Combiner Box (3kw)

solarnewzealand · April 2014

Hello,

I'm moving onto the combiner box part of my self taught solar expertise and can only call on the forum experts for more advice. As the title suggests I'm installing a 3kw solar system that is in 6 strings of 5 modules. Why would I need to connect the positive and negative to individual breakers? That means I would need to purchase 6 breakers, why not just one? Is it a just a safety measure? Correct me if I'm wrong but I could just purchase an empty box and attach the universal breaker attachments to said box then install my wiring? Why do some boxes have a busbar installed? New to CB's so any advice would be greatly appreciated and thank you in advance.

Cariboocoot · April 2014

Re: Combiner Box (3kw)

It is indeed a safety measure. It works like this: if one or more of those strings develops a short the remaining ones can dump their full Isc into it. So one shorted string of six can have 5X Isc applied to it, which is likely well above its conductor current rating (look for the specification "maximum series fuse rating").

In fact this is more important than putting over-current protection on the whole array's output to the charge controller (or GTI) because that is self-limiting; you know the combined Isc off all strings and they will not exceed that amount. So as long as the wiring can handle that full current (and it should be sized to) there is no possibility of an over-current situation.

solarnewzealand · April 2014

Re: Combiner Box (3kw)

Thank you again Cariboo that's just the answer I was looking for.

westbranch · April 2014

Re: Combiner Box (3kw)

You need a CB on each Pos lead from the string, the Neg does not need a CB and will be combined with a buss bar.
here is the type of combiner you will want, and it just happens to have space for 6 CBs http://www.solar-electric.com/mnpv6.html

solarnewzealand · April 2014

Re: Combiner Box (3kw)

Thank you west branch, I checked out that combiner but doubt it will handle the special type of fuses I need which are 500v and 12a, very hard to find let alone banking on them being attachable to that box.

Cariboocoot · April 2014

Re: Combiner Box (3kw)

solarnewzealand wrote: »

Thank you west branch, I checked out that combiner but doubt it will handle the special type of fuses I need which are 500v and 12a, very hard to find let alone banking on them being attachable to that box.

Uh, why do you need such fuses? As a rule you would be using 600 Volt (standard) 15 Amp (standard) fuse like these: http://www.solar-electric.com/mnatm.html

vtmaps · April 2014

Re: Combiner Box (3kw)

solarnewzealand wrote: »

Thank you west branch, I checked out that combiner but doubt it will handle the special type of fuses I need which are 500v and 12a, very hard to find let alone banking on them being attachable to that box.

Midnite makes excellent combiners. You can get them with 150 volt breakers or 600 volt fuses. The fuses are 50% wider than the breakers, so a combiner that holds 6 breakers only holds 4 fuses.

With 5 modules per series string, I suspect the voltage is too high for breakers.

--vtMaps

westbranch · April 2014

Re: Combiner Box (3kw)

solarnewzealand wrote: »

Hello,

I'm installing a 3kw solar system that is in 6 strings of 5 modules.

the special type of fuses I need which are 500v and 12a

I am making some assumptions here:

From post #1
3000w /6 strings = 500w per string, 500W /5 panels =100W per panel

A 100W panel (12v nominal) will have an average amperage of ~ 6A and need a ~10A CB

5 panels will have an output of 5 x ~17.0 V = 85Vmp

where did you get the 500V and 12A?

solarnewzealand · April 2014

Re: Combiner Box (3kw)

Hey cariboo cheers for that, my VOC total after temperature correction is 540v and 15.4a Isc (after 156% correction) Would like to fuse it under those ratings just to be safe.

vtmaps · April 2014

Re: Combiner Box (3kw)

westbranch wrote: »

I am making some assumptions here:

From post #1
3000w /6 strings = 500w per string, 500W /5 panels =100W per panel

I agree with your assumptions.... probably 12 volt panels. Maybe he can use breakers :cool: --vtMaps

Cariboocoot · April 2014

Re: Combiner Box (3kw)

solarnewzealand wrote: »

Hey cariboo cheers for that, my VOC total after temperature correction is 540v and 15.4a Isc (after 156% correction) Would like to fuse it under those ratings just to be safe.

