Load Factor to calculate PV cables ampacity

Electrical PE

I have been asked by a client to submit ampacity calculations for PV cables in underground trench to make sure that the cables' ampacity don't get derated by much due to temperature build up. When using softwares that model cable ampacities, one of the inputs is load factor.
The uncertainty I have is what should be used for load factor. One recommendation I received is to use total number of hours of sunshine at summer solstice and divide it by 24. The number I get is around 62.5%. But a PV system, obviously, does not continuously generate its maximum output all day.

By definition, the load factor is calculated by dividing the average system output (over a certain period of time - assuming it is 24hr for solar) by the maximum output (over the same period of time). Can the output of the production modeling software be used to calculate the maximum daily load factor for the whole year, and use it in the ampacity calcs?

I am wondering which of the above methods the solar industry is using to calculate the load factor, or if there is any other method that is being used?


Staff member
Without direct experience with the software and calculations involved, it seems to me that for thermal calculations the relevance of a load factor is that over whatever time period is significant for the environment (probably a couple of hours at most?) the average heat dissipation will be the peak heat generation reduced by the load factor. This can also be applicable over a longer time period than the thermal time constant if the load factor also represents, at least in part, a time diversity of individual loads.
In the case of PE, all of the array sources are likely to be producing full output for the same several hour period each day. To me that would mean that we are not justified in reducing the heating effect based on the load factor.
Are you referring the allowance to simulate/model current for systems over 100KW in 690.8? I think the appropriateness of adjusting ampacity depends on the design of the system and what conductors we are talking about. Most larger systems these days tend to have high DC/Ac ratios and thus the inverter output WILL be at max for hours on end. I cant see how applying any load factor would be appropriate there. On the other hand, PV, source circuits (and DC combiner outputs) will likely never/rarely see nameplate current so some reduction may be justified there.


Senior Member
I'm not sure how load factor would go into an ampacity calculation. Load factor has more to do with how efficiently a generator is being used than ampacity. Are you sure you are not actually looking for the diversity of the load?