When to use a combiner box?

System · October 2007

Hi there,

We are laying out the design of and array using 8 Suntech STP 175 Watt panels to power a solar water pump. We would like to configure the panels with two sets of 4 panels wired in series and each set connected together in parallel. Do we need to use a combiner box to do this? We are working on a project in South America and are having difficulty tracking down combiner boxes commonly available from North American solar suppliers.

Because we are only wiring 2 series in parallel, can we wire the panels safely without a combiner box? If a combiner box is necessary, are there alternatives to standard PV combiner boxes that we might be able to find down here?

Here are the specs for the panels:

STP 175-24/Ab-1
Pmax=175W
Voc=44.2V
Vmp=35.2V
Isc=5.2V
Imp=4.95A

Thank you.

niel · October 2007

Re: When to use a combiner box?

sooner or later those leads from each string are going to have to be fed into the controller and unless you are using 2 seperate controllers you will need to combine them so as to allow as single set of larger wire leads to be connected to the controller. the combiner box itself does not necessarilly have to be made for pvs for you could buy a cheap box with qo breakers meant for a small ac application. think of it as a miniature ac breaker panel like those used in homes, but that the power is being inputted through the many small breakers and outputted through the 1 larger main breaker. i cite qo breakers only because they are dc rated to 48v. be advised that the pvs being 24v would necessitate the strings be no larger than 2 pvs in series because of that particular breaker's rating thusly giving you 4 strings to work with and this breaker may not be suitable for your application due to that. fuses are another option too though.
if you don't need to be able to switch the power and protect the strings(highly recommended) you could make one. buy a suitable box of suuficient size and put in it something like this: http://store.solar-electric.com/16220-1.html
in some cases you could use neutral bus bars that accomodate different wire sizes and mount 2 of them to a suitable nonconductive base (wood or plastic with plastics prefered as wood absorbs moisture) that could then be attached to the inside of a standard electrical box both of sufficient size for safety and workability. the base would be needed to insulate each buss from each other while giving good mechanical strength and arranged so that they are also not in contact with the boxes metal housing in the case of metal electrical boxes. here's an example buss bar that has only one size hole as i couldn't find the multi-sized one on the site, but it gives you an idea of what i'm talking about: http://www.lowes.com/lowes/lkn?action=productDetail&productId=93985-82364-GBK10P&lpage=none

westbranch · October 2007

Re: When to use a combiner box?

you can search EBay for "Power Distribution Block" tha can be used for combining different wire sizes. The results are mainly for auto Boom Box installations and they will handle a variety of sizes. For interior installations only!
cheers,

Eric

niel · October 2007

Re: When to use a combiner box?

westbranch,
yes, i had forgotten about those, but i don't know if such auto boom boxes or their supplies are available down there as a walkin store might be prefered. if they are down there that does give you another option, UNLESS you can or feel like dealing with the likes of ebay.

crewzer · October 2007

Re: When to use a combiner box?

aurora,

We need to understand more about what you're trying to do. Configuring "two sets of 4 panels wired in series and each set connected together in parallel" will result in an array rated at 176.8 Voc / 140.8 Vmp x 10.4 A Isc / 9.9 A Imp.

Is this what you're trying to do? If so, these array voltage specs exceed the 150 VDC voltage specs for combiner boxes and circuit breakers available from Midnite Solar and OutBack Power.

Note: The inexpensive Square D QO breakers are rated at 48 VDC max, including temperature compensated Voc specs for PV modules. In other words, they're limited to "24 V" modules (Voc = ~42 V) used in moderate climates.

Considering (what I believe are) your high array voltages, you may need to look into a conmbination of fuses and a combiner block.

There are combiner blocks available to connect sub-arrays in parallel. They do not provide overcurrent protection. See: http://store.solar-electric.com/podibl2x4ci.html

HTH,
Jim / crewzer

Roderick · October 2007

Re: When to use a combiner box?

Try taking a look at this thread http://forum.solar-electric.com/showthread.php?t=1379 for ideas.

This might be all you really need to do. With more than 2 strings, you would want blocking diodes, but with just 2 strings, the worst that can happen is one string puts its full current the wrong way through the other string - not even enough current to blow embedded fuses on the panels.

System2 · October 2007

Re: When to use a combiner box?

Thanks for all the suggestions, everyone. We are in fact trying to do as Crewzer suggests and configure "two sets of 4 panels wired in series and each set connected together in parallel" to result in an array rated at 176.8 Voc / 140.8 Vmp x 10.4 A Isc / 9.9 A Imp. The reason for this is that our Grundfos pumps will give us a higher performance with higher voltages but cannot take any more than 6 of our panels wired in series.

So this has led us to configure the panels with 2 series of 4 panels wired in parallel which assumes the necessity for a combiner box with fuses rated for higher voltages. We are not electrical engineers and this is our first installation so it has taken us awhile to wrap our heads around some of these ideas. We are trying to track down these specialized DC fuses down here and have contacted some local solar installers for advice but are still waiting for a response.

In the meanwhile, we are curious about the idea the Roderick proposed above. It sounds like a simple solution, but we're not completely clear as to what is going on inside that junction box. Is it just a matter of connecting the 2 positives together with a wire nut and doing the same with the negatives and then sending them on to control unit? How can we determine if we can safely do that or not? What about breakers/fuses?

Also, we we're hoping for some general help on breakers/fuses as we are finding a plethora of options designed for AC voltages but nothing for DC. A few electricians have assured us we can use the AC breakers for our application but we're not clear on what are the differences and what are the consequences, if any, of using AC breakers rated to higher voltages, rather than DC breakers. Any input here?

Thanks again for all the help. We really appreciate it!

- The Aurora Project Team

Roderick · November 2007

Re: When to use a combiner box?

auroraproject wrote: »

...
In the meanwhile, we are curious about the idea the Roderick proposed above. It sounds like a simple solution, but we're not completely clear as to what is going on inside that junction box. Is it just a matter of connecting the 2 positives together with a wire nut and doing the same with the negatives and then sending them on to control unit? How can we determine if we can safely do that or not? What about breakers/fuses?
...
- The Aurora Project Team

All I did was wire the positives together and the negatives together, yes. There is generally no danger to the panels from this configuration, even if one set is fully shaded, or shorted, or the output is shorted (but don't just go shorting it to see, the spark can eat away metal from the MC connector.)

There is not that huge an amount of current flowing that you would have to worry about wires burning up if something was shorted.

If you're going from the panels to a charge controller, you might want to have a disconnect switch as a convenience, and if so, you could get a disconnect switch that accepts fuses. Or maybe that's already part of the controller, I don't know.

I'm also ignoring any code considerations for your area, of course.

When to use a combiner box?

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