Correct connections for 2s2p rv system (junction box location?)

Skyko · February 2014

I am getting ready to order some parts from NAWS and want to make sure I am correct on what is needed. Any help is appreciated.

The RV system will have four 270 watt panels powering a Midnite Classic 150 controller that is charging a 24V 300AH AGM battery bank consisting of four 6V cells.

It seems like the MPPT for the panels is around 30V at 9 amps, so I would like to run two in series and those in parallel, for 60V 18 amps. I would use 6 gauge wire for the roughly 15 foot run (30 feet total electrical path length). Total wire resistance 0.012 ohms, power loss = 4 watts.

The main questions I have are:

1) Does 2s2p make good sense?

2) Is it ok to just fuse each series panel group, at say 15 amps, for a total of two 15 amp breakers or fuses.

3) Do I need to have this combiner box containing the fuses or breakers mounted near the panels on the roof of the RV, exposed to all of the wind and weather or can it be inside the electrical cabinet in the RV.

4) Can you just use some sort of inline fuse in the cabling instead of a combiner box?

5) In addition to fusing the panels, it seems as if some designs fuse the combined solar output (like with a 30 amp breaker)...no idea why?

6) In most designs, there is also a fuse after the MPPT controller, between the controller and the batteries. Seems strange that the Midnite classic would not have something built in for this, but if needed, would I size this fuse at about 50 or 60 amps? At 100% solar and 100% conversion efficiency, the most the MPPT controller can output would be 24V at 45 amps.

BB. · February 2014

Re: Correct connections for 2s2p rv system (junction box location?)

Skyko wrote: »

The RV system will have four 270 watt panels powering a Midnite Classic 150 controller that is charging a 24V 300AH AGM battery bank consisting of four 6V cells.

Sounds like goo choice/configuration.

It seems like the MPPT for the panels is around 30V at 9 amps, so I would like to run two in series and those in parallel, for 60V 18 amps. I would use 6 gauge wire for the roughly 15 foot run (30 feet total electrical path length). Total wire resistance 0.012 ohms, power loss = 4 watts.

Very nice.

The main questions I have are:

1) Does 2s2p make good sense?

Yes--This is probably the only configuration that will work with your number of panels, battery bank, controller choice. There is no reason to choose anything different. If size/price is an issue (and no plans for expansion of the array), the Midnite Kid 30 amp MPPT controller might be worth a look (may still be hard to get--Just being released to market now).

2) Is it ok to just fuse each series panel group, at say 15 amps, for a total of two 15 amp breakers or fuses.

With 2s2p panels--You do not even need any fuses/breakers (three or more parallel strings require fusing/breaker per series string usually). You could put one breaker (or good quality switch) on the wire from the array to the charge controller so you can switch stuff off when/if servicing. Of course, there is no problem with two breakers (one per string). It can make diagnosing problems a bit easier (turn off one string and see if the current flow is equal between strings). Of course, you can get an inexpensive DC Current Clamp DMM and simple measure the current in each wire.

3) Do I need to have this combiner box containing the fuses or breakers mounted near the panels on the roof of the RV, exposed to all of the wind and weather or can it be inside the electrical cabinet in the RV.

Keeping breakers/fuses out of the sun/weather is always a good idea. They are affected by water and thermal cycling reducing long term life.

4) Can you just use some sort of inline fuse in the cabling instead of a combiner box?

Yes--But sometimes that is not good... One poster here (Wayne from Nova Scotia) used automotive fuse holders and found that they melted when operated well within their specifications (voltage/current). He had installed them in a box and it may have saved him from a fire. Fuses and breakers need good air circulation or they can get hot.

5) In addition to fusing the panels, it seems as if some designs fuse the combined solar output (like with a 30 amp breaker)...no idea why?

You only need breakers/fuses if the available current through the wire/cabling exceeds the rated current capacity of the copper/insulation. You would almost never have this issue with solar power... Your design has heavier than required copper to keep the voltage drop low. No "extra" breaker is needed (and fuses/breakers are "unreliable" by design--Extras tend to just make for problems down the road).

6) In most designs, there is also a fuse after the MPPT controller, between the controller and the batteries. Seems strange that the Midnite classic would not have something built in for this, but if needed, would I size this fuse at about 50 or 60 amps? At 100% solar and 100% conversion efficiency, the most the MPPT controller can output would be 24V at 45 amps.

There are "two limits" to look at... First the practical limits of your array. In cold weather and snow on the ground reflecting a tilted array, the maximum output current would be (on the order of):

4x 270 watt panels * 1.25 NEC wiring derating * 1.25 NEC solar derating * 1/23.0 volts dead battery = 80.35 amp branch circuit

The other limit (for an MPPT type charge controller) is its rated output limit (and many MPPT controllers, you can program them for less output current too). The Midnite on a 24 volt system has a rated output of 94 amps:

94 amps * 1.25 NEC wiring limit = 117.5 amp minimum branch circuit

Either method of calculation is fine. You just need to pick the wiring+breaker to support your desired limit (>80 amps or over 118 amps).

-Bill

Cariboocoot · February 2014

Re: Correct connections for 2s2p rv system (junction box location?)

