circuit breakers

[Julianna]

I'm installing solar on my bus and was wondering what size circuit breaker I should put in between my panels and growatt 24v all in one system, and also what size should I put between my 4 100ah liFePO4 battery bank if I need one?

https://store.santansolar.com/product/rec-350w/

I have 2 of these solar panels mounted to the roof

 

[John Frum]

I'm installing solar on my bus and was wondering what size circuit breaker I should put in between my panels and growatt 24v all in one system, and also what size should I put between my 4 100ah liFePO4 battery bank if I need one?

https://store.santansolar.com/product/rec-350w/

I have 2 of these solar panels mounted to the roof

For 2 panels no over-current protection is required whether they are configured in series or parallel.
You only need over-current protection with 3 or more strings.
It is nice to have a means of disconnect between the panels and the solar charge controller.
If the wire is 10awg pure copper with insulation rated for 105C(which is likely) you can use a 30 amp breaker rated for no less than 120 volts for series and 60 volts for parallel.

 

[Dzl]

Here is a good resource to get you started:

DC Fuse and breaker Sizing and Placement

To get the paper, click on the orange button at the top of the page. The attached presentation gives the fundamentals for picking fuse/breaker sizes and describes where in the circuit they should be placed. You can download the presentation...

diysolarforum.com

Some basic pointers:

• Buy from reputable brands, don't buy amazon knock-offs when it comes to fuses/brakers/safety equpiment. Blue Sea, Bussman, Littelfuse, Midnite, Outback, etc are good brands.
• The fuse/breaker protects the wire. always make sure the fuse is sized to protect the wire.
• For DC make sure you are buying DC rated fuses/breakers for the proper voltage. If a breaker is DC rated it will make that clear.
• For your main battery bank fuse/breaker. You want something with a high "AIC" (that is: Ampere Interrupt Capacity). This rating tells you how large of a surge the device can protect against without failing unsafely. For lifepo4 you need a substantial AIC rating. The suggested minimum is an AIC of 20,000A (20kA). A class T fuse will do this, there are a few breakers that will do this as well but they are expensive.

 

[Julianna]

Thank you so much!
So I don't really need the breaker between the panels and system but a cut-off switch instead?
And as for the batteries I used 1/0 awg copper wire and was also going to put a cut-off switch between them and the system too, would this require a breaker as well?

Thank you for all your help!

 

[John Frum]

Thank you so much!
So I don't really need the breaker between the panels and system but a cut-off switch instead?
And as for the batteries I used 1/0 awg copper wire and was also going to put a cut-off switch between them and the system too, would this require a breaker as well?

Thank you for all your help!

You don't need either but, either would be nice to have as a means of disconnect.
Makes it easier to work on the system.

 

[Julianna]

Here is a good resource to get you started:

DC Fuse and breaker Sizing and Placement

To get the paper, click on the orange button at the top of the page. The attached presentation gives the fundamentals for picking fuse/breaker sizes and describes where in the circuit they should be placed. You can download the presentation...

diysolarforum.com

Some basic pointers:

• Buy from reputable brands, don't buy amazon knock-offs when it comes to fuses/brakers/safety equpiment. Blue Sea, Bussman, Littelfuse, Midnite, Outback, etc are good brands.
• The fuse/breaker protects the wire. always make sure the fuse is sized to protect the wire.
• For DC make sure you are buying DC rated fuses/breakers for the proper voltage. If a breaker is DC rated it will make that clear.
• For your main battery bank fuse/breaker. You want something with a high "AIC" (that is: Ampere Interrupt Capacity). This rating tells you how large of a surge the device can protect against without failing unsafely. For lifepo4 you need a substantial AIC rating. The suggested minimum is an AIC of 20,000A (20kA). A class T fuse will do this, there are a few breakers that will do this as well but they are expensive.

