rccapps Posted March 9, 2021 Share Posted March 9, 2021 I can't see anything wrong with wiring the DC inputs of two inverters in parallel to the main + and - of the same battery bank. But I know just enough about electrical stuff to get me in trouble, so I wanted to seek confirmation (or condemnation) from wiser heads before proceeding. Quote Link to comment Share on other sites More sharing options...
Sid Genetry Solar Posted March 9, 2021 Share Posted March 9, 2021 Absolutely nothing wrong with connecting 2 inverters to the same battery bank. HOWEVER... I highly recommend (no, demand!) that a properly rated DC breaker be used on EACH device connecting to an off-grid battery bank...for safety purposes. For 12 / 24v systems, you usually can use pretty much any breaker (regardless of DC or AC rating...most AC breakers also have a 32vDC rating). However, for 36 / 48v (or higher) systems, you will need to find a dedicated DC breaker. And yes, breaker polarity often matters (you can find YT videos that show what happens if a properly rated DC breaker is wired backwards--it turns into a fire hazard.) A properly-rated DC breaker is extremely important: if a breaker isn't rated for the DC voltage, what was supposed to protect things...will actually turn into a fire hazard. Not funny. I personally have 3 MPPT charge controllers, one inverter, and a number of DC loads connected to a single 12kwh LFP battery bank...but each device has a separate properly-rated DC breaker. (Most of my breakers are rated to 80vDC.) 1 Quote Link to comment Share on other sites More sharing options...
Sid Genetry Solar Posted March 9, 2021 Share Posted March 9, 2021 I've had a breaker "save the day" on one occasion when I had an MPPT fail. Pretty big blowout from it, but the 80A breaker tripped, cutting power. And I prefer breakers over fuses, 'cause they aren't throwaway devices 😉 Quote Link to comment Share on other sites More sharing options...
Waterman Posted March 9, 2021 Share Posted March 9, 2021 5 minutes ago, Sid Genetry Solar said: Absolutely nothing wrong with connecting 2 inverters to the same battery bank. HOWEVER... I highly recommend (no, demand!) that a properly rated DC breaker be used on EACH device connecting to an off-grid battery bank...for safety purposes. For 12 / 24v systems, you usually can use pretty much any breaker (regardless of DC or AC rating...most AC breakers also have a 32vDC rating). However, for 36 / 48v (or higher) systems, you will need to find a dedicated DC breaker. And yes, breaker polarity often matters (you can find YT videos that show what happens if a properly rated DC breaker is wired backwards--it turns into a fire hazard.) A properly-rated DC breaker is extremely important: if a breaker isn't rated for the DC voltage, what was supposed to protect things...will actually turn into a fire hazard. Not funny. I personally have 3 MPPT charge controllers, one inverter, and a number of DC loads connected to a single 12kwh LFP battery bank...but each device has a separate properly-rated DC breaker. (Most of my breakers are rated to 80vDC.) I second that. All of mine also are on the same battery bank and they have their own breakers. Just like all my charge controllers are on different breakers. And light duty circuits all have fuses. With the fuse boxes having a fuse on the input also. As I run a 12/24V system, all of the 24V breakers are rated at 42VDC as a minimum. Quote Link to comment Share on other sites More sharing options...
rccapps Posted March 9, 2021 Author Share Posted March 9, 2021 Thanks for the prompt reply, Sid! If my system is 48V (16S x 3.2V LiFPeO4), I think the typical discharge voltage would be in the mid to high 50s, right? If so, could I get away with a breaker rated for 60 or 70V? And if each inverter were 3kW continuous, would a 120A breaker for each be suitable (to allow for 2X surges)? Quote Link to comment Share on other sites More sharing options...
Sid Genetry Solar Posted March 9, 2021 Share Posted March 9, 2021 6 minutes ago, rccapps said: If my system is 48V (16S x 3.2V LiFPeO4), I think the typical discharge voltage would be in the mid to high 50s, right? Yes. My 16S LFP bank ranges from 52-56v depending on whether at FLOAT in the daytime, or resting at night. 6 minutes ago, rccapps said: If so, could I get away with a breaker rated for 60 or 70V? You'll probably be fine with something like that. The ones I could find for cheap on eBay were 80v, so I'm happy with that ;-). If you can find magnetic trip breakers, those are the best (as opposed to thermal trip). 6 minutes ago, rccapps said: And if each inverter were 3kW continuous, would a 120A breaker for each be suitable (to allow for 2X surges)? 3kw continuous output @ 48v is roughly 68A (3,300W / 48)...so I'd say a 70-80A breaker should be more than plenty. Most commonly available breakers have a "B" trip curve, which means that they have a characterized trip delay based on the overload--so they will handle surge loads without tripping. ("A" curve usually is very quick, "B" is slower, and "C" usually the slowest.) I personally have a 150A DC breaker on my inverter...expecting to be able to run up to 7kw continuous out of it. (May well need to upgrade it sometime soon haha!) How about something like this? Magnetic trip, 80v, 100A: https://www.ebay.com/itm/Airpax-LMLK1-1RLS4-29928-100-100A-DC-BREAKER-52F-DELAY-125A-TRIP-80V-Max/124122811290 Note the "Line" and "Load" indications..."Line" goes to battery positive, and "Load" to the inverter. Quote Link to comment Share on other sites More sharing options...
