.

*Voltage Drop Per 100 FT Run of Paired Wire

*Calculating Voltage DropsBy matching load current (in amps) across the top of the chart with wire gauge (AWG) down the left side of the chart, one can determine voltage drop per 100 feet of paired wire run.

NOTE: A paired wire run represents the feed and return line to the load. Therefore, a 500 foot wire pair is equivalent to 1000 feet of total wire.

*Calculating Voltage DropsEXAMPLE ONE:

Given a load current of 1 AMP, and using 18 AWG wire, how much voltage drop can we expect at the load end for a 350 foot run of paired wire?

*Using the chart, we match the row for 18 AWG and the column for 1 AMP and determine that voltage drop per 100 feet is 1.27 Volts.

*By dividing the paired wire length by 100, we get the factor by which we need to multiply voltage drop per 100 feet to determine total voltage drop. Therefore, 350 feet divided by 100 equals 3.5. Multiply 3.5 by 1.27 volts drop per 100 feet to get your total voltage drop. Thus the total voltage drop is 3.5 times 1.27, or 4.445 voltage drop for 350 feet.

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*Good Web Sitehttp://www.electrician.com/slide_show/blackbd.htm

*The End!!!!