How to Calculate Volt Drop in Copper Wire

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Although copper metal is considered an excellent conductor, it still has a slight amount of electrical resistance. When electricity travels from a power source to an appliance, some of the initial voltage is lost as the energy passes through the copper wire en route. Known as "voltage drop," this small portion of drained energy depends not only on the length and width of the wire, but on the amperage of the current and the nature of the power source, i.e. a single-phase system vs. a three-phase system.

Single-Phase System

  • Measure the length of the wire (in feet).

  • Calculate the cross-section area of the wire in circular mils (Cm). (See "Resources" for a link to a chart containing the cross-section areas of all the standard America Wire Gauge (AWG) sizes.)

  • Multiply the current or "load" of the circuit (in amps) by the length of the wire (from Step 1).

  • Multiply the result by 2.

  • Multiply the result by 12.9, which is the voltage-drop constant for copper (expressed in units of ohms per mil foot).

  • Divide the result by the cross-sectional area of the wire (from Step 2).

  • Multiply the result by 100 percent. This value is the voltage drop (expressed as a percentage of total voltage). For example, if the voltage drop on a wire is 10 percent, it means that only 90 percent of the power source's energy will be transmitted at the other end of the wire. To determine the actual amount of voltage lost, multiply the decimal form of the voltage drop (the result from Step 6) by the total input voltage.

Three-Phase System

  • Perform Steps 1, 2 and 3 as with the one-phase system. You will again have two figures: the cross-section area, and the load multiplied by the length of the wire.

  • Multiply the load result by 1.73.

  • Multiply that result by 12.9, which is the voltage-drop constant for copper.

  • Divide the result by the cross-sectional area of the wire.

  • Multiply the result by 100 percent to determine the voltage drop. To determine the actual amount of voltage lost, multiply the decimal form of the voltage drop by the total input voltage.

Tips & Warnings

  • The formulas for voltage drop involve variables that are calibrated specifically to certain U.S. measurement units. Therefore, always measure in the units specified.
  • One Cm unit is the area of a circle one-thousandth (0.001) of an inch wide.

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  • Photo Credit Zedcor Wholly Owned/PhotoObjects.net/Getty Images
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