The Complete Guide to Voltage Drop and Wire Sizing
When wiring a new electrical circuit, many people assume that if they use the correct wire gauge for the amperage (e.g., 12 AWG for a 20-Amp breaker), everything will work perfectly. However, this assumption completely ignores a silent killer of electrical efficiency: Voltage Drop.
Voltage drop occurs because no wire is a perfect conductor. Even pure copper wire has a tiny amount of natural electrical resistance. Over short distances, this resistance is negligible. But when you run a wire over a long distance—such as out to a detached garage, a barn, or an irrigation pump—that tiny resistance adds up. As the current pushes through the long wire, some of the electrical pressure (voltage) is lost along the way.
Our free Voltage Drop Calculator allows electricians and DIYers to instantly calculate exactly how much voltage will be lost over a specific distance, ensuring the equipment at the end of the line receives enough power to operate safely.
Why is Voltage Drop Dangerous?
When voltage drops too low, the equipment plugged into the circuit is starved of the electrical pressure it needs to function correctly. This causes several severe issues:
- Motor Burnout: Electric motors (like in air conditioners, refrigerators, and power tools) are designed to run at a specific voltage. If the voltage drops, the motor tries to compensate by drawing more current (Amps) to maintain its power output. This excess current causes the motor coils to overheat and eventually burn out entirely.
- Flickering Lights: Incandescent and halogen bulbs will noticeably dim if the voltage drops. LED lights may flicker rapidly or fail to turn on at all.
- Wasted Energy (Heat): The voltage that is "lost" doesn't just disappear. According to the laws of physics, it is converted directly into heat along the entire length of the wire. In extreme cases, this heat can melt the insulation and start a fire. Even if it doesn't cause a fire, you are paying the utility company for electricity that is literally just warming up the dirt in your yard.
What does the NEC (National Electrical Code) say?
The NEC strongly recommends (via Informational Note in Section 210.19(A)) that the maximum voltage drop should not exceed 3% for a branch circuit, or a total of 5% for both the feeder and branch circuit combined.
For example, on a standard 120V circuit, a 3% drop means you lose 3.6V. The voltage at the end of the wire should be no less than 116.4V.
How to Fix a High Voltage Drop
If you run the calculator and find that your voltage drop is 8%, you have a problem. Fortunately, the solution is mathematically simple: you must decrease the resistance of the wire. There are two ways to do this:
- Upsize the Wire Gauge: Thicker wires have less resistance. If a 12 AWG wire gives you an 8% drop, bumping up to a thicker 10 AWG or 8 AWG wire will significantly lower the resistance and bring the voltage drop back into the safe 3% zone. This is the most common solution.
- Increase the Source Voltage: Higher voltages suffer less percentage drop over the same distance. This is why power companies transmit electricity across the country at 345,000 Volts instead of 120 Volts. If you are running power 300 feet to a workshop, it is vastly more efficient to run a 240V circuit and install a subpanel, rather than trying to run a 120V circuit that far.
Copper vs. Aluminum Wire
When calculating voltage drop, the material of the conductor matters immensely. Copper is an excellent conductor of electricity. Aluminum is also a good conductor, but it has roughly 60% more electrical resistance than copper.
Because of this higher resistance, if you choose to use aluminum wire (which is much cheaper and lighter than copper), you must use a significantly thicker gauge to achieve the same voltage drop. For example, if a run requires a 6 AWG copper wire, you would likely need to jump up to a 4 AWG aluminum wire to safely carry the same load over the same distance.
Frequently Asked Questions (FAQs)
1. Does the length of the neutral wire count?
Yes! In a standard single-phase 120V circuit, the electricity has to travel out to the device on the hot wire, and all the way back to the panel on the neutral wire. Our calculator automatically handles this by multiplying the one-way distance by two.
2. Why is a 5% drop allowed for feeders?
The NEC allows a maximum 5% total drop from the main panel to the final device. This is usually split as 2% loss in the heavy feeder wires running to a subpanel, and 3% loss in the smaller branch circuit wires running from the subpanel to the outlet.
3. Is it illegal to have a voltage drop greater than 5%?
In most cases, the NEC 5% rule is an "Informational Note," meaning it is a strong engineering recommendation rather than a strict, enforceable law. However, local inspectors can and will fail your installation if they deem the extreme voltage drop to be a fire hazard or detrimental to the equipment.
4. Should I calculate voltage drop based on the breaker size or the actual load?
You should calculate it based on the maximum continuous load you expect the circuit to carry. However, to be perfectly safe and future-proof the circuit, many electricians calculate the voltage drop based on 80% of the breaker's rating (e.g., calculating for 16 Amps on a 20-Amp breaker).
5. Why do my lights dim when the AC turns on?
When an air conditioner compressor starts, it pulls a massive surge of current (called Inrush Current or Locked Rotor Amps) for a fraction of a second. This massive current spike causes an immediate, severe voltage drop across your home's electrical panel, which causes the lights to momentarily dim.
6. Does voltage drop affect DC circuits like solar panels?
Absolutely. In fact, voltage drop is often a much bigger problem in low-voltage DC systems (like 12V RV solar setups) because losing just 1 Volt out of 12V is a massive 8.3% drop, severely crippling the efficiency of the solar charging system.
7. How does Three-Phase power reduce voltage drop?
Three-phase systems are more efficient at transmitting power. Because the three phases are offset by 120 degrees, the current returning on the neutral wire cancels out in a perfectly balanced system. The voltage drop formula multiplier for three-phase is roughly 1.732 (the square root of 3), which results in less drop compared to a single-phase system.