Need formulas for fuses/ dc breakers

[hclarkx]

Originally Posted by TacomaJoe

Two thoughts, what is the max voltage your controller can accept and have you factored the low temperature rating into the equation?

For the experts, where do you install a fuse between the controller and the battery. If the fuse is at the controller and there is a short between the fuse and the battery, there is no protection. Fuse both ends?

Good point on the voltage. The solar controller I have is advertised as having 150 volt rating but in the fine print, that's at 25C. At higher temperatures the panel voltage must not exceed 138 volts. Always read the fine print.

Since the controller can only deliver current up to it's output rating when healthy and not much above if output protection fails (i.e., it can't output more power than it gets at the input), protection is not necessary close to the controller output terminals. The battery on the other hand, can deliver thousands of amperes and that makes protection close to the battery important. The BMS provides built-in protection for short-circuits outside of the battery. Most BMS have both software and "second line of defense" "hardware" protection but the BMS is still a single finite unit and surely has failure modes that could cause it to fail to operate for a short-circuit outside of the battery. Given that possibility, and the potentially catastrophic consequences of a short-circuit at such high currents (possibly 20,000 amps or more), it's wise to provide a second "line of defense" independent of the BMS. This is usually done with a Class T fuse at the battery terminal. This type of fuse can be had with enough short-circuit clearing capability to handle the high current a LiFePO4 battery can produce.

The Class T fuse is "current limiting." The short-circuit current from a LiFePO4 battery takes about a millisecond to build to its maximum level (one thousandth of a second). The Class T fuse can open well within that millisecond and thereby interrupt the short-circuit current while it is well below it's maximum level. Further, if one uses circuit breakers on a bus for switching purposes or as "feeders" to devices in the RV and the solar controller, the fuse will open far more quickly than the circuit breakers and thus prevent the circuit breakers from opening with more current flowing than they can handle (3600 amps for the common CB-285 circuit breakers).

It is possible for the battery or batteries to deliver lower currents to a device such as the solar controller in the case of a short-circuit inside of the solar controller. I.e., currents limited by wire between the battery and controller and moreover resistance within the solar controller circuit board. Such currents could be under 100 amps and thus too low for the Class T fuse at the battery to detect. For this reason it's desirable to have a circuit breaker rated not much above the solar controller maximum output current placed near the battery, where the larger wire from the battery steps down to smaller wire running to the solar controller (wire sized for the solar controller output).

In my case the Class T fuses are at the battery terminals. A 150 amp CB-285 circuit breaker is on a busbar and connects the battery cables to the busbar (serves mostly as a switch). And a 50 amp circuit breaker for each of my 40 amps solar controllers is on that same busbar feeding #8 wire to the solar controller.

Tomorrow I'll try to work up a sketch showing the above.

[Persistent]

Originally Posted by TacomaJoe

Two thoughts, what is the max voltage your controller can accept and have you factored the low temperature rating into the equation?

For the experts, where do you install a fuse between the controller and the battery. If the fuse is at the controller and there is a short between the fuse and the battery, there is no protection. Fuse both ends?

A fuse near the battery to protect the wire is required. A fuse near the solar controller is require if wire exceeds a certain length. I don't know that length.

A Li BMS in a drop-in battery may or may not protect anything. Some only provide catastrophic protection to the battery itself. Some have both catastrophic and additional protection

[hclarkx]

Originally Posted by Persistent

A fuse near the battery to protect the wire is required. A fuse near the solar controller is require if wire exceeds a certain length. I don't know that length.

I'm at a loss as to how wire length is a factor. Where did you see this?

There is a lot of bizarre information out there regarding fusing. E.g. the Renogy website admonishes using a fuse between panels and the controller and panel rated 25% above panel load current. This is nuts when the load current can never get that high and short-circuit current is only about 10% above maximum load current. Such fusing is useless and adds a failure point. https://www.renogy.com/blog/how-to-f...-solar-system/

Curiously Renogy does not say where the fuse should be placed in the circuit (if it in fact did serve any purpose).

