Full Product Manual

Document: September1, 2009 Page 4of 10

Measured Voltage Drop Method

Before we get into the standardization method of a branch circuit it would be good to go

over the difference of Voltage drop {VD} verses % of Voltage Drop {%VD}. In a branch

circuit there is a Current* Resistance {IR} voltage drop across the wire. This is due to the

resistance of each conductor as a function of wire size, length, and device connections.

For most branch circuits the resistance or impedance stays constant while the current

varies. The higher the current the greater the voltage drops. As an example to perform a

voltage drop test and check the % of voltage drop you would need to perform the

following test in the field.

Test equipment needed:

• True RMS Digital multimeter with ACV accuracy of 0.25% and ACC measurement

accuracy of 0.5% or better

• 8 to 12 Ampere, 1500W load with accuracy of .25% or better

Measurements:

• Measure the Branch circuit voltage at no load

• Measure the Branch circuit voltage and current under load

o The current measurement is very critical use the best method possible

o The VD and %VD below would be for the load current measurement.

Calculate Voltage drop for any load we would use the following

 Vd= rZ * A

 %Vd=(Vd/V)*100

For the following example the no load Voltage is 120V, loaded Voltage 115 with an 8

amp load.

V = Voltage no-load

V = Voltage with load

VD = Voltage no-load - Voltage w-Load

I = Load Current

rZ = Total circuit Impedance

Total circuit Impedance is:

• rZ =(V(no-load)-

V (w-load))/I

• rZ =(120-115)/8

• rZ =5/8

• rZ =.625