Questions regarding combiner and overcurrent protection?

PNW_Steve · February 2018

Hey Everyone,

I am planning out my solar install for my RV and have a few questions regarding combiners, overcurrent protection and installation.

I am installing 6 250 watt panels (Voc=38.1v, Isc=8.98A) on the roof of my RV. I will be using a Victron 150/75 controller and wiring two series strings of 3 panels.
Questions:
1) With just two strings should I use Branch Connectors (https://www.solar-electric.com/multibranch-connector-h4-ffm.html) or a combiner with O/C protection in the combiner?

2)Would it be appropriate to place O/C protection for the strings inside the RV as opposed to putting it on the roof in something like this: https://www.solar-electric.com/sogrcobox.html . If I place it inside it would be in very close proximity to the roof penetration for the panel wiring.

3) Breaker sizing? Depending on whether I protect each string (using combiner box) or put a single breaker after the branch connector if I go that route. I will either have 2x 8.98A strings or a combined 17.96A. I am of a mind that 10A and 20A breaker respectively may be too small. The next steps up would be 12A and 30A. What do you recommend?

Thanks.

S.

PNW_Steve · February 2018

Well I don't see the edit button to fix my type.... They are 260 Watt panels not 250.

PNW_Steve · February 2018

Well I may have found one answer. I saw this on another site:

Let's say we're using a SolarWorld 315 Watt module with an ISC rationg of 9.12. To calculate the fuse size required between the string and the inverter's DC input you take 9.12 x 1.56 = 14.7 and round up to the next trade size of 15A.

If that is correct then I would use a pair of 15A or a single 30A. Am I on the right track?

Thanks again.

mcgivor · February 2018

Since distances would be reletivaly short, perhaps it would be better to run each string to a circuit breaker mounted near the controller as a means of protection, service disconnect and combination. This would minimize on roof conections as well as providing the ability to check each strings output, emergency or service shutdown in one convenient location. Breaker size would be 10-15 A each string.

EDIT. To edit a post click the star upper right corner, allows editing for 1 week

PNW_Steve · February 2018

mcgivor said:

Since distances would be reletivaly short, perhaps it would be better to run each string to a circuit breaker mounted near the controller as a means of protection, service disconnect and combination. This would minimize on roof conections as well as providing the ability to check each strings output, emergency or service shutdown in one convenient location. Breaker size would be 10-15 A each string.

EDIT. To edit a post click the star upper right corner, allows editing for 1 week

Thank you for the reply.

If I understand correctly you are saying that I should run the wiring from the two strings through the roof to the combiner inside the RV. The combiner should have a breaker for each string. Using the example I found on the other site tells me that I should use 15A breakers.

Is this correct?

Thanks again.

S.

mcgivor · February 2018

Correct, wiring this way would be convenient as all equipment is in a single location.
Usually on the label in the back of the panel, it will state maximum fuse size, I have 10A on my 250W strings with no issues for over a year now, FWIW

Photowhit · February 2018

With 2 string, you don't need to protection for each string, just a breaker near the power center mostly to turn them on and off for other things. The protection on each string is just to protect the other string, since they have no more power than a single string, they can't over power the other string.

I would use branch connecters to combine them on the roof and bring them to a single breaker just before the charge controller. It's nice to have a breaker after the charge controller as well, so it can be isolated. A 30 amp DC rated breaker should be fine. I'm pretty sure they make a 25 amp, but couldn't find it on NAWS new web site.

https://www.solar-electric.com/midnite-solar-mnepv-30-amps-circuit-breaker.html

mcgivor · February 2018

Agreed there is no need to have separate breakers, but it is a convenient way to monitor each strings output once in a while, without having to climb up on the roof, a few bird bombs could drag down a string and go unnoticed.

PNW_Steve · February 2018

THANK YOU!

I appreciate the assistance.

Is there any down side to using branch connectors and a single breaker?

