distant from the array to the DC Combiner box

[wyboy]

Is there a requirement for maximum distant from the array to the DC Combiner box? Specifically, can the photovoltaic source circuits on a small stand alone system be ran from the roof mounted array along the outside of the building to the DC Combiner box mounted on the side of the house, a distance of about 20 feet without conduit? The source circuits do not go inside the building. 690.31(B) does not seem clear to me on this. The location of disconnects does also not seem clear to me on this.

 

[SolarPro]

There is no limit to the distance between the array and the combiner, but best practice is to transition to conductors in conduit as soon as you leave the vicinity of array. It's a workmanship issue, as well as a conductor protection issue. Unprotected single-conductor cables are undoubtedly the #1 cause of ground faults and arcing faults in PV systems.

While it's not uncommon to find combiners on the roof, accessibility is better at ground level, which is important if you need to install and maintain OCPDs. While the NEC requires the use of listed combiner boxes (690.4(D)), no such requirement applies to transition boxes (for transitioning from USE-2 or PV Wire in free air to THWN-2 in conduit), meaning you can site-build these using listed components, like a NEMA 4 box and polaris connectors. Disconnect devices are almost always installed at ground level.

Note that in a stand-alone system, even very short distances may require you to upsize conductors for voltage drop. You might want to pick up the "PV Design and Installation Manual" from SEI (Solar Energy International) as this contains good design and construction guidance, as well as voltage drop tables for common voltages (12, 24, 48 Vdc, etc).

 

[SolarPro]

(Sorry I just realized you're probably inspecting, rather than installing this system.) FWIW: 690.31(B) means what it says. You can use single conductor cable—typically USE-2 or PV Wire—"within the PV array." Once you leave the array you typically need to be in a raceway; it's hard to meet Code otherwise. The second paragraph in 690.31(A) is interesting if you are looking at a 12 Vdc system. If the max system voltage is under 30 Vdc, then you may be able to meet the Code without the conductors being in a raceway. (A 24 Vdc nominal system will have a maximum system voltage that is greater than 30 Vdc.) Whatever wiring method is used, it needs to be meet applicable requirements in Chapter 3. Also, the circuit routing, identification and grouping requirements in 690.4 will apply; these are expanded under the 2011 NEC.