The maximum voltage drop allowed in a circuit is 5% from the service entrance to the furthest outlet. This total is typically split between the feeder (the wiring from your electrical panel to a subpanel or junction) and the branch circuit (the wiring from there to your outlet or device). The most common split is roughly 2% on the feeder and 3% on the branch circuit, though any combination that stays within 5% total is acceptable.
Where the 3% and 5% Rules Come From
The National Electrical Code (NEC) addresses voltage drop in two key sections: 210.19 for branch circuits and 215.2 for feeders. Both include Informational Notes recommending that conductors be sized to limit voltage drop to about 3% on any individual branch circuit and 5% for the combined feeder-plus-branch-circuit path.
Here’s an important distinction: these are recommendations, not hard requirements. The NEC places them in Informational Notes rather than in the enforceable code text itself. That said, most electrical engineers, inspectors, and contractors treat them as practical minimums. The U.S. Army Corps of Engineers, for instance, enforces these same limits as firm requirements on its projects: 3% on the secondary service, 2% on any feeder or branch circuit, and no more than 5% combined.
Your local jurisdiction may adopt stricter rules. Always check with your local building department, since some areas enforce voltage drop limits that go beyond the NEC’s advisory language.
What Voltage Drop Actually Means
Every wire has resistance. As electrical current flows through that resistance, some voltage is lost as heat before it ever reaches your device. On a 120-volt circuit with a 5% drop, the outlet at the far end of the run would deliver only about 114 volts instead of 120. On a 240-volt circuit, that same percentage means you’d lose 12 volts.
The longer the wire run and the smaller the wire gauge, the more voltage you lose. Higher current loads also increase the drop. This is why a short 15-amp circuit to a nearby bedroom outlet rarely has voltage drop issues, while a long run to a detached garage or workshop often does.
Why Staying Under 5% Matters
Excessive voltage drop doesn’t just waste energy. It causes real problems with the equipment connected to the circuit.
- Motors lose torque. When a motor receives less voltage than it’s designed for, it struggles to start and runs hotter than normal. Over time, this accelerates wear and can lead to premature failure. Air conditioners, well pumps, and shop tools are all vulnerable.
- Lights dim or flicker. This is especially noticeable on long lighting runs or with LED fixtures, which can behave erratically when input voltage sags below their operating range.
- Sensitive electronics malfunction. Computers, audio equipment, and smart home devices may reset, glitch, or operate unreliably when voltage fluctuates due to drop in the supply wiring.
- Energy is wasted as heat. The voltage lost in the wire doesn’t disappear. It converts to heat in the conductor itself, meaning you’re paying for electricity that never reaches your equipment.
When to Upsize Your Wiring
A common rule of thumb among electricians is to increase the wire size by one gauge for every 100 feet of circuit length. Some are more conservative and bump up a size every 75 feet. These shortcuts work reasonably well for standard 120-volt, 15- to 30-amp circuits, but they’re rough estimates.
If a 120-volt circuit is under 100 feet from panel to outlet, voltage drop is rarely a concern with properly sized wire. Once you get close to or beyond 100 feet, it’s worth running an actual calculation. For 240-volt circuits, the math is more forgiving because the higher voltage means the same percentage drop translates to a larger absolute number of volts before you hit the limit.
The calculation itself is straightforward. You need four numbers: the wire’s resistance per foot (available in standard tables by gauge), the one-way length of the run, the current in amps, and the supply voltage. Multiply resistance per foot by the round-trip distance (twice the one-way length) by the current, then divide by the supply voltage to get the percentage drop. If it exceeds 3% for the branch circuit alone or pushes the total system past 5%, go up a wire size and recalculate.
How to Measure Voltage Drop on an Existing Circuit
If you suspect voltage drop on a circuit that’s already installed, you can confirm it with a digital multimeter. The key is that you need to measure under load, not with everything turned off. Without a load drawing current through the wire, there’s no current flow and therefore no measurable drop.
Start by measuring the voltage at the electrical panel or the source end of the circuit. Then measure the voltage at the furthest outlet or device on that same circuit while it’s powering its normal load. The difference between those two readings is your actual voltage drop. Divide that difference by the source voltage and multiply by 100 to get the percentage.
For example, if you read 121 volts at the panel and 115 volts at the far outlet under load, the drop is 6 volts. That’s about 5%, right at the upper limit. A clamp meter can also help by showing you the actual current flowing through the wire, which lets you verify whether the load you’re testing under is representative of real-world use.
Practical Examples by Circuit Type
On a standard 120-volt, 20-amp kitchen circuit running 50 feet from the panel, voltage drop is minimal with 12-gauge wire, typically well under 2%. The same wire stretched to 150 feet for an outbuilding could easily exceed 5%, making 10-gauge or even 8-gauge wire necessary.
For a 240-volt, 30-amp circuit feeding a workshop subpanel 200 feet away, the higher voltage works in your favor, but the distance and current still demand careful sizing. Running the calculation often reveals that jumping from 10-gauge to 8-gauge wire keeps you comfortably within the 5% total limit, while the smaller wire would put you over.
The cost of upsizing wire during installation is almost always less than the cost of replacing it later. If a circuit is borderline in your calculations, going one size larger is cheap insurance against flickering lights, tripped breakers, and equipment that underperforms for years.