Pls help me with system design

[Zwy]

thank you for your time , the inverter came with its own fuse , but i have no fuse from the battery to the house wiring or anything else , the only component of my system that has a fuse is the inverter-battery

i think im overthinking this

if the inverter already has a fuse , do i need any other terminal fuse to protect the battery?

I assumed that aside from the inverter i would need a class T fuse to protect the battery itself from all the other positive sources of power

Always an ANL fuse on positive cable as close to the battery as possible. This is for system protection. Shunt is on the left, negative cable; ANL is on the right side, positive cable.

 

[rmaddy]

2 x 32A double breakers from panels-scc and scc-battery

You can't have a 32A breaker between the SCC and battery if you are planning on the 100/50 SCC. The 50 means it outputs up to 50A to the battery. First, make sure you choose wire that can handle the 50A. Then you want 65A breaker, not 32A.

It's really really important you remember the most important basics. Based on the needed current in the wire you must choose a wire than can safely carry that current and not cause too much voltage drop. Once the wire is properly chosen you then pick the fuse/breaker size. The size you choose must be between the expected current and the max safe size for the chosen wire. A simple rule of thumb is 1.25 times the needed current then rounded up to a size you can actually get. But you must make sure this size is not greater than the max safe size for the chosen wire.

You must follow these steps for every single wire and fuse/breaker in your system. If you choose a wire that is too small then it can be very dangerous. Or at least it can cause problems. If you choose a fuse that is too large for the wire then it does no good. The wire will fail before the fuse. That's bad. Choose a fuse that is too small and it will needlessly trip.

The breaker you use between the panels and the SCC must be chosen based on the wire and the expected current which is determined by the panel configuration and their specs (Isc and Impp). You must also remember that the breaker must be able to handle the Voc of the panels based on their configuration. It's common for the Voc of the panels to be higher than many typical breakers are designed for so be sure to verify that too.

320a Class T fuse

That's a bad idea. You can't just choose an overly large fuse. If you plan on the 4AWG wire then putting a 320A fuse is as bad as having no fuse. The 4AWG wire will burn long before you get anywhere near 320A. You must be using at least 2/0AWG wire to safely use a 320A fuse.

 

[moonlitsouls]

You can't have a 32A breaker between the SCC and battery if you are planning on the 100/50 SCC. The 50 means it outputs up to 50A to the battery. First, make sure you choose wire that can handle the 50A. Then you want 65A breaker, not 32A.

It's really really important you remember the most important basics. Based on the needed current in the wire you must choose a wire than can safely carry that current and not cause too much voltage drop. Once the wire is properly chosen you then pick the fuse/breaker size. The size you choose must be between the expected current and the max safe size for the chosen wire. A simple rule of thumb is 1.25 times the needed current then rounded up to a size you can actually get. But you must make sure this size is not greater than the max safe size for the chosen wire.

You must follow these steps for every single wire and fuse/breaker in your system. If you choose a wire that is too small then it can be very dangerous. Or at least it can cause problems. If you choose a fuse that is too large for the wire then it does no good. The wire will fail before the fuse. That's bad. Choose a fuse that is too small and it will needlessly trip.

The breaker you use between the panels and the SCC must be chosen based on the wire and the expected current which is determined by the panel configuration and their specs (Isc and Impp). You must also remember that the breaker must be able to handle the Voc of the panels based on their configuration. It's common for the Voc of the panels to be higher than many typical breakers are designed for so be sure to verify that too.

That's a bad idea. You can't just choose an overly large fuse. If you plan on the 4AWG wire then putting a 320A fuse is as bad as having no fuse. The 4AWG wire will burn long before you get anywhere near 320A. You must be using at least 2/0AWG wire to safely use a 320A fuse.

ok i misread what you said earlier "Nothing wrong with using bigger wire or bigger bus bars."

As far as the 32a breaker for the SCC , we came to this number yesterday based on this

"If you wire them in series using Vmp you'd have: 20.4v + 20.4v + 16.77v + 16.77v + 17v = 91.34v * (lowest amps = 9.45a) = 863w. This would be 863w/940w (max) = 91.8% of the max."

so with the higher voltage , the max amps would be 9.5 , thats why i thought the 32a would be sufficient

the fuse between the inverter and battery is already installed and came with the inverter so i know thats good

the fuse between the dc charger to battery is recommended based on this

ill stick with the 250a bus bars


as far as the class t , i will prob use 2 guage wire from the class t to the battery , or 4 guage , only reason i selected that 320a class T was because it was priced well and its hard for me to find 150a class t fuses on amazon that have good reviews ,

im going to do more research on how to pick fuses , youve been very helpful

 

[rmaddy]

ok i misread what you said earlier "Nothing wrong with using bigger wire or bigger bus bars."

