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What is a schedule?
With any project involving various material and components there should be a schedule. A schedule lays out what parts/material are needed, what are the quantities, where they are going and who is responsible for their installation.
In this blog we look at a DC cable schedule for a commercial solar project that has been constructed using a spreadsheet. Once the schedule has been completed it can be provided to the site project manager/foreman to be used as a working document (preferably soft copies, but print it out if it suits your needs better).
What are the headings
In this case we have a title being DC Cable Schedule, then the individual columns that include: cable size, brand, panel wattage, string current, Vmpp* of panel, number of strings etc.
* Voltage at maximum power point
The installer/designer fills out most of the information with some cells performing calculations based on inputted data.
In many cases data validation is used as a reminder to input the correct values such as inverter/MPPT/String DC isolator signage or a reminder to check the panel data sheet. This is achieved by using the input message area after clicking on the data validation tab.
Calculations are performed in some cells such as the voltage of the DC string using simple Excel formulas.
Also the determination of the Vmpp of the string based on inputted data concerning the number of panels and the Vmpp of the panel in question.
Vlookup to the rescue
Another example is the actual DC volt drop that uses a formula that includes Vlookup.
The Vlookup references a cable resistance table that looks at a certain value and extracts another value based on it.
It looks at the value in B4 which is the cable size (in this case 4mm2) which is selected from a drop down list, then it looks at a table that contains different DC cable sizes and the corresponding resistance figure which is in the second column.
It then takes this figure and multiply it by the string current, 9.5A, then by the distance to the inverter, 35 metres x 2 (there and back) then divides by 1000 as the resistance figure is in ohms per km then divides by J4 which is the string voltage a Vmpp to get a % figure.
In addition there is a formula that gives a visual warning should the string voltage, that uses Voc max of the panel with temperature compensation, exceed 1000 Volts
The last two columns are there for checks and balances. They provide a record of who performed the cable to DC isolator connection and also what was the serial number.
The success of a commercial solar project lies in attention to detail and this ideally should apply to the documentation used both on site and as part of the whole process. Using a spreadsheet allows the automation of some calculations and once everything is checked over should be printed out and placed in the site folder.