Skyko wrote: »

It seems like the MPPT for the panels is around 30V at 9 amps, so I would like to run two in series and those in parallel, for 60V 18 amps. I would use 6 gauge wire for the roughly 15 foot run (30 feet total electrical path length). Total wire resistance 0.012 ohms, power loss = 4 watts.

You don't need that large of wire for that run at the Voltage and current involved. 10 AWG would be less than 3% loss, and remember the loss is at maximum power.

1) Does 2s2p make good sense?

It certainly does. Voltage is high enough to charge without being too far above system to cause significant controller efficiency loss.

2) Is it ok to just fuse each series panel group, at say 15 amps, for a total of two 15 amp breakers or fuses.

Only two parallel connections: no fuse is required at all.

3) Do I need to have this combiner box containing the fuses or breakers mounted near the panels on the roof of the RV, exposed to all of the wind and weather or can it be inside the electrical cabinet in the RV.

You don't need a combiner box unless you plan on expanding later. You can use the MC4 'Y' connectors instead. In fact you can put those in now and install a box later if you expand.

4) Can you just use some sort of inline fuse in the cabling instead of a combiner box?

As above.

5) In addition to fusing the panels, it seems as if some designs fuse the combined solar output (like with a 30 amp breaker)...no idea why?

Fusing of the combined output is also not necessary. You may want some form of disconnect though.

6) In most designs, there is also a fuse after the MPPT controller, between the controller and the batteries. Seems strange that the Midnite classic would not have something built in for this, but if needed, would I size this fuse at about 50 or 60 amps? At 100% solar and 100% conversion efficiency, the most the MPPT controller can output would be 24V at 45 amps.

There most definitely should be a fuse or breaker on the output of the charge controller. Preferably as close to the battery as possible. They don't build them in to the charge controller due to the numerous ways systems can be constructed. It is not the controller's output that would be the problem, but rather the batteries feeding back to it in the even of a short.

You can size this circuit protection based on either the expected output from the existing panels (about 35 Amps) or according to the controller's maximum current ability (80 Amps). Just be sure the fuse/breaker is lower current than the wiring connecting the two.

Skyko · February 2014

Re: Correct connections for 2s2p rv system (junction box location?)

Great info guys, thanks!

I am surprised by the lack of need for fusing the panels. I went to the NAWS site and looked at the wiring diagram for some of their RV "kits", most of which use two or four panels of smaller size than I plan. All of the kits had a little junction box with breakers fusing the panels or series pairs of panels. Maybe they are just being cautious?

Cariboocoot · February 2014

Re: Correct connections for 2s2p rv system (junction box location?)

Skyko wrote: »

Great info guys, thanks!

I am surprised by the lack of need for fusing the panels. I went to the NAWS site and looked at the wiring diagram for some of their RV "kits", most of which use two or four panels of smaller size than I plan. All of the kits had a little junction box with breakers fusing the panels or series pairs of panels. Maybe they are just being cautious?

Yes, cautious.
The thing is with two matched panels (or strings) in parallel the maximum current that can be put to one if it should short dead (zero Ohms resistance) is the Isc rating of the panel. Since as a conductor it's designed to take more than that it really can't heat up and start a fire. Put three (or more) in parallel and the picture changes as now the maximum current to the shorted panel can be 2X (or more) Isc, which it was not designed to handle.

Vic · February 2014

Re: Correct connections for 2s2p rv system (junction box location?)

Skyko wrote: »

5) In addition to fusing the panels, it seems as if some designs fuse the combined solar output (like with a 30 amp breaker)...no idea why?

6) In most designs, there is also a fuse after the MPPT controller, between the controller and the batteries. Seems strange that the Midnite classic would not have something built in for this, but if needed, would I size this fuse at about 50 or 60 amps? At 100% solar and 100% conversion efficiency, the most the MPPT controller can output would be 24V at 45 amps.

Skyko,

Using Circuit Breakers, instead of fuses on the input and output of Charge Controllers is a very good idea. One often needs a Disconnect as well as circuit protection. MidNite circuit breakers are inexpensive, and are rated to be used as switches, as well as circuit breakers.

AND, when Updating the FirmWare on the Classic, you will need to disconnect the PV input before switching OFF the CC to battery connection. Removing the battery power and then repowering the Classic is what is needed to do the FW Update. It is a very good idea to switch OFF the PV input before switching Off the Battery connection, and switching On the PV input after switching On the battery breaker for the best protection of any CC (in my opinion). So Breakers are by far the best device to use, instead of just a fuse on the input and output of any CC, and being able to switch off and on the connections to the Classic is almost essential.

In the appendix of the Classic CC Manual, there is a table of recommended cable and breaker sizes for different sized systems. Generally, the cable into and out of a CC are sized for the maximum current flowing in these circuits , plus a safety factor, as a beginning, and often the cable size is increased to reduce the voltage drop in that cable. Then the breaker is sized to protect each cable from excessive heating due to these maximum currents.

Believe that you have the Classic Manual, but it is here, if you need it:
http://www.midnitesolar.com/pdfs/classicManual.pdf

FWIW, opinions, Good Luck, Vic

Correct connections for 2s2p rv system (junction box location?)

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