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Confusing online voltage drop calculators

April 13, 2020 — BarryK

I received an email from Gerard, claiming that my voltage-drop formula is wrong, by a factor of 2. That is, where I calculated 1.11V drop, Gerard said it should be 2.22V. See this post from a couple of days ago:

The factor of 2 difference should have been the big hint. I was confused for awhile, as some online calculators seemed to support what Gerard claimed. But then we both realised the reason for the difference...

Most of the online calculators are for AC cable, with ISO sizes, and use of twin-core. The online calculators require the length of the actual cable to be entered, not the total "round trip" through each core. So for my 15m cable from the solar panel, "15" has to be entered into the online calculator, not "30".

In other words, where it asks for "cable length" it means the length of the twin-core cable, not adding the return path.

Regarding ISO sizes, these are used for electrical building wiring in Australia and just about everywhere internationally. There is a nice table here that shows AWG versus ISO sizes:

In fact, that page is an excellent read to understand about all of the different cable sizes -- and in particular how confusing "auto cable" is: 

...that page has an online calculator, that requires entry of the length of one core, so in the case of 15m twin-core, enter "30". 

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