Voltage drop is what happens when electricity travels through a long wire: some of the electrical pressure (voltage) is lost to the resistance of the conductor before it reaches the load. Too much voltage drop, and your lights are dim, your motors run hot and slow, and your electronics behave erratically.
It's one of the most commonly overlooked problems in residential and commercial electrical work — because circuits that are technically code-compliant for ampacity can still have unacceptable voltage drop if the run is long enough.
What Causes Voltage Drop?
All conductors have electrical resistance, even copper. When current flows through a conductor with resistance, a voltage drop occurs proportional to both the current and the resistance (Ohm's Law: V = I × R). The longer the wire and the more current flowing through it, the greater the voltage drop.
Wire resistance is expressed in ohms per thousand feet and varies by material and gauge. Copper 12 AWG has a resistance of about 1.59 ohms per 1,000 feet. Aluminum 12 AWG has about 2.61 ohms per 1,000 feet — about 64% more resistance than copper.
In a typical branch circuit, current travels through the hot conductor to the load and back through the neutral, so you're dealing with the resistance of two conductors for the total round-trip distance. This is why the voltage drop formula uses 2 × length.
NEC Recommendations for Voltage Drop
The NEC doesn't mandate maximum voltage drop for most branch circuits — it's informational (found in 210.19(A) FPN No. 4 and 215.2(A) FPN No. 2), recommending a maximum of 3% on branch circuits and 5% total from the service entrance to the final outlet.
While not mandatory in the code, voltage drop limits are enforced by many local jurisdictions, and exceeding them causes real problems. A 5% voltage drop on a 120V circuit means the outlet delivers only 114V. Motors are particularly sensitive — a 10% voltage drop can reduce a motor's efficiency by 20% and significantly shorten its life.
For sensitive electronic equipment, data centers, and medical facilities, voltage drop should be kept under 2%. For LED lighting, 3% is generally acceptable. For motor loads, especially in commercial and industrial applications, staying under 3% is strongly recommended.
The Voltage Drop Formula
The standard formula is: VD = (2 × K × I × L) / CM. K is the resistivity constant — 12.9 for copper, 21.2 for aluminum. I is the load current in amperes. L is the one-way length of the circuit in feet. CM is the circular mil area of the conductor.
Common circular mil areas: 14 AWG = 4,110 CM. 12 AWG = 6,530 CM. 10 AWG = 10,380 CM. 8 AWG = 16,510 CM. 6 AWG = 26,240 CM. 4 AWG = 41,740 CM. 2 AWG = 66,360 CM.
Example: 20-amp circuit, 150 feet, 12 AWG copper. VD = (2 × 12.9 × 20 × 150) / 6,530 = 77,400 / 6,530 = 11.85 volts. On a 120V circuit, that's 9.9% voltage drop — far too high. To fix it: upsize to 8 AWG (VD = 77,400 / 16,510 = 4.7V = 3.9%) or to 6 AWG (VD = 77,400 / 26,240 = 2.95V = 2.5% ✓).
When Voltage Drop Is Most Critical
Long runs from a subpanel to outbuildings (garages, workshops, barns) are where voltage drop most frequently causes problems. A 200-foot run to a detached garage needs to be calculated carefully — what's fine for lighting might be inadequate if you later add a table saw or air compressor.
Outdoor lighting circuits, especially those running long distances in residential landscaping, often require upsizing. A 12 AWG circuit running 200 feet to a string of landscape lights will have noticeable dimming even at modest loads.
EV charger installations are another common problem area. A 48-amp Level 2 charger on a 200-foot run requires careful conductor sizing — at those currents, the voltage drop on undersized wire wastes charging efficiency and can trigger charger faults.
Solving Voltage Drop
The fix is almost always upsizing the conductor. Increasing from 12 AWG to 10 AWG cuts resistance by 37%. Going from 12 to 8 AWG cuts resistance by 60%. The additional copper cost is usually minor compared to the cost of calling an electrician back to redo the work.
For very long runs, adding a subpanel closer to the load is more cost-effective than running oversized conductors the entire distance. A small 60-amp subpanel in a detached garage solves voltage drop problems for everything fed from it.
Use our free Voltage Drop Calculator to instantly check any circuit. Enter the conductor material, gauge, current, and one-way distance, and it calculates the voltage drop, percentage, and whether you're within the recommended 3% limit — along with conductor size recommendations to fix any issues.