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On 2016/01/15 at 7:15 AM, Chris Hobson said:

to de-rate them by 1.25 and round to nearest size whereas you calculation is higher (1.56).

Hi Chris you have answered your own question as to why the factor is 1.56 and not 1.25. Like TTT says it's all in the maths. Standard calculation for a breaker is 1.25, if that is then 'derated' for a solar application by 1.25 the calculation is 1.25*1.25 = ??

The reason that we 'derate' the calculation is that unlike in a normal situation with utility power where the amps with a given power draw can be fairly accurately determined (factor of 1.25) in a solar situation the figures can be a lot more unpredictable. The Isc of a particular panel is measured at STC conditions. Especially here in SA it is not uncommon to have conditions better than STC( standard test conditions) I have seen a 330w rated panel for instance comfortably running at 345w for instance. Hence the industry recommendation of the 1.56 factor for highly variable solar applications. 

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1 hour ago, BDL said:

... for instance comfortably running at 345w for instance ...

Ditto. Like my 2.1kw Canadian Kumax array. Have seen on the MPPT like in excess of 2700 watts generated on a good special day. 😜

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