Safety First - locating correct fuses & breakers?

[BobbyP]

Collectively my system, when constructed, will have 4 - 275w panels, a 100a MPPT SCC, 24v 3000k inverter. Having considered series/parallel connections for both batteries and panels I'm having a hard time understanding what size fuses and breakers would be most closely advantageous to the parameters my input and output power can supply.
My b-bank is 8 - 6v/225amh lead acid, and my panels are rated: Vmp - 32.1/ Voc-39v, Imp ~ 8.5 amps/ Isc ~ 9.20 amps.
My charge controller is: max 2400v PV input Voc ~ 96 / Vmp ~ 72.
I've got my 24v b-bank wired up and fully charged. I'm 3/4 finished panel frame/construction and now designing a mounting board for components,
connections and (my present delima) fuses/breakers.
At present, because this is like Greek I am hesitant but in time will all become clear as day.
Any help would be most appreciated and a thank you in advance is sincerely put forth.

 

[WYtreasure]

Collectively my system, when constructed, will have 4 - 275w panels, a 100a MPPT SCC, 24v 3000k inverter. Having considered series/parallel connections for both batteries and panels I'm having a hard time understanding what size fuses and breakers would be most closely advantageous to the parameters my input and output power can supply.
My b-bank is 8 - 6v/225amh lead acid, and my panels are rated: Vmp - 32.1/ Voc-39v, Imp ~ 8.5 amps/ Isc ~ 9.20 amps.
My charge controller is: max 2400v PV input Voc ~ 96 / Vmp ~ 72.
I've got my 24v b-bank wired up and fully charged. I'm 3/4 finished panel frame/construction and now designing a mounting board for components,
connections and (my present delima) fuses/breakers.
At present, because this is like Greek I am hesitant but in time will all become clear as day.
Any help would be most appreciated and a thank you in advance is sincerely put forth.

Hey BobbyP, just guessing here, you are not in Greece are you? Adding your location will help.

What controller do you have? What inverter do you have?
What is your inverter output, in volts?
Can we get confirmation on your charge controller input and "b-bank" numbers please?

How, specifically, are your batteries and panels connected?

If possible you may want to consider a non-combustible mounting board, like the cement board one would find behind tile in a shower or ceiling tiles which have a Class A fire rating.

 

[rmaddy]

4 275W panels is 1100W. 1100W at 24V means about 45A of possible charge current. Why do you plan on a 100A charge controller? That's more than twice what you need with those four panels. Do you plan on doubling the amount of solar in the future?

Put your 4 solar panels in a 2S2P arrangement. Add a PV disconnect breaker near the SCC. Make sure it is a two-pole DC breaker capable of handling at least 100V and at least 30A. This breaker is only there as a disconnect. It will not be there to protect any wiring.

When choosing the fuse between the SCC and the battery bank you base it on the SCC's output current. Multiply that by 125% to get the correct fuse size. If you stay with the 100A SCC then you would want 4AWG or 25mm² with a 125A fuse. If you go with a 45A controller then you would use 6AWG or 16mm² with a 60A fuse.

You have a 3000W inverter on a 24V system. That can result in about 3000W / 24V / 85% efficiency which is roughly 160A. I would use 1AWG (or maybe 1/0AWG) or 50mm² for all of your battery wiring and the wires to the inverter. Use a 200A fuse near the battery.

Do you have any pure DC loads that will be run off of the battery?

 

[BobbyP]

Hey BobbyP, just guessing here, you are not in Greece are you? Adding your location will help.

What controller do you have? What inverter do you have?
What is your inverter output, in volts?
Can we get confirmation on your charge controller input and "b-bank" numbers please?

How, specifically, are your batteries and panels connected?

If possible you may want to consider a non-combustible mounting board, like the cement board one would find behind tile in a shower or ceiling tiles which have a Class A fire rating.

No, not in Greece, but it feels like it when I try to understand "sparks" and the flow thereof! I'm in the PNW.
My b-bank is 8 - 6v/225amh, lead acid, wired in series/parallel to = 24v / 450amh, connected w/ #2 cables.
My Inverter is a low budgit Chinese type, Cumsetzer 3000w, 24v, Pure sine wave. The onboard sticker states: AC output 120v
My MPPT Solar Charge Controller is a PowMr, @ 24v solar panel - 96v Voc, 72v Vmp.
I'm planning to series/parallel wire my panels so as to not exceed the mfg specs, giving me 78v Voc or 64v Vmp. Can't check panel array for output yet as the concrete posts/frame assembly, are drying as we speak.
The information I've listed is directly off the components as the manuals offer little a layman can understand.(ie the Greek)
I have some of that cement board, thanks.
If there's any more clarification you need let me know.
Thanks bro

 

[HRTKD]

I picked up on the same thing that rmaddy did. Your solar charge controller is bigger than it needs to be. That's not always a bad thing, but if you're looking to control costs, there is one place to do it.

