Wire Size Voltage Drop Calculator – NEC AWG Sizing Tool

Now maybe you’ve been noticing how dim the lights are these days, or how the air conditioner drops voltage on the whole house. You’re experiencing voltage drop in action. It’s annoying, it’s invisible, and thankfully, we can plan for it and make it go away.

The wire size voltage drop calculator does all the hard work for us here. It makes complex math easy and gives a simple pass/fail decision at the end.

How to Use a Voltage Drop Calculator

All wires has some amount of resistance. Resistance uses voltage before it gets to the item you want it powering. At short runs, you hardly even notice. Stretch the run to a subpanel or a shed, though, and those missing volts becomes very important. It’s simple, just Physics at work. While both copper and aluminum wires does conduct electricity well, they’re not perfect. There’s a specific rating for how much each size of wire resists electricity, measured in ohms per thousand feet. The thinner the wire, the greater amount of resistance; the higher the resistance, the more voltage that’s wasted along the way as heat. This means that a large six AWG cable will drop less voltage then a small skinny twelve AWG cable carrying equal current.

The calculator simply takes that into account with standard formula automatically, no need to remember the coefficients. On single-phase circuits, the current travels out and then returns, so length is doubled. For three-phase, it multiplies by 1.732. Just select type of phase you want to calculate, and let the tool do the math.

Basically what we’re looking at here are our two main targets from the National Electrical Code… Three percent for branch circuits and five percent for the entire path (service to load). While it’s not a hard law everywhere, it is a strong recommendation for a good reason. Motors running low on voltage will pull more current as compensation, overheating their windings in the process. Sensitive electronics may simply stop working altogether. By keeping the drop below three percent you’ll ensure your equipment runs efficiently and has a longer life.

The tool tells you instantly whether or not the wire size you have chosen will realy work with your specific set-up. This is where folks get hung up. They choose their wire solely by current capacity. Yep, that twelve AWG wire can handle twenty amps without melting. So what? That run is two hundred feet long. There’ll be a ton of voltage drop. Your lights will dim but the wire won’t melt. To meet requirements of both safety and performance you have to check voltage drop in addition to ampacity. When ampacity looks good but the voltage drop calculator shows a fail, time to upsize the conductor. Sometimes just jumping from twelve AWG to ten AWG will pull those percentages into an acceptable range.

Material also matters. For the same size, copper has roughly 60 percent lower resistance than aluminum. (Aluminum is also less expensive and lighter.) You won’t be able to replace them one-for-one without accounting for that fact. If you choose aluminum from the dropdown, the calculator will do this automatically for you. It’s a simple toggle, but it would of saved you some costly mistakes out in the field.

Keep in mind that heat ratings are not the same as voltage drop. Your wire needs to be large enough to transport current safely, and then large enough to reduce loss even more. Whichever is the greater requirement dictate your wire size. This is meant to be used in conjunction with a good engineer, NOT AS A replacement for one! Local amendments, temperature corrections, and conduit fill factors affects real-world installs. However, this is a great starting point for most residential or light commercial applications.

This will show you why larger wire should of been used for longer runs. It helps you avoid those red-faced “I’ve got a new piece of equipment, but it’s not performing well” situations caused by insufficient power at the start of the install. You’ll save yourself some headaches down the road by doing it right up front.