Over the years, we have seen just about every combination of equations and numbers used to calculate the NEC required values on a DC disconnect switch. NEC requires these values to be calculated and shown on the DC disconnect switch, but it doesn’t give the equations which leads to many interpretations of the calculations should be.
Here are the equations Pure Power uses:
- Operating Current = Imp * # of strings
- Operating Voltage = Vmp * # of modules per string
- Maximum Current = Isc * # of strings * 1.25
- Maximum Voltage = 1 – [Beta * Voc * (25 – ASHRAE low temp)] / 100
We treat Operating Current and Operating Voltage as the values that would be present in STC conditions. This is a good representation of the average operating values that would be expected on a cool sunny day.
The calculation we most often see as incorrect is the Maximum Current. Many engineers use a 1.56 (1.25 x 1.25) factor in their calculation. But that is not correct, because only the first 1.25 represents possible current (for conditions better than STC), but the second 1.25 factor is for loading the overcurrent protection device to 80% (which isn’t 1.25 x more current). Therefore, only use a single 1.25 factor for calculating maximum current.
Maximum voltage uses the same equation used to calculate the number of modules per string using the ASHRAE design low temperature for the site location.