We receive a lot of questions about wire amp ratings, or current ratings, and with good reason. Most manufacturers don't provide them. Although a wire or cable's ampacity can be calculated from a few key factors in the item's specs and your intended use, we know most folks making the actual purchase are not electrical experts. So here's a rundown of what amps are all about, including a helpful chart.
While the article typically uses the word "wire," you can usually substitute the word "cable" in its place. Technically speaking, wires are the individual strands of metal that are bunched or twisted together to form one or more conductors in a cable. It's common to refer to cables as wires, even though wires are often just one component of a cable.
Please note: This information does not constitute a recommendation as to which wire is right for your job. While Pro Wire and Cable stands behind the accuracy of our product listings, we cannot be held responsible for product decisions. We always recommend consulting the National Electrical Code, your local building inspector, a professional electrician or electrical engineer to properly select materials for your application that adhere to local electrical codes and relevant industry standards.
What is Wire Ampacity?
Ampacity is the maximum current (measured in amperes, or "amps") that a wire or cable can carry while remaining within its temperature rating. In other words, amps measure how much electrical current is flowing, and ampacity is the safe operating limit of current. Using a wire below its maximum current carrying capacity ensures the conductor doesn't melt itself, or its protective insulation. Conductors heat up when carrying electricity because the electrons are basically colliding with their neighbors as they flow along the length of the wire. This heat is a byproduct of resistance, which is inversely proportional to the diameter or gauge of the wire, usually represented as AWG (or MCM for large-diameter wire).
So, the smaller the wire diameter, the more resistance there is, and therefore the more it will heat up for a given current, compared to a larger diameter wire. Smaller wires tend to have lower ampacity than larger ones.
What Affects Ampacity Besides Wire Size?
In addition to the size of a wire conductor, there are four main factors that affect a given wire's safe operating limits: conductor material, insulation material, length of wire and installation environment. Things can get complicated quickly! The reason has to do with heat dissipation. A safe amp limit depends on the wire's ability to stay below its temperature rating under various loads.
Copper has less resistance to electrical current than aluminum, so it will heat up less at a given current. The same size wire made with copper will have a higher ampacity compared to an aluminum alternative. Aluminum also has a higher thermal expansion rate, so it is more prone to failure due to repeated thermal expansion/contraction cycles. However, aluminum tends to be less expensive than copper, so while a smaller copper wire could get the job done, a larger aluminum one might be more cost effective. Silver and gold are also used as electrical conductors, but only in very specialized, small-scale applications.
Some types of insulating materials are more resistant to heat than others, allowing for higher amp ratings. For example, Teflon-insulated wire can safely carry much more current than the exact same wire with PVC insulation. This is why temperature rating of a wire can affect amp rating. If the insulation maintains its integrity even as the wire and ambient temperature heat up, then more current can safely flow.
Length of Installed Wire
The longer the wire is, the more resistance there is to impede the flow of electricity. Every addition to a conductor's length adds more collision points between electrons as they pass through the material. That's why a given amp rating might not apply for a particularly long run. Generally speaking, the wire should only be as long as necessary, but longer runs may require increasing the size, to maintain the amp rating. Because extra length adds extra resistance that can increase heat, it can also limit how much power reaches the other end, unless you size up the wire. Even if your wire is not overheating, power loss over a long run could damage equipment at the other end.
Installed Environment: Free Air, Conduit & Ambient Temperature
Once again, it's all about heat dissipation. Although resistance creates heat, it doesn't necessarily destroy the conductor or its insulation. It is heat itself that causes damage. Naturally, wire that is located in free air will dissipate its heat much more effectively than if it is enclosed in a conduit, raceway, or covered with other materials like wood, vinyl siding, or carpeting. Material surrounding a wire acts as an insulator, not a conductor of heat, so ampacity is reduced. The same is true if multiple wires are run through a conduit or other enclosure. More wires = more heat to dissipate.
NEC provides tables for adjusting allowable ampacities when the ambient temperature deviates from an expected range for the temp rating of the conductor, as well as for number of conductors in raceways or cable trays, when part of the wire is exposed to sun or wind, etc.
What's the Difference between Amperage and Voltage?
If amps are so important to safe wire use, why are amp ratings optional in wire and cable print legends while voltage ratings are required? As we discussed, amperage (measured in "amps") describes the flow rate of electricity. Voltage (measured in volts) is the force that causes current to flow. Although it is possible to have voltage without amps, amps don't flow without voltage. With that in mind, voltage is the required rating because it can be dangerous by itself. For example, if you touch a wire that has no current flowing through it, but is hooked up to a 240 volt circuit, it will still hurt. Current (amps) is only dangerous when voltage is applied. Wires are voltage rated because that sets a limit on the force that can be safely applied to a wire, before other factors are considered.
Don't Forget About Connectors and Terminals
Choosing the correct wire at the appropriate amp rating is only part of the system. Because electricity flows through connectors and terminals, they need to be given the same consideration. There are many ways to connect wires to each other, and to their respective components. Typically, connectors that are made for the size of wire you're using are rated to handle one size larger, for safety. As a rule of thumb, if you're not sure, connectors should be sized up from the wire size used, to be certain they don't create a point of failure in your circuit.
Once again, we'll caution: if you're not sure, it's better to consult a professional than to make a potentially dangerous decision.
Tray Cable & Building Wire Amp Ratings:
Conductor Temperature Rating
|60°C (140°F)||75°C (167°F)||90°C (194°F)||60°C (140°F)||75°C (167°F)||90°C (194°F)|
|TW, UF||RHW, THHW, THW, THWN, XHHW, USE, ZW||SIS, FEP, RHH, RHW-2, THHN, THHW, THW-2, THWN-2, USE-2, XHH, XHHW, XHHW-2, ZW-2||TW, UF||RHW, THHW, THW, THWN, XHHW, USE||SIS, THHN, THHW, THW-2, RHH, RHW-2, USE-2, XHH, XHHW, XHHW-2, ZW-2|
|Copper||Aluminum or Copper-Clad Aluminum|