The Value of Calculating Voltage Drop

Posted on 1/16/18 2:05 PM

Determining if the wiring in a power cord or cord set will provide acceptable efficiency is important when considering the length of that power cord or cord set. Long cord assemblies may have potential safety risks due to voltage drop incurring because of that length.

Definition of Voltage Drop

Wires carrying current always have inherent resistance, or impedance, to current flow. Power cord or cord set voltage drop is defined as the amount of voltage loss that occurs between the cord’s connection to a power source and the equipment it is supplying the power to on the opposite end.

Voltage Drop Needs to be Determined on Long Cord Assemblies

For long cord assemblies (over 50 feet), it’s important to calculate voltage drop due to potential safety risks. Reasons include: loss of power to equipment, potential damage to cords and wires, and safety issues.

Loss of Power to Equipment

“The loss of power to equipment reduces the voltage and overall power available to the equipment,” said Dan Ford, Technical Support Specialist at Interpower. “If too much voltage is lost, the equipment may not function correctly or may not work at all. For some equipment, such as devices that employ compressors, large motors, or pumps, even a small loss of voltage may cause a problem.”

Potential Damage to Cords and Wires

The occurrence of the voltage drop also generates heat. Too much heat build-up in a wire or cable can damage insulation. “The type of damage can range from a degradation (breakdown) of the insulation material, to a softening of the material, which could result in tearing if the cable or wire is bent or moved, to outright melting of the insulation and exposure of the conductor wire(s) beneath,” Dan explained. “Regardless, this reduces the life of the wire or cable and can lead to other safety issues. Excessive heat build-up can also degrade the conductor material and result in an increase in resistance, which compounds the issue, causing more voltage drop and more heat.”

Safety Issues of Shock and Fire

Two of the safety issues include shock and fire. “If the insulation is damaged to the point it tears or melts away exposing conductor wires, a user can inadvertently touch the exposed area and receive a shock,” Dan said. “As voltage drop increases, so does the heat build-up. Too much heat can cause the weakening of the material to the point where arcing between conductors could occur, further increasing the heat, until a fire is ignited with the insulation and/or adjacent combustible material (e.g. carpet).”

Regulatory Information

Besides safety issues on long cord assemblies, there are also standards and other regulatory information that need to be followed.

NEC Rule 210.19

This requires no more than a 5% voltage drop at the outlet.
CSA requires no more than 5% from service entrance to point of use.

UL 817

This requires cord sets to be derated after 50 feet and 100 feet to minimize issues with over-heating due to voltage drop. Some specialized cord assemblies also have length restrictions in place.

Other Country Requirements

Some countries, such as the UK (BS 7671), also have requirements in place. Therefore, it is wise to check with safety agencies if planning to send equipment with long cords to another country.

Interpower’s Policy in Calculating Voltage Drop

“Interpower’s policy is to calculate voltage drop for requested long cord assemblies and notify the customer of any potential issues if the voltage drop could be over 5%. In these cases, we will let the customer know our findings which are calculated at a full rated load and ask for details regarding their application’s voltage and current usage,” Dan explained. “We will then recalculate. If there is still an issue, we can assist the customer in trying to decrease the voltage drop by suggesting alternate, larger cable sizes, where possible. Increasing cable size or decreasing length are the only two options for decreasing voltage drop. In cases where the voltage drop is too severe—in excess of 8%—Interpower reserves the right to refuse manufacture of such items due to safety concerns.”

Calculating Voltage Drop


Dan gave two ways for someone to calculate voltage drop—manual calculation and with a voltage drop calculator.

Manual Calculation

In order to calculate voltage drop, one must first obtain the expected direct current resistance from the applicable standard or measure the resistance through the length of cable in question. Standards supply this information in ohms/km and sometimes ohms/1000ft. If taking a value from the standards, the value given must be converted/reduced to ohms/ft. For North American cable, Tables 5 and 6 in UL 62 should be used to obtain the value. For international cable types, such as H05VV-F, IEC 60228 Table 1 should be used. These numbers will not give exact voltage drop numbers, but will be close enough to identify potential issues.

Once the ohms/ft of a cable size is known, one must multiply this value by the length of the assembly in feet. This gives the total expected resistance for that assembly. Then it is simply a matter of applying Ohm’s Law: multiply the resistance value of the assembly by the steady state current to get the voltage drop (if the current is going to fluctuate a lot based on parts of the application shutting on and off, use the highest value). One can then use this value, with the mains voltage value, to determine the remaining voltage and percentage loss, if desired.

Voltage Drop Calculator

An easier way is to go online and search for a “voltage drop calculator.” Supply the details requested and it will automatically calculate the voltage drop, percentage loss, and remaining voltage. Be warned, however, that most of these are set up for North American AWG cable. For international sizes use the closest corresponding AWG size. This can be obtained by looking at UL 62 or contacting a cable manufacturer for assistance. An example of a good voltage drop calculator is the Voltage Drop Calculator on
This one also provides a cable chart on the same page that can be helpful to the user as well.

Additional Resources

Interpower offers free technical support. For further assistance, please see Interpower’s contact information below.

Topics: voltage drop, cord set, product design, power cord

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