Circuit breaker question- Voltage rating?

loac

New Member
There is a lot of info on sizing a circuit breaker per amperage. It seems the consensus is that the breaker amperage rating should be about 1.25 times the conductor sizing. But I can’t find anything telling me where the voltage rating of circuit breakers comes in.
Also, there don’t seem to be a ton of voltage options for dc dipole breakers.
Thanks for your help in clarifying this.
 

iamrich

Solar Addict
I just make sure the voltage rating is much higher than my working voltage. I am using the midnight solar breakers that are rated for 150vdc MNEPV15.

Here is more than you will ever want to know on breakers :p

 

RCinFLA

Photon Sorcerer
Any breaker for higher voltage rating has an arc suppressor mechanism to chop off any arc that is created by breaking contacts on higher voltages.

AC arc is easier to break then DC arc which is why you should not use an AC rated breaker for DC circuit.

It is usually a bunch of metal fins that creates a field to push the arc away, breaking it quicker. For DC breakers this also means there is polarity on terminals for source and load side connections. Getting these backwards defeats the arc suppression resulting in more contact burning when breaking connection.

Since any arc is hot plasma it tends to flare upwards. Some breakers require the orientation of breaker to be a particular way because the arc suppression takes advantage of the upward flare of the arc. If you mount a DC breaker to inverter so any text on breaker is upside-down reading you likely have orientation wrong. There is likely more then a few PV DC breaker in use mounted upside down.

Most breakers have a thermal triggered latch for slow blow action near breaker current rating and a magnetic triggered latch for quick high current (like short circuit) disconnect. There is always a manufacturing make tolerance so typically the slow blow thermal break is 10% to 20% over breaker rated current to ensure it can hold at rated current. Short circuit break is very much higher current over rated current and has a wide tolerance range. The actual spec is complicated as it also includes current vs. time factor.
 
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rmaddy

Solar Addict
It seems the consensus is that the breaker amperage rating should be about 1.25 times the conductor sizing.
Not quite. It's typical to size the breaker at 1.25 times the amount of current needed by the load. The most important thing to remember is that the chosen wire MUST be able to safely handle more current than the selected breaker. You always want the breaker to be the weakest point in the circuit.
 
Not quite. It's typical to size the breaker at 1.25 times the amount of current needed by the load. The most important thing to remember is that the chosen wire MUST be able to safely handle more current than the selected breaker. You always want the breaker to be the weakest point in the circuit.
So for my 24v 620ah bank going into a growatt 3k inverter. If I'm using 1 wire from my bus bars where the two packs connect to the inverter for the final run, what size breaker should I put in that run between the bus bar and the inverter? I was initially thinking 150a but wanted to stay under the max constant rating of the inverter at 3k or 25a ac and keep my draw down to 2600w or 22a ac.
Hope that makes sense.
 

TomC4306

Solar Enthusiast
Circuit breaker should never trip.
If it were me...
3000 /25.6 = 117 *1.25 = 146.
I'd use a 150 amp.

With 2/0 wire
 
Ok awesome thanks. I was initially thinking 150a.
On my crude drawing below, I have my 2 separate 8s packs that will join together at 2 3 post bus bars.
The wire off my daly bms is 6ga which will have to be spliced or connected to whatever size wire I use to lengthen it so it can reach the bars. Is 2ga wire also sufficient for that extension to the bus bars? Then from the bars to the inverter you said 2 as well? Trying to size my stuff accordingly on paper before I get the wrong stuff and waste time.
 

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TomC4306

Solar Enthusiast
Previous answer applies to bus bar to inverter.
You should fuse your batteries too. Make it part of the battery assembly. Bms to it, extension cable from it to bus.
 
Ok thanks.
So bms direct to battery, fuse/breaker inline on both - and + then to the bus bar?
2 fuses per pack total ore just one on the -/+?
 

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