Voltage drop in a house happens when electrical current loses pressure as it travels through wiring, connections, and devices between your electrical panel and the outlets where you plug things in. Every wire has some natural resistance, and that resistance converts a small amount of electrical energy into heat instead of delivering it to your appliances. When the drop is excessive, you’ll notice dimming lights, sluggish motors, flickering screens, or appliances that don’t perform the way they should. The National Electrical Code recommends no more than 3% voltage drop on any single branch circuit and no more than 5% total from your service entrance to the furthest outlet.
Wire Size and Circuit Length
The most fundamental cause of voltage drop is the resistance built into the wire itself. Thinner wire has more resistance, and longer runs of wire accumulate more resistance. A standard 14-gauge copper wire, commonly used for 15-amp lighting circuits, has a resistance of about 3.07 ohms per 1,000 feet. Step up to 12-gauge wire on a 20-amp circuit and resistance drops to 1.93 ohms per 1,000 feet. That difference matters more than you might expect when a circuit runs 75 or 100 feet from the panel to a far corner of the house, a detached garage, or an outbuilding.
This is pure physics: the farther electricity has to travel and the narrower the path, the more voltage it loses along the way. Homes with long circuit runs, especially to workshops, garages, or additions built far from the main panel, are the most common candidates for noticeable voltage drop. The fix in these cases is straightforward: using a larger gauge wire reduces resistance and brings the drop back within acceptable limits.
Aluminum Wiring vs. Copper
If your home was built in the late 1960s or 1970s, some or all of the branch circuits may use aluminum wiring instead of copper. Aluminum carries about 61% of the conductivity of copper, which means it has significantly higher resistance at every wire size. A 12-gauge aluminum wire, for example, has a resistance of 3.18 ohms per 1,000 feet, compared to 1.93 ohms for 12-gauge copper. That’s roughly 65% more resistance for the same size conductor.
This higher resistance means aluminum circuits produce more voltage drop over the same distance. Aluminum wiring also expands and contracts more with temperature changes, which can loosen connections over time and create additional resistance at terminals and splice points.
Loose and Corroded Connections
Every electrical circuit has dozens of connection points: wire nuts in junction boxes, terminal screws on outlets and switches, breaker lugs in the panel, and splice points where wires join. Each of these is a potential weak link. When a connection loosens even slightly, it creates a high-resistance point that chokes the flow of current.
Corrosion does the same thing. Moisture, chemical exposure, or simply decades of oxidation can build up a thin resistive layer on copper or aluminum surfaces. Improper crimps and worn fittings also limit conductivity. These high-resistance points don’t just waste energy. They generate heat, and that heat makes them degrade faster, creating a feedback loop. In severe cases, an overheated connection can melt insulation or ignite surrounding materials, making this one of the more dangerous causes of voltage drop.
The tricky part is that these problems hide inside walls, behind cover plates, and inside sealed junction boxes. A circuit can test fine with no load and then drop voltage significantly when you turn on an appliance, because the current flowing through that bad connection is what generates the heat and resistance.
Overloaded Circuits
Voltage drop increases proportionally with the amount of current flowing through a wire. A circuit that performs perfectly when running a few lights will drop noticeably more voltage when you plug in a space heater, a hair dryer, or a window air conditioner. If multiple high-draw appliances share a single circuit, the combined current can push voltage drop well past the 3% threshold.
Older homes are especially vulnerable here. Many were wired with fewer circuits than modern electrical demands require. A kitchen that was designed for a toaster and a coffee maker may now be running a microwave, a dishwasher, an instant pot, and a refrigerator. Each added load increases the current on those circuits and amplifies the voltage drop.
High Inrush Current From Motors
Some of the most dramatic voltage drops happen not during steady operation but in the first fraction of a second when certain appliances start up. Electric motors, the kind found in air conditioners, refrigerator compressors, sump pumps, and garage door openers, can draw inrush current up to 10 times their normal running current when they kick on. That sudden spike in demand creates a momentary but significant voltage drop across the circuit.
This is what causes lights to briefly dim when your AC compressor starts or when your well pump cycles on. Even switch-mode power supplies in computers and LED lighting systems can draw surprisingly large inrush currents. If the flicker is brief and only happens at startup, it’s usually a normal consequence of inrush current on a circuit that’s slightly undersized for the load. If it’s persistent or severe, the circuit may need to be dedicated to that appliance or upsized.
Problems Outside Your House
Not all voltage drop originates inside your walls. The utility company’s infrastructure, including the transformer on the pole, the service drop wires running to your house, and the meter connection, can all contribute. An undersized utility transformer will sag under heavy demand, especially when multiple homes on the same transformer are drawing large loads simultaneously. Long secondary runs from the transformer to your home compound the problem.
You can check whether the issue is internal or external by measuring voltage at your main panel. If voltage at the panel is already low (well below 120 volts on a single leg), the problem is likely on the utility side. If panel voltage looks normal but drops significantly at outlets under load, the issue is in your house wiring. Your utility company is responsible for maintaining adequate voltage to your meter, and they’ll typically investigate if you report consistently low readings.
Why Voltage Drop Matters
A small amount of voltage drop is inevitable and harmless. But when it crosses the 5% total threshold, the consequences go beyond flickering lights. Motors in appliances like refrigerators, air conditioners, and washing machines draw more current to compensate for the lower voltage, which makes them run hotter and wear out faster. Heating elements in ovens, dryers, and water heaters produce less heat and take longer to reach temperature, wasting energy. Electronics may behave erratically or reset unexpectedly.
The most serious risk is fire. High-resistance connections that cause voltage drop also generate concentrated heat. That heat can degrade wire insulation, char wooden framing, and eventually ignite surrounding materials. Excessive voltage drop is both a performance problem and a safety signal that shouldn’t be ignored.
How to Identify the Source
A basic voltage test requires only a multimeter set to AC volts. Measure the voltage at an outlet with nothing plugged in, then measure again while running the appliance that seems affected. If the voltage drops more than about 3 to 4 volts (from a normal 120-volt supply), the circuit has excessive drop. Repeating this test at the main panel helps narrow down whether the loss is happening in the branch circuit wiring, at the panel connections, or upstream from the utility.
If you find a problem on a specific circuit, the most common culprits are, in order: a loose connection at an outlet, switch, or junction box along that circuit; wire that’s undersized for the load; or a circuit run that’s simply too long for its wire gauge. An electrician can pinpoint the exact location by testing voltage at each connection point along the circuit under load.
Common Fixes
The right solution depends on what’s causing the drop. For loose or corroded connections, the fix is straightforward: tighten terminals, replace damaged wire nuts, and clean or replace corroded contact points. This is often the cheapest and most effective repair, since a single bad connection can account for most of the drop on a circuit.
For circuits that are too long or too heavily loaded, upgrading to a larger gauge wire reduces resistance and brings voltage drop back within limits. If a distant part of the house, like a detached garage or a new addition, consistently has low voltage, installing a sub-panel closer to that area shortens the branch circuit runs and improves delivery. Splitting an overloaded circuit into two dedicated circuits is another common solution, particularly in kitchens and workshops where multiple high-draw tools or appliances compete for the same wiring.