Photowhit · February 2018

As @mcgivor said, it would be easier to determine if you have an obstruction on one series of panels. I suspect you'll have them flat mounted and it will be hard to see if you had dirtied a panel on one string. If they are oriented the same direction, you could 'see' if there was a problem by comparing output.

mcgivor · February 2018

There is no advantage to using two breakers over one, other than the aforementioned ease of checking each strings output. As part of regular maintenance I check weekly with such a setup, the reward was finding a bad MC4 connector which was cracked and high resistance, it only took a few seconds to reveal there was a problem. A similar problem with a single breaker may have gone unnoticed until there was a complete failure, having said that I'm maintenance oriented, believing maintenance is cheaper than repair, many years of experience has taught me this.FWIW

PNW_Steve · February 2018

mcgivor said:

There is no advantage to using two breakers over one, other than the aforementioned ease of checking each strings output. As part of regular maintenance I check weekly with such a setup, the reward was finding a bad MC4 connector which was cracked and high resistance, it only took a few seconds to reveal there was a problem. A similar problem with a single breaker may have gone unnoticed until there was a complete failure, having said that I'm maintenance oriented, believing maintenance is cheaper than repair, many years of experience has taught me this.FWIW

That sounds like a good argument for two breakers.

Thank you all very much for your time and knowledge. Much appreciated.

S.

mike95490 · February 2018

With a breaker for each string, you have an easy way to detect fauts

PNW_Steve · February 2018

Thank you.

I think that I am going to go that route. I'll likely spend a few dollars more but I expect that will be a good investment.

BB. · February 2018

With one breaker per string (parallel connected solar panel(s)), you will not need another breaker on the main run back to the charge controller (solar panel input).

The basic current requirements for NEC (there are more details in the NEC manual), is here:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Using the table as a minimum wire gauge (based on insulation type--Other items like fill in conduit, ambient temperature like on a sunny roof, can derate.

For solar power (arrays, solar charge controllers, AC inverter DC inputs, AC and DC connections for AC to DC battery charger), I suggest a minimum of 1.25x the maximum continuous current (for example, a 20 amp current * 1.25 derate = 25 amp minimum branch circuit rating--both wire and fuse/breaker rating). NEC ratings are for typical house hold and light industrial power needs. When you have a circuit that is running many hours per day, you need the 1.25x up rating for fuses or breakers--Your system will be much more reliable (no false fuse/breaker trips) and last longer (wiring gets hot and cooks insulation, binding screws get hot and ruin wire ends/breaker screws, etc.).

At a minimum, I suggest that you do two calculations. One based on voltage drop using a calculator (for solar arrays, typically 1% to 3% drop at Vmp-array voltage--For the charge controller to battery bank, I suggest 0.05 to 0.10 volt drop for 12 volts, 0.1 to 0.2 for 24 volt, and 0.2 to 0.4 volt drop for 48 volts).

For DC bus drops (to lights, DC input to AC inverter, etc.), I suggest 0.5 volt max drop for 12 volt bank (1.0 volt for 24 volt bank, 2.0 volt drop for 48 volt bank).

http://www.calculator.net/voltage-drop-calculator.html (one way distance for wiring)

The 1.56x solar derating you see--I think that is double derating (1.25x NEC derating * 1.25x solar = 1.5625x derating for variable solar harvest). We do not do that in industry (the double derating). The solar mfg. should already have the (for example) 60 amps "backed into" their specifications (60a/1.56=38.4 amps "solar" max current). There are various reasons why I think this is overkill (keeping the 1.25x derating for wiring+breakers--Very useful/good).

There is the Isc rating of panels--That is (roughly) the maximum current you see from the panels ever (short circuited). Double check that the Isc of all the panels added up is less than the rating of the single cable from the array (combiner box) to the solar charge controller input. Usually, you will see that one of the other calculations (wire diameter for voltage drop as an example) already is bigger than n*Isc of your parallel array. If the cable is larger diameter than the Isc-array rated current, then you do not need the separate "master breaker" on the solar charger panel input.

However, you still need the breaker/fuse on the solar charger output to the battery bank (located near the battery to stop short circuit current from battery to "any" wiring running off the battery bus)--sized for the wire/branch circuit rating or downsized to the charge controller rating (i.e, 60 amp controller * 1.25 NEC derating = 75 Amp minimum, round up to 80 Amp branch circuit+breaker rating).

-Bill

mike95490 · February 2018

Yes, I have 1 redundant breaker, and it adds another device into the system - adding another failure point. But it also does not hurt to have a plainly labeled PV-Controller single switch. All these midnight breakers are rated for switch duty, they don't wear out if you cycle them a few times. The breakers are mostly for my convieance, but they are sized correctly for the wires & panels.

At the battery, I have a blue seas Dual MRBF fuse block, with a 40 & 120A fuses in it.

https://www.bluesea.com/products/2151/Dual_MRBF_Terminal_Fuse_Block_-_30_to_300A

Questions regarding combiner and overcurrent protection?

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