Sid Genetry Solar Posted March 9, 2021 Share Posted March 9, 2021 This one's a tad cheaper if you want to buy the last 2 😉 https://www.ebay.com/itm/Carling-Technologies-80-Amp-DC-Volt-Breakers-1REC5/402378611072 Quote Link to comment Share on other sites More sharing options...
rccapps Posted March 9, 2021 Author Share Posted March 9, 2021 Looks like I've got a plan now. Thanks again for the help! Quote Link to comment Share on other sites More sharing options...
The Blind Wolf Posted March 9, 2021 Share Posted March 9, 2021 Uhm, Call me crazy, but, I only used a T Fuse 400A in line slow burning fuse, is that good enought for two 6k 24v? I have no fuse or breaker between my chargers or batts, cause ever time I try a resetble breaker, I would trip it even if it was rated higher thenwhat the battery should be able to push I would still trip the thing, causeing issues with having to go turn it back on. Quote Link to comment Share on other sites More sharing options...
The Blind Wolf Posted March 9, 2021 Share Posted March 9, 2021 Also I have two t fuse 400A, so each 6k would have its own. I used it on a 6k modfied sign wave for two years with no issue, it takes a 0/2 aug cable 😛 Quote Link to comment Share on other sites More sharing options...
rccapps Posted March 9, 2021 Author Share Posted March 9, 2021 One more question, since we're on the subject of breakers: I was planning to put one between the charge controller and the batteries, too. Would the "line" side still connect to the battery, or would it be reversed (since the current is flowing toward the battery from the charger)? Quote Link to comment Share on other sites More sharing options...
Sid Genetry Solar Posted March 9, 2021 Share Posted March 9, 2021 4 minutes ago, rccapps said: One more question, since we're on the subject of breakers: I was planning to put one between the charge controller and the batteries, too. Would the "line" side still connect to the battery, or would it be reversed (since the current is flowing toward the battery from the charger)? That is a VERY good question; I'm a bit undecided myself on exactly how that's supposed to work. At first glance, it would seem that the breaker should be polarized "line to the MPPT" (due to the general power flow). HOWEVER...it will be impossible for the charge controller to trip the breaker in normal operation. If it malfunctions, it won't be able to "multiply" amperage...so wiring the breaker "line to MPPT" is really kinda useless. The only way I've had an MPPT trip a breaker is by catastrophic internal failure, which resulted in a short circuit across the battery. In which case the breaker must go LINE to the battery, LOAD to the MPPT--in order to properly break the DC arc. (The arc resulting from opening a DC circuit is why polarity & DC rating is extremely important.) My conclusion: I would recommend connecting the breaker "Line" to the battery positive terminal, "Load" to the MPPT. Personally, I use tandem (2-pole) breakers on the MPPTs...one side for the battery, and the other side for the solar wires coming in. Some manufacturers don't recommend it anymore (Morningstar Corp for one), but I don't see quite why. Because the solar maximum power amperage is so close to the short circuit amperage, it is practically impossible for a regular thermal or magnetic breaker to trip if the panels are shorted out--and only then possible if direct full sunshine at the time. And if the 2 breakers are separate...then if the MPPT malfunctions and trips the battery breaker, you still have the full MPPT array connected to the damaged unit. If the 2 breakers are a tandem unit, then any MPPT breaker fault will cut both power sources simultaneously. But that's a different discussion 😉 Quote Link to comment Share on other sites More sharing options...
rccapps Posted March 9, 2021 Author Share Posted March 9, 2021 Good enough for me. Thanks, Sid! Quote Link to comment Share on other sites More sharing options...
InPhase Posted March 10, 2021 Share Posted March 10, 2021 Code-wise, you size a breaker (generally.. there are exceptions) to 125% of the load... This is because the code limits the continuous conductor loading to 80% of the rated ampacity. So if you have a 100 amp load, you need a 125 amp conductor and breaker. As long as it's under 800 amps, you can select the next highest standard size if you end up with an oddball number. Quote Link to comment Share on other sites More sharing options...
wpg205 Posted March 16, 2021 Share Posted March 16, 2021 They also have nonpolarized DC breakers now. Not as easy to find and more expensive, but I think worth it in solar applications. Quote Link to comment Share on other sites More sharing options...
TheButcher Posted March 19, 2021 Share Posted March 19, 2021 (edited) The form factor starts to become an issue. People like their MCBs and there just isn't enough space inside an MCB to move the contacts far enough apart in order to be able to reliably break a high current 'high' voltage DC supply with a non-polarised MCB. When it comes to those sorts of conditions non-polarised MCBs are usually just ganged polarised MCBs (they have a magnet inside to push the arc into the trap) with the current going the 'wrong' way through one side to ensure that regardless of which way the current is going at the time of fault (or the user flipping the breaker to off) one breaker will always force any arc into the trap and therefore be able to interrupt the current. Edited March 19, 2021 by TheButcher 1 Quote Link to comment Share on other sites More sharing options...
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