How to Calculate Voltage Drop for Conductors

Good cable management is an important part of maintaining safe and effective power systems. Part of proper cable management is calculating voltage drop. By calculating voltage drop, you can determine the correct cable type for an electrical circuit or circuit design.

What is voltage drop?

Voltage is the force that drives the electrical current between two points. Wires that carry voltage resist that flow of current. Resistance increases in proportion to the length of the wire (or cable). Therefore, voltage is lost, or dropped, as it travels along the cable. Voltage drop can become a problem in long cable runs.

Why managing voltage drop is important

For equipment to run properly, there must be a consistent and correct amount of power supply. Equipment that runs on electricity is rated for power, which is measured in watts. Watts are calculated by multiplying amps x volts. If voltage drop happens while a tool, machine or generator is running, it can cause that equipment to malfunction or fail, or cause a hazardous situation due to an electrical short or fire. Voltage drop is especially risky with regard to modern electrical equipment that contains sensitive electronic components. These components are especially susceptible to damage due to voltage drop.

Understanding how to calculate voltage drop and using correct cable routing and connection procedures can ensure that equipment is supplied with the correct voltage for optimal efficiency.

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How to calculate voltage drop

For AC circuits, how much the voltage drops depends on the resistance of the wire or cable and the length of the run. Therefore, it is important to choose the correct cable type for the length of the run to prevent voltage drop. Here’s how to calculate voltage drop:

Vd = (1.73 x K x L x I)/Cm

where K=resistance in ohms of one circular mil (12.9 for copper)

L= length of cable run in feet

I=amperes

Cm=cross-sectional area of conductor in circular mils (211600 for 4/0 cable)

For a 300-foot long run of cable with a load of 300 amps, the calculation would look like this:

Vd = (1.73 x 12.9 ohms x 300 amps x 300 feet)/211600 = 9.492 volts

The application determines how critical the result will be. If a load-bank test is being performed, for example, the contractor would want to calculate how this impacts the equipment. A load bank rated for 600 kW at 480V will provide only 575 kW at 470 V, 10V less. If the contractor needs the full 600 kW for their test, they could use a larger load bank that provides 625 kW of load, which would derate to approximately 600 kW. 

Another option would be to add a conductor—three additional cables, one per phase—to decrease the voltage drop. In a generator application, the contractor has a little more leeway. The National Electric Code (NEC) allows up to 10 percent voltage drop in distribution systems, so most equipment is designed to handle voltage +/- 10 percent of the rated voltage. 

Sunbelt Rentals safety promise

Safety is Sunbelt Rentals’ primary priority. That’s why we provide durable cable designed never to fail and optimized for full carrying capacity. Are you using your own cables? Bring them to us to test and inspect. We’ll perform quality control, check every inch for cuts and scrapes, conduct low-voltage testing, clean the cable and re-roll it for you.

If you have any questions about the best procedures for cable routing and connections in an application, please contact a Sunbelt Rentals representative.

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