Fusing to protect parallel panels from each other is a different problem and has some purpose.

[AlanTerry]

I have already purchased the Victron MPPT 150/100 TR Ve-can controller.
Shows max Open circuit PV voyage is 150.max cells in series 360 12V panels.I have 4-6V lifeline agm’s house batteries.

As far as the max Voc of 146 isn’t that only in perfect conditions under the equator?

Any other thoughts please advise.
AlanTerry

[hclarkx]

Originally Posted by AlanTerry

I have already purchased the Victron MPPT 150/100 TR Ve-can controller.
Shows max Open circuit PV voyage is 150.max cells in series 360 12V panels.I have 4-6V lifeline agm’s house batteries.

As far as the max Voc of 146 isn’t that only in perfect conditions under the equator?

Any other thoughts please advise.
AlanTerry

Voltage generated under load is very much limited by how much sun the panels are receiving. But, open circuit voltage is reached with only modest sun. I've measured my open circuit voltage on quite cloudy days and still gotten full rated Voc from the panels.

[AlanTerry]

Wanted to say I installed my Solar system and it is working great so far. As far as cost approximately $2000.00 got items on sale and did it ALL by myself as well as reading all threads on iRV2.

[astrocamper]

I use a disconnect switch between each of my panel groups and the MPPT controller. I use a breaker/disconnect between the MPPT controllers and the battery bus bars.

The solar disconnect switch allows me to shut of the solar power so I can work on my DC power system while not energized. I always shut my solar off first and then disconnect the MPPT controller from the bus bars.

[Pappion]

Solar panels have diodes to bypass shaded cell (groups). They do not prevent reverse flow.
https://www.pveducation.org/pvcdrom/.../bypass-diodes


More than 2 panel (strings) in parallel should have over current protection for each string. Typically a fuse in MC4 compatible holder.

https://ca.renogy.com/renogy-mc4-wat...holder-w-fuse/


I do use a DC circuit breaker for the panels near the solar controller. It must be DC rated so it survives arcing.
https://www.amazon.com/MidNite-Solar.../dp/B004EQK8SA


A handy wire and fuse size charts.
https://www.bluesea.com/products/200...election_Chart

[ArtJoyce]

I put a fuse on each side of my controller equal to 125% of the max current of my array and controller output. The fuse from the battery to the controller protects the batteries from a controller short. The fuse from the panels protests from a short at the lugs on the controller.

[twinboat]

Fuses protect wires from burning.

The fuse/breaker from the battery to controller should be as near to the battery as possable. The battery has the energy potential to cause a burning wire. You need to protect the wiring from the battery.

Properly wired, the solar shouldn't ever overload the wiring. It is the power producer, limited to it's max output and there is no stored energy.

[Pappion]

Originally Posted by twinboat

Fuses protect wires from burning.

The fuse/breaker from the battery to controller should be as near to the battery as possable. The battery has the energy potential to cause a burning wire. You need to protect the wiring from the battery.

Properly wired, the solar shouldn't ever overload the wiring. It is the power producer, limited to it's max output and there is no stored energy.

That is too brief. Many fuses are used at the destination, value is tailored to it. The device manufacturer has no control over the Source fuse and wire sizing. They still have liability for fire.

The solar controller has an internal fuse for the battery connector.
https://diysolarforum.com/threads/do...ts-kind.23289/
I would rather replace an external fuse.

[paul65k]

Originally Posted by DavidEM

BTW five panels wired in series is rather dangerous IMO. 146 volts can kill you.

David

Here is a chart showing the volts for 5 panels in series compensated for a range of temperature conditions.

I'm planning an installation with 3 rows of 5 of these panels (actually the new 200-260 Hightech panels) and it works for me but I have a 24V system, at 12V the voltage is the same but the amps are doubled.

My plan is to use a 150/100 and a 150/70 MPPT controller(s).