385KW PV System Design

[djr3203]

I am assisting a senior engineer on an installation of a PV system for one of our commercial projects. I have been given the task for calculating the voltage drop and I have a few concerns. The following is the basic outline of the design:

- 4 separate PV arrays on different parts of the south and east sloping roofs.
- Each array is comprised of 7 panels per string with an average of about 20 strings per array (some arrays have more strings and some have less).
- The PV Panels have a Vmpp = 54.7V and Impp = 5.86A and Temp Coefficient Voc = -176.6mV/K

In calculating the voltage drop, I have come up with the following questions:

1. What temperature should I assume for temperature adjustment corrections for the ampacity and resistance of the conductors? The project is located in the San Jose, CA region, and I am not sure what assumptions I should make to adjust the numbers. Do I take the hottest recorded day? Do I take an average temperature number? Are there any stipulations in the NEC for choosing this value?

2. Is it common to use multiple combiner boxes before going to the DC disconnect and the Inverter? I am assuming a combiner box per array, and then having these combiner boxes feed into 1 combiner box at the DC disconnect and inverter. Is this an acceptable/common practice?

3. Do I neglect the combiner boxes while performing the voltage drop calculations? Or is there some way to calculate any increase in voltage drop due to the connection through the combiner boxes?

4. The current design has the arrays feeding into an inverter which then feeds into the main switchgear at the basement without any battery backup. When calculating voltage drop, do I want to keep the voltage drop less than 5% from the array all the way to the switchgear?

I appreciate any help and insight you all can add! If there any other concerns I have not thought about, please feel free to express them.

DJR

 

[jfpe]

DJR,
1. For conductor ampacity and resistance, I would reccomend using the July or August ASHRAE 2% dry bulb temperature with the NEC temperature adders for conduit in sun light.
2. Multiple combiner boxes are standard for an installation of this size. Typically, a "master combiner" or "subarray combiner" is used to combine the combiner box outputs before entering the inverter. A couple potential vendors are SolarBOS and Amtec. Also, many inverter manufacturers offer integrated subarray combiners.
3. I think you can safely omit the voltage drop in the combiner box.
4. Check with the inverter manufacturer on the recomended AC voltage "drop". Typical recomendations are 1-2%. Since the inverter is a source, rather than a load this will actually be a voltage rise. If the voltage rise is too high, the inverter may trip off line due to high AC voltage.

Good luck,
JFPE