However, how long is the run between your panels and your solar charge controller? If it's a long-ish distance you may want to configure the panels 4s to keep the amps down, lower voltage loss and keep the cost of cable down.

 

[BobbyP]

4 275W panels is 1100W. 1100W at 24V means about 45A of possible charge current. Why do you plan on a 100A charge controller? That's more than twice what you need with those four panels. Do you plan on doubling the amount of solar in the future?

Put your 4 solar panels in a 2S2P arrangement. Add a PV disconnect breaker near the SCC. Make sure it is a two-pole DC breaker capable of handling at least 100V and at least 30A. This breaker is only there as a disconnect. It will not be there to protect any wiring.

When choosing the fuse between the SCC and the battery bank you base it on the SCC's output current. Multiply that by 125% to get the correct fuse size. If you stay with the 100A SCC then you would want 4AWG or 25mm² with a 125A fuse. If you go with a 45A controller then you would use 6AWG or 16mm² with a 60A fuse.

You have a 3000W inverter on a 24V system. That can result in about 3000W / 24V / 85% efficiency which is roughly 160A. I would use 1AWG (or maybe 1/0AWG) or 50mm² for all of your battery wiring and the wires to the inverter. Use a 200A fuse near the battery.

Do you have any pure DC loads that will be run off of the battery?

 

[BobbyP]

Thanks rmaddy... I purchased the 100amp controller some time back and may expand down the road but it's what I've got for now.
I do have a dual pole breaker but it's rated to 500v / 63amp. Too much????
I've done some calculations based on your numbers and I'm seeing some light down the tunnel.
As my b-bank is already wired with #2 awg I've added a pic of the SCC manual which is vague, at best. Can you suggest a fuse for my K 100 model. (40A,???)
No DC hooks ups.

 

[rmaddy]

I do have a dual pole breaker but it's rated to 500v / 63amp. Too much?

That's fine. Supporting lots of voltage is a good thing. Supporting too many amps is normally a bad thing but in this case the breaker is only acting as a disconnect. It's not meant to protect anything. So being rated at a higher amp rating is actually what you need in this case.

I've added a pic of the SCC manual which is vague, at best. Can you suggest a fuse for my K 100 model. (40A,???)

You didn't add any picture. But if it really is a 100A SCC then you need a 125A fuse between the SCC and battery along with 4AWG (2AWG would be even better) wire.

 

[BobbyP]

I picked up on the same thing that rmaddy did. Your solar charge controller is bigger than it needs to be. That's not always a bad thing, but if you're looking to control costs, there is one place to do it.

However, how long is the run between your panels and your solar charge controller? If it's a long-ish distance you may want to configure the panels 4s to keep the amps down, lower voltage loss and keep the cost of cable down.

Thanks HRTKD...Distance from panels to SCC no more than 30'. Got to live with present SCC, it's what I've already purchased.

 

[BobbyP]

That's fine. Supporting lots of voltage is a good thing. Supporting too many amps is normally a bad thing but in this case the breaker is only acting as a disconnect. It's not meant to protect anything. So being rated at a higher amp rating is actually what you need in this case.


You didn't add any picture. But if it really is a 100A SCC then you need a 125A fuse between the SCC and battery along with 4AWG (2AWG would be even better) wire.

Thanks as this helps alot. I'll try the pic again.

 

[rmaddy]

Thanks as this helps alot. I'll try the pic again.View attachment 93417

Looks like it is recommending 4AWG wire to the battery. That matches what I said.

Note that you will not be using the load output of 40A so ignore that line. The 100A output of the K-100 is what you need to deal with. Though with your current panel arrangement you will never go over 45A. It's best to wire the hardware for what it can do. That way if you add more panels later you don't need to replace the wire or fuse.

 

[BobbyP]

I picked up on the same thing that rmaddy did. Your solar charge controller is bigger than it needs to be. That's not always a bad thing, but if you're looking to control costs, there is one place to do it.

However, how long is the run between your panels and your solar charge controller? If it's a long-ish distance you may want to configure the panels 4s to keep the amps down, lower voltage loss and keep the cost of cable down.

