What size circuit breakers for this setup?

johnny5

New Member
Quick question: what size circuit breakers and fuses for this setup?

2 100a 12v lithium batteries
2 100w solar panels
Renogy 50a DC to DC MPPT Charger
1000w pure sine inverter

I'm looking at replicating Will's schematic here:


The specs of the components in his example don't match mine (he's using 400w solar here and 1100-1200w inverter I think), so I would like to know what size circuit breakers I should be using for the same setup but with my components. I am also wondering what size mc4 inline fuse to get for the solar panels.
 
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Deleted member 23531

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Good rule of thumb is 1.2x maximum nominal current. There will often be some rounding and approximation due to available breakers and the current of your particular equipment.

If in parallel at 12V, 2x 100W / 12V = 16.7A and 1.2x is an even 20A.

If battery is also 12V then same size between charge controller and battery. If you are stepping down from PV to battery voltage the battery current will be larger than PV current.

between the battery and the inverter, you use the load current for the calculation.
 

Supervstech

Administrator
Staff member
Moderator
To the inverter, you need to protect the wire, but prevent nuisance trips.
Surge loads come into play.
Figure 2000W, and a 25% over. 208A, drop back to a 200.
 
D

Deleted member 23531

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One follow up (first post was from mobile so was brief):

You might want to plug in your actual panel info. You didn't share that but let's assume it is this 100W Rich Solar panel. Then you want to size your breakers off of Isc = short circuit current = 5.93A. Again, if 2 of them are in parallel, that would be 2*5.93A = 11.86A, then 1.2x gives 14.2A. So actually a 15A breaker would be more appropriate. However, since this power is so low I don't think 20A is a big deal either. Sorry for the revision!
 

johnny5

New Member
One follow up (first post was from mobile so was brief):

You might want to plug in your actual panel info. You didn't share that but let's assume it is this 100W Rich Solar panel. Then you want to size your breakers off of Isc = short circuit current = 5.93A. Again, if 2 of them are in parallel, that would be 2*5.93A = 11.86A, then 1.2x gives 14.2A. So actually a 15A breaker would be more appropriate. However, since this power is so low I don't think 20A is a big deal either. Sorry for the revision!
Thanks for the follow up. They are renogy panels: https://www.renogy.com/100-watt-12-...TJuXItIO9vkQgBXfr4SQWPoXSUIDX9IhoC3tcQAvD_BwE

Also, in the photo of Will's setup, he is using a 250a bus bar. Would that also be appropriate for my setup?
 
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Deleted member 23531

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250a bus bar. Would that also be appropriate for my setup?
It's probably a bit more than you need, but wouldn't hurt. You need roughly 1000W / 12V / 0.8 (assuming 80% efficiency) so that's 104A. If your inverter can briefly surge to twice that output power (2000W) like some of them can, then yes you probably want a ~200A bus bar, and 250A would be fine.
 

johnny5

New Member
It's probably a bit more than you need, but wouldn't hurt. You need roughly 1000W / 12V / 0.8 (assuming 80% efficiency) so that's 104A. If your inverter can briefly surge to twice that output power (2000W) like some of them can, then yes you probably want a ~200A bus bar, and 250A would be fine.
Awesome, thanks. Better to be on the overkill side than the underkill side!
 

johnny5

New Member
To the inverter, you need to protect the wire, but prevent nuisance trips.
Surge loads come into play.
Figure 2000W, and a 25% over. 208A, drop back to a 200.
Hi, just want to follow up on this. Before I posted here, I had also sent an email to renogy asking them this same question, but I had asked about fuses instead of circuit breakers. I saw Will's setup (the photo I posted of his diagram) and thought circuit breakers would be easier to work with. Anyhow, renogy got back to me and said they recommend a 100a fuse in-between the battery and inverter. Whether it's a fuse or circuit breaker, I have two differing recommendations now that are quite far apart... 200a and 100a. It sounds like 100a would be safer, but at the expense of possibly more nuisance trips as you mentioned. I'm not really expecting to use any large appliances other than a dometic fridge, most just charging devices, lighting, fan, and maybe some sort of water pump eventually. Just want to see what you think of renogy's 100a recommendation.
 

Supervstech

Administrator
Staff member
Moderator
100AMPS breaker size won’t cover surge current, so, size the wire to handle 166A 100AMPS breaker should handle full 1000W load, plus a bit of heat from continuous loads, so would protect the inverter from harm.
more than likely, the inverter cannot supply more than a fraction of a second surge current anyway.
100A breaker will be OK.
 

Supervstech

Administrator
Staff member
Moderator
That seems low to me. Even just the nominal wattage at 80% efficiency would [maybe] trip that breaker: 1000W/12V/0.8 = 104A
Oh, I agree, running full loads for long Will heat up the breaker, but they don’t make a 105A breaker... as long as the wire is sized for 166A, it should run fairly cool... maybe split the difference and put in a 125A?
 

johnny5

New Member
Oh, I agree, running full loads for long Will heat up the breaker, but they don’t make a 105A breaker... as long as the wire is sized for 166A, it should run fairly cool... maybe split the difference and put in a 125A?
That seems reasonable. What are some good brands? A lot of the stuff on Amazon looks kinda generic, don't really want to skimp on protection stuff.
 
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