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Thread: Breaker vs Fuse

  1. #1
    Join Date
    Sep 2010
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    Muscle Shoals, AL
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    Default Breaker vs Fuse

    Is a DC breaker and a fuse the same thing? I think of a breaker as one with a flip/trip lever and a fuse as automotive type maxi, mini, glass tube or ANL type. Given the same amp rating, can one be used instead of the other? I realize, in most cases, current through a breaker would be directional with highest potential on the line side. I cant see where it would make any difference with a fuse. But the question is, can I use a breaker where instructions say "fuse"?
    4 Kyocera KD135 in series, Tristar 45 MPPT, Exeltech XP600 24
    volt, 4 - 6 volt 210AH AGM's in series, Iota 25 amp/24 volt charger, 6kw cheap noisy generator.

  2. #2

    Default Re: Breaker vs Fuse

    Quote Originally Posted by rgk1 View Post
    Is a DC breaker and a fuse the same thing? I think of a breaker as one with a flip/trip lever and a fuse as automotive type maxi, mini, glass tube or ANL type. Given the same amp rating, can one be used instead of the other? I realize, in most cases, current through a breaker would be directional with highest potential on the line side. I cant see where it would make any difference with a fuse. But the question is, can I use a breaker where instructions say "fuse"?
    Two different things. A fuse is a "use once and replace" device; if it blows you have to get a new one. The breaker is a thermally (or electromagnetic) controlled switch: too much current causes it to shut off. You can reset it and be back in business over and over. Of course if you do you'd better look around to find out what's causing the repeated trips: that's not supposed to happen.

    Current through a fuse or a breaker is the same on in as out, so long as contact is made. They are not directional, per se, although some breakers have a "preference" due to internal design factors.

    For the most part they are interchangeable, so long as the device in question meets the specifications for the circuit. That means Voltage (maximum it's safe to interrupt), current (peak it can carry), and AC or DC rating. The DC rating is very important as it is more difficult to interrupt DC current flow.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  3. #3
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    Muscle Shoals, AL
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    Default Re: Breaker vs Fuse

    That,s what I needed to know. I wanted to replace the inline fuse to my inverter and put it on a breaker instead. The breaker is rated 150 vdc so no problem there. Not sure why my small 300 watt inverter calls for a 100 amp fuse. I have a 40amp on it now and have pushed the inverter about to its limit (500w or so) for a few minutes and it didn't blow. Does a 50amp breaker sound acceptable?
    4 Kyocera KD135 in series, Tristar 45 MPPT, Exeltech XP600 24
    volt, 4 - 6 volt 210AH AGM's in series, Iota 25 amp/24 volt charger, 6kw cheap noisy generator.

  4. #4

    Default Re: Breaker vs Fuse

    I use a 60 Amp marine breaker, sold by our host, for my 300 watt Morningstar SureSine inverter. It was recommended to me by several people on the board here and has worked very well.

    http://www.solar-electric.com/mr60ampdccib.html


  5. #5

    Default Re: Breaker vs Fuse

    Quote Originally Posted by rgk1 View Post
    That,s what I needed to know. I wanted to replace the inline fuse to my inverter and put it on a breaker instead. The breaker is rated 150 vdc so no problem there. Not sure why my small 300 watt inverter calls for a 100 amp fuse. I have a 40amp on it now and have pushed the inverter about to its limit (500w or so) for a few minutes and it didn't blow. Does a 50amp breaker sound acceptable?
    Why does the inverter call for a 100 Amp fuse? Because it is rated by the maximum output power of the inverter vs. the minimum input Voltage before shut-down. If this is a 12 Volt inverter with a 600 Watt surge capacity, that's 600 Watts divided by 10.5 Volts: 57 Amps. The NEC calls for the rating to be upped 25%, so that's 71 Amps. Then you round up to the nearest available fuse size: most likely 100 Amps.

    Do you need to go that far? For the wire current rating, yes; it should always be able to handle the "worst case scenario" in current draw. The fuse or breaker is sized to be the thing that goes before the wire burns up, so you can actually use a smaller breaker if you want. But ask yourself; how comfortable am I with "nuisance" trips? That 50 Amp breaker will be tripping around 525 Watts, which may occur momentarily some times. How long it takes to trip the breaker is another issue; some will trip quickly, others are a bit slower.

    So the only downside to the smaller breaker is that it may trip when you don't want it to. That and the wires may not fit (it's probably going to be 2 AWG or larger).
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  6. #6
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    Default Re: Breaker vs Fuse

    This is where the confusion for me starts. I am using the recommended 4awg on + and - from the battery. Looking at charts for amp capacity, it varies. Chassis wiring, transmission, open air, bundled, etc. I'm guessing that 50amp on 4awg in these conditions is ok. I personally am ok with the possibility of a nuisance trip. A little off the subject, but I am the noob and sure I'm wrong but would like to understand why. If the worst case is about 60A, wouldn't you want the breaker protection to kick in at that point? Just sounds dangerous for a protection device to allow an additional 25% current to get pulled to something that has already past its limit.
    4 Kyocera KD135 in series, Tristar 45 MPPT, Exeltech XP600 24
    volt, 4 - 6 volt 210AH AGM's in series, Iota 25 amp/24 volt charger, 6kw cheap noisy generator.

