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In this blog we will be looking at a basic spreadsheet approach to determine an AC cable schedule that calculates voltage rise. For more detail see our video presentation.
What is an AC cable schedule?
An AC cable schedule outlines what AC cables are being used on a commercial solar project, their lengths, what current they are carrying, where they start and end and the necessary voltage rise calculations that are required to be adhered to; in the case of 3 phase, a maximum of 2% (according to Australian Standards, please check your own local, state and Federal acceptable voltage rise).
The spreadsheet approach
Using a spreadsheet allows the designer to automate some of the calculations in addition to the required inputs.
In regards to the above table we have the cable run number, start and end position of the cable, type, in this case, XLPE and size.
I have also included a section which asks can the cable carry the current, yes or no. The option here is to use one of the lookup functions that automatically “fetches” the correct answer but in this case it is up to the user to check the current carrying capacity (CCC) of the cable in question.
This is an incredibly important calculation in conjunction with the CCC of the cable and I have automated this part using a vlookup that references jCalc.net and they use the cable impedance method to determine the voltage drop/rise.
In the first screenshot the v lookup formula contained in the cell showing 0.9000% volt rise looks at the value in column F under cable size, in this case 400 mm, searches the table referenced as $T$3:$U$18, looking in the second column, cable impedance, where it extracts the figure of 0.096.
Then the calculation multiplies 0.096 x 1.732 (the square root of three as it is three phase) then multiplies by the current, in this case, 144.34 A, multiplies again by the cable run in metres, 150, divides by 1000 to get resistance per metre and divides again by the voltage to achieve a voltage rise expressed as a percentage.
Phew. A lot going on but the maths is fairly basic.
All of this can be done manually but by using a spreadsheet you have a record of the project cable details and it can be used time and time again.
See our video presentation for more details.
In future presentations we will be looking at how to construct more complex spreadsheets that automate more calculations.
Any commercial solar project requires documentation and the AC cable schedule is no exception. When it comes to calculation and assessing factors such as CCC and voltage rise this can be performed manually, use an online tool and/or create your own spreadsheet but whatever the method accuracy and correct detail is of the utmost importance.
It is up to you as the designer installer to ensure that all information is correct.