Community Forums

R.B · 02-12-2018, 06:45 PM

I'm hoping to get a sanity check on the solar system I'm looking to put in, specifically when it comes to the sizing to allow for future capacity expansion. I'd like to avoid needing to upgrade equipment by buying for the immediate capacity goals. I've never done this before, so everything I'm basing this on is my understanding of what I have read.

I'm planning on six 180W panels in series-parallel groups of two for a total of 1080W (eventually). The panels are rated at 19.25 Vmpp/9.35A Impp. I'm opting to go series-parallel for convenience: the prewiring for solar is 8AWG. If my math is correct, at this voltage and amperage I should be able to manage somewhere between 2-3% loss at the length I think the prewiring is without exceeding the ampacity of 8 AWG wiring. It would be fairly difficult to upgrade that wiring if I had to.

The raw power will be taken down to an MPPT controller mounted near the batteries and inverter. The battery system is 12V (currently four 6V GC-2 SLA in series-parallel for a total of 450 Ah, likely lithium in the future). From the controller to the batteries I calculated a max amperage of 81.8A (I don't believe the current set of batteries will accept over 58.5A). Based on that I selected a charge controller rated for a max of 85A: Victron SmartSolar MPPT 150/85.

Does this seem reasonable or am I missing something?

Searching_Ut · 02-12-2018, 07:58 PM

Does you pre wiring include 3 parallel legs? Your battery acceptance calculations look to be in the right for generic flooded batts in your capacity range. Your calculated max charge current will probably be rarely if ever hit with flat mounted panels but you should be able to stay in the current acceptance rate you’re looking at for a reasonable part of the day in the summer months.

I haven’t ever played with the charger your looking at so I don’t know how well it handles parallel series wiring in partial shade issues. With most of the ones I did play with you pretty much lost a whole leg in a string if one panel was shaded in a series parallel setup. If you don’t anticipate partial sanding it’s probably not worth looking in to. So far looks like you’re planning a great system. Good luck.

R.B · 02-12-2018, 08:05 PM

Quote:

Originally Posted by Searching_Ut

Does you pre wiring include 3 parallel legs? Your battery acceptance calculations look to be in the right for generic flooded batts in your capacity range. Your calculated max charge current will probably be rarely if ever hit with flat mounted panels but you should be able to stay in the current acceptance rate you’re looking at for a reasonable part of the day in the summer months.

I haven’t ever played with the charger your looking at so I don’t know how well it handles parallel series wiring in partial shade issues. With most of the ones I did play with you pretty much lost a whole leg in a string if one panel was shaded in a series parallel setup. If you don’t anticipate partial sanding it’s probably not worth looking in to. So far looks like you’re planning a great system. Good luck.

Thanks. It's a standard Zamp 3-port combiner. The single 8 AWG pair comes down from that to the power junction next to the batteries.

C.Martin · 02-13-2018, 08:27 AM

Number eight won’t go any higher than your six panels, unless you wire them in strings of three.

The spec used for determining wire gauge and circuit protection is the Isc (short circuit amperage) Each string will add voltage...and current will remain the same. The number of strings you have will multiply amperage...

I believe the standard safety factors for planning wire size and circuit protections are...

>= Isc x 1.56
>= Voc x 1.2
And string fuse of Iz

So if you had and Isc for each string of 9.77a

If I get you correctly, you said parallel-series...

If you did three strings of two...the numbers would be...

3 x 9.77 x 1.56 = 45.7 amps for wire and fuse planning... closest fuse/breaker will be 50 most likely...and 8ga wire with 75 degree or better insulation will work. In the two panel string configuration this is maxed out for Your 8 gauge wire. No room to expand unless you keep the number of strings to 3...and place more panels on each string in series.

Let’s assume the Voc is 24.06v

You have two panels in each string so your voltage rating for the wire and fuse or breaker would need to be..

2 x 24.06 x 1.2 = 57.74v. Most wire is rated for 600v but when you get to higher voltages you have to be more selective for protection devices.

The inline string fuse should be paired to the Iz rating which I would guess is 15a.

So, if you want to go bigger in the future...and you insist on 6 gauge wire...the next logical step would be three strings of 3...

3 x 24.06Voc x 1.2 = 86.62v Rating or better.

So, plan you protective device for the higher voltage. Which will probably make you choose Solar specific products...

180w x 6 / 14.4v (nominal Absorbtion voltage). = 75A of Charge on a perfect STC rated panel day. Which is what I would plan for...but not what you should expect. Expect about 80 - 83 % of that. Like 60 amps of charge...

If you go to three strings of three... 180 x 9 x / 14.4 = 112.5 A... I believe you could still get away with a 150/100 Controller...but Inthink the 150/85 is too small.

