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How to Not Start an Electrical Fire: Calculating Wire Gauges for Your Next DIY Project

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For most hobbyists, wire choice is an under-considered aspect of project design. Using too thin of a wire can have disastrous effects, ranging from voltage drops resulting in poor performance to electrical fires.

Despite its importance, thicker wire is more expensive. So, hobbyists will often skimp on this material.

Our goal is to figure out the thinnest wire we can use, while keeping our voltage drops to a reasonable margin and avoiding any risk of an electrical fire.

By taking the time to go through the math, we will save ourselves a huge amount of work down the road. Let’s dive in!

Untangling Electricity, With Plumbing

Electricity can be difficult to visualize, so let’s look at a classic metaphor for understanding electricity. Instead of electrons in a wire, picture water in a pipe:

Diagram illustrating water metaphor to explain electricity basics
  • Current would be the water in the pipe.
  • Voltage would be the pressure pushing the water through the pipe.
  • Resistance would be the diameter of the pipe itself.

In many ways, this metaphor is perfect for thinking about wire gauges. Much like the diameter of a pipe, the diameter of a wire is directly proportional to its internal resistance. The less space there is for electricity (or water) to flow, the more resistance there will be to its flow.

Thinking back to the pipe full of water, what would happen if we tried to force 100 liters of water into a pipe that can only hold one liter? As you can imagine, we would find ourselves very quickly on the phone to the plumber, as that pipe would have burst!

A similar phenomenon occurs when we try to send too much current down too thin of a wire; only instead of water flowing out, the wire overheats and melts. With a large enough voltage pushing that current, this melted wire can lead very rapidly to an electrical fire.

Don't worry if you've already bought your wires and they don't fit your needs; you can use them for several fun DIY projects.

Voltage Drop, and Why Too Much Can Ruin a Circuit

In any DC electrical circuit, we will experience voltage drops; this is a fundamental rule of circuits. The issue arises, however, when too much voltage has been lost, specifically due to the resistance of the wire itself.

If we think of our pipes again, imagine a very large pipe bottlenecking into a much smaller one. Suddenly, our torrent of water is slowed down, and the pressure behind the water is decreased. This slowed-down water does not have the pressure to turn a water wheel, and hence we have decreased performance.

Bottlenecking Pipes, illustrating slowed rate of flow

So, short of using a much smaller water wheel, it seems like we need to use larger pipes. This would allow the water to flow relatively uninhibited, keeping the pressure high enough to turn our wheel.

Moving to wires and electrons, the idea is much the same! In electrical terms, we can very easily explain this with one of the fundamental laws of physics, Ohm’s Law:

Ohms Law diagram; V=I*R

Ohm’s Law dictates the relationships between Current, Voltage, and Resistance. In a DC circuit, our current is constant throughout, but our voltage and resistance are inversely related to one another.

To put that more simply, as resistance increases, voltage decreases.

To bring that to our real-world application: as wires get smaller, their resistance increases. Therefore, as wires get smaller, the voltage across them decreases, just like how thinner pipes decrease the flow of water!

Hence, using thicker wires in your circuit allows the voltage to be applied where it is needed—and not wasted on wire resistance.

Calculating Optimal Wire Gauge

By far, the best way to figure out your wire gauge is to use an online calculator; these applications allow users to input their voltage, amperage (current), the length of their wire runs, and their allowable voltage drop. After crunching the numbers, the calculator outputs the optimal wire gauge for that run of wire.

For most applications, a voltage drop of around three percent is allowable—anything more than that, and more sensitive electronic equipment may begin to function improperly.

We also always want to use the maximum possible current draw of our device—this will generally be written on the device. While it likely will not have a sustained current draw of that value, we need to make sure we are protected from current spikes.

Here is one example of an online wire gauge calculator, found at wiresizecalculator.net:

Wire Size Calculator example

As you can see, this calculator also allows for further variable input, such as the heat rating of the insulation, what kind of conductor the wires are made of, and the type of installation the wire is being put into.

In this example, we were given a minimum safe wire gauge of 3 American Wire Gauge (AWG).

For safety’s sake, it is always best practice to over gauge your wires by one or two, so we would end up with a wire of 2/0 AWG. Note, the standard of wire gauge is totally backward; the thicker the wire, the smaller the wire gauge! So, 2/0 AWG is, in fact, thicker wire than 3 AWG.

Related: Calculate Your Electricity Costs With These Handy Tools

Which Wire Should I Choose?

Outside of wire gauge, the other variables to consider are strand count—i.e. how many strands of wire comprise the cable—and wire material.

As a rule of thumb, more strands of wire yield greater flexibility. If you are running wire in tight spaces, a greater strand count would be something worth investing in.

Regarding the actual material of the wire, copper is the standard for most applications, though aluminum can be used as well.

The key thing to remember is consistency. If you are using copper connectors, and copper wire elsewhere in the circuit, use copper wire. Mixing and matching materials can lead to corrosion and broken connectors, as the materials will expand and contract at different rates.

Related: Not Just Solder: Different Methods for Connecting Wires, Explained

Saving Money and Staying Safe

There are a fair few variables to consider when choosing the right wire for your build. The number one rule of electricity, however, is to bring in a professional if you feel uncomfortable.

Otherwise, calculate your wire gauge based on the maximum current you intend to run through it, buy wire one size up from that, and get building! Always maintain material consistency between wires and connectors and do the math!

With a bit of forethought, you can put out any risk of fire and save yourself a few bucks.

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