Optimizer output conductor sizing

[hhsting]

I have 3 strings of 15 modules one unit.
Each string of modules has 15 optimizer so it’s one optimizer per string.

I have total of three units from inverter. That’s total 18 DC conductors in one conduit.

The maximum output current of the optimizer is 15A according to datasheet. Optimizer is Power Optimizer P400.

Question:

1. I am using #8 THWN -2 up to junction box. So based on roof temp correction factor local temp is 34 Celsius and rooftop is add 22 Celsius so it’s 56 Celsius which is 71%. Conduit fill is total 18 so it’s 50%. #8 THWN-2 is 50A so 50x.71x.50 = 17.75A. So now in order to check #8 is ok should I take maximum output current optimizer 15A or should do max output currentx1.25 which is 15x1.25=18.825?

 

[jaggedben]

I think its 15x125%. 2014 NEC?

 

[Carultch]

I think its 15x125%. 2014 NEC?

It is.

To understand how this works, review what you do for PV circuits without power-conditioning electronics. PV circuits whose current is uncontrolled, whether source circuits, or combiner output circuits, will have two 125% factors that govern design. The first 125% factor is the enhancement factor, anticipating the event that the sunlight exceeds the standard 1kW/m^2. The second 125% factor is the continuous load factor, applicable to loads in general that qualify as continuous.

By contrast, if power electronics are involved, that are current-limited in their output capacity, the first 125% enhancement factor no longer applies. The maximum output current of the power-conditioning device (call it Imax), without any additional safety factors, takes the place of 1.25*Isc in your calculations. 1.25*Imax then takes the place of 1.56*Isc, in places where the 125% factors would ordinarily compound to form 156%. This reasoning applies for DC-to-DC converters (optimizers) whether module-level or string-level, microinverters, and inverters. No matter the irradiance, the device has a maximum current it can output, and it will "leave sun on the module" in order to stay within the limit.

 

[Designer101]

I think its optimizer current x 1.25=15*1.25=18.75A
looks like jurisdiction is following NEC 2014, So you have to add 22 degrees, else if you follow NEC 2017, the distance above roof top is 0.5",3.5" or 12". you can use different temperature derate factor, and looks like installing the conduit 3.5 inch above roof top is easily achievable. Please visit http://www.solarabcs.org for reference to use interactive solar reference map and respective temperature.
Also I have often used 90 degrees temperature column instead of 75 degrees for wire sizing and AHJ have no objections. if we can use 55A for #8 wire instead 50A , I am positive this might not be the option for your case.
we can use 3 separate conduits if space and aesthetic is not a problem and get rid of conduit fill factor value too.