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Wire Ampacity.

Maximum continuous current limits per NEC Table 310.16. Ensure your breaker protection matches your conductor thermal rating.

Maximum Safe Current 35 Amps
SAFE LIMIT

What Is Wire Ampacity?

In electrical engineering, ampacity (a portmanteau of "ampere capacity") is the absolute maximum amount of electrical current a conductor can carry continuously without exceeding its temperature rating. Unlike voltage drop—which is about efficiency and equipment performance—ampacity is strictly about fire safety.

When electrical current (Amps) flows through a wire, the natural resistance of the copper or aluminum generates heat. If too much current flows through a wire that is too thin, the heat will accumulate faster than it can dissipate into the surrounding air. Eventually, the plastic insulation surrounding the wire will melt, exposing live copper, causing short circuits, and potentially starting an electrical fire.

A wire ampacity calculator is used to determine the exact safety limit for a given wire size (AWG), material, and insulation type, ensuring that the circuit breaker protecting the wire is sized correctly. This calculator uses data strictly derived from the National Electrical Code (NEC) Table 310.16 (formerly Table 310.15(B)(16)).

How to Use This Wire Ampacity Calculator

This tool eliminates the need to cross-reference complex NEC tables manually. Follow these steps to find the safe current limit for your conductor:

  1. Select Conductor Material: Choose Copper (Cu) or Aluminum (Al). Copper is more conductive and can carry more amps for a given size than aluminum.
  2. Select Insulation Temperature Rating: Look at the printing on the wire's jacket.
    60°C: Common for older wiring, TW, UF, and often used as the default for Romex (NM-B) under 100A.
    75°C: Common for THW, THWN, and many equipment terminals.
    90°C: Common for modern commercial/industrial wire like THHN, XHHW-2, and USE-2.
  3. Select Wire Size (AWG): Choose the gauge of the wire you intend to use. Remember that in the American Wire Gauge system, smaller numbers mean thicker wire (e.g., 8 AWG is thicker than 12 AWG).

The dashboard will instantly display the Maximum Safe Current. Your circuit breaker or fuse must be sized at or below this number to adequately protect the wire.

The 60°C, 75°C, and 90°C "Weakest Link" Rule

One of the most common mistakes made by DIYers and junior electricians is assuming that because they bought "90°C THHN wire," they can automatically use the ampacities in the 90°C column of the NEC table. This is often incorrect and dangerous.

The Principle of Terminations

The NEC (Article 110.14(C)) states that an electrical circuit is only as strong as its weakest thermal link. While the wire itself might be rated to withstand 90°C without melting, the terminals it connects to (like the circuit breaker in your panel or the screws on a wall outlet) are usually only tested and rated for 60°C or 75°C.

If you allow enough current to push the wire to 90°C, that heat will transfer directly into the 75°C breaker, potentially damaging the breaker's internal thermal-magnetic trip mechanism.

General Rules of Thumb:

Ampacity Derating: Ambient Temperature Corrections

The baseline values in NEC Table 310.16 (and in this calculator) assume an ambient room temperature of 30°C (86°F). If you are running wires through an area that gets much hotter than 86°F—such as an unventilated attic in the summer, or a conduit exposed to direct rooftop sunlight—the wire's ability to dissipate its own internal heat is severely compromised.

In these situations, the NEC requires you to apply an ambient temperature correction factor. You must multiply the wire's base ampacity by a decimal percentage (less than 1.0) found in NEC Table 310.15(B)(1).

Example: You are running a 10 AWG THHN (90°C) wire through an attic that reaches 50°C (122°F). The base ampacity of 10 AWG at 90°C is 40 Amps. However, the correction factor for a 90°C wire in a 50°C environment is 0.82. Therefore, the derated ampacity is 40A × 0.82 = 32.8 Amps.

Ampacity Derating: Conduit Fill Adjustments

Similarly, if you bundle too many current-carrying wires together in a single conduit or cable tray, they heat each other up. The NEC (Table 310.15(C)(1)) requires you to derate the ampacity of all wires in the bundle if there are more than three current-carrying conductors.

  • 4 to 6 conductors: Multiply ampacity by 80% (0.80)
  • 7 to 9 conductors: Multiply ampacity by 70% (0.70)
  • 10 to 20 conductors: Multiply ampacity by 50% (0.50)

Pro Tip: This is where 90°C wire shines. You are allowed to apply these derating percentages to the higher 90°C ampacity value, even if you are ultimately restricted by a 75°C breaker. As long as the final derated number is equal to or greater than your breaker size, the installation is code-compliant.

Ampacity vs. Voltage Drop: Which Governs?

It is crucial to understand that ampacity and voltage drop are two separate calculations, and you must always size your wire to satisfy the worst-case scenario.

Ampacity only cares about heat. It does not care how long the wire is. A 10 AWG copper wire at 75°C can safely carry 35 Amps whether it is 5 feet long or 500 feet long without catching fire.

However, if you run 35 Amps through 500 feet of 10 AWG wire, the voltage drop will be astronomical (the equipment at the end won't work). While a 10 AWG wire is perfectly safe (ampacity) for a 35A load, you would likely need to upgrade to a massive 1/0 AWG wire to make the circuit functional (voltage drop). Always use our Wire Size Calculator alongside this ampacity tool to ensure both safety and performance.

Frequently Asked Questions

What is the difference between ampacity and voltage drop? +
Ampacity is the maximum current a wire can handle before it overheats and melts its insulation (a fire safety limit). Voltage drop is the amount of electrical pressure lost over a distance due to the wire's resistance (an equipment performance issue). Both must be calculated, and the larger wire size dictated by either calculation must be used.
Can a 12 AWG wire handle 30 amps? +
Technically, according to the 90°C column of NEC Table 310.16, a 12 AWG THHN wire has an ampacity of 30 Amps. However, NEC Section 240.4(D) explicitly limits 12 AWG copper wire to a 20 Amp overcurrent protection device (breaker) under standard conditions. Therefore, you cannot put a 12 AWG wire on a 30 Amp breaker.
Why does Aluminum have a lower ampacity than Copper? +
Aluminum has approximately 61% of the electrical conductivity of copper. Because it resists the flow of electricity more than copper does, an aluminum wire generates more heat for the same amount of current. Therefore, an aluminum wire must be physically thicker (a lower AWG number) to safely handle the same ampacity as a copper wire.
How do I determine the temperature rating of my wire? +
The temperature and insulation rating are physically printed or embossed on the outer jacket of the wire. Common residential NM-B (Romex) must be treated as 60°C for ampacity limits. Individual wires pulled through conduit, like THHN or THWN, are generally rated for 90°C (though their terminations might restrict you to the 75°C column).
What is continuous vs non-continuous load? +
The NEC defines a "continuous load" as any load where the maximum current is expected to continue for 3 hours or more (e.g., commercial lighting, EV chargers, water heaters). For continuous loads, you must size the wire and breaker to handle 125% of the actual load to prevent gradual heat buildup from tripping the thermal breaker.