Combiner box with fuses for each string

[harryn]

Hi,

Working on a friends Class C.

Normally I try to wire roof panels all in series, but in this case it needs to tie into the existing system.

It is going to end up being:
- 2 arrays, each with its own bogart pwm charge controller
- each array will have 3 panels wired in parallel
- The panels are typical 100 watt RV size / nominal 12 volt type panels.

We are doing it this way not because it is ideal, but because we are re-using the existing bogart pwm charge controller and adding a second.

I would like to find a combiner box for mounting on the roof that can:
- support the wiring for both arrays, so two independent setups.
- include the hardware for a fuse on each panel. (6 fuses)

The ones I am finding for RV use are just straight combiners - no fuses or breakers built in.

We can build one if needed, but if we can find a commercial product that is convenient.

Thanks

Harry

[AFfiredawg]

Why not use inline mc4 fuses? Then parallel them into a bussman resettable breaker?

[harryn]

Thanks - that is a good idea.

We are considering that approach as an option if the other path doesn't work out. There are some odd space constraints that make the MC4 fuse approach imperfect.

It might end up being easier to just build one inside of a nema box and we can do that relatively easily. As a practical matter, crimped on ring terminals are more reliable than a screw down connection anyway.

[CountryFit]

i made my combiner box with atc fuse on each leg inside.

[vsheetz]

I have a six circuit combiner box from Midnight Solar - using four DC circuit breakers in it. Dies double duty as service disconnects for the panels. Three strings of panels on the roof and a portable string I setup if desired. The combiner box is installed near the batteries along with a Midnight Solar Classic 150 controller. So three pairs of wire coming down from the roof.

I would not want fuses or circuit breakers on the roof.

[baphenatem]

Originally Posted by harryn

Hi,

Working on a friends Class C.

Normally I try to wire roof panels all in series, but in this case it needs to tie into the existing system.

It is going to end up being:
- 2 arrays, each with its own bogart pwm charge controller
- each array will have 3 panels wired in parallel
- The panels are typical 100 watt RV size / nominal 12 volt type panels.

We are doing it this way not because it is ideal, but because we are re-using the existing bogart pwm charge controller and adding a second.

I would like to find a combiner box for mounting on the roof that can:
- support the wiring for both arrays, so two independent setups.
- include the hardware for a fuse on each panel. (6 fuses)

The ones I am finding for RV use are just straight combiners - no fuses or breakers built in.

We can build one if needed, but if we can find a commercial product that is convenient.

Thanks

Harry

The rule of thumb is that any more than two modules in parallel requires overcurrent protection for each string. However, there are some rare exceptions, and 100 Watt “12 volt” units such as we use on RV’s may or may not need overcurrrnt protection when connecting three modules in parallel. At issue is the maximum nameplate rating for the series protective device, which can be 8, 10, 15, or 20 amps, and is an indication of how much backfeed current the internal connections within a module can withstand.

Divide the series fuse rating by 1.25, and then divide by the number of other modules to be connected in parallel, since it is only the other modules that can backfeed a given module. For example, an HQST-100D module has a series OCPD rating of 15A. 15/1.25/2 = 6. As long as the short circuit current (Isc) for each module is less than 6A they can be paralleled without overcurrent protection. The HQST-100D’s have an Isc of 5.75, which means that three of them can be paralleled with individual protection IF (there’s always a caveat) there is no possibility of a backfeed from the charge controller. Most MPPT controllers will not backfeed, but some PWM’s will. You have to check with the manufactuer to be sure.

[Rvlegaleagle]

I plan on having either 2 sets of three panels in series or three sets of two panels in series with the Magnum PT-100. Will be following this thread as I am interested to see what others have to say about the OP combiner box setup.

[rarebear.nm]

For the combiner box with six fuses try a SolarDeck
https://www.altestore.com/store/encl...SABEgK_r_D_BwE

I've used it with great success. In the picture they've fuse both + & - leads of three circuits. You could fuse the hot side of six panels instead.

