What is Ampacity?

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  • Written By: Felicia Dye
  • Edited By: Heather Bailey
  • Last Modified Date: 20 January 2020
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Ampacity is an electrical term used interchangeably with the term current-carrying capacity. These words refer to the maximum load of current a cable can carry. Ampacity can vary greatly from one cable to the next. The variance can be based on internal factors, external factors, or both.

To understand this, it may be helpful to have a basic understanding of electrical conduction. Copper and aluminum are considered good conductors of heat. This means that large amounts of electricity can travel through them from one point to another without them disintegrating. Of the two, copper is the better conductor. This is why it is so commonly used in electric cables.

The electricity that moves through a copper wire is actually a group of electrons being pushed by voltage. The higher the voltage, the more electrons that can be sent flowing through the wire. Ampacity refers to the maximum number of electrons that can be pushed.

It could be easy to mistakenly think the maximum amount of electricity that can flow through copper is equal to the ampacity of the cable it is in. This conclusion, however, is false. There are several factors that can result in a cable having lower capabilities than its inner wire.


A cable’s conductor is insulated. If it wasn’t, the part of the cable that can be touched, also known as the jacket, would burn up. When current travels within that insulation, heat is generated. That insulation can tolerate less heat than the conductor inside. If this is ignored, the insulation and perhaps the entire cable will burn up.

Ampacity is also affected by external factors. A cable lying in the sun is also impacted from a high level of exterior heating. The cable must be able to withstand this and the heat generated by the current flowing through it. A cable in a cool area, such as underground, can have a higher ampacity because the external temperatures can help lower the cable’s temperature. The amount of heat a cable can sustain while functioning normally and safely is its temperature rating.

Another factor that affects ampacity is the size of the conductor. When carrying electricity, a piece of copper is like a tunnel. The smaller the tunnel, the more limited the flow. If a wider tunnel is made available, the flow can increase. Cables that are larger in diameter tend to be that way because the conductor inside is larger. This results in higher ampacity.


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Post 2

@allenJo - I would think that those factors might affect your connection. I have cable myself, and I’ve read that the cable wiring inside and outside the house would have some impact on connection speed. I would call a technician and have them check to see if indeed that is the problem.

Post 1

This is a very eye opening article. I wonder if it would explain how Internet connection speed might be affected by the ampacity of cable?

The reason that I ask is that I have cable for Internet, but I often have slow Internet speeds. I realize that if you have cable, you have other people in the neighborhood who are using the same connection, and that could create congestion and therefore reduce your Internet speed.

However, I wonder if having some coaxial cable outdoors, or in the heat, or in various gauges or lengths – factors which would affect its ampacity according to this article – would play a factor in the speed of the transmission as well.

I am beginning to think that it would, although I don’t know for sure whether the specs on my cable are “good” or “bad” all things considered.

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