Array wiring, conduit, and grounding questions

[sunnyguy]

Thanks. So I'll plan on doing the 2 runs of conduit 1/2" EMT, butt splices from USE-2 to THWN-2 in NEMA 4 junction boxes, EMT grounding lugs.

Do my wire size calculations look right? 12AWG DC conductors and 8AWG EGC? What type of connectors are recommended for combining/transitioning ground wires in the junction boxes?

Any suggestions on conduit supports on tile roofs? These: http://www.ecofastensolar.com/store/...p?idproduct=27 ?

Thanks

[SunEagle]

Each MPPT input is totally separate and you will not have a problem with backfeeding between the strings.

So the way the inverter is set up it looks like you can put 2 strings in parallel to one MPPT and 2 strings to the second MPPT. So technically you are not exceeding the max of 2 panels/strings to the inverter and may not need OCPDs.

[solarix]

Each MPPT input is totally separate and you will not have a problem with backfeeding between the strings.

[solarix]

Each MPPT input is totally separate and you will not have a problem with backfeeding between the strings.

[sunnyguy]

Because 2 strings are paralleled to each separate MPPT input then I am unsure whether it is necessary to consider them as possible backfeed sources for one another.

If they are then a fault could result in up to 3 strings backfeeding current through a DC conductor meaning OCPD devices are necessary because the current exceeds the fuse rating of the module and to avoid conductor upsizing.

If they are not then a fault could result in only 1 string backfeeding current through a DC conductor meaning OCPD devices are not necessary because conductor size can accommodate for this and it does not exceed the fuse rating of the module.

[sunnyguy]

So am I on the right track or do I have it wrong and need OCPDs?

[sunnyguy]

Sorry for the omission of details.

Panels:
Pmax = 295W
Imp = 8.13A
Isc = 8.62A
Voc = 44.7V

Inverter
Fronius Primo 8.2 Transformerless Dual MPPT
2 sets of DC input terminals per MPPT (4 total)
Max DC input 18A per MPPT
Ground Fault Protection : Yes

String B1 & B2 are in series, resulting in 2 strings of 8 and 2 strings of 9.

Wondering about EGC sizing, combining array EGC in junction boxes, ability to use standard NEMA 4 junction boxes since there is no string combining, and other relevant 2011 NEC requirements.

Adjustment factors (NEC 310.15):

Conduit 0.5-3.5in above roof temperature adder: 22C
Number of conductors ampacity derating (4-6): 0.8
Number of conductors ampacity derating (7-9): 0.7
Temperature derating factor of 90C conductor @ 65C (43C+22C): 0.65

Non-Adjusted Ampacities @ 30C:
12AWG 90C copper: 30A
10AWG 90C copper: 40A
8AWG 90C copper: 55A
6AWG 90C copper: 75A
4AWG 90C copper: 95A

Adjusted Ampacities @ 65C:
12AWG 90C copper (1-3 conductors): 19.5A
12AWG 90C copper (4-6 conductors): 15.6A
12AWG 90C copper (7-9 conductors): 13.6A

10AWG 90C copper (1-3 conductors): 26.0A
10AWG 90C copper (4-6 conductors): 20.8A
10AWG 90C copper (7-9 conductors): 18.2A

8AWG 90C copper (1-3 conductors): 35.7A
8AWG 90C copper (4-6 conductors): 28.6A
8AWG 90C copper (7-9 conductors): 25.0A

6AWG 90C copper (4-6 conductors): 39.0A
6AWG 90C copper (7-9 conductors): 34.1A

4AWG 90C copper (4-6 conductors): 49.4A
4AWG 90C copper (7-9 conductors): 43.2A



Max EGC Current = Isc * 4 * 1.25 = 43.1A ?
Max String Current = Isc * 1.25 = 10.8A ?

So how does this apply to the conductors and different segments of conduit?

For example, the only conduit carrying 4 pairs of DC wires would be from array 4 to the inverter and would require a 4AWG EGC. However, the inverter only accepts up to 6AWG, so it seems 2 conduits are required.

So I guess I can go with 2 conduits, one for strings A&B and one for strings C&D.

1/2" EMT 37% fill
DC Conductors - 4 x 12AWG (upsized for <1% voltage drop)
EGC - 1 x 8AWG

However in the conduit segments that carry wiring from only 1 string can I use a 12AWG EGC?

Since I am not combining strings I can use standard NEMA 4 JBs and not expensive "listed for purpose" solar boxes, right?

[solarix]

What inverter are you using?
Most inverters work with strings of between 8 and 13 panels.
You'll probably be dividing up your arrays to meet the inverter's string requirements instead of trying to connect a bunch of odd sized strings to the inverter.

[SunEagle]

Unless your inverter already has a built in "combiner box" I don't believe you can run 4 different "strings" to it without installing some type of over-current device for each string.

As the others have asked, what are your panel and inverter specs?

[solarfrank]

looks like you can run all 4 string with no 10 and also ground no 10 or 8 depends of your inverter specs in a 3/4" conduit from the roof , but we need more details for more accurate information.

[sensij]

You are going to need to share more info for help. What are the electrical specs of your panels? What inverter (or charge controller) are you using?

[sunnyguy]

My layout has 4 arrays.

1 - 15 panels (String A = 9 panels, String B1 = 6 panels)
2 - 3 panels (String B2 = 3 panels)
3 - 8 panels (String C)
4 - 8 panels (String D)

What is the best way to design the wiring, taking into account grounding and conduit issues?

If I want to skip the combiner box and just wire all 4 strings to the inverter, are there advantages to running the wiring in separate or shared conduits?

The physical layout lends itself to combining wiring to a single conduit in the following way:

Array 1 & Array 3 conduits combine at Array 2. Single conduit from Array 2 to Array 4. Single conduit from Array 4 to inverter.