System Design help

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mnlara

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I have an initial design concept of one Grid-tied Photovoltaic system (residential) consisting of 36 Sunpower SPR-327NE-WHT-D (327W) in 6 strings of 6 modules and a Fronius IG Plus 11.4-1 UNI (240V) inverter. The array layout is 4 strings of 6 modules mounted in a pitched roof of 42 degrees and 2 strings of 6 modules mounted on a flat roof tilted 10 degrees (both layout has azimuth = 213 degrees). How many wires and what size of wires needed from:
1. Pullbox to inverter ( can we use a combiner box in between array pullbox and inverter?)
2. Inverter to Service Panel.
I believe 60 amp breaker will be used at the service panel.
 
Your basic design options on the dc side of the system are described in Diagram 2a & 2b (which uses a Fronius product in the example):

http://solarprofessional.com/article/?file=SP2_3_pg28_Jacoby&search=

as are some basic design considerations:

* What is the cost difference between a transition box?plus the necessary internal components?and a combiner box?
* What is the cost difference between installing multiple PV source circuits or one PV output circuit?
* Will using a separate combiner box require a separate dc disconnect switch? If yes, what is the additional cost to install?
* Will using a separate combiner box mean that the PV output circuit current rating exceeds the input rating of the inverter input combiner terminals or fuses?
* Does bypassing the integrated fusible combiner violate the product listing?
* Can the combined output circuit be installed per the manufacturer?s specifications, as required in NEC 110.3(B)?
* Will combining the conductors change the required conduit size? If yes, how will this affect material and labor costs?
* Are there any other benefits to combining? Will the wire pull be easier or harder? Will fewer or more dc disconnects be required by the AHJ or the Code?

Your situation is complicated by the fact that you are transitioning conductors off of wo separate roof surfaces, but the basic thought process is similar.

The logic behind Code-compliant conductor sizing is discussed in this article, which also includes examples:

http://solarprofessional.com/article/?file=SP4_3_pg14_QA_1&search=

RE: the AC OCPD. Yes, the max continuous output current from the inverter (47.5 A @ 240 V, 1-P) is intentionally set to let you get the most PV capacity possible on a 60 A OCPD (47.5 A x 1.25 = 59.4 A, which is less than or equal to 60 A).
 
You do not supply enough information for a proper design. There are at least four considerations, note that your location (temperature) is important.

DC array wiring size is based on the procedures in the 2011 NEC, Art 690.8"

1. 2011 NEC Art 690.8(B)(2) "Circuit conductors shall be sized to to carry not less than the larger of 690.8(B)(2)(a) or (2)(b).".

2. 690.8(B)(2)(a) Calculation is 125% x 125% of the sum of the parallel module short circuit currents without any additional correction factors for conditions of use (temperature, conduit fill, etc.).

3. 690.8(B)(2)(b) Calculation is 125% of the sum of the parallel module short circuit currents with correction factors for conditions of use (temperature, conduit fill, etc.).

4. Also, in many cases the temperature rating of the terminals at the ends of the wiring have to considered and may affect wire size. This calculation is based on 125% x 125% of the sum of the parallel module short circuit currents and the 30?C ampacity of the conductor using the 75?C ratings of assumed 90?C rated conductor to get the conductor ampacity.

The temperature of the conductors is affected by exposure to sunshine on the roof. [Table 310.15(B)(3)(c)]

For the AC wiring the distance (length of wire) is important as you need to keep the voltage drop in these wire below 1.5% (actually the voltage rise above the utility service to your main panel). Fronius has some tables on this.

You should consider two smaller inverters to eliminate the mismatch loss you will get with modules at different tilts.
 
mnlara said:
I have an initial design concept of one Grid-tied Photovoltaic system (residential) consisting of 36 Sunpower SPR-327NE-WHT-D (327W) in 6 strings of 6 modules and a Fronius IG Plus 11.4-1 UNI (240V) inverter. The array layout is 4 strings of 6 modules mounted in a pitched roof of 42 degrees and 2 strings of 6 modules mounted on a flat roof tilted 10 degrees (both layout has azimuth = 213 degrees). How many wires and what size of wires needed from:
1. Pullbox to inverter ( can we use a combiner box in between array pullbox and inverter?)
2. Inverter to Service Panel.
I believe 60 amp breaker will be used at the service panel.
Click to expand...

An alternative, now that everyone has explained just how complex this install is going to be, are these guys --

http://us.sunpowercorp.com/homes/products-services/solar-panels/AC-home-solar-panel/

One concern I have, given the you have two different pitches and the roofs seem close enough that people are discussing combiner boxes, is self-shading from one roof to the next. How's the shade situation? Do you have a SunEye photo you could share?

(Disclaimer: I work for the microinverter manufacturer.)
 
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