This page is to provide a single place to look to for what the safe rated capacities of various size wires in general use. These are general guidelines - check with the wire manufacturer or standards body controlling your installation for any additional specifications. Keep in mind that temperature and environment have a dramatic effect on these ratings, and that for wiring it's much better to err on the side of too large a wire than too small.
This page started as a page for 12V DC automotive use, but has grown over time to include a more general set of information on wire sizing. I've tried to add some basic explanations of what matters when sizing a wire and to avoid using too many details specific to certain applications. The actual formulas used to figure this out can be very complex - for example the National Electrical Code specifies the wire sizes to be used in excruciating detail based on years of actual research on what happens to wires in The Real World. Keeping up with all those details can be very hard, but the basic principles are pretty straightforward. My goal for this page is to expose you to those basic concepts, and at the end to give a basic "rule of thumb" chart for folks to start out with.
This page was created to help explain concepts and give an overview of wire capacity and what is factored into deciding on the wire size to use in a given application. This page should not to be considered an authoritative source of exact numbers on what wire size to use. Consult other sources such as wiring codes and manufacturers recommendations on the piece of equipment you are installing for more details. I am not telling you what wire size to use - the information here is provided as-is and without any guarantee as to it's accuracy or completeness. Any issues caused by the use of this information are not my fault - be smart, use common sense, and use this information at your own risk.
Measuring Wire Capacity
The amount of power a wire can safely carry is related to how hot it can safely get. All wires have resistance, and as power flows through a wire that resistance causes heat - and it can be quite a bit of heat. The more power you put through a wire, the hotter it gets. Insulation breaks down as it gets hot, and at some point it will melt away leaving the wire exposed to whatever is around it - other wires, grounded metal, people, etc. The heat can even be enough to start a fire in the surrounding material in some cases. Electrical fires are nasty and tend to start in the hardest to reach places - where the most heat builds up back in dark corners and tight spaces. This is why using the right size wires is important for your safety and for safety of others using your wiring work.
In some respects, the capacity of a wire is actually best measured in watts, not amperage. Why? Because a watt is a unit or power that is a combination of amperage (volume), voltage (pressure), and resistance to the power flowing through that wire. Watts measure the amount of power (aka, heat) a wire can safely dissipate. However, most wire charts are done in amps. This is unfortunate because it means the wire chart is sort of assumed to be at a single voltage level. For most usage, this is fine because the chart has an assumed usage. As an example, charts for amperage ratings of of various sizes wires for 110V AC house current charts are popular and reasonably well-known. On the other hand, the amperage ratings are very different for common/typical 12V DC automotive usage. For example, a 12 gauge wire is commonly rated at 20A for 110V AC home usage, but in automotive 12V DC use 12 gauge wire is commonly used for circuits carrying 60A! A prime example would be the main charging wire from the alternator to the battery and out to the main electrical circuits of the car. I thought I had a satisfactory explanation posted here previously, but a few folks took aim at it and blew gaping holes in my understanding - without actually explaining what I was trying to understand or explain here. As of yet, I have not gotten a satisfactory explanation for this discrepancy. No one I've talked to as of yet has been able to explain it to me, but if you think you know the magic answer, please let me know. Maybe I'm missing something obvious. Maybe I'm just not understanding this as well I as think I am. Who knows... At any rate, the chart below reflects the difference in 110V AC vs. 12V DC usage, even though I'm still at a loss to explain the details.
Remember, if in doubt, it's always better to put in too big of a wire than too small of a wire.
Stranded vs. Solid Wire
This one is a bit of a mind-boggler, but it's important. When electricity flows through a wire, it mostly flows on the surface of the wire, not through the middle. This effect is more pronounced on high frequency AC than it is on DC or low frequency AC. This means that a "wire" of a given size that made up of many smaller strands can carry more power than a solid wire - simply because the stranded wire has more surface area. This is one reason why battery cables in your car and welding cables are made up of many very fine strands of smaller wire - it allows them to safely carry more power with less of that power being dissipated as heat. However, this "skin" effect is not as pronounced in a typical 12V DC automotive application, and the wire and cable used there is stranded for flexibility reasons.
When looking at a chart or description of wire capacity, take note of whether it is referring to stranded or solid wire - some charts may not specify but instead assume a default based on the typical wiring used in a given application. For example, almost all automotive wiring is stranded while almost all home wiring is solid. For most applications, flexibility or the lack thereof will be more important, but for very high frequency AC applications, stranded wire might be a requirement.
Open Air vs. Bundles and/or Conduits
Heat is the primary determiner of the maximum amount of power any wire can carry, and the ability of that wire to dissipate that heat has a large impact on the final rating. Wires that are run in bundles (such as in a wiring harness or wiring conduit) cannot dissipate heat as easily as a single wire run in "open air", and as such must be "de-rated" to less than their maximum value to account for this. Also, wires that are run in areas that are unusually hot (such as in an attic or in an engine compartment) may need similar de-ratings. If both situations are encountered together (bundled wires in an unusually hot environment) then you need to de-rate for both factors and the capacity is further reduced.
In a car, almost all wiring is run in a bundle, and much of it runs near the engine. In a house, a lot of wiring typically runs through the attic, often in a bundle/group and sometimes in a conduit. Pay attention to this and size your wires appropriately.
Since all wires have resistance, the longer the wire, the greater the resistance. This means that for longer wiring runs you need to use a larger wire to compensate. This phenomenon is often referred to as "voltage drop", and for lower voltage automotive systems, the loss of 2V or even 1V can be significant. On longer wire runs, plan on using a larger size wire. There are specific voltage drop calculations that depend on the wire size in use, the length of the wire, the load applied, and the voltage in use. The National Electric Code has tons of charts for this, but there's a nifty online voltage drop calculator that one of my readers pointed out to me that does 120V AC as well as 12V DC - and even 6V DC. You'd be surprised at some of the voltage drops you can find just form the wiring in use, so experiment with the calculator a bit to see if it's worth going to the next highest size wire in your application. On automotive applications of only 12V, losing a single volt of power in the wire is a whopping 8% loss, so it can be a big deal for voltage critical applications like your headlights where more voltage = more light. Kudos to Ron White for providing me with the link to that calculator, and kudos to the folks over at PowerStream.com for putting that calculator and other data online.
Duration of Usage
Some electrical loads are continuous for long periods of times (like a light in your house or the headlights on your car) and some are much more intermittent (like a garbage disposal in your house or the starter in your car). This affects the wire size used - the longer a wire is in use, the more heat it will tend to retain. A wire for something that is only used for short periods (like the starter in your car) does not need quite as large of a wire as something that will be in use for very long periods of time. This means that for long-duration uses, you must de-rate the wire even further and use a larger size.
There are four basic units of measurement for electricity:
There are a number of formulas that relate each of these four things - they all change in relationship to one another such that if you know any two you can calculate the other two. Lots of folks on the Internet have easy-to use calculators that allow you to do this online - http://www.sengpielaudio.com/calculator-ohm.htm is one. The formula wheel below was on their website and presents the info in a pretty easy to understand format.
This chart is a simple "max capacity" chart for a short wire run. Increase the wire size for long runs - for example the wires running to the back of a vehicle to power the taillights may need to be one size larger to account for the length.
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Page last updated 12/27/2011 10:23:21 AM