For wire and protection on the battery side...

Amp x 1.25 is what they use for 100% rated breakers.

75 x 1.25 = 93.75. So, 100A protection 3awg wire...

If you go with the other ...logically the 150/100 Controller will limit you to 100a... so 100 x 1.25 = 125a. 125A protection and 2 awg wire if wire and terminations are rated for 90degrees or better...otherwise 1awg wire if 75 degree rated...

Here’s a link to a planning page...

http://www.cooperindustries.com/cont...-app-guide.pdf

If you have AGM or Lithium...charge rate is not something to worry about. Flooded Lead Acid batteries...you do.

My interstates require a max of C/10 where C is the back capacity.

If you have 4 6v batteries rated at 225aH for example and wired to make 2 12v battery equivalent...

Your capacity would be 2 x 225 = 450 aH

C/10. Would be 450/10=45.0 AH. Max rate.

This doesn’t mean you need to stop adding Solar at 45A...it just means that that’s all the battery can handle.

If you stick with Flooded Lead Acid...and have no way to regulate charge controller output based on actual battery shunt measurement. The 85a controller would be good enough...until you upgrade to AGM batteries.

I have the SmartSolar MPPT 150/100-TR with 8x 180w...and eight 232 aH FLA batteries. I have a 92.8A max rate for my FLA batteries. With an STC rating of 100A of Solar charge...I rarely see more than 86 amps...so no problem there.

R.B · 02-13-2018, 10:16 AM

Quote:

Originally Posted by C.Martin

Number eight won’t go any higher than your six panels, unless you wire them in strings of three.

The spec used for determining wire gauge and circuit protection is the Isc (short circuit amperage) Each string will add voltage...and current will remain the same. The number of strings you have will multiply amperage...

I believe the standard safety factors for planning wire size and circuit protections are...

>= Isc x 1.56
>= Voc x 1.2
And string fuse of Iz

So if you had and Isc for each string of 9.77a

If I get you correctly, you said parallel-series...

If you did three strings of two...the numbers would be...

3 x 9.77 x 1.56 = 45.7 amps for wire and fuse planning... closest fuse/breaker will be 50 most likely...and 8ga wire with 75 degree or better insulation will work. In the two panel string configuration this is maxed out for Your 8 gauge wire. No room to expand unless you keep the number of strings to 3...and place more panels on each string in series.

Let’s assume the Voc is 24.06v

You have two panels in each string so your voltage rating for the wire and fuse or breaker would need to be..

2 x 24.06 x 1.2 = 57.74v. Most wire is rated for 600v but when you get to higher voltages you have to be more selective for protection devices.

The inline string fuse should be paired to the Iz rating which I would guess is 15a.

So, if you want to go bigger in the future...and you insist on 6 gauge wire...the next logical step would be three strings of 3...

3 x 24.06Voc x 1.2 = 86.62v Rating or better.

So, plan you protective device for the higher voltage. Which will probably make you choose Solar specific products...

180w x 6 / 14.4v (nominal Absorbtion voltage). = 75A of Charge on a perfect STC rated panel day. Which is what I would plan for...but not what you should expect. Expect about 80 - 83 % of that. Like 60 amps of charge...

If you go to three strings of three... 180 x 9 x / 14.4 = 112.5 A... I believe you could still get away with a 150/100 Controller...but Inthink the 150/85 is too small.

For wire and protection on the battery side...

Amp x 1.25 is what they use for 100% rated breakers.

75 x 1.25 = 93.75. So, 100A protection 3awg wire...

If you go with the other ...logically the 150/100 Controller will limit you to 100a... so 100 x 1.25 = 125a. 125A protection and 2 awg wire if wire and terminations are rated for 90degrees or better...otherwise 1awg wire if 75 degree rated...

Here’s a link to a planning page...

http://www.cooperindustries.com/cont...-app-guide.pdf

If you have AGM or Lithium...charge rate is not something to worry about. Flooded Lead Acid batteries...you do.

My interstates require a max of C/10 where C is the back capacity.

If you have 4 6v batteries rated at 225aH for example and wired to make 2 12v battery equivalent...

Your capacity would be 2 x 225 = 450 aH

C/10. Would be 450/10=45.0 AH. Max rate.

This doesn’t mean you need to stop adding Solar at 45A...it just means that that’s all the battery can handle.

If you stick with Flooded Lead Acid...and have no way to regulate charge controller output based on actual battery shunt measurement. The 85a controller would be good enough...until you upgrade to AGM batteries.

