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DC breaker capacity ?

Bossrox

Solar tinkerer
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May 21, 2021
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Columbia, SC
If you have for example a DC 2 pole 200 amp breaker & combine both poles, my thinking is it'll now trip @ 400 amps. Would that be a correct assumption?

Update: OK, I had to do this test on my own to find the true answer.

Got the breaker capacity test done. This is the way it is... A double breaker with both sides combined will be whatever the rating capacity of the breaker is. It doesn't double it like I was thinking. However from a reply to my earlier post about this, combining 2 identical independent breakers in the same way, WILL double the amp capacity. I then removed 1 of the jumpers & tested again & capacity was reduced in half as expected. My inverter at full output will draw 300 amps so bad news is my 200 amp double breaker isn't strong enough to carry it above 65% load. Here ya go with the video of that test.
 
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Maybe, maybe not. It depends on several factors; it is a risky assumption. Indeed dangerous with a directional DC MCB.
 
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Breakers generally have two methods they use to break circuit. The 'rated' current breaking mechanism is a bimetallic latching release. As it heats up it flexes, finally reaching a point where it releases hold in latch. This has a time-current dependent action. In typically about half an hour at rated load, +/- a little make tolerance, it with trip. In the short term it typically takes 3-4 times the rated current for it to trip in a minute or two.

Second method is short circuit trip mechanism. This is an electro-magnetic solenoid with coil winding in series with breaker current. A sudden heavy load current, many times the breaker's rated current, will cause the solenoid to pull in, immediately tripping the breaker latch release.

All this 'stuff' in a breaker, particularly the solenoid coil, adds series resistance to breaker. A high amperage breaker has typically 3-5 milliohms of terminal to terminal resistance. This causes high amperage breakers to get very hot under heavy load current. Be careful of connecting wires insulation. Cheap wire with vinyl insulation on wires close to breaker may melt.

Better to use a fuse instead of high amperage breaker for 12v system as the voltage drop of breaker is too much for 12v systems. Fuse and its connections are typically in 0.5 to 1 milliohm range.
 
If you have for example a DC 2 pole 200 amp breaker & combine both poles, my thinking is it'll now trip @ 400 amps. Would that be a correct assumption?
If I'm following you, which I may not be, combing breakers in series is typically done to be able to use a breaker at a higher voltage. But as has already been pointed out it's not something I'd do without specific direction from the manufacturer.

For example Midnite solar combiner use 150v breakers in series to get to 300v.


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Not a great idea to use two breakers in parallel.

Tolerance difference in their breaking trip will cause one to trip before other. This puts more stress on the second breaker contacts carrying much more current when it follows a small amount of time later.

Contact resistance will also not be identical so one breaker will take more of the current. Result will be a trip current below twice the single breaker rating. This delta grows the more the breaker is used degrading contact resistance at different rates.
 
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