Breaker box design and configuration.

nobody928 · February 2014

I am trying to design a combiner box out of a square d outdoor electrical box from home depot. I have bought 2 tandem breakers one 20 amp and one 15 amp. Im not sure which one to use.

I have a total of 4x 140 watt 12volt panels that I would like to run in series in order to get 48v. I have a 12 volt nominal voltage battery bank. I have a midnite mppt charge controller.

What size breaker should I install in the combiner box?
What is the best way to install the mc4 cables to the breaker?
How do I install the PV wire that connects to the charge controller from the breaker?

Im looking at the midnite combiner boxes. It looks like they have all their mc4 cables cut in to about 1 foot lengthts. I was also wondering where the main mc4 cables that run to the charge controller plug in to??

Vic · February 2014

Re: Breaker box design and configuration.

Generally, you do not need a Combiner box for a single string or two strings of PV modules.

If you are planning expansion of your system in the future, then it is a good idea to start with a combiner that is sufficiently large for that later expansion -- this saves some on rewiring, etc.

I DO believe that the Square D Tandem breakers are not rated for DC current ... will check.

Normally, string breakers/fuses can be sized by following the PV manufacturer's Maximum Fuse size spec, on the label on the back of the PV.

Many of the MidNite PV combiners have conduit knockouts, and NO MC connectors installed. Often it works well to get MC 4 extension cables that are twice the required length of the longest run, cut them in half, plug the MC ends into the PV MC connectors, and run the other end into the combiner. OR, run the cut ends into a splice box, and from that box, make the home runs to the Charge Controller's Input DC breaker.

You will also want an output DC breaker for the Classic. You will need to switch off the power to the Classic, and then back on in updating process.

More later, Vic

vtmaps · February 2014

Re: Breaker box design and configuration.

Vic wrote: »

You will also want an output DC breaker for the Classic. You will need to switch off the power to the Classic, and then back on in updating process.

True the breaker is a handy switch, but the MAIN PURPOSE of that breaker is for safety. When controllers fail, they may become a direct short circuit across the battery.

If you use a polarized breaker, be sure the battery (+) goes to (+) side of the breaker, and the controller's Battery (+) output goes to the (-) side of the breaker.

--vtMaps

Vic · February 2014

Re: Breaker box design and configuration.

vt, yes of course this is the main purpose of any OCPD, and then it is protecting the cable/conductors not the equipment .. (not to get into a protracted discussion).

And in studying the 2008 Square D spec, there is not really a specific prohibition on using the Tandem breakers on DC circuits,. Personally, I would never use them on DC.

The spec states that Single Pole and Double Pole QO and QOB breakers with ratings above 10 A, and to 70 A ARE DC rated, but Tandem breakers are noted as QOTs.

In the application description for the QOT Tandems, it states,

"QOT tandem circuit breakers ... are used in applications where circuit loading is light and/or
noncontinuous, as in residential applications ..."

Would classify PV circuits as a Continuous use, and therefore IMO, the QOTs are not a good fit, and probably NOT DC rated.

Furthermore, all of these breakers are rated at 5,000 AIC, which a number of experts feel would be too low a rating on circuits connected to batteries used in battery-based RE systems ... SO, it is inadvisable to use these QOT breakers on the output side of the CC, which connects to the battery, IMO.

EDIT: Download this doc, and see only 120/240 VAC ratings for QOTs, no more time to study this now:
http://products.schneider-electric.us/technical-library/?event=detail&oid=09008926800a38dc&cat=0b008926801142d9

The MidNite MNEPV and MNEDC breakers are very inexpensive and are suitably-rated for PV and battery DC circuits:

http://www.solar-electric.com/pvaranddcrab.html

But, the MN breakers need to be mounted in a suitable MN box, that is not available in a BigBox store.

The above are just my opinions. Good Luck, Vic

zoneblue · February 2014

Re: Breaker box design and configuration.

I know full well the temptation to try to make do with what you can find at the local store. But for 11 bucks (right here) you just buy a proper rated DC breaker with bullet proof interupt and arc supression capacity. Thats darn cheap for a good nites sleep aint it?

And while your there you can do an awful lot with a big baby box.

jcheil · February 2014

Re: Breaker box design and configuration.

There was a really cool link someone posted here a while ago to some youtube videos of what happens to AC rated breakers when you try to use them for DC.

BB. · February 2014

Re: Breaker box design and configuration.

Here are some threads with DC breaker testing:

Interesting videos on breakers and fuses

BB. wrote: »

In general, the answer is no, you are not supposed to use AC breakers in a DC application.

The reason is that DC current sustains and Arc very nicely vs AC current. If you look at switches (and some breakers) you will find that they have both AC and DC ratings and the DC ratings (maximum current/maximum voltage) are usually much lower than the AC ratings.

In the US, we have some home AC panel breakers that are also rated for DC use (something like 48 VDC maximum vs the normal 120/240 VAC ratings)--But I don't know what you have available there.

Fuses also have DC and AC ratings.

Lastly, there is a value called the AIC rating (maximum Ampere Interrupting Current ratings--or something like that). Basically, that is the maximum current the fuse/breaker can interrupt with a dead short current through it. Turns out that a a medium sized DC battery bank can output thousands of amps into a dead short (heavy/short cable, etc.).

This Youtube video is an example of an breaker for US house wiring switching switching off relatively high DC voltage (~290 Vmp array @ 12 amps)... and you can see that it breaks out in fire

http://www.youtube.com/watch?v=HLX9cdB4TFQ

This video (2nd half) shows what 300 amps at 12 volts can do to a nail/screw/bolt in a short circuit:

http://www.youtube.com/watch?v=Avg5mJw0qHc

So--What is the take away from all of this? More or less, arcing starts to be a problem around 12 VDC... And becomes a larger problem at higher voltages. Does this tell you that a 12 VDC battery is safe on an AC rated fuse/breaker--No. Does it mean that it is safer than a 290 VDC circuit on an AC fuse/breaker--probably.

Vic wrote: »

It appears the the venerable Delta units are a spark-gap surrounded by sand in an ABS or PVC case. Fairly rundimentary "protection".

Found this Video on the Midnite site. It is a bit lengthy, but MN had the Delta and MN SPD protectors tested at a testing lab. That is the heart of this video FWIW:
http://www.midnitesolar.com/video/spd/spd.php

Have used Deltas in the past, and am thankful that no lightning strikes for them to "protect".

Here is the SPD (Lightning Protector) Topic on the Midnite site:
http://midnitesolar.com/smf_forum/index.php?board=13.0

Vic

-Bill

Breaker box design and configuration.

Comments

Categories