What Voltage Drop Is and Why It Matters
Voltage drop is the reduction in electrical pressure as current travels through a wire. When electricity moves from your panel to a light fixture, appliance, or outlet, it loses some voltage along the way—the longer the wire or the more current flowing through it, the greater the loss. A small drop is normal and harmless, but a large one causes lights to dim, motors to run hot, and appliances to work poorly or fail early.
The National Electrical Code (NEC) sets limits: voltage drop on branch circuits (the wires serving individual outlets or fixtures) should not exceed 3 percent, and the combined drop from the main panel to the farthest outlet should not exceed 5 percent. Knowing how to calculate voltage drop helps you choose the right wire size when running new circuits, troubleshoot dimming lights, or understand why an outlet far from the panel isn't delivering full power.
Key Takeaways
- Voltage drop is calculated using the formula: VD = (2 × L × I × R) ÷ 1000, where L is wire length in feet, I is current in amps, and R is the resistance per 1,000 feet of the wire material and size.
- Copper wire has lower resistance than aluminum, so it causes less voltage drop; larger wire gauges (thicker wires) also reduce drop significantly.
- The NEC recommends keeping voltage drop under 3 percent on branch circuits and under 5 percent on the combined path from panel to load.
- You can find wire resistance values in the NEC tables or on wire manufacturer datasheets; these values change based on wire gauge and material.
- If your calculation shows excessive voltage drop, the solution is to use a larger wire gauge, run a separate circuit closer to the load, or reduce the current demand.
The Voltage Drop Formula and What Each Part Means
The standard formula for calculating voltage drop in a single-phase circuit is:
VD = (2 × L × I × R) ÷ 1000
Here is what each variable represents:
- VD = voltage drop, measured in volts
- L = one-way wire length in feet (the distance from the source to the load, not the round trip)
- I = current flowing through the wire in amps
- R = resistance of the wire per 1,000 feet, measured in ohms (Ω)
- 2 = a constant that accounts for the round-trip path (current goes out and returns)
- 1000 = a divisor that converts the result to volts
The formula works because voltage drop depends on three things: how far the electricity travels, how much current is moving, and how much the wire resists that flow. Doubling any of these increases the drop.
Finding Wire Resistance Values
Wire resistance is not something you calculate—it is a property of the material and size, and manufacturers publish it. The NEC provides resistance tables in Chapter 9, Table 8, which lists resistance per 1,000 feet for copper and aluminum wires of every standard gauge at 68°F (20°C).
For example, a 12 AWG copper wire has a resistance of about 1.98 ohms per 1,000 feet. A 10 AWG copper wire has about 0.99 ohms per 1,000 feet—notice that going one size larger cuts the resistance roughly in half. Aluminum wire has higher resistance: a 12 AWG aluminum wire is about 3.16 ohms per 1,000 feet.
You can also find resistance values on the wire manufacturer's datasheet or on the spool label. If you are working with conduit or cable that contains multiple conductors, use the resistance of a single conductor, not the bundle.
Step-by-Step Calculation Example
Suppose you are running a 20-amp circuit to a workshop outlet 150 feet away from the panel using 12 AWG copper wire. Here is how to calculate the voltage drop:
- Identify your values: L = 150 feet (one-way distance), I = 20 amps, R = 1.98 ohms per 1,000 feet (from the NEC table for 12 AWG copper)
- Plug into the formula: VD = (2 × 150 × 20 × 1.98) ÷ 1000
- Multiply the numerator: 2 × 150 = 300; 300 × 20 = 6,000; 6,000 × 1.98 = 11,880
- Divide by 1,000: 11,880 ÷ 1,000 = 11.88 volts
A voltage drop of 11.88 volts on a 120-volt circuit is about 9.9 percent—far above the 3 percent limit. This wire is too small for this distance and load. You would need to use 10 AWG copper (resistance 0.99 ohms per 1,000 feet), which would give you a drop of about 5.94 volts, or roughly 5 percent—still slightly high but closer to acceptable. A 8 AWG wire would bring it well under 3 percent.
How to Interpret Your Result
Once you have calculated voltage drop in volts, convert it to a percentage to compare against the NEC standard. Divide the voltage drop by the circuit voltage and multiply by 100:
Percent VD = (VD in volts ÷ Circuit voltage) × 100
For a 120-volt circuit with a 3.6-volt drop: (3.6 ÷ 120) × 100 = 3 percent. For a 240-volt circuit with the same 3.6-volt drop: (3.6 ÷ 240) × 100 = 1.5 percent. The same voltage drop is less significant on a higher-voltage circuit.
If your calculation shows voltage drop under 3 percent on a branch circuit, the wire size is acceptable. If it exceeds 3 percent, you must use a larger wire gauge. If the combined drop from the main panel through feeder wires and then to the final load exceeds 5 percent, you need to upsize the feeder or install a subpanel closer to the load.
When to Use Larger Wire to Reduce Voltage Drop
If your calculation shows excessive voltage drop, the most practical solution is to move up one or more wire sizes. Each step up in gauge (from 12 to 10, or 10 to 8) roughly halves the resistance, which cuts the voltage drop in half.
Larger wire is more expensive and harder to work with, but it is the standard fix for long runs or high-current circuits. For example, if you are running a 30-amp subpanel feeder 200 feet from the main panel, 8 AWG copper may not be enough—you might need 6 AWG or even 4 AWG depending on the voltage and acceptable drop.
Another option is to run a separate circuit from the panel to the load, shortening the wire run. If the workshop outlet is 150 feet away, but you can install a small subpanel 50 feet away and run the final 100 feet from there, you reduce the voltage drop on the long run significantly. This approach costs more upfront but is sometimes the only practical solution for very distant loads.
Voltage Drop in Three-Phase Circuits
Three-phase circuits, common in industrial and commercial settings, use a different formula because current flows through three conductors instead of two:
VD = (√3 × L × I × R) ÷ 1000
The only difference is that the constant 2 is replaced by √3 (the square root of 3, approximately 1.732). Everything else—wire length, current, and resistance—works the same way. If you are working with three-phase power, use this formula instead of the single-phase version.
Frequently Asked Questions
What is an acceptable voltage drop?
The NEC recommends no more than 3 percent on branch circuits and no more than 5 percent on the combined path from the main panel to the farthest load. For a 120-volt circuit, 3 percent equals 3.6 volts; for 240 volts, it is 7.2 volts. Most electricians aim for 2 percent or less on branch circuits to leave a safety margin.
Does voltage drop matter for low-current circuits?
Yes, but the effect is smaller. A 5-amp circuit at 150 feet will have much less voltage drop than a 20-amp circuit at the same distance. However, if the wire is very long or very thin, even low-current circuits can exceed the 3 percent limit. Always calculate rather than guess.
Can I use aluminum wire instead of copper to save money?
Aluminum wire has higher resistance than copper, so it causes more voltage drop. If you choose aluminum, you must use a larger gauge than you would with copper—often two sizes larger. This usually costs more than using copper, so aluminum is rarely the cost-saving choice for residential circuits.
What if I do not know the current my circuit will carry?
Use the breaker size as a worst-case estimate. A 20-amp breaker means the circuit could carry up to 20 amps, so calculate voltage drop at 20 amps. In practice, the circuit may draw less, but designing for the maximum protects you against future overload.
Do I need to account for temperature when calculating voltage drop?
The NEC tables assume 68°F (20°C). In hot environments, wire resistance increases slightly, so voltage drop will be a bit higher than your calculation. For most residential work, this difference is small enough to ignore, but in very hot locations or critical circuits, you may want to upsize the wire by one additional gauge as a safety margin.