Okay, first of all the Voltage rating of a fuse or breaker is the maximum Voltage it can safely interrupt. You do not want that to be less than the Voc of the string.

Second the Isc doesn't really need a 1.56 correction factor as it is the short circuit current. If possible go by the "maximum series fuse rating" of the panel. That means the fuse (or breaker) should be no larger than that, and no smaller than the Imp (with a bit of 'headroom' to allow for rare instances of higher than normal insolation).

Can you give us the actual panels specs?

solarnewzealand · April 2014

Re: Combiner Box (3kw)

Sorry 500v and 15a was the grand total of my system that I WAS going to fuse it at as a collective, before the helpful advice from Cariboo and West on CB's and breaking the strings individually, you guys are too fast for me I'm going to have to re mathematize my figures into their respective output values in individual strings of 6. Sorry for the noob like confusion this is unchartered territory for me.

solarnewzealand · April 2014

Re: Combiner Box (3kw)

Attachment not found.

Hey cariboo please find attached specs, mine are the ASF100 model.

Cariboocoot · April 2014

Re: Combiner Box (3kw)

I knew it was going to be oddball amorphous panels with the weird Vmp's!

So if you make a string of five in series (500 Watts) you have Imp 1.29 Vmp 385 Isc 1.65 and Voc 495. This is why you can't use 150 Volt breakers. Ideally these would be protected by 600 Volt 2 Amp fuses. There aren't any 600 Volt 2 Amp fuses. 1 Amp is too small. But the good news is the Isc of five strings feeding into one if it shorts will exceed 4 Amps and there are 600 Volt 4 Amp fuses.

It looks like the temp coefficient is 0.33% per degree C. So the 600 Volt fuses should be safe to -40C (unless I did the math wrong).

The six strings together, btw, will be 7.74 Imp @ 385 Vmp or 2979.9 Watts. Close enough to 3kW I'd say.

I take it this is feeding a grid tie inverter? The only other choice would be a 600 Volt MPPT controller from a long ways away.

zoneblue · April 2014

Re: Combiner Box (3kw)

A 3kW 500+V system is no learner system. Please tell me you will be getting an installer involved? I see from your previous posts that this is a grid tie system. Grid tie cant as a rule be done by yourself. The power companys will require engineers, approved solar contractors and mega red tape, much of which is designed to protect you. And youre not thinking of connecting without approval? Dont even think about trying that!

Why amorphous especially?

solarnewzealand · April 2014

Re: Combiner Box (3kw)

Sorry for the delay in getting back to this thread, why Amorphous? Because they were cheap (60NZD per panel for 30 of them) And they seem pretty well made. Thanks Zoneblue your personal site was very informative albeit technical.

| | | | | |
| | | | | | These represent 6 outputs into the fuses, rated at 2.57a each after NEC correction of 1.56% just to be safe and a VOC value of 495v per string.
O O O O O The 0's represent fuses which is where I'm stuck at the moment can't find 3a, 600v fuses and am a bit confused as to what to do here.
\ \ || / /
. . | |.. They merge into one positive connection via combiner box at 15.46a Isc total (2.57 x 6 strings) And 544v after temp correction factor.
. . | |..
The negatives are connected to an earthed busbar and fed via negative connection to the Inverter (grounding is a safety measure in case of GFCI being installed?)

Sorry for the noob questions.

stephendv · April 2014

Re: Combiner Box (3kw)

solarnewzealand wrote: »

The negatives are connected to an earthed busbar and fed via negative connection to the Inverter (grounding is a safety measure in case of GFCI being installed?)

Check the panel installation guidelines, because many amorphous panels require grounding of their negatives for correct installation and warranty. Also check that your inverter supports this! Many newer transformerless inverters do not allow negative grounding.

inetdog · April 2014

Re: Combiner Box (3kw)

stephendv wrote: »

Check the panel installation guidelines, because many amorphous panels require grounding of their negatives for correct installation and warranty.

And some panels require grounding of their positive side (some Grape Solar panels, for example.) The result of running them ungrounded or negative grounded is that output falls off time. The effect is reversible though and is corrected by proper grounding.

Combiner Box (3kw)

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