Temp replacement if FM80 with MX60 - Breaker size

I have a defective Outback FM80. Outback will be replacing it (excellent customer service by the way:D).

In the meantime I have an old MX60 I need to use in it's place. The MX60 is rated at 60amps. FM80 @ 80amps. I've shut the breakers at the combiner box to some panels to decrease my input to the MX60 to 700 watts (MX60 is rated for 800 at 12v).

The MX60 is rated at 60 amps and correspondingly uses 60 amp breakers. Since the system was designed for the FM80 it has 80 amp breakers. Am I looking at any potential problems using 80 amp breakers temporarily on the 60 amp MX60?:confused:

Comments

  • BB.BB. Super Moderators Posts: 26,872 admin
    Re: Temp replacement if FM80 with MX60 - Breaker size

    Not that I can see...

    First, you do not have to disable part of your array... Virtually all (that I know of) MPPT charge controllers will safely limit their output current to rated maximum... No danger or damage.

    Second, if I recall correctly, you can program the old MX 60's to a maximum of 72 amps output current.

    Third, the wiring is already "sized" for the 80 amp controller--and breakers are there (for the most part) to protect the wiring.

    I would not worry at all.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BigwoooBigwooo Solar Expert Posts: 60 ✭✭✭✭
    Re: Temp replacement if FM80 with MX60 - Breaker size

    Thank you BB., that was the answer I was hoping for. I thought the breaker was sized for the wire, just wanted to be sure I wasn't missing something.
  • BB.BB. Super Moderators Posts: 26,872 admin
    Re: Temp replacement if FM80 with MX60 - Breaker size

    Technically, there should be a maximum breaker rating for the device too (in the manual)... Obviously, a 1,000-10,000 amp capable battery bank on the output of 60 amp solar charge controller has never been evaluated/tested for failure modes when connected to a huge current source (those types of currents can cause internal wiring to break loose and burn a hole through the case--start a fire, etc.)...

    But the difference between a 72 amp output and the 80 amp output is not worth worrying about.

    One obscure point... The NEC and various US agencies use a 1.25 safety factor for wiring/fuses vs loads... So, for a circuit designed to carry 80 amps continuously will have wiring/fuses rated for:
    • 80 amps * 1.25 = 100 Amp fuse/breaker/wiring rating
    A 100 amp breaker carrying 80 amps should never trip. An 80 amp breaker carrying 80 amps can eventually trip (in minutes/hours/etc.).

    Outback, as I understand, does their rating differently... The 80 amp rated output FM 80 uses an Outback 80 Amp Rated breaker. However, their rating is for 100% continuous rated load (not the same rating method as the NEC usually uses) (this is my limited understanding--contact NAWS or Outback directly if you have any questions).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 3,329 ✭✭✭✭
    Re: Temp replacement if FM80 with MX60 - Breaker size

    There is also a screen on the MX-60 that allowed the unit to operate at 70 amps.
    I am not sure if the op's model firmware allowed it but my old, old, old one did.
    Either way as Bill says it will current limit. What failed on the -80? I still have a few out there...:cry:
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail offgridsolar@sti.net

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