© Thomas Perkins | Dreamstime.com
Electrical calculations
Electrical calculations
Electrical calculations
Electrical calculations
Electrical calculations

The Basics of Calculating Voltage Drop

March 1, 1999
How do you know if your wiring will provide a reasonable efficiency of operation?

Thank you for visiting one of our most popular classic articles. If you’d like to see updated information on this topic, please check out this recently published article, Don’t Let Voltage Drop Get Your System Down.

How do you know if your wiring will provide a reasonable efficiency of operation? The National Electrical Code, 210-19(a) (FPN 4) and 215-2 (b) (FPN 3), recommends 5% voltage drop for feeder circuits and 3% for branch circuits. Let's work some examples, using the equations in the sidebar (right). Our examples use uncoated copper wire in steel conduit, for 480V branch circuits; we'll use NEC Table 9's power factor column.

Example 1: Determine voltage drop

Run a No. 10 stranded wire 200 ft at 20A.

Per Table 9, our "ohms to neutral per 1,000 ft" is 1.1 ohms. To complete the numerator, multiply as follows: (2 x 0.866) x 200 ft x 1.1 ohms x 20A = 7,620.8 Dividing 7,621 by 1,000 ft gives a voltage drop of 7.7V. This drop is acceptable for our 480V circuit. A No. 12 would drop 11.8V. Boost the length to 500 ft, and that No. 10 drops 18V; the No. 12 drops 29V.

Example 2: Determine wire size

Run a stranded copper wire 200 ft at 20A.

You can find the wire size by algebraically altering the first equation, or you can use the following method. To complete the numerator, multiply as follows: 1.73 x 212.9 ohms x 200 ft x 20A = 89,371.2. Dividing the 89,371.2 by the acceptable voltage drop of 14.4V gives you 6,207 circular mils. NEC Table 8 shows that a No. 12 wire satisfies the voltage drop recommendation.

Example 3: Determine wire length

Run a stranded copper No. 10 wire for a 20A circuit.

To complete the numerator, multiply as follows: 1,000 x 14.4V = 14,400. To complete the denominator, multiply as follows: (2 x 0.866) x 1.1ohms x 20A = 38.104. Finally, divide the numerator by the denominator, as follows: 14,400 ÷ 38.104 = 378 ft. If you ran the No. 12 wire for the same circuit, you could run it 244 ft.

Example 4: Determine maximum load

Run a stranded copper No. 10 wire for a 200 ft circuit.

To complete the numerator, multiply as follows: 1,000 x 14.4V = 14,400. To complete the denominator, multiply as follows: (2 x 0.866) x 1.1 ohms x 200 ft = 381.04. Finally, divide the numerator by the denominator, as follows: 14,400 ÷ 381.04 = 37A. This circuit could handle 37A on each phase conductor. A 200 ft No. 2 could handle 24A. 

* The number "0.866" is for 3-phase only. It converts the number "2" to "1.732" (the square root of 3). For single-phase circuits, don't use the "0.866" in the calculations.

* "CM"denotes wire size in circular mils, as shown in Table 8.

* To calculate wire size, use 12.9 as your K for copper and 21.2 as your K for aluminum.

* "L" is the one-way wire length in ft.

* "R" is the resistance per 1,000 ft. Use NEC Table 9 for AC wiring. If you have non-linear loads, use the column that helps account for power factor.

Equation 1: Calculating the actual Voltage Drop I[Volts dropped = (2 x 0.866) x L x R x Amps/1,000]

Equation 2: Calculating the Wire Size in circular mils [CM = 2 x K x L x Amps/Acceptable Voltage Drop]. Alternatively, you can algebraically manipulate Equation 1 to: Acceptable Voltage Drop ÷ 1.732 x L x Amps and then look up the wire size according to its AC resistance.

Equation 3: Calculating the Length in ft [Length = 1,000 x Acceptable Voltage Drop ÷ (2 x 0.866) x R x Amps]

Equation 4: Calculating the Load in amps [Amps = 1,000 x Acceptable Voltage Drop ÷ (2 x 0.866) x R x L]

About the Author

Mark Lamendola

Mark is an expert in maintenance management, having racked up an impressive track record during his time working in the field. He also has extensive knowledge of, and practical expertise with, the National Electrical Code (NEC). Through his consulting business, he provides articles and training materials on electrical topics, specializing in making difficult subjects easy to understand and focusing on the practical aspects of electrical work.

Prior to starting his own business, Mark served as the Technical Editor on EC&M for six years, worked three years in nuclear maintenance, six years as a contract project engineer/project manager, three years as a systems engineer, and three years in plant maintenance management.

Mark earned an AAS degree from Rock Valley College, a BSEET from Columbia Pacific University, and an MBA from Lake Erie College. He’s also completed several related certifications over the years and even was formerly licensed as a Master Electrician. He is a Senior Member of the IEEE and past Chairman of the Kansas City Chapters of both the IEEE and the IEEE Computer Society. Mark also served as the program director for, a board member of, and webmaster of, the Midwest Chapter of the 7x24 Exchange. He has also held memberships with the following organizations: NETA, NFPA, International Association of Webmasters, and Institute of Certified Professional Managers.

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Fiberglass Electrical Conduit Chemical Resistance Chart

This information is provided solely as a guide since it is impossible to anticipate all individual site conditions. For specific applications which are not covered in this guide...

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.