Using bigger wire (or bus bars) is fine. But that is not at all true of fuses/breakers.

As far as the 32a breaker for the SCC , we came to this number yesterday based on this

"If you wire them in series using Vmp you'd have: 20.4v + 20.4v + 16.77v + 16.77v + 17v = 91.34v * (lowest amps = 9.45a) = 863w. This would be 863w/940w (max) = 91.8% of the max."

so with the higher voltage , the max amps would be 9.5 , thats why i thought the 32a would be sufficient

32A is more than 3x the 9.5A load. As I stated, 1.25 (25%) is a much better guideline. But you're not going to find a 12.5A fuse. 15A probably. Though in reality 32A would be OK since your PV wires will likely be 10AWG and that's safe with 32A. Even if you have 12AWG PV wires, 32A would be safe. But it's definitely overkill for a 10A load.

only reason i selected that 320a class T was because it was priced well and its hard for me to find 150a class t fuses on amazon that have good reviews

Never choose a fuse for those reasons. You must pick a proper sized fuse for the reasons I've described. Picking one that's too big just because it's $5 cheaper will cost you many thousands of dollars when a fire breaks out.

When it comes to electrical components do not try to save a few dollars on cheap junk. Buy high quality well known named brand items. Spend an extra $100 on your system and be able to sleep well knowing a fire isn't going to break out.

 

[rmaddy]

BTW - I urge you to create your own wiring diagram showing every wire, every fuse/breaker, every component. Clearly label everything with their sizes. Get one last look over by a few people to verify your choices before you purchase anything. You don't want a catastrophe because you chose one wrong sized fuse.

 

[moonlitsouls]

BTW - I urge you to create your own wiring diagram showing every wire, every fuse/breaker, every component. Clearly label everything with their sizes. Get one last look over by a few people to verify your choices before you purchase anything. You don't want a catastrophe because you chose one wrong sized fuse.

Rmaddy how are ya? thanks for the help yesterday

I am watching videos today on how to properly fuse my electrical system to educate myself a bit more, i found a guy on youtube who has a fuse calculator on his website

Fuse Sizing Calculator

www.explorist.life

right now im trying to calculate the proper class t fuse size for my terminal fuse or "catastrophic fuse" which will live between my positive busbar and my lithium battery ,

i will use 2awg wire , the first question asked "device amps" , now in the case of the terminal fuse , should i use the amps for the BMS ? which is 120a rated capacity ?

i will defff create the diagram to get some feedback before i buy anything.

thanks

 

[moonlitsouls]

32A is more than 3x the 9.5A load. As I stated, 1.25 (25%) is a much better guideline. But you're not going to find a 12.5A fuse. 15A probably. Though in reality 32A would be OK since your PV wires will likely be 10AWG and that's safe with 32A. Even if you have 12AWG PV wires, 32A would be safe. But it's definitely overkill for a 10A load.

switched these to 16amp

Amazon.com

question regarding the double pole breakers . if i wire in series and keep the amps under 10 from the panels to the scc , wouldnt i need a higher amperage breaker from the scc-bus bars if the scc reduces the volts down to 12? or is this second breaker from scc-busbars no longer needed if im going to use bus bars and then a final class t fuse to the battery

 

[corn18]

Once it sunk into my head that the fuse is for the wires, life was simple. Pick the wire size for the load, pick the circuit breaker/fuse for the wire size. In some cases, that means I have a larger fuze than the planned load. For instance, I have a 50A converter charger that I used 2 ga wire on. I put a 100A circuit breaker on that.

 

[rmaddy]

In some cases, that means I have a larger fuze than the planned load.

The fuse is always larger than the load. If it’s not then the fuse will blow needlessly from a normal load.

I have a 50A converter charger that I used 2 ga wire on. I put a 100A circuit breaker on that.

Why twice? For a 50A load the breaker only needs to be 65A.

 

[corn18]

The fuse is always larger than the load. If it’s not then the fuse will blow needlessly from a normal load.

Why twice? For a 50A load the breaker only needs to be 65A.

