Outback FM 80 Solar Input Breaker Size

RWB · July 2010

hey guys I am wanting your opinion on the size breaker I should be using with the Flex Max 80's Solar input side.

I'm used to using Bussman thermal Breakers which need to be sized 10-15% larger than your actual load to keep them from tripping when temperature rises. But those are only good for 42-48V max depending on which one you are using.

I want to be able to use the higher voltage array inputs the flex max offers but need to know if the Outback breakers need to be oversized the same way as the thermal breakers if we are going to max out the solar input ratings of 80 Amps.

I have read that the Outback breakers are rated for a 80A constant load, but what exactly does that mean??

Any help is appreciated on this subject.

Cariboocoot · July 2010

Re: Outback FM 80 Solar Input Breaker Size

There's a little bit of confusion here. Either thee or me or both.

The FM80's "80 Amp" is its maximum output current, at any given Voltage. The input current is dependent on your array's Imp. Since Vmp is always higher than system Voltage, the input current should be less than the output current.

As in: 12 Volt system; output 14.2 V @ 80 Amp = 1136 Watts, input 1136 Watts / 17.7 Vmp = 64 Amps. That on a "straightforward" system, no MPPT function et cetera.

Some further detail regarding your array's specifications would help determine what would be the proper fuse/breaker(s) to use.

The Outback 80 Amp breaker's "continuous load" means that; it can run at 80A without tripping (in theory, at least).

I'm not sure I'm explaining this well. I'm not sure I understood the question either.

AntronX · July 2010

Re: Outback FM 80 Solar Input Breaker Size

RWB wrote: »

hey guys I am wanting your opinion on the size breaker I should be using with the Flex Max 80's Solar input side.

I don't think you need to have the breaker on PV input side. On battery side, you do need fuse or breaker in case if controller shorts and whatever current battery can supply, will be dumped into this shorted controller which results in a fire. But shorted PV array will stay at maximum short circuit current, while dropping voltage to almost zero. Not much power will be dissipated in wires, so no chance of fire due to wire overheating. Now, when you connect more than 2 solar arrays in parallel, you do need to have a fuse in line with each array before common tie point. That is what combiner boxes are for.

Windsun · July 2010

Re: Outback FM 80 Solar Input Breaker Size

A general rule for input breaker is about 125% of rated amps (Imp) or 110% of short circuit amps (Isc). A breaker on the input side does not really offer a lot of protection - that should be handled by the output side breaker, but it often makes troubleshooting and maintenance a lot easier.

mike95490 · July 2010

Re: Outback FM 80 Solar Input Breaker Size

Arent' there breakers or fuses in the combiner box ?

RWB · July 2010

Re: Outback FM 80 Solar Input Breaker Size

Cariboocoot wrote: »

There's a little bit of confusion here. Either thee or me or both.

The FM80's "80 Amp" is its maximum output current, at any given Voltage. The input current is dependent on your array's Imp. Since Vmp is always higher than system Voltage, the input current should be less than the output current.

As in: 12 Volt system; output 14.2 V @ 80 Amp = 1136 Watts, input 1136 Watts / 17.7 Vmp = 64 Amps. That on a "straightforward" system, no MPPT function et cetera.

Some further detail regarding your array's specifications would help determine what would be the proper fuse/breaker(s) to use.

The Outback 80 Amp breaker's "continuous load" means that; it can run at 80A without tripping (in theory, at least).

I'm not sure I'm explaining this well. I'm not sure I understood the question either.

Yes I understand what your saying 100%.

But You never know what the Flex Max is going to operate the panels at so I will assume that it could pull 80Amps from the panels at 14.6V. The solar input can and will change depending on what the setup calls for. This is a portable system not a home install.

So if the Flex max could be hooked up to a solar panel array that could be operated with little MPPT Conversion, meaning that it could be running close to 80 Amps continuously for a while I need to fuse the Solar Input Breaker so it is able to run at or close to 80A continuously without nusance tripping.

But I agree with you that most of the time the panels voltage is going to be a good amount above the batteries charge voltage.

RWB · July 2010

Re: Outback FM 80 Solar Input Breaker Size

AntronX wrote: »

I don't think you need to have the breaker on PV input side. On battery side, you do need fuse or breaker in case if controller shorts and whatever current battery can supply, will be dumped into this shorted controller which results in a fire. But shorted PV array will stay at maximum short circuit current, while dropping voltage to almost zero. Not much power will be dissipated in wires, so no chance of fire due to wire overheating. Now, when you connect more than 2 solar arrays in parallel, you do need to have a fuse in line with each array before common tie point. That is what combiner boxes are for.

Good info AntronX, Thank you.

RWB · July 2010

Re: Outback FM 80 Solar Input Breaker Size

Windsun wrote: »

A general rule for input breaker is about 125% of rated amps (Imp) or 110% of short circuit amps (Isc). A breaker on the input side does not really offer a lot of protection - that should be handled by the output side breaker, but it often makes troubleshooting and maintenance a lot easier.

Exactly, Its mainly a extra feature I was thinking I wanted to add to this project but I'm not sure I want to go that route now.

And thanks for the reply.

Cariboocoot · July 2010

Re: Outback FM 80 Solar Input Breaker Size

RWB wrote: »

Yes I understand what your saying 100%.

