Solar Panel to Combiner Box Wiring

I have four 170 watt panels that will be installed end to end on the port side of the roof.

I intend to wire in a 2S2P configuration with #10 (possibly #8) wire. Panels 1 and 2 will be in series and a cable run to a combiner box. Panel 3 and 4 likewise.
I'm looking at a cable length of 10' and 23' respectively.

Using the panel specs for voltage and current at max power (17.8v & 9.55a) see a voltage drop of 0.24v (0.67%) on the 10' run and 0.59v (1.66%) on the 23' run.

Is this difference of any real consequence?

Thanks,
Tom

Greetings

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Heli_av8tor said:

I have four 170 watt panels that will be installed end to end on the port side of the roof.

I intend to wire in a 2S2P configuration with #10 (possibly #8) wire. Panels 1 and 2 will be in series and a cable run to a combiner box. Panel 3 and 4 likewise.
I'm looking at a cable length of 10' and 23' respectively.

Using the panel specs for voltage and current at max power (17.8v & 9.55a) see a voltage drop of 0.24v (0.67%) on the 10' run and 0.59v (1.66%) on the 23' run.

Is this difference of any real consequence?

Thanks,
Tom

Click to expand...

what was the reason you did not connect them all in series ?  is this a voltage limitation of the controller ?

10 AWG is more than enough for that wattage, and no, there is no consequence with such a small voltage difference.


what is your controller make ?

Heli_av8tor said:

I have four 170 watt panels that will be installed end to end on the port side of the roof.

I intend to wire in a 2S2P configuration with #10 (possibly #8) wire. Panels 1 and 2 will be in series and a cable run to a combiner box. Panel 3 and 4 likewise.
I'm looking at a cable length of 10' and 23' respectively.

Using the panel specs for voltage and current at max power (17.8v & 9.55a) see a voltage drop of 0.24v (0.67%) on the 10' run and 0.59v (1.66%) on the 23' run.

Is this difference of any real consequence?

Thanks,
Tom

Click to expand...

#10 wire to the combiner will be fine, you have less than a 1% voltage drop.  Also remember you won't be getting the full 9.55amps except under laboratory conditions.

Be sure to use a wire size calculator for the run from the combiner box to the controller.  Try to use heavy enough wire for a 1% loss.  The cost of the wire for a new install isn't very much more for a 1% loss than the cost of smaller wire for a 2% or 3% loss, especially considering the total cost of your solar system. 

Just in case you are not aware of it, if you cut the connector off of the solar panel to hard wire it in, you will void the warranty.  Always use a jumper cable with the proper connector on one end to attach to the panel, then cut the other end of the jumper to hard wire to the combiner box. 

As mentioned in an earlier posting, Jack Mayer's website has lots of great info: http://www.jackdanmayer.com/rv_electrical_and_solar.htm

solarman said:

what was the reason you did not connect them all in series ?  is this a voltage limitation of the controller ?

10 AWG is more than enough for that wattage, and no, there is no consequence with such a small voltage difference.


what is your controller make ?

Click to expand...

I can't speak to Tom's reasons.  However my reason would be with 4 panels all in series it just takes covering a single small 6" x 6" solar square to loose just about all the output from all 4 panels. 

With the two different distances in wire runs that Tom mentioned (23' and 10') if you are parked anywhere near a tree, or even close by a telephone pole, the possibility is increased to have a single panel shaded.

On my 29' Class A I have a pair of 325watt, 38V residential panels.  One mounted in the front and the other in the rear.  There have been quite a few times when early in the day or later in the afternoon when one of the panels has some shade. 

I'm using a TriStar MPPT-45.  The controller would have no problem with full series voltage. It's rated for 150 volt max input.

I wondered if the four in series would be a better option but thought with the possibility of shading on panels 1 & 2 with less than ideal coach orientation It would be better in 2S2P configuration. Am I wrong? It would certainly help with keeping wire size from the combiner box to the controller more manageable (#6 wire = 0.23% loss).

The attached drawing is roughly to scale. The dashed line is the wiring path from the panels to the combiner box. There's junk in the front and right side of the roof not shown in the drawing that precludes putting panels there. The air conditioner units are about 14" high and panels 1 & 2 are about 5" from them.

Though it's been awhile, I've read the the references cited.

Tom

Hi Tom. The voltage differential you mentioned will be un-noticeable. As to which method is best for wiring panels - series, or parallel - I doubt you'll notice significant difference at all, but the shading issue Al mentioned is something to consider, depending on your boondocking environment.

I don't know about your Morningstar's specs, but FWIW, a tech rep at Outback (the manufacturer of our controller) told me that our MPPT controller is most efficient at converting volts to amps when the input voltage is between 40 and 44 volts (approximately.) That's pretty much the sum of two of our panel's output when wired in series, so based on Outback's recommendation, I rewired our six panels so that every two panels were in series, then three sets of two panels were paralleled.

It took me about 30 minutes to rewire them at the junction box, but the fact is, I noticed no difference at all. There may indeed be a slight difference, but nothing that was noticeable on our battery monitor. Say hi to Theresa for us.

Kev

Heli_av8tor said:

I'm using a TriStar MPPT-45.  The controller would have no problem with full series voltage. It's rated for 150 volt max input.

I wondered if the four in series would be a better option but thought with the possibility of shading on panels 1 & 2 with less than ideal coach orientation It would be better in 2S2P configuration. Am I wrong? It would certainly help with keeping wire size from the combiner box to the controller more manageable (#6 wire = 0.23% loss).

The attached drawing is roughly to scale. The dashed line is the wiring path from the panels to the combiner box. There's junk in the front and right side of the roof not shown in the drawing that precludes putting panels there. The air conditioner units are about 14" high and panels 1 & 2 are about 5" from them.

Though it's been awhile, I've read the the references cited.

Tom

Click to expand...

OK.. well if you have any doubts with shading then stay 2S2P otherwise go 4S

as Kevin stated, his particular controller according to the rep has a sweet spot at a particular input voltage. I may be a percent or two
better, but it's not an issue..

Thanks guys. Guess I?m down to picking nits on this install.

Kev, hope things are going well for you and Cyndi. We are looking forward to meeting up with you guys in January.