Thank you so much for the resources and pointers! I'm new to this type of thing and really appreciate the help and advice

 

[Cdkipp]

Thank you so much for the resources and pointers! I'm new to this type of thing and really appreciate the help and advice

For the main battery disconnect, I would look at the Blue sea line. There are others similar but blue sea is often used. look at this link and then shop for the type and size you need based on your expected battery currents.

These are not breakers, but fuses, but for the best protection use a class T fuse. I don't know what current you are planning on pulling but for 1/0 I would figure a 150 amp class T fuse. They are annoyingly expensive but the best.

Here are examples. I did not shop prices so look around for similar. If you want to know things for sure give us more info about what you plan.

TFB1-150, 150A Class T Fuse Kit | DonRowe.com

The TFB1-150 offers 150 amps of over-current protection and provides safe operation of your inverter.

www.donrowe.com

NC 300A Power Switch M Series Battery Switch, Used for Battery Switch Single Circuit Marine OnOff Red: Amazon.com: Industrial & Scientific

NC 300A Power Switch M Series Battery Switch, Used for Battery Switch Single Circuit Marine OnOff Red: Amazon.com: Industrial & Scientific

www.amazon.com

 

[Dzl]

Thank you so much!
So I don't really need the breaker between the panels and system but a cut-off switch instead?

And as for the batteries I used 1/0 awg copper wire and was also going to put a cut-off switch between them and the system too, would this require a breaker as well?

What is the wire temperature rating of the wire? That is important for knowing its current rating

To do it right, there are two options (probably more, but 2 that I am aware of) for main circuit protection with lithium batteries:

1. A properly rated Class T fuse and a manual disconnect switch
2. A properly rated high AIC DC circuit breaker (no need for the disconnect switch)

By properly rated, I am referring to A. The fuse is DC rated for the voltage of your circuit B. the proper current rating for your circuit C. a high AIC rating (20kA+ seems to be the best practice)

Neither option is cheap (expect $50-100), because they must be rated for the much higher currents that lithium batteries can produce in a dead short situation. Circuit breakers that meet these conditions can be hard to find in a convenient form factor.

The fuse @Cdkipp posted above meets all these conditions, not sure if that is the right size (current rating) or not without knowing your max expected load and your wire type, but its available in multiple sizes.

 

[Cdkipp]

What is the wire temperature rating of the wire? That is important for knowing its current rating

To do it right, there are two options (probably more, but 2 that I am aware of) for main circuit protection with lithium batteries:

1. A properly rated Class T fuse and a manual disconnect switch
2. A properly rated high AIC DC circuit breaker (no need for the disconnect switch)

By properly rated, I am referring to A. The fuse is DC rated for the voltage of your circuit B. the proper current rating for your circuit C. a high AIC rating (20kA+ seems to be the best practice)

Neither option is cheap (expect $50-100), because they must be rated for the much higher currents that lithium batteries can produce in a dead short situation. Circuit breakers that meet these conditions can be hard to find in a convenient form factor.

The fuse @Cdkipp posted above meets all these conditions, not sure if that is the right size (current rating) or not without knowing your max expected load and your wire type, but its available in multiple sizes.

I would vastly prefer a DC breaker set up just for space convenience and limiting connection points. As you say they are not easy to find, especially at a reasonable cost, although if you are starting fresh and need to buy both a switch and class T anyway, then maybe a breaker makes sense.

 

[John Frum]

I would vastly prefer a DC breaker set up just for space convenience and limiting connection points. As you say they are not easy to find, especially at a reasonable cost, although if you are starting fresh and need to buy both a switch and class T anyway, then maybe a breaker makes sense.

Another alternative is a fused busbar on the positive side.
This is my favorite
https://www.waytekwire.com/item/45547/Littelfuse-0FHZ0001Z-FHZ-Series-ZCASE-Fuse/
It works with zcase and mrbf fuses.

