Did you ever use a heavy tool at the end of a long extension cord and feel like it wasn't quite up to par? Did you also notice that when the air compressor turns on, the lights in your workshop dim? You're not making it up. That slow performance and those flickering lights are common signs of voltage drop, a problem that many electrical systems have.
It's not enough to just match the wire to the circuit switch when picking wire for a job. Making sure the "pressure" from the panel goes all the way to the exit, no matter how far away it is, is part of it. Otherwise, you could be cutting off power to a device or, even worse, you could be making a danger. Don't worry, though—we'll explain this so you can wire your next job with confidence.
What’s the Big Deal with Voltage Drop Anyway?
If you really want to understand how it works, let me use plumbing from your home as an example. In this case, voltage is like water pressure and current (amps) is like flow rate. Wires are also like pipes. Let's say you want to send a lot of water down a very long and narrow pipe. Isn't it going to be pretty weak against the other end?
The same is true for electricity. There is more resistance in wires that are longer and smaller. Because of the resistance, the electricity is losing some of its energy or "pressure" as it moves along the wire. This is why the voltage at the end of the wire is lower than the voltage at the beginning of the wire in the breaker panel. Some drops are normal and to be expected, but a big drop can cause a lot of problems, from your tools and gadgets not working right to breaking them.
The NEC’s “Rule of Thumb” for a Healthy Circuit
There may not be a rule in the NEC that says, "You shall not have voltage drop," but the code is there to make sure things work and are safe. Several NEC Informational Notes say that a voltage drop of no more than 3% for a single branch line (like the one that goes to your new workshop) is generally thought to be the best practice. This drop shouldn't be more than 5% for the whole system, from the service link to the last outlet.
So, this rule doesn't just answer a question of code; it also answers questions of speed and durability. When the voltage is low, motors that draw current get extra hot, which is bad because it shortens the life of the motor and, in the worst case, causes the lines to become more isolated.
Step 1: Sizing for Amps (The Bare Minimum)
Safety is the first and most important thing that any wire should be made for. There is current in the connection, and the wire needs to be strong enough to handle it. This is known as Wire ampacity. For each circuit, there is a breaker or fuse with a certain amp value, like 15-amp or 20-amp. Because of this, the wire must have the same ampacity or more than the grade.
A 15-amp circuit needs at least 14-gauge copper wire, and a 20-amp circuit needs at least 12 gauge copper wire. These are the simple rules that apply to most home circuits. The ampacity table in the NEC would be used for higher loads or other situations. A good wire sizing calculator can help you make sure you'll cover the basics before you even think about distance, so you can be sure you're following the rules.
Step 2: Sizing for Distance (The Pro Move)
Meeting the minimum ampacity requirement is only the beginning, especially for wire runs longer than about 50 feet. While that 12-gauge wire worked fine for a 20-amp outlet in the living room, it will not work at all for a 20-amp outlet 150 feet away from your panel. Here is where power drop comes into play.
There is a method for figuring out the voltage drop that takes into account the type of wire, the flow of current, and, most importantly, the circuit's one-way distance. You could work it out with a pencil and paper, but why make things more difficult? A voltage drop calculator is an important tool to have in order to stay within the suggested 3% drop. You choose the power (like 120V), the load (in amps), the length of the wire, and the gauge of the wire you want to use. It instantly tells you what percentage of voltage loss you'll be getting, so you can decide if you want to upsize to thicker gauge wire depending on the distance.
A Real-World Example: Wiring a Workshop
Let's say you want to connect your new workshop, which is 150 feet away from your home's main electrical panel, to a 120V, 20-amp line.
In the beginning, you check the ampacity. At least 12-gauge wire is needed for a 20-amp connection. The voltage would drop more than 5.7% if you ran 150 feet of 12-gauge copper wire, though. That is almost twice the 3% limit that is suggested. The outlet would have less than 113V, down from 120V at the panel. It might be hard to get your table saw to turn on, and the lights might blink.
How to solve it? The wire needs to be bigger. You could use a calculator to find that the drop would be about 3.6% less if you used a larger 10-gauge wire. This is a much better result. You would need to use an 8-gauge wire to get it well below 3%. Even though the circuit is only 20 amps and is covered by a 20-amp breaker, the longer distance means that the wire needs to be thicker to get through it.
Conclusion: Wire for Success
Getting the right size wire takes two steps: first, get the right size for the breaker's current, and then, make the size bigger as needed for the length of the run. You can make sure that your electricity projects are safe, effective, and dependable by planning for voltage drop from the start. You keep your tools and machines safe from damage and make sure they work the way the maker intended.