  7. #7

    Default Re: Breaker vs Fuse

    It's confusing, you're right.

    Start with the fact that the wire is capable of handling different maximum current depending on how it is used. This has to do with the ability to dissipate heat. In the open air it is easier to lose heat than if the same wire were confined to conduit.

    The other thing is the question of how long the current will be above maximum "allowed". Little spikes in current can and do occur without any danger. If the circuit protection were designed around "nominals" you'd be flipping the breaker back on too often for your own comfort. So the goal is to determine a point when something has definitely gone wrong and stop current at that point.

    The more you load a wire, the hotter it gets, the poorer the conductivity, the lower the Voltage at the "load", the higher the current, et cetera. Thus the safety specs are set for given values that seem to be quite high if you look at the basics but are nonetheless designed to keep the system operating even at the fringes of "normal".

    If you did this: 300 Watts / 12 Volts = 25 Amps (install 25 Amp fuse) what would happen?
    This: 300 Watts / 10.5 minimum Volts = 28 Amps, fuse blows.
    Or this: 600 Watt surge / 12 Volts = 50 Amps, fuse blows.
    Or this (worst case scenario, but within operating parameters): 600 Watt surge / 10.5 minimum Volts = 57 Amps, fuse blows.

    It may seem silly, but if something is meant to supply up to 'X' Watts on a minimum Voltage of 'Y' Volts that is considered the operating parameters. Even though this together with the NEC derating factor puts the "ideal" maximum circuit protector at 4X the expected nominal maximum.

    Frankly, NEC's 80% derating factor is one of the things that has never been satisfactorily explained to me. I think most of us here have a disagreement with one part or another of the regs.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  8. #8
    Join Date
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    Nova Scotia canada
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    Default Re: Breaker vs Fuse

    FYI, I'm running two SureSine-300 inverters for about 3 years now. One is fed through a 50 amp Slo-Blo fuse, while the other through a 40 amp Slo Blow. Why different size fuses? Because that's what I had at the time and they worked, so they stayed. In all the cases where they have been accidentally overloaded, they have instantly shut down. They try 3 times to restart, and if after the 3'rd time the excess load has not been removed, that's it, they stay shut down, requiring a manual restart. In no case has either one of them ever blown a fuse, and if they were fed through 100 amp fuses, I'd have trouble sleeping at night. Even so, the manuals with them do indeed call for 100 amp fuses. Guess my thinking was I'd rather have a fuse blow than have 100 amps surging through that poor little inverter. :) Even my Xantrex 1800/12 only has a 250 amp internal fuse and some of the loads I've had on that are unreal considering what it is. Interesting.
    1900 watts PV, (1000 watts PV feeding MidNite Classic 150; 900 watts PV + 160 watts micro hydro both feeding into a single shared Morningstar TS-MPPT-60) ; Xantrex Pure Sine 1800/12 for heavy loads; Xantrex Pure Sine 1000/12 on 24/7 for everything else; six Rolls Surrette 2 volt L16 @ 12 volts.
    Domestic hot water totally provided by the sun 8 months out of every year via thermal panel.

  9. #9
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    Muscle Shoals, AL
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    Default Re: Breaker vs Fuse

    Cariboocoot, thanks for the explanation. That makes sense. But, will err on the side of caution and go with lower amp breaker. Heck, only blew that tiny 3amp AC fuse once..on purpose. Waynefromnscanada, that is a great feature. I am ashamed to say I left wire nuts off on the AC side and had them touch. Inverter immediatly shut down. I didn't give it the chance to try again. Just out of curiosity, what kind of AC voltage do you see on your SureSine? No load, I see about 114v. Load to 2-250 watts and it drops to 109v +-1v. I know it's within spec. for the inverter, but dont want to fry the LCD TV or Satellite receiver. Anybody know what is acceptable voltage for AC appliances that just say "Input: 120 volts"?
    4 Kyocera KD135 in series, Tristar 45 MPPT, Exeltech XP600 24
    volt, 4 - 6 volt 210AH AGM's in series, Iota 25 amp/24 volt charger, 6kw cheap noisy generator.

  10. #10
    Join Date
    Mar 2006
    Location
    SF Bay Area (California)
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    20,532

    Default Re: Breaker vs Fuse

    Interestingly enough... there is not really a hard specification for AC Line voltage in the US...

    The range I would plan on:
    120 VAC +/- 10% = 108-132 VAC
    240 VAC +/- 10% = 216-264 VAC

    Those seem to be the voltage ranges plugged into the GT inverters.

    MSW inverters will read different because of the square wave output. And "simple/inexpensive" DVMs will read different than true RMS reading meters.

    And because of the lower peak voltage on the MSW waveform--A few appliances may also not work quite right either.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

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