I have the SmartSolar MPPT 150/100-TR with 8x 180w...and eight 232 aH FLA batteries. I have a 92.8A max rate for my FLA batteries. With an STC rating of 100A of Solar charge...I rarely see more than 86 amps...so no problem there.

Thanks for the very detailed response. I don't actually have room on the roof for anything more than the six panels, so maxing out at that count is acceptable. I'm also okay with having more solar than the current batteries can handle because I anticipate upgrading those to something larger when the costs are acceptable to me.

The prewiring is 8AWG, 600V, 105ºC, so it definitely sounds like that will work for what I have in mind. My reasoning for choosing strings of two was to minimize the effects of shade and keeping series internal wire lengths the same -- their placement on the roof will basically have them positioned in groups of two anyway.

As for the rest of the numbers, I'll definitely reference those when selecting the protective devices.

C.Martin · 02-13-2018, 12:17 PM

Quote:

Originally Posted by R.B

Thanks for the very detailed response. I don't actually have room on the roof for anything more than the six panels, so maxing out at that count is acceptable. I'm also okay with having more solar than the current batteries can handle because I anticipate upgrading those to something larger when the costs are acceptable to me.

The prewiring is 8AWG, 600V, 105ºC, so it definitely sounds like that will work for what I have in mind. My reasoning for choosing strings of two was to minimize the effects of shade and keeping series internal wire lengths the same -- their placement on the roof will basically have them positioned in groups of two anyway.

As for the rest of the numbers, I'll definitely reference those when selecting the protective devices.

105 degree wire ...that will work nicely.

Strings of two are perfect. Single panels all paralleled aren’t producing enough voltage to take advantage of MPPT. With pairs...your voltage will be high enough to wake up the controller early in the day and give it optimum room to find max power point. With the three strings of two your voltage won’t be crazy high...so a renogy fuse or blue seas dc switchable Breaker should work perfectly.

R.B · 05-07-2018, 09:36 AM

Quote:

Originally Posted by C.Martin

Let’s assume the Voc is 24.06v

You have two panels in each string so your voltage rating for the wire and fuse or breaker would need to be..

2 x 24.06 x 1.2 = 57.74v. Most wire is rated for 600v but when you get to higher voltages you have to be more selective for protection devices.

......

So, plan you protective device for the higher voltage. Which will probably make you choose Solar specific products...

I'm finally getting to the installation portion of this since the weather is warm enough to seal the roof. I have a question about the protection devices specifically: am I correct that a disconnect switch needs to support this voltage as well? I'd love to use something like this (https://www.bluesea.com/products/900...Battery_Switch), but it's only rated to 48V. The few higher voltage options I can find seem to be mostly bulky and/or ugly.

C.Martin · 05-07-2018, 02:07 PM

Yes...it's an arcing thing. The higher the voltage the more arcing that takes place. I went with M-series disconnect switches and fuses...my PV is a 72volt/60amp Renogy fuse and fuse holder...

R.B · 05-07-2018, 02:52 PM

Thanks. I ended up settling on a couple Midnite Solar products for a disconnect switch:

- MidNite Solar MNE Baby Box
- MidNite Solar MNEPV50-300 (50 amp, 300 VDC breaker)

This is just prior to the charge controller, between it and the combiner box. I'm still a little bit off from doing the actual panels and combiner box, so I'm not overly concerned about those at the moment, but I wanted to get this bit right so that I don't have to try swapping anything out after it's installed in a much more restricted space inside the trailer.

C.Martin · 05-07-2018, 03:24 PM

I really like the Midnite baby box...

R.B · 05-07-2018, 09:02 PM

At least while I’m waiting for that last bit I can finish the rest. I did a test fit tonight and drilled a couple of the holes where the AC wiring will come through.

C.Martin · 05-08-2018, 06:21 AM

Quote:

Originally Posted by R.B

At least while I’m waiting for that last bit I can finish the rest. I did a test fit tonight and drilled a couple of the holes where the AC wiring will come through.

Very nice!

Your are gonna love the CCGX with your SmartSolar Controller...

If you haven’t already...if you add a WiFi dongle and have a connected RV, thru WiFi or cellular...the VRM portal that Victron provides is such a great system tool. Under the Advanced tab, you can add the “widgets” you want. The widgets have graphs...and by clicking on a graph, the data point values appear. So you can study the loads on your system and monitor the health of your system. I can even check my system from hotel internet while on layovers in South America.

I don’t have the CCGX...but use the Venus GX (CCGX without a display panel) to wirelessly do the same thing. Venus gets feeds from all the other Victron products that we have... VE.Direct cable links it to our BMV-712 and the MPPT 150/100-TR Charge Controller.

Looks like your having a good time.