Voltage Drop Calculator

Unique Menus

• Edit
• Pass to Size Selection: Set related items on the Size Selection Calculator to the values of their counterparts here.
• Options
• Custom Materials: Opens a form where custom conductor materials can be edited
• Custom Conductor Sizes: Opens a form where custom conductor sizes can be edited

Menu Buttons

• Size Units/Specification
• AWG: Size pop-up menu will use American Wire Gauge sizes
• Metric: Size pop-up menu will use metric sizes
• Custom: Size pop-up menu will use user-specified custom sizes (if any have been defined)

Other

• Info Buttons
• Material: Displays the specific resistance (rho value) and temperature coeffecient (alpha value) for the selected material
• Size: Displays the size of selected conductor in circular mils (cmils)
• Other Buttons
• DC Res: Displays the DC resistance of the conductor as specified by the supplied user input

Finding voltage drop Vd, percentage of drop, and voltage at load V @ Load

• Write in the amount of current I
• Use the Phases pop-up to select the number of AC phases or a DC circuit
• Select the conductor material
• Select the conductor size and select number of parallel conductors with the pop-up
• Write in the one way length of the circuit, selecting the unit of measure with the pop-up
• Use the Amb Temp pop-up to select the maximum ambient temperature
• OPTIONAL: Write in source voltage V_s

   When all values are entered tap the Calc button to display the voltage drop in the field at the top of the form. If you have also entered the source voltage Vs then voltage at load V @ Load and the percentage of voltage drop will be displayed at the top of the form as well.

A couple of users have noted that Electrist uses a 10.09 ohm/cmil-ft specific resistance for copper and have inquired why this is the case when they are more accustomed to seeing a "K value" of 12.9.    The familiar 12.9 "K value" represents the direct current resistance for a one thousand circular mils conductor that is one thousand feet long, at an operating temperature of 75º C    When referring to specific resistance the unit is ohm/cmil-ft and is generally given at an ambient temperature of 20º C and will use a temperature coeffecient to adjust for temperatures other than that.    If you were to convert the Electrist rho and alpha values of 10.09 ohm/cmil-ft at 20º C to 1000 foot of 1000 cmil wire at 75º C it comes quite close to the 12.9 "K value" that is used in some forms of voltage drop calculations. You can test this very quickly by selecting 20 AWG (1020 cmils), entering a length of 1000 feet, and an ambient temperature of 75º C. Tapping the DC Resistance menu button will show you that this calculates to 12.09 ohms. If you wish an even closer test you can try the same calculation using a custom conductor size of 1000 cmil which will yield a result of 12.33 ohms for the same conditions.    I would attribute the 0.57 ohm difference of these 2 values in these conditions as most likely a product of rounding error and the lack of any temperature compensation in the "K value", and I further maintain that the specific resistance used in Electrist is the more accurate.    If you're absolutely determined to use the 12.9 "K value" it is a trivial matter to configure a custom material - you can call it "Copper 12.9" or "My Copper" - with a specific resistance of 12.9 and a temperature coeffecient of 0.00000001.

Finding DC resistance of conductor

• Select the conductor material
• Select the conductor size
• Write in the one way length of the circuit, selecting the unit of measure with the pop-up

   When all values are entered tap the DC Res button to display the resistance of the conductor in a pop-up form.



