12 v Circuit Breaker Advice, Please

[kernowsolar]

Circuit Brake my 12v System​

Hello, I would like to add the following DC circuit breakers to my solar system. I have researched and worked out what I think is safe , but I would appreciate any opinions :

12 v System
440ah Lead Acid Batteries
*I understand Fuses and Breakers are there to protect the cable

Inverter Breaker (Battery to Inverter) (40amp Breaker)
10mm2 Cable
375 Victron Phoenix Inverter (12v)
(375/12 X 1.25 = 39 amps)

Solar Panel to Charge Controller Breaker (20amp Breaker)
4mm2 Solar Flex (55amps)
2 x 250w Panels in Parallel. Voc = 37.3v . Isc = 9amps
2 x 9amps - 18 amps x 1.25 = 22.5amps

Charge Controller to Battery Breaker (40amp Breaker)
Tracer 40amp Charge Controller
6mm2 Solar Flex (79amps)

Thank you for your input

 

[MisterSandals]

Your breakers should be 20-50% more amps than your loads. A 40A breaker on 39A load will probably trip quite often and unnecessarily. Same with your array breaker.
Why a 40A breaker on 79A SCC to battery? Seems oddly small.

Is there a reason (shading?) why you are putting your panels in parallel? What is SCC max input voltage and MPPT range?

 

[kernowsolar]

Thank you for your reply.

1). I thought the Breaker Size from the Charge Controller (40amp) to the battery , should match the amp rating of the charge controller. What circuit breaker size would you suggest? I did not understand the 79A part of your reply, what amp rating is that referring to?

2). I have always connected my panels in parallel. Would you suggest having them in series? The max open circuit voltage of the controller is : 138v. The max pv array power is 1560w/12v

 

[Supervstech]

I see, he read your definition of the 6mm2 79A as the rating of the scc…

 

[kernowsolar]

I see too now, thank you for your reply and help Supervstech, that has clarified that part for me

 

[MisterSandals]

Sorry, i misread most of what your wrote. Looks like you are using 1.25 to upsize breaker for your expected loads.
Do your panels have a "string fuse rating" on them?

Do you see your inverter's MPPT range in your manual? This is usually the voltage range where it is optimized. Since your max input voltage is 138Voc i suspect its about 50-80% of that. I dunno which is why i am asking if you can find it.

 

[kernowsolar]

Thank you for your help. I have a Tracer 4215BN. I have been through the manual online but could not see an MPPT range. I have a photo of the sticker on the back of my panel, attached to this message.

I guess I am trying to just get the right DC breaker size for peace of mind, maybe I am being too cautious on my calculations?

 

[Supervstech]

Your breakers should be 20-50% more amps than your loads. A 40A breaker on 39A load will probably trip quite often and unnecessarily. Same with your array breaker.
Why a 40A breaker on 79A SCC to battery? Seems oddly small.

Is there a reason (shading?) why you are putting your panels in parallel? What is SCC max input voltage and MPPT range?

To further clarify the breaker sizing… if an inverter can sustain 40A, it can surge 80A some inverters can surge 3x the amperage rating for minutes! If your LOAD is 39A, the wire needs a 1.25% ampacity increase. Or 48ish amps MINIMUM wire rating. I would recommend doubling the amp rating so, use wire rated double the expected ampacity, and OCP for THAT size.

Remember, nuisance trip over fusing is for surge startup load protection. Like a motor load. Not just to prevent nuisance trips. You have to follow guidelines for breaker vs conductor sizing when dealing with a panel feeder.
The amperage of the specific loads get oversized… not the feeder.
I mean that if a breaker is feeding just a motor load, THAT circuit gets oversized… not the feeder feeding the main panel. A panel feeder has to be fused to prevent the feeder circuit from sustained heat overload.

 

[kernowsolar]

Thank you for your reply. Yes, i just checked and my Victron 12/375 has a peak power of 700w. So, 700/12x1.25 = 72amps, so maybe I should increase my breaker size. I have a 64amp breaker somewhere , so I might use that in the first instance for my inverter to save money. I will buy the other breakers I require tomorrow.

I see you did the MCSE, many years ago I did the MCSA. It is hamburger night here in the UK, so i will have to disappear and cook hamburgers for my family...I will reply to any more replies, tomorrow.

Thank you for your help

 

[Supervstech]

Eat well!