The breaker is for the wire, not the load. A 2ga wire 3 ft long can handle 100A easy. Don't care if the load is 25A or 75A, the CB is for the wire. I should have just used 4ga wire, but I had more 2ga wire laying around so I used that.

 

[moonlitsouls]

The breaker is for the wire, not the load. A 2ga wire 3 ft long can handle 100A easy. Don't care if the load is 25A or 75A, the CB is for the wire. I should have just used 4ga wire, but I had more 2ga wire laying around so I used that.

take a look at this , this is the recommendation from renogy for the fuse for the dc-dc charger

the load is 60a , they recommend a fuse of 9a amp from the vehicle starter to the dc -dc charger


however in this chart below for the cable itself, it says the wire is rated to 150a , it would seem renogy is taking both into account with their recommendation of 90a fuse

 

[John Frum]

Generally the biggest wires are along the path from the battery to the inverter.
I tend to oversize these wires to keep this path cool and size the fuse for the wire.
For the rest of the core I want a common wire size to accommodate the rest of the circuits.
Fuse blocks are generally rated for 100 amps so it gets 6 awg and a 100 amp fuse.
Unless there is a charge source rated over 80 amps then everything gets 6 awg and a 100 amp fuse.
The common wire size means I can buy less different types of wire and don't have to keep as many spare fuses.

A typical system might have 2/0 awg with 300 amp fuses from the battery to the inverter and 6 awg with 100 amp fuses for the rest of the core connections.

 

[corn18]

Never hurts to undersize the fuse/CB for the wire. Never good to undersize the fuse/CB for the load. Never hurts to oversize the fuse/CB for the load. Never good to oversize the fuse/CB for the wire.

 

[rmaddy]

The breaker is for the wire, not the load. A 2ga wire 3 ft long can handle 100A easy. Don't care if the load is 25A or 75A, the CB is for the wire. I should have just used 4ga wire, but I had more 2ga wire laying around so I used that.

Yes, the fuse/breaker is always for the wire. Since the wire is sized for the load (normally) this means the fuse/breaker is bigger (normally) than the load.

But just because you might happen to use a much bigger wire than needed doesn’t mean you should also use a much bigger fuse than needed. You can but there’s no need. I can use a giant wire and a small fuse. But the fuse still needs to be bigger than the load to avoid nuisance trips.

 

[John Frum]

You can but there’s no need.

If you fuse for the wire and the wire is sufficient for an upgrade or replacement component then you don't need to swap out the fuse.

 

[rmaddy]

Never hurts to oversize the fuse/CB for the load.

Assuming you don’t oversize too much for the wire.

Never hurts to undersize the fuse/CB for the wire.

Assuming you don’t undersized too low for the load.

 

[rmaddy]

If you fuse for the wire and the wire is sufficient for an upgrade or replacement component then you don't need to swap out the fuse.

Planning ahead for upgrades is a useful approach.

 

[moonlitsouls]

ok thanks a lot guys

so according to this , if i use 2 awg wire from positive bus bar to class t fuse , max amps for 2 guage wire is 205a ,

so ill use a 200amp class t fuse

 

[corn18]

Planning ahead for upgrades is a useful approach.

And using what you have laying around is good, too. I may go to an 80A charger converter in the future. Wire and CB are all ready for that upgrade.

I will say I did size my main breaker differently. I have 3000W of inverter on my 2x206Ah SOK LFP's. The SOK LFPs are rated for 200A discharge, but I have drawn 115A from a single battery. So I sized the breaker @ 250A. The 4/0 wire can handle 300A+. The inverters can draw 300A+. But my batteries can only deliver 230A. So a 250A breaker. If I add another SOK 206Ah, I will upgrade the breaker to 350A and my wires will be fine. I doubt I ever will upgrade.

 

[moonlitsouls]

And using what you have laying around is good, too. I may go to an 80A charger converter in the future. Wire and CB are all ready for that upgrade.

I will say I did size my main breaker differently. I have 3000W of inverter on my 2x206Ah SOK LFP's. The SOK LFPs are rated for 200A discharge, but I have drawn 115A from a single battery. So I sized the breaker @ 250A. The 4/0 wire can handle 300A+. The inverters can draw 300A+. But my batteries can only deliver 230A. So a 250A breaker. If I add another SOK 206Ah, I will upgrade the breaker to 350A and my wires will be fine. I doubt I ever will upgrade.

what are your thoughts on my post above