Everywhere I go the standard operating procedure for safely sizing electrical wires can be completely summarized as ‘pick a really big one’. When provided guidance like that I’m sorely tempted to do something stupid just to make a point, like building a flashlight with welding wire or connecting strain gauges with power line cables.
You don’t have to be an electrician to figure out that over-design on that scale costs big money and can make you look like a joke.
I’ve worked on multiple projects involving extreme temperatures and high power transmission including an electric motorcycle conversion and custom high temperature test chambers. In cases like these picking the appropriate wire size is not just an efficiency issue, but also one that could kill someone very important (me) if done incorrectly.
Wires sized correctly: Check. Wiring reminiscent of spaghetti: Check.
When I started those projects I did a little research expecting to easily find a standardized table relating wire gauge to ampacity (current carrying capacity). There isn’t one. Instead I found discrepancies between sources with one wire manufacturer claiming a capacity 20 amps and another listing 25 for the same gauge…. a 20% difference!
Eventually I figured out why it was hard to get a straight answer.There are multiple variables at play which simultaneously affect the ampacity of electrical wires.
Shockingly, system voltage is not one of those variables. Voltage has no effect on the ampacity of electrical wire! In fact, if you increase the voltage you can transfer more wattage (power) through a given wire (Watts = Volts * Amps). That’s why you find little bitty wires running into large electric motors, because they operate on 480VAC power and draw very little current.
While the voltage doesn’t affect the ampacity of the wire, it does have a significant effect on how long that wire can safely be run. The resistance of a given length of wire is constant, so the voltage drop through that wire will be constant for any voltage. However, the percentage voltage drop is much greater for low voltage systems. The difference between 120vac and 12vdc is significant and worth looking into before designing your low voltage system. If you’re not careful, that DIY solar system on your roof could give you a brown out.
The National Electrical Code (NEC) recommends that electrical systems be designed with a voltage drop of 3% at most.
There’s another reason you’ll need to be careful with your wire selection; heat generated from wires can raise ambient temperatures and cause electronic devices to fail. According to the Uptime Institute, for every 18 degrees Fahrenheit (10 degrees Celsius) that internal electrical cabinet temperatures rise above normal room temperature, the life expectancy of the enclosed electronics drops by 50%.
In any case with the ever growing popularity of DIY and the maker community, I think this subject deserves a better explanation that what is currently out there so I went to the trouble of creating a visual aid to try to make things a bit clearer. I have also created a wire sizing calculator here. (If you like that one, see my other free resources here!)
Wire Ampacity: Current carrying capacity of the wire. If you exceed this rating then your wire will generate heat faster than it can dissipate and it will eventually start a fire.
Total Safe Length of Wire: How long you can run the wire before the voltage drop becomes too significant to ignore. The rule of thumb is not to a exceed 3% drop in your operating voltage.
Rated Wattage: How much power you can safely transmit through the wire, volts times amps.
Resistance: The measurable obstruction to power transfer through a wire in ohms.
Heat Generated: The heat created due to the resistance of the wire.
Voltage Drop/Loss: How many volts you lose over the given length of wire. Voltage drop is caused by the wire itself acting like a resistor in series with your load.
Wire Gauge: The diameter of the wire measured in AWG size.
Length of wire: Length of the wire used.
System Voltage: Voltage you are trying to pass through the wire.
Rated Wire Temperature: What ambient temperature the wire is rated to operate at. Common ratings are: 60C, 75C or 90C, the higher the better and more expensive.
# Wires Bundled: How many wires are crammed into the same sheath or conduit. Running lots of wires together concentrates the heat that the wires generate by limiting each wires exposure to ambient air which slows the wires cooling rate.
Ambient Temperature: This has the same effect as bundling many wires together. If the air around the wires is too hot then the wires cannot quickly transfer heat to their surroundings.