12V Solar/Battery Q: Can I use glass fuse?

Discussion in 'General Electronics Chat' started by foolios, Jul 31, 2014.

  1. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    I was hoping I could use this switch in-between the 12v 100watt panel and the charge controller:
    http://www.menards.com/main/electri...-plug-fuse-safety-switch/p-1773843-c-6435.htm
    with this fuse:
    http://www.menards.com/main/electri...p-medium-duty-fuse-box-4/p-1449458-c-9543.htm
    But I am reading comments about AC breakers being ok to use on low voltage DC circuits and yet there are comments that say they shouldn't.
    My question is would it be ok to use an AC fuse on a 12v DC circuit?

    One reference:
    http://control.com/thread/985805467

    Thanks in advance.
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Hm... 12 volts, 100 watts... that gives us current of 8.33 amperes. Is the fuse rated for 8.33 amperes?

    What the general rule of thumb for the fuses? Should they be rated for twice the current you expect or just 1.5x?
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Unless the fuse (and switch) is rated to interrupt DC then it is NOT suitable to use.
    Simple as that.. :)
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    higher voltage fuses are ok, just dont use lower voltage fuses than the origional. someone used a 120 volt fues out here in a 480 volt motor starter, made a mess, minor explosion.
     
  5. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Just to clarify..
    The voltage rating on the fuse must be greater than the working voltage of the circuit.. But it still MUST be rated for AC if used on AC and DC if used on DC.

    It is not OK to use a 120V AC rated fuse to protect a 12V DC circuit.
     
  6. NorthGuy

    Active Member

    Jun 28, 2014
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    You don't need a fuse between controller and the panel. The panel won't produce much more than its rated current even if the controller shorts, so the fuse is completely useless.

    As to the switches, most AC switches are also rated up to 24V DC. Or you can buy a dedicated DC switch, e.g. http://www.bluesea.com/
     
  7. alfacliff

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    Dec 13, 2013
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    why is it not proper to use a 120 volt ac fuse on a 12 volt dc circuit? the voltage rating on a fuse is determined by how much voltage it will block when it blows, the current rating is determined by the heat generated by the current flowing through the fuse melting the fuse. a lower volt rated fuse might not open up far enough to break a higher voltage, but a higher voltage fuse will open enough to break a lower volltage.
     
  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    because its NOT rated to break DC currents.
    DC is harder to break as it does not cross zero,etc...

    In general a dual rated fuse will be rated to 1/2 the AC voltage rating.
    Ex.. 600VAC/300VDC

    Now will it work.. Probably
    But is it "proper" NO its not because its not rated at all for DC no matter what the voltage.
     
  9. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Ok, I have that switch. I'll put that switch between the panels and the charge controller.

    Do I need to position fuses between the controller and the batteries instead?
    Or where would you put the fuses?
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    so #12 wire is rated differnt for ac than dc? not in any of my refferences. a fuse is just a peice of metal calibrated to melt in two when a certain current flows through it. an agc3 fuse works just as good on 12 volts as 120. ac or dc. the heat generated to melt the fuse element dosnt change during zero crossing, if it did, fuses would hum. fuse elements respond to average current through them, just like bolometers.
     
  11. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Most charts I've seen for AC circuits state that I can use 10AWG wire up to 25-30 amps.
    But this chart for 12V DC circuits says I need 8AWG for 25-30 amps.
    http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html
     
  12. NorthGuy

    Active Member

    Jun 28, 2014
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    Absolutely. If the controller fails short it'll destroy the battery very quickly. Fuse or DC breaker will protect against this. Breaker might be better because it replaces both the fuse and the disconnect.
     
  13. alfacliff

    Well-Known Member

    Dec 13, 2013
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    foolios, that chart is for lengths of wire and size of wire. since fuses arent 25 feet long, they wont have as much voltage drop at 20 amps as a piece of 10 guage wire has. and the chart is for completely different subjece, voltage drop per length of wire. of course 12 volts cant take as much drop as 120, but thats the same for ac as dc.
     
  14. foolios

    Thread Starter Active Member

    Feb 4, 2009
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    Ok, that's what I was wanting to do, have a switch to turn off the panels before I unhook a battery so as to avoid damaging the charge controller. I was thinking a fused switch would be even better.

    I'm kinda confused now with options. But what I really need to know is if that switch with old style glass fuse is ok for using on this solar circuit. I'm gonna reread the posts. I got off on a tangent about wire thickness and amperage and that's not my concern right now.
    I just want to know if that switch will work. I do have the other switch but I like the idea of the fuses built into the one I posted about.
     
  15. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Well that depends on the freq of the AC and if the skin effect comes into play
    But in general the industry uses the same tables for AC (50/60hz) and DC as far as wire ampacity is concerned.

    BUT regardless..
    I'm from the professional world which is dictated by standards/ratings/approvals.
    If I went into UL with a product intended for DC operation and protected by an AC rated fuse it would immediately be rejected.
    Hence why its not "proper"..

    DC fuse and circuit breakers are typically also designed differently from AC devices. With AC there is essentially no arc on each half cycle.
    In general the DC rated circuit breakers are larger as more arc suppression (blow out coils,etc..)/contact material,etc... is built into them again because its harder to interrupt DC current flow than AC.


    Out of curiosity.. You did/don't work for Nasa do you.. You sound just like a guy I talked to over there once that said he couldn't use this fuse we provided him with because it said 300VDC on it and he was only using it in a 12VDC circuit.. After my explanation as to the what/why he said "Thats not how it works in my book" and hung up the phone on me.
     
    Last edited: Jul 31, 2014
  16. AlphaDesign888

    New Member

    Jul 27, 2014
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    It is rated different for an reason. Could be politics.

    Either way who the hell are you to argue?

    You need to learn how to just follow instructions.
     
  17. foolios

    Thread Starter Active Member

    Feb 4, 2009
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  18. mcgyvr

    AAC Fanatic!

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    I have not seen screw in DC rated fuses.. I'll check my bussmann book though.
    (edit.. nope... none of the screw in plug fuses carry dc ratings)
     
    Sensacell likes this.
  19. AlphaDesign888

    New Member

    Jul 27, 2014
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    He works for Nasa redefining all the rules based on opinion. God help us.
     
  20. foolios

    Thread Starter Active Member

    Feb 4, 2009
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