Number of panels to a PSPV

Discussion about the Power System PV Combiner (PSPV)

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Number of panels to a PSPV

Post by discoverpower » Mon Apr 10, 2006 9:29 am

Hello, my name is Kevin. I recently started working for a solar company and am trying to find information for complete home systems. We have one tech, and he's always extremely busy (and not on-site most of the time), so I don't get much chance to ask questions....

My questions...

1. How many panels do you need before you need a combiner box? How many panels can go to one combiner box? I noticed there are many different types of fuses, are the fuses for each individual panel? Are they based off panel rating? Basically I don't quite know how the combiner boxes work... I believe the largest panels that we're working with are the BP170's (24v, 5amp), lowest the Uni-solar 58watt panels (12V, 5amp).

2. This may be posting off topic, but how do you determine your DC disconnect ratings? Where specifically do the disconnects go (ie. before batteries after panels). Also, where would the AC disconnect go? Is a PV breaker box the same as a DC breaker (or can it be used in the same way)? Do the disconnect placements change whith higher or lower voltage.

3. In general, when I'm sizing a system, should I look at the short circut current (Isc), or the Pmax current to determine the controller size (and other components)?

Thanks for any and all help


Jim L.
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Post by Jim L. » Mon Apr 10, 2006 11:54 am

Hi Kevin

Let me suggest that this would be helpful: ... es2005.pdf

It is some suggested practices for PV using the NEC 2005. It will answer a lot of questions to come.

Jim L.

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How many 48 V strings to a PSPV; V and current leaving PSPV?

Post by uniquebobc » Thu Jun 22, 2006 8:19 pm

:???: :???:

Outback Power System 2 - GVFX3648D (Dual inverter); 7.2KW, 120/240V, 48 VDC; 42 SHARP 170 W panels wired in series of two for a 48 V array; 2PSPVs; 2MX60 charge controllers; 48 V battery bank consisting of 3, 4-battery series strings of 12 V Concorde SunXtender 2580L batteries.

Here are my basic questions (this is after reading the above-referenced document several times, recently and over the last year)--please refer to page numbers if the answers to the following questions are directly and specifically ansered--otherwise, I have extreme difficulty understanding "codese":

1. Can up to 12 series-wired pairs of SHARP 170 W modules be wired into each of the two PSPVs? Plan to use 2 PSPVs with 10 pairs in one and 11 pairs in the other.

2. The breaker for each of these pairs would be 10 A, correct?

3. What size wire should come out of the 2 PSPVs and connect first through the utilliity-required DC disconnect and then to the (?) amp breakers (positive conductors) and to the DC negative bus bar (negative conductors) in the PS2DC? The estimated, maximum one-way distance between the PSPV and the PS2DC is 30 feet. Desire a no greater than 2% voltage loss in this distance. I am having great difficulty conceptualizing the resultant voltage and amperage that come out of the PSPVs. Would these be too large for each of the MX60s?

The client absolutely wants grid-interactive with back-up. This is absolutely to largest system I have attempted to design to date. I want to get it right.

Kent Osterberg
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My RE system: SMA Sunny Boy 2500 with eight Mobil Solar Ra 180 modules
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Post by Kent Osterberg » Thu Jun 22, 2006 10:18 pm


When there will be more than two parallel strings of modules, overcurrent protection is usually required for each string. With some manufacturer's modules you can get away with three stings without overcurrent protection.

Components for your PV wiring should be selected per NEC 690.8. Multiply the short circuit current by 1.56 to determine the minimum ampacity of the wire. Same with breakers unless they are rated for continuous duty, in which case multiply by 1.25.


There is room for 12 breakers in the PSPV. However, getting PV wiring from 12 strings into the PSPV can be difficult because there are only eight 1/2" KOs on the bottom of the PSPV. If you use strain relief connectors that each hold three wires, you can get 24 wires though the eight 1/2" KOs. Also, there is room to put in more KOs on the sides and back. Get the mechanical drawing of the PSPV and plan your wire entries before deciding on the number of PSPVs.

Sharp's literature clearly says that the maximum series fuse for the 170 is 10 amps.

The power going through one of your PSPVs is quite a bit more than the maximum that Outback recommends for a MX 60.

#4 copper will meet your requirement for a 2% voltage drop, however it probably won't meet the ampacity requirements of 85 amps for the 10 sting set and 94 amps for the 11 string set. But that depends on the insulation type, ambient temperature, and the number of conductors in the conduit. A 2% voltage drop means that 2% of the power from the PV array never gets to the charge controller. It really is worth the extra money to use #2 copper.

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