Custom Materials

To create a custom material: Select Custom Materials from the Options menu to open the list of available custom materials. Tap the New button. Enter the name of your new custom material for example: "Nichrome®" Enter the specific resistance of the material (aka 'rho'). The unit of measure for rho is Ω-cmil/ft (ohms-cmil/ft):the resistance in ohms of a one foot length of the material with a diameter of 1 circular mil at 20 degrees celsius (actually it is also quite common to see rho expressed in the SI/metric units Ω-m, Ω-cm, µΩ-m, and µΩ-cm. See table below for conversion to Ω-cmil/ft) for example: the rho value of Nichrome® is 675 ohm-cmil/ft Next enter the temperature coefficient of the material at 20 degrees celsius (aka 'alpha') for example: the alpha value of Nichrome® is 0.00017 Click the Done button The material you just entered will now be available in the Material pop-up of the Voltage Drop calculator
RHO AND ALPHA VALUES OF CONDUCTORS  These values are presented as examples only and are not to be considered definitive or absolute Material Rho Alpha Ω-cmil/ft   Material Rho Alpha Ω-cmil/ft Silver 9.546 0.003819 Nickel 41.69 0.005866 Copper 10.09 0.004041 Lithium 55.88 0.00435 Gold 13.32 0.003715 Iron 57.81 0.005671 Aluminum 15.94 0.004308 Platinum 63.16 0.003729 Beryllium 19.85 0.0090 Phosphor Bronze [Cu94/Sn6] 66.17 0.0006 Calcium 22.26 0.00457 Lead 123.92 0.0042 Magnesium 25.26 0.00425 Uranium 162.41 0.0034 Rhodium 28.27 0.0044 Nickel Silver [Cu62/Ni18/Zn20] 174.44 0.0003 Sodium 29.48 0.0055 Constantan® [Cu55/Ni45] 312.80 0.00003 Iridium 30.67 0.0045 Manganin® [Cu86/Mn12/Ni2] 290 0.000015 Cobalt 38.14 0.0066 Titanium 324.83 0.0038 Cadmium 43.91 0.0043 Neodymium 384.98 0.00164 Tungsten 31.76 0.004403 Mercury 576.87 0.001 Molybdenum 32.12 0.004579 Nichrome V® [Ni80/Cr20] 650 0.00011 Zinc 35.49 0.003847 Nichrome® 675 0.00017 CONVERSION FACTORS To Convert Multiply By To Get Ω-m 601529086.939 Ω-cmil/ft µΩ-m 601529.086939 Ω-cmil/ft Ω-cm 6015.29086939 Ω-cmil/ft µΩ-cm 6.01529086939 Ω-cmil/ft

Custom Conductor Sizes

To create a custom Conductor Size: Select Custom Conductor Sizes from the Options menu to open the list of available custom conductor sizes. Tap the New button. Enter the name of your new custom size for example: "8 mil by 0.7 mil PCB trace" Enter the cross-sectional area of the new size in circular mils (aka 'circular mil area' or 'CMA'). for example: the CMA of an 8 mil by 0.7 mil conductor would be 71.301384 cmils (see below for information on this conversion) Click the Done button To use the conductor size you just entered select Custom from the pop-up at the top of the Voltage Drop calculator after which it will be available in the Size pop-up below.
Converting Measurements: To convert the cross-sectional area of a rectangular conductor from square mils to CMA (circular mil area) just multiply by 1.273239 If you have a measurement of the cross-sectional area in a unit other than cmils, such as square inches or square mm, you can convert to cmils with a single factor. (See area conversion factors listed below) Similarly, you can convert the cross-sectional area of any rectangular conductor into circular mils by: Converting it's width and thickness measurements into mils (See linear conversion factors listed below) Multiplying these converted measurements to get the cross-sectional area in square mils Multiplying this result by 1.273239 to get the CMA You can also convert the measurement of any cylindrically-shaped conductors diameter into circular mils by: Converting the measurement into mils (See linear conversion factors listed below) Square this result to get the CMA Some Examples: For the example above of a PCB trace that is 8 mils wide and 0.7 mils thick: Find the cross-sectional area in square mils: 8 * 0.7 = 56 square mils Convert the square mils to circular mils: 56 * 1.273239 = 71.301384 CMA A bus bar that is 2 inches wide and 3/4 inch thick: Find the cross-sectional area in square inches: 2 * 0.75 = 1.5 square inches Convert the square inches to circular mils: 1.5 * 1273239 = 1909858.5 CMA - OR - Convert the linear measurements from inches to mils: 2 inches = 2000 mils, 0.75 inch = 750 mils Find the cross-sectional area in square mils: 2000 * 750 = 1500000 square mils Convert the square mils to circular mils: 1500000 * 1.273239 = 1909858.5 CMA A heater band that is 10 mm wide and 0.5 mm thick: Find the cross-sectional area in square mm: 10 * 0.5 = 5 square mm Convert the square mm to circular mils: 5 * 1973.525 = 9867.625 CMA - OR - Convert the linear measurements from mm to mils: 10 mm = 393.7008 mils, 0.5 mm = 19.685 mils Find the cross-sectional area in square mils: 393.7008 * 19.685 = 7750.0157 square mils Convert the square mils to circular mils: 7750.0157 * 1.273239 = 9867.622 CMA CONVERSION FACTORS AREA   LINEAR To Convert Multiply By To Get To Convert Multiply By To Get square mils 1.273239 circular mils       square inches 1273239 circular mils inches 1000 mils square cm 197352.5 circular mils cm 393.70078740157 mils square mm 1973.525 circular mils mm 39.370078740157 mils       microns 0.0393700787402 mils       nm 0.0000393700787 mils       furlongs 7920000 mils