*Using the Wire Size Calculator*
By: Spitfire
02 November 2008

When choosing wire and cable for alternative power systems it's important to keep safety and efficiency in mind. Safety is simple - choose a wire that can carry more Amperage than you're going to need. Efficiency is usually expressed as how much power (Wattage) is lot over a given cable distance. The calculations are well understood but tedious and time consuming to do "what if" scenarios like "what if I changed the system voltage" or "what if I wanted to move the batteries farther away", so I created a spreadsheet to make life easier.

Link To Wire Size Calculator Spreadsheet

To use the spreadsheet you simply enter your data into the Blue colored boxes and the read the results in the table below. Here's an explanation of the data that goes into the Blue boxes.

  1. Load (Watts). Put the maximum peak load (not surge load) you expect to see over this circuit.
  2. Nominal Voltage. This is your system Voltage, usually 12, 24 or 48VDC.
  3. Wire Length. Put in the one way length of your circuit.

The table automatically adjusts based on your Blue box inputs.

There are a lot of columns that I needed to make the calculator but you don't care about. This is all you usually need to know.

  1. Wire AWG. This is the wire / cable size of a single conductor.
  2. Wattage Lost (Efficiency). Based on the Load, Nominal Voltage and Wire Length, you can see what percentage of Watts will be lost. As a general goal, we try to design systems with a maximum loss of 3%. You'll notice that this column is color coded and changes colors as you change your input parameters. The color is Green if you're losing <3%, Yellow if >3% ~ 5%, and Red if greater than 5%. NOTE: This calculation has *already* added the distance back to the source so this is the *total* out and back circuit loss.
  3. Remaining Amps (Safety). This column shows the Max Cable Amps - These Amps as calculated from your input parameters. If the result leaves some Ampacity in the wire the color is Green. If the current exceeds the wires current rating the color is Red, which you should avoid using.

The object is to find a combination that gives you a Green color in Remaining Amps and a Green color in Wattage Lost.

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