Ampacity and What it Means to Me

Posted on Wednesday, October 15, 2014 by Dave Watson

Dave Watson, Director of Engineering, also known as Alpha Wire's resident wire and cable expert was prompted with the question below regarding ampacity.  Check out Professor Watson's explanation!

Q. What is ampacity? What does it mean to me and how is it measured?

A. In electronic wire and cable, ampacity is the maximum amount of electric current that the product can safely carry.  The entomology of the word is “amp”, the unit of measurement for electrical current and “acity”, which is short for capacity. So the easy way to think of ampacity is how many amps of electrical current a wire of cable is capable of carrying.

The primary importance of ampacity is that it is often the main driver for determining the conductor AWG that is required for a given application. The bigger the conductor, the more current it can carry. But of course bigger conductors are more expensive, so generally one would want to ‘right size’ the conductor AWG by selecting the size that is just big enough to handle the current.

You can measure or determine ampacity in a lot of different ways.  Look at it this way, when current is traveling through a conductor it meets the opposition of electrical resistance. The result is that this resistance causes the wire to get hot. The more current (more Amps), the more heat is generated. Copper conductors can actually take a heck of a lot of current before they will melt (called fusing current). So the question is, how hot do you want your wire to get during operation?  This leads to several considerations; the temperature rating of the insulation material, the ambient temperature of the area where the cable will be installed, the safety aspect of someone coming into contact with a hot wire or cable, etc.  The best starting point for sorting all this out are the tables contained in the National Electric Code, NFPA 70.  The Code does not cover all the AWG sizes that we use in electronic wire and cable, but the logic and calculations used can be applied in a broader context.

There are 2 important disclaimers I need to add.

First, (and likely a topic for a future ask Prof Watson) one must consider the voltage drop.  For now, just remember that the resistance of a wire means that voltage will be lost over the length of the wire and the longer the wire and greater the current, the greater the voltage drop.  There are certainly cases where the AWG size selection is more driven by the voltage drop and not the ampacity.

Secondly, and most importantly, recognize that correctly sizing a wire for the current it will carry has safety consideration.  It is very possible to have fires, damage to equipment and so on due to incorrectly sized wire. There are many tools available to determine or calculate ampacity.  Before implementing anything for a particular application, the results of any ampacity calculations should be reviewed and validated by a qualified Engineer and/or Electrician.



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