[harryn]

Originally Posted by baphenatem

The rule of thumb is that any more than two modules in parallel requires overcurrent protection for each string. However, there are some rare exceptions, and 100 Watt “12 volt” units such as we use on RV’s may or may not need overcurrrnt protection when connecting three modules in parallel. At issue is the maximum nameplate rating for the series protective device, which can be 8, 10, 15, or 20 amps, and is an indication of how much backfeed current the internal connections within a module can withstand.

Divide the series fuse rating by 1.25, and then divide by the number of other modules to be connected in parallel, since it is only the other modules that can backfeed a given module. For example, an HQST-100D module has a series OCPD rating of 15A. 15/1.25/2 = 6. As long as the short circuit current (Isc) for each module is less than 6A they can be paralleled without overcurrent protection. The HQST-100D’s have an Isc of 5.75, which means that three of them can be paralleled with individual protection IF (there’s always a caveat) there is no possibility of a backfeed from the charge controller. Most MPPT controllers will not backfeed, but some PWM’s will. You have to check with the manufactuer to be sure.

Thanks for the great advice - it makes a lot of sense.

I looked for the maximum series fuse rating on a number of different panel data sheets and only a few list it.

Is this a relatively new rating spec or is it just becoming important enough for panel makers to put in the documentation?

[harryn]

Originally Posted by rarebear.nm

For the combiner box with six fuses try a SolarDeck
https://www.altestore.com/store/encl...SABEgK_r_D_BwE

I've used it with great success. In the picture they've fuse both + & - leads of three circuits. You could fuse the hot side of six panels instead.

Thanks, that is an interesting combiner product.

I will have to study it a bit more to see if there is room for turning it into 2 parallel arrays vs combining 6 strings into one array.

[harryn]

Originally Posted by baphenatem

The rule of thumb is that any more than two modules in parallel requires overcurrent protection for each string. However, there are some rare exceptions, and 100 Watt “12 volt” units such as we use on RV’s may or may not need overcurrrnt protection when connecting three modules in parallel.

Just double checking:
- in case these over current protection devices are needed, is this protection per string supposed to be on the roof near the panels or can it be inside of the RV?
- Is this subject to the concept of intrinsically safe voltages - example if under 60 volts it is ok? For example if a 60 volt Vmp is used?

[harryn]

Originally Posted by Rvlegaleagle

I plan on having either 2 sets of three panels in series or three sets of two panels in series with the Magnum PT-100. Will be following this thread as I am interested to see what others have to say about the OP combiner box setup.

My understanding (subject to further corrections and Tom's inputs, are:
- Since you have multiple panels in series, you will need some type of over current protection on each "string".

- I am not yet sure if it is the string voltage that matters or the exception provided for for 12 volt nominal RV panels that allows the exceptions but we can wait for that answer.

- Your set up will be either 2 or 3 "strings", so 2 or 3 over current protection devices.

- Your setup will be simpler though, because it is "one array" feeding "one controller" vs I this one is "two arrays" feeding "two controllers".

- Fuses act as an over current device, but you still have to be a bit cautious on this point, because they don't act as a "disconnect under load".

In other words, if your solar panels are generating power, and power is flowing into the charge controller, removing that fuse will make a big arc. Somehow the current flow has to be stopped first or it takes a DC breaker, as those are capable of stopping current flow under load.

[baphenatem]

Originally Posted by harryn

Thanks for the great advice - it makes a lot of sense.

I looked for the maximum series fuse rating on a number of different panel data sheets and only a few list it.

Is this a relatively new rating spec or is it just becoming important enough for panel makers to put in the documentation?

The maximum series fuse specification has been around for ages. The problem is that many panel manufacturers do not put their panels through the listing process, and consequently are not compelled to provide the information that would normally be part of a listed product’s testing.

[baphenatem]

Originally Posted by harryn

Just double checking:
- in case these over current protection devices are needed, is this protection per string supposed to be on the roof near the panels or can it be inside of the RV?
- Is this subject to the concept of intrinsically safe voltages - example if under 60 volts it is ok? For example if a 60 volt Vmp is used?

The modules/wiring "shall be protected at the higher current source connection" per the 2017 NEC.

Intrinsically safe systems are another animal entirely, as they are both voltage and current limited to prevent ignition of specified explosive gas, dust, and vapor containing atmospheres. There are no equivalents in PV.