But You never know what the Flex Max is going to operate the panels at so I will assume that it could pull 80Amps from the panels at 14.6V. The solar input can and will change depending on what the setup calls for. This is a portable system not a home install.

So if the Flex max could be hooked up to a solar panel array that could be operated with little MPPT Conversion, meaning that it could be running close to 80 Amps continuously for a while I need to fuse the Solar Input Breaker so it is able to run at or close to 80A continuously without nusance tripping.

But I agree with you that most of the time the panels voltage is going to be a good amount above the batteries charge voltage.

Not to nitpick, but the FM80 doesn't operate the panels; the panels operate the charge controller. The input current can never exceed the panels' maximum potential. So the question of circuit protection between the panels and the controller is dependent on how your array is configured. With MPPT, you can increase the array Voltage above the system Voltage and keep the current to the controller low. As per (for comparative example only, not exact numbers):

960 Watts from panels @ "12 Volts" = 80 Amps to the controller
960 Watts from panels @ "24 Volts" = 40 Amps to the controller
960 Watts from panels @ "48 Volts" = 20 Amps to the controller

The controller's output remains the same, as it is based on Watts in divided by Voltage out (basically - not including losses).

As AntronX said, the only time you really need CP on an array is if there are two or more panels/strings of panels in parallel, as there is the potential for one segment to damage another if something goes wrong.

RWB · July 2010

Re: Outback FM 80 Solar Input Breaker Size

Cariboocoot wrote: »

Not to nitpick, but the FM80 doesn't operate the panels; the panels operate the charge controller. The input current can never exceed the panels' maximum potential. So the question of circuit protection between the panels and the controller is dependent on how your array is configured. With MPPT, you can increase the array Voltage above the system Voltage and keep the current to the controller low. As per (for comparative example only, not exact numbers):

960 Watts from panels @ "12 Volts" = 80 Amps to the controller
960 Watts from panels @ "24 Volts" = 40 Amps to the controller
960 Watts from panels @ "48 Volts" = 20 Amps to the controller

The controller's output remains the same, as it is based on Watts in divided by Voltage out (basically - not including losses).

As AntronX said, the only time you really need CP on an array is if there are two or more panels/strings of panels in parallel, as there is the potential for one segment to damage another if something goes wrong.

Yes I understand 100% what you are saying.

All I'm trying to say is that when were bulk charging at 14.6V and this Flex Max 80 is hooked up to an array that is 18V nominal and when its hot I am seeing the MPPT Charge controllers operate 1 specific brand of panels we have at 14.8-14.9V in MPPT mode, which is giving us very little current boost and therefore keeping the input current real close to the output current.

Sometimes this system could be used with panels that operate close to the charging voltage and little MPPT conversion will be going on. Most of the time the solar array will be operating at 60V and the input current will be much lower as you are trying to explain to me which I do understand and its why I'm excited to get my hands on the Midnight Classic when its available one of these days.

Thanks for everyone's input, its helped me make my decision as far as this design goes.

Next question is will the Outback FM 80 Limit the Output current to 80 Amps like the Morningstar MPPT Controllers do when you plug them into a slightly larger array then they are rated for?

Cariboocoot · July 2010

Re: Outback FM 80 Solar Input Breaker Size

Yes the FM 80's output is limited to 80 Amps and you can run slightly larger array to keep the output up. But, the closer to the limit you run any controller the hotter it will run and the efficiency will suffer. Some of us find 75% of output capacity ideal, others disagree.

What kind of array do you have that varies so much in output?
Another efficiency tip is to limit the nominal array Voltage to no more than 2X the nominal system Voltage. Others may disagree with this as well, as personal experiences vary greatly according to individual site installs.

The long-promised Midnight Classic still sounds great (no "sweeping"), but how long can we wait?

gav · July 2018

So that means u need a 80 amp breaker then?

BB. · July 2018

The circuit breakers are there to protect the wiring. If there is a short circuit somewhere, the battery bank will output way more than 80 amps, causing the wiring to overheat and possibly start a fire.

So, you need a circuit breaker to protect the wiring.

Circuit breaker in the US (for "house type wiring" ) are generally sized to trip at rated current (may take hours) and at less than 80% of rated current, the breaker/fuse should not trip.

So, the process is... That is the long term maximum current you expect (is it 80 amps, or is your solar array smaller?). Then figure out the wire needed to carry that rated current (times 1/0.80 or x 1.25 -- The NEC factor for continuous loads (80% of breaker/wiring rating)--including insulation type, conduit fill factor, ambient temperatures, etc.), and then the breaker needed to protect the wiring.

Most household loads run for a few minutes to perhaps 20-40 minutes (such as an electric water heater). However, when you have a large battery bank and are charging it, the maximum continuous current draw can be for hours at a time--And even NEC design specifications for typical house wiring is not really "good enough"--For reliability, I always suggest the 1.25x derating factor.

Both for safety sake, and just avoiding the issues of random breaker/fuse trips (random breaker trip on solar charger, rest of system still runs, and family takes the battery bank dead--And you may need a new battery bank for want of an "undersized" charge controller breaker).

-Bill

Outback FM 80 Solar Input Breaker Size

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