Fuses and disconnect switches for 12v packs?
- Thread starter LVLAaron
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Two resources that I have found helpful are @FilterGuy 's "DC Fuse and breaker Sizing and Placement" and the "Wire and Fuse Selection Quick Reference (Blue Sea Systems)". Others may be better at directly linking to those.
Have spent way too much time looking at those and wondering if I've got it right.
Have spent way too much time looking at those and wondering if I've got it right.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
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 by...
diysolarforum.com
Wire and Fuse Selection Quick Reference (Blue Sea Systems)
This PDF is a good quick reference guide for wire size and fuse selection. The wire size chart is based on 12v DC, but is safe to use for higher voltages (in which case it will be more conservative). It should be noted that circuit length is the...
diysolarforum.com
I like your resources and have move I've come across downloaded.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 by...Wire and Fuse Selection Quick Reference (Blue Sea Systems)
This PDF is a good quick reference guide for wire size and fuse selection. The wire size chart is based on 12v DC, but is safe to use for higher voltages (in which case it will be more conservative). It should be noted that circuit length is the...
Something I'm curious on is what size ocpd would be recommended for my hybrid inverter that has this in the description.
"Maximum Charging Current: 40A when Utility/Grid Charging, 0-80A when PV Charging."
From that I take that when plugged into AC and charging from the mains it can charge at a max of 40a, and I can set the PV charge 0-80a.
By using the 1.25 rule in your document on the most current the charger can provide, 80a, it suggests a 100a fuse which is what I have planned and will match my 4 wire. Since the bank will be charged and discharged by the same wires and at 3000w/29a ac gives me 80a dc @ 48v the 4 wire with a 100a fuse also matched my needs there I think. Bms is also a 100a.
Should I set the PV charge profile for my setup to match the 40a that it can provide when using the mains or it wont matter? Bank will be 16s 310ah.
On page 2 of the Overcurrent Protection Devices (OCPD) on Solar Arrays document it says fuses become necessary when "3 or more Parallel Strings of 1 or more series panels". My intention is to use 6 250w panels in a 2s3p configuration giving me around 74v and 28a. If I understand it correctly I don't fall within the above parameter as I only have 2 in series not 3. Or am I way off?
Appreciate your thoughts.
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FilterGuy
Solar Engineering Consultant - EG4 and Consumers
I am trying to pick through the various comments to understand the set-up.
It sounds like it is a 48V, 3000W system with a capability of charging at 80A from PV or 40 from AC Mains. Is that correct?
Assuming a 90% efficientcy of the inverter, it could draw up to 3000/.9 = 3333W. If the battery is low that works out to be 3333/48V = 69A.
The driving factor for sizing the fuse and wire is the largest current source which is the 80A PV charge. Consequently, the fuse size should be 1.25 x 80 = 100A. You had it correct. ?
No. What counts is the number of Parallel Strings. If I am following you correctly, you will have this:
3 strings in Parallel need to have OCPD on each string as shown above.
If, on the other hand, you had 2 strings of 3 panels in series, OCPDs would not be required.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
The thread title mentions disconnect switches but it was not discussed in your questions. I do like to put in a disconnect switch between the inverter and the battery for ease of working on the system, but I make it one with a pre-charge circuit.
diysolarforum.com
As you can see from the comment from @time2roll, others have different points of view. There is no right or wrong answer on this. As long as you do something to pre-charge the inverter before it is connected to the battery each time, you will be fine.
I also like to put a disconnect switch between the panels and the charge controller. (If you are trying to build to code a PV disconnect is required)
Inverter Disconnect Switch with Precharge
To get the paper, click on the orange button at the top of this page. This circuit is designed as a disconnect switch that allows the user to pre-charge the inverter capacitors before turning the switch completely on. 26 June 2020 update: At...
diysolarforum.com
I also like to put a disconnect switch between the panels and the charge controller. (If you are trying to build to code a PV disconnect is required)
Awesome, thanks!
I'm ambivalent and have no real preference other than what you think would be better. I've been told that 2s3p or 3s2p should preform somewhat similar through a mppt. Your thoughts or recommendations on 2s3p or 3s2p?
The 2s3p would give me 74v and 25a and the 3s2p would be 111v and 16v roughly. The 2s3p would need the fuse because the amperage is increased and the fuse is for the amps not the voltage?
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
more Parallel strings are better if you have shading issues (and blocking diodes).
More Serial strings are better because your current is lower so your wires can be smaller.
In general, I try to maximise the number of serial panels unless there is a potential problem with shading because it minimizes the current and makes it easier and cheaper to run the wires for low loss. Most panels have one or more bypass diodes that minimize the impact of shading so if I have a long run to the charge controller I will still favor serial over parallel.
