Ampacity
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Ampacity, also described as current rating, is the RMS electric current which a device can continuously carry while remaining within its temperature rating. The ampacity of a cable depends on:
- its insulation temperature rating
- cable power dissipation (copper loss)
- ability to dissipate heat, which depends on its surroundings.
- ambient temperature
The ampacity for a power cable is thus based on physical and electrical properties of the material & construction of the conductor, the composition of the insulation, ambient temperature, and environmental conditions adjacent to the cable. Having a large overall surface area may dissipate heat well if the environment can absorb the heat.
In a long run of cable, different conditions govern, and installation regulations normally specify that the most severe condition along the run governs the cable's rating. Cables run in wet or oily locations may carry a lower temperature rating than in a dry installation. Derating is necessary for multiple circuits in proximity in adjacent raceways. When multiple cables are bundled together, each contributes heat to the bundle and diminishes the amount of cooling air that can flow past the individual cables. The overall ampacity of the insulated conductors in a bundle of more than 3 must be derated, whether in a raceway or cable. Usually the de-rating factor is tabulated in a nation's wiring regulations.
Depending on the type of insulating material, common maximum allowable temperatures at the surface of the conductor are 60, 60, 75 and 90 degrees Celsius, often with an ambient air temperature of 30°C. In the US 105°C is allowed with ambient of 40C, for larger power cables, especially those operating at more than 2kV.
The allowed current in cables generally needs to be decreased (derated) when the cable is covered with fireproofing material.
Electric wires have some resistance, and electric current flowing through them causes voltage drop and power dissipation, which heats the cable. Copper or aluminum can conduct a large amount of current before melting, but long before the conductors melt, their insulation would be damaged by the heat.
For example, the United States National Electric Code, Table 310-16, specifies that up to three 8 AWG copper wires having a common insulating material (THWN) in a raceway, cable, or direct burial has an ampacity of 50A when the ambient air is 30°C, the conductor surface temperature allowed to be 75°C. A single insulated conductor in air has 70A rating.
Ampacity rating is normally for continuous current, and short periods of overcurrent occur without harm in most cabling systems. The acceptable magnitude and duration of overcurrent is a more complex topic than ampacity.
When designing an electrical system, one will normally need to know the current rating for the following:
Some devices are limited by power rating, and when this power rating occurs below their current limit, it is not necessary to know the current limit to design a system. A common example of this is lightbulb holders.
[edit] Current rating
For electronic components (such as transistors, voltage regulators, and the like), the term current rating is more-commonly used than ampacity, but the considerations are broadly similar. However the tolerance of short term overcurrent is near zero for active devices, as the thermal capacities are extremely small.
