Basic cable sizing and ampacity determination

This is one of the first things an electrical engineer in the nuclear power industry will probably do: sizing a cable by determining, among other things, its ampacity. . . .

New cables, like most things, must be sized in accordance with whatever code, standard, or procedure(s) the plant is committed to. Two common standards used by nuclear plants are the National Electric Code (NEC) and the IPCEA standards. If they don’t use one of those, they might have their own internal procedures instead, but often even those will reference one — or both — of these two documents as their basis. The National Electric Code is prevalent inside the industry and out.

A cable must be sized so that it can adequately handle, first, the full-load current it’s designed to carry and second, any future capacity that may be added to it. Cable life and personal safety are the primary concerns driving these requirements. The amount of full-load current a cable can safely carry is called its “ampacity.”

Go ahead and add this word to the dictionary of your word-processor of choice. You’ll be using and seeing it a lot.

Cables are typically sized so that their ampacity (“current-carrying capacity”) is 1.25 times their designed full-load current. If there will be any intermittent loads operating on top of the standard load, 100% of their value is added to the ampacity requirement. The NEC distinguishes intermittent or noncontinuous loads from “continuous” loads. Continuous loads are defined as loads expected to operate for 3 hours or more. The NEC states the ampacity requirement like this:

Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load. [Article 210.19(A)(1) of NEC 2014]

Oversizing the cable makes sure that the cable will not overheat during normal use. When cables overheat, their insulation degrades. Given enough time, degraded cables can lead to shorts if the metal conductor inside burns through its insulation and comes into contact with some nearby conductor, such as a conduit, a cable tray, or a panel.