It sounds like you have a 48V AIO. They usually have a fairly high max PV input voltage. Can you do all 4 in series? You appear to have panels with 37V Voc so 4 in a series would be 148V before temperature compensation. If you live in an area that gets cold, you would need something like 200V max PV input voltage on your charge controller...... Your controller probably has a 150V limit so now that I have done the math I am guessing 3 in series is the most you can do.
The solar panel fusing (or not) has to do with what happens if a panel shorts out and the amount of current a panel can handle. Panels are designed to be able to handle something greater than Isc. The current they are rated for is called the Series Fusing rating in the panels specs. This is usually just under 2 x Isc and is the current they can handle without creating a fire hazard.
The diagrams below show two parallel panels with shorts in a couple places
Notice that the only place that sees the 2xImax is the short itself. Everyplace else in the circuit only sees Imax. Therefor, there is no place you could put an OCP device on the parallel circuits that would see the double current. Furthermore, the main circuit wires must be sized to handle the 2x 1.25 X Imax ( 2 x 1.25 x 1.25 x Isc) and a short would not be harmful on them.
However, if you have 3 in parallel, then you could see 2 x Imax in parts of the panel and that would exceed the Series Fuse rating of the panel.
I do have some shade but not a ton. So more parallel might be the way to go.
Yea the max is 145 but Im planning for six panels not 4. I suppose I could just get 8 and do 4s2p. Giving me voltage around 148 at 16 amps. Wasn't really sure and how hard of a number the 145 was but I was advised another threads to stay under it always.
Yea so looking like three is the max I can do in series to stay within my voltage and amperage parameter.
So to confirm I'm a safe with 3s 2p. Giving me voltage around 111 and 16 amps. And not needing a fuse in that array?
This
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
yes That looks reasonable. You have (145V-111V)/111V = 30.6% Voc headroom for low temperature conditions so you should be good for even extreme cold temps
It does not take much shade to really mess things up. Your panels probably already have bypass diodes. You may want to consider a blocking diode on each string so if one string is shaded it won't impact the others.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
Correct.
Fusing & Wire Sizing guidelines for solar panels
To get the paper, click on the orange'' button at the top of the screen. This resource is intended to help the user understand when they need to add Over Current Protection Devices (OCPDs - Fuses or Breakers) to the solar panel array and what...
diysolarforum.com
Awesome thanks. Depending on when I buy my panels I may end up getting 8 if they're the right price. If that happens I'll just be sure to stay within my v and a input parameters. And if I bust over on the v side and have to go in parallel more than one string I will refer to the above and put in the correct fuse.
Appreciate your help.
Correct.
Fusing & Wire Sizing guidelines for solar panels
To get the paper, click on the orange'' button at the top of the screen. This resource is intended to help the user understand when they need to add Over Current Protection Devices (OCPDs - Fuses or Breakers) to the solar panel array and what...
Referencing this post again as I'm about to buy my panels tomorrow. They're 8 yr old 250w photon solar brand panels for $40 each with mounting rails and brackets. Short circuit current is 8.7 and max fuse rating is 15. If I stick to my 3s2p like I wanted to above will I need 2 fuses since I'll have 90 or so v @ 16.6a?
Thanks
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
With 2 parallel strings of 3-Serial panels. No fuses are needed. However, you should put a disconnect switch in before the charge controller. Some people use breakers as disconnect but a regular 2 pole switch would work as well.
The current you should use for wire sizing is 8.7 x 2 x 1.56 = 27.144. I would use a minimum of 10AWG. However, if your run is long, you should go to 8AWG.
If you decide to use breakers as the disconnect, make it a 30A breaker.
Very good thank you. I don't understand it all but I appreciate it.
I plan on 10' my run isn't long at all. 15' maybe 20.
So even though my hybrid inverter can take in up to 50a still use a 30a as it's unlikely 6 250w panels in a 3s2p config can put out 50a? Trying to understand why 30a.
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
The breakers are just being used as switches. The panels will never produce 30A even with a dead short in the most optimal solar conditions. Consequently, anything over 28A will work. In fact, A dual pole switch is acceptable. People use breakers as solar disconnects because they are an easy and cost-effective way to do the disconnect.
However, since they are breakers, I like to make them small so if something weird happened they would pop sooner than later. (The only weird thing I could imagine where they would pop is some kind of short through the SCC that dumps the full battery current through the SCC output... That seems highly unlikely, but why NOT use the smaller breaker)
Makes sense now thank you. Nearing the end of my questions unless when I put this together Saturday something goes arie.
If I opt for a breaker and not a two pole it should go on the + line?
FilterGuy
Solar Engineering Consultant - EG4 and Consumers
The NEC requires both lines for a disconnect, so I always recommend a dual breaker. Something like this:
Note: Sinc it is polarized, I would put the + on the positive line towards the SCC.
Note: Sinc it is polarized, I would put the + on the positive line towards the SCC.