Thanks HRTDK... Distance from panels to the SCC is 30'. Might go larger down the road but I've got to live with my SCC as that's what I've got

 

[HRTKD]

As rmaddy said, 2s2p configuration of the PV is the way to go. The 96 volt Max PV Input limit dictates that. 10, 12 or 14 gauge PV wire will work for that distance.

 

[BobbyP]

Looks like it is recommending 4AWG wire to the battery. That matches what I said.

Note that you will not be using the load output of 40A so ignore that line. The 100A output of the K-100 is what you need to deal with. Though with your current panel arrangement you will never go over 45A. It's best to wire the hardware for what it can do. That way if you add more panels later you don't need to replace the wire or fuse.

I've had this #2 awg before I got the SCC.
I read somewhere that larger wires are a safer application.
So as to understand, a slightly larger fuse than the max power flowing is what we want???

 

[BobbyP]

As rmaddy said, 2s2p configuration of the PV is the way to go. The 96 volt Max PV Input limit dictates that. 10, 12 or 14 gauge PV wire will work for that distance.

Thanks Boondocker... I have #10 ready to go.
When installing a fuse does the length of the run have any bearing?

 

[rmaddy]

I've had this #2 awg before I got the SCC.

2AWG may not fit in the SCC. Try it. If not, you'll need 4AWG.

I read somewhere that larger wires are a safer application.

Larger wire is always good unless it doesn't physically fit in the connectors. Larger wire will run cooler and have less voltage drop. It's also more expensive and hard to work with so don't get carried away.

So as to understand, a slightly larger fuse than the max power flowing is what we want?

The basic approach is to look at the max load (current) that will be run through a wire. You choose a wire that can safely handle that current and avoid too much voltage drop. Then you pick a fuse to protect that wire. There is a range of currents that the fuse must fall between. At one end is the actual max load. At the other end is the largest current rating of the chosen wire. You need a fuse between those two values. In most cases that value is roughly 125% of the max load. If you choose a fuse that matches the max load then you will have nuisance trips.

In this example you have a 100A max load coming through the SCC. 4AWG wire is good choice for that current and it provides minimal voltage drop if the length is kept under 10' round trip (so keep your SCC less than 5' from the battery). 4AWG can be fused up to 150A. So you need a fuse between 100A and 150A. A 100A fuse could trip even under normal conditions. 125% of 100A is 125A. That falls between 100A and 150A so it's a good choice.

 

[rmaddy]

The 96 volt Max PV Input limit dictates that. 10, 12 or 14 gauge PV wire will work for that distance.

I would base the wire size on the actual voltage of the panel array. While the SCC can handle 96V, let's say the OP was doing 4P instead of 2S2P. That would only be 40V which would require much bigger wire than higher voltage. So the SCC max input voltage really has no bearing in the wire size.

 

[BobbyP]

I've had this #2 awg before I got the SCC.
I read somewhere that larger wires are a safer application.
So as to understand, a slightly larger fuse than the max power flowing is what we want???

I will use the 4awg from the Scc to the battery and install the fuse there.
Oh happy days are on the way!

 

[BobbyP]

2AWG may not fit in the SCC. Try it. If not, you'll need 4AWG.


Larger wire is always good unless it doesn't physically fit in the connectors. Larger wire will run cooler and have less voltage drop. It's also more expensive and hard to work with so don't get carried away.


The basic approach is to look at the max load (current) that will be run through a wire. You choose a wire that can safely handle that current and avoid too much voltage drop. Then you pick a fuse to protect that wire. There is a range of currents that the fuse must fall between. At one end is the actual max load. At the other end is the largest current rating of the chosen wire. You need a fuse between those two values. In most cases that value is roughly 125% of the max load. If you choose a fuse that matches the max load then you will have nuisance trips.

In this example you have a 100A max load coming through the SCC. 4AWG wire is good choice for that current and it provides minimal voltage drop if the length is kept under 10' round trip (so keep your SCC less than 5' from the battery). 4AWG can be fused up to 150A. So you need a fuse between 100A and 150A. A 100A fuse could trip even under normal conditions. 125% of 100A is 125A. That falls between 100A and 150A so it's a good choice.

I have 150v fuses but I can get 125v. Now all we need is sunshine.
Thank you very much

 

[rmaddy]

I have 150v fuses but I can get 125v.

As long as you mean 150A and not 150V then that will work with 4AWG.