ZCASE-Series: Blade Fuses | Waytek

Bolt-down fuses provide protection from 30A to 600A and are ideal for battery and alternator circuit protection. Order from Waytek today!

www.waytekwire.com

MRBF-Series: Blade Fuses | Waytek

Bolt-down fuses provide protection from 30A to 600A and are ideal for battery and alternator circuit protection. Order from Waytek today!

www.waytekwire.com

I also use one of these busbars on the negative side as the zcase fuses are so inexpensive I just put them under the busbar to get a common un-fused negative busbar.

 

[Dzl]

Another alternative is a fused busbar on the positive side.
This is my favorite
https://www.waytekwire.com/item/45547/Littelfuse-0FHZ0001Z-FHZ-Series-ZCASE-Fuse/
It works with zcase and mrbf fuses.

ZCASE-Series: Blade Fuses | Waytek

Bolt-down fuses provide protection from 30A to 600A and are ideal for battery and alternator circuit protection. Order from Waytek today!

www.waytekwire.com

MRBF-Series: Blade Fuses | Waytek

Bolt-down fuses provide protection from 30A to 600A and are ideal for battery and alternator circuit protection. Order from Waytek today!

www.waytekwire.com

I also use one of these busbars on the negative side as the zcase fuses are so inexpensive I just put them under the busbar to get a common un-fused negative busbar.

I think the AIC rating for the Zcase fuses (2,000A) is much too low for a main battery fuse, even MRBF falls short of the typical recommendation of >20,000A AIC. MRBF may well be fine for smallish LFP banks, but its a bit of an unknown i think, Class T seems to be the cautious choice.

My understanding is the ABYC (marine specific standards body) recommends 20,000A minimum for traditional lead acid battery banks over 500Ah, or any battery bank where the short circuit current of the battery bank is unknown. And a disclaimer that lithium battery banks may necessitate much higher AIC rated fuses above their general recommendations. (but in all honesty, even in the most recent publications, the ABYC is still frustratingly vague on lithium specific considerations, and manufacturers are little to no help either).

 

[John Frum]

I think the AIC rating for the Zcase fuses (2,000A) is much too low for a main battery fuse, even MRBF falls short of the typical recommendation of >20,000A AIC. MRBF may well be fine for smallish LFP banks, but its a bit of an unknown i think, Class T seems to be the cautious choice.

My understanding is the ABYC (marine specific standards body) recommends 20,000A minimum for traditional lead acid battery banks over 500Ah, or any battery bank where the short circuit current of the battery bank is unknown. And a disclaimer that lithium battery banks may necessitate much higher AIC rated fuses above their general recommendations.

The busbar is for branch circuits where a ~10000 amp AIC breaker is there to isolate the fault if it can.
If not the class-t fuse on the feeder circuit(as close to the battery positive terminal as possible) should prevent catastrophe.
IMO the high cost of class-t fuses makes them impractical for use on branch circuits at this time.
Also Victron's lynx distributor uses mega fuses which from memory would have a lower AIC than the MRBFs.

I have been coaching that is was ok to use MRBF as the main fuse in 12 volt LFP systems up to 300 amp hours and basically all lead acid systems.
You have convinced me to change that policy.

 

[Cdkipp]

The busbar is for branch circuits where a ~10000 amp AIC breaker is there to isolate the fault if it can.
If not the class-t fuse on the feeder circuit(as close to the battery positive terminal as possible) should prevent catastrophe.
IMO the high cost of class-t fuses makes them impractical for use on branch circuits at this time.
Also Victron's lynx distributor uses mega fuses which from memory would have a lower AIC than the MRBFs.

I have been coaching that is was ok to use MRBF as the main fuse in 12 volt LFP systems up to 300 amp hours and basically all lead acid systems.
You have convinced me to change that policy.

It is a great idea for branch circuits. I had not seen them before.

 

[Snubber]

Thank you so much for the resources and pointers! I'm new to this type of thing and really appreciate the help and advice

New as well. I'm following.