Sizing Feeders, Part 4

In Annex D3(a), the neutrals complicate feeder sizing because we cannot assume the load is evenly distributed. Let's see just how much more complicated this becomes.

After that first long paragraph, Annex D3(a) provides a summary of the loads. The first one listed is discharge lighting. This type of load has high nonlinear content, so there's substantial current flowing through the neutral. That means you must consider the neutral to be a current-carrying conductor.

Annex D3(a) references the adjustment factors listed in 310.15(B)(5)(c), and if you refer back to that summary of loads you see that the VA represented by lighting is the majority of the total load on that feeder.

So now you're not running a relatively small neutral conductor, you're running a current-carrying conductor. One way around this would be to use a local separately derived transformer system at each building. The larger conductor is easily a less expensive way to go. In this example, you have a cost-savings advantage.

In the high temperature area, the terminations don't limit the temperature rating of the conductor because they are located outside that area. The load is 177A, and if the terminations limited you to the 60°C column you'd be running 4/0. But you can use the 90°C column and run 2/0.

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.