Voltage Drop & Cord Length Calculation

 While Configuring Your Electrical Designs, Remember Cord Length & Voltage Drop

Using longer cord lengths in your electrical design may become problematic enough to pose potential dangers. When cable length is 50 feet or longer, voltage drop occurs—the resistance in the copper measured per foot—causing heat buildup. Though the maximum length per UL is 15 feet for hospital extension cord sets (NEMA to NEMA), there is no definitive lengths for hospital-grade power supply cords and cord sets in agency standards. Longer cords typically result in the degradation of voltage, and when plugging into an accessory power strip that is almost certainly the case.

Voltage pushes electrons through a cable like water rushes through a hose, both cable and hose offering resistance, i.e., its thickness and diameter helping impede the flow. The impedance of Alternating Current (AC) has a two-dimensional quantity creating both resistance and reactance, the latter due to an electric field resulting from changing current—the “pushed” current must be greater than the amount of resistance. With cord length at or beyond 50 feet, the heat build-up due to voltage drop can melt cords. UL testing confirms this, and thus derates current rating for cords of 50 feet or longer per UL 817. Deration is also addressed in NEC Rule 210.19 (Minimum Ampacity and Size—Branch Circuits Not More Than 600 Volts). By exceeding the 50-foot cord length, you could create a potential fire hazard.

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Voltage Drop and Cord Length Calculation

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“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.” Also prudent would be to check the standards and regulations of the countries you’re exporting equipment to, and the safety agencies representing those countries.

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 those cases,” Ford said, “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. 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.” Voltage drops may cause lights to flicker or appliances to overheat—the load works harder with less voltage pushing it through. As a general rule, voltage drop should never exceed 5%. This is done by selecting the right size of wire, and by taking care in the use of extension cords and similar components. Heat buildup in a wire or cable can damage insulation.

“The type of damage can range from a breakdown of the insulation material to a softening of the material,” Ford said, “which could result in tearing if the cable or wire is bent or moved, or outright melting of the insulation and exposure of the conductor wire beneath. Regardless, this reduces the life of the wire or cable and can lead to additional 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.”

Using Manual Calculation or the Voltage Drop Calculator

The easiest way to calculate voltage drop is to use one of the many online calculators, such as the one here: Voltage Drop Calculator. In order to manually 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. 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’s 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.

Calculating Cord Length and EMI

Longer cords also tend to have antennalike qualities by attracting the wavelengths of certain signals. This Electromagnetic or Radio Frequency Interference (EMI/RFI) can be troublesome in applications where current leakage is measured in microamps vs. milliamps for safety purposes, which is often the case in sensitive medical applications use.

“In regard to medical equipment,” said Dan Ford, Interpower Technical Support Specialist, “if you go beyond the 12–15 feet length for medical cords, it becomes much more difficult to meet the equipment standard requirements for resistance and leakage current for the entire system. You don’t want high levels of leakage in equipment attached to the patient.” While medical applications standards allow minute leakage current in proximity of the patient, many cords in hospital applications are not “patient connected,” and can therefore be measured in milliamps if the testing equipment is a safe distance from the patient, or if used in medical office applications.

Still, cutting back on the length of the cord connected to the power main may not solve your EMI issues. Let’s say you even add an EMI/RFI filter. Now you’ve added a layer of redundancy to lower your interference. You should be good to go, right? Maybe.

Ford presses home the point of “cord calculation,” for every electrical design. “If you plug your main power cord into an Accessory Power System, (APS), then plug additional accessory cords into peripheral equipment, you must calculate total cord length (not just the main power cord) to get total resistance and leakage current,” Ford said. That is, a 50-foot cord, plus 4 ten-foot cords into the same system would calculate to 90 feet of resistance and current leakage. This may affect equipment performance, or take you beyond the limits of the specific standards you are complying with.