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Thread: Fuse size for "Convenience" Disconnect on 20A Circuit?

  1. #1
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    Question Fuse size for "Convenience" Disconnect on 20A Circuit?

    Is it okay to use 30A fuses in a disconnect switch on a 20A circuit if the disconnect switch is not there to protect anything--just for convenience? The fused disconnect was just cheaper and more available at my local hardware store than a non-fused disconnect. My thinking is that the 20A circuit breaker protects all the 12/2 wiring and the tools are fine on their own. If something should trip the breaker (nothing has thus far on circuit) 20A fuses would or could just need replacing for no real reason--that's why I initially got the 30A fuses.

    Some more details in case they affect answer. I'm running 12/2 wire for my new table saw by extending an existing 20A 220v circuit for my jointer (all wire is 12/2 and breaker is 20A DP). Table saw and jointer will be only tools on circuit and will not operate at same time as it's just me in my basement shop. I bought a "Light Duty" 30A fused disconnect (Square D L211N). It won't be the primary disconnect as the saw plugs into an outlet (L6-20P/R). The switch is just so I can cut off the power to the table saw (not turn it off) with a switch rather than unplug it for changing blades or making adjustments and maybe a lockout point for children as they get older. Mismatching the 30A fuses with a 20A circuit seems odd (maybe a NEC code violation?), but putting 20A fuses in seems redundant with the circuit breaker since I don't really want/need them to blow to protect anything. As I type this I think that maybe 20A fuses would "backup" the circuit breaker, but is that necessary or practical? Would the 20A breaker trip before 20A fuses (type TL) would blow?

    Thanks for any advice or suggestions. -Rick

  2. #2
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    Not an electrician but been spending a bit of time on a couple of DIY electrical forums. I think it would be okay and I am doing the same thing, but not sure what the NEC requirements are. Electrically it should be safe but may need to be labeled or something so future users know it's not a 30 amp circuit.
    You're never too old to have a happy childhood.

  3. #3
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    Hi Rick, why not replace the fuses with shorting links?

    You can buy them at the electrical wholesalers.

    Regards, Rod.

    P.S. Fuses give you the most accurate over current protection possible, the cheap breaker in your panel doesn't have great accuracy or consistancy. It is possible that the fuse may open before the breaker does, or not as the case may be.

  4. The circuit breaker and/or fuse protects the wire and not directly the machinery. It keeps it from overheating and possibly causing a fire. Go with what the wire is rated for.

  5. #5
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    I've got 2 in my shop that are for convenience like yours--I just put the same size fuse in them and the breaker. I've never blown a fuse or popped the breaker, but I do keep spare fuses around just in case since that's what's likely to go first.

    Rod, do they make Shorting Links for type TL fuses? A quick Google search didn't turn up anything.
    Last edited by Matt Meiser; 05-12-2011 at 9:10 AM.


  6. #6
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    You may think about a 20a dp switch, just toggle it on and off, not necessary to fuse/protect it. Some are made with a lockout device which would help with your small children issue, or you can get a separate device that fits on the switchplate and save that disconnect for something that needs 30 amp protection ............

    or..................................put 20amp fuses in your disconnect...................

    Rich
    "The best way to get better is to leave your ego in the parking lot."----Eddie Wood, 1994

    We discovered that he had been educated beyond his intelligence........

    Student of Rigonomics & Gizmology

    Waste Knot Woods
    Rice, VA

  7. #7
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    I needed to replace the disconnect (box rotted out) on an outside air conditioning compressor. I had no problem finding one and it wasn't expensive.

  8. #8
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    Thanks for the input!

    I was thinking the 30A fuses would function like the "shorting links" Rod mentioned (though I didn't actually know about shorting links). I'll just get some 20A fuses since it's only a few bucks and they seem unlikely to blow anyway.

  9. #9
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    Quote Originally Posted by Matt Meiser View Post
    I've got 2 in my shop that are for convenience like yours--I just put the same size fuse in them and the breaker. I've never blown a fuse or popped the breaker, but I do keep spare fuses around just in case since that's what's likely to go first.

    Rod, do they make Shorting Links for type TL fuses? A quick Google search didn't turn up anything.
    Matt, I missed the TL fuse part completely as it was a 240 volt application, I assumed cartridge fuses.

    Can you use a TL fuse on 240 volts? I thought they were 125 volta only..............Rod.

  10. #10
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    That's what all the 240v disconnects sold in big box/hardware stores use from what I've seen.


  11. #11
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    Hi Matt, yes I've seen them, as well as the cartridge fuse types, however I thought that the TL fuse was for line to neutral loads?

    Regards, Rod.

  12. There's no need to replace the fuses, and doing so may lead to nuisance trips if they are not time delay on a motor circuit. Just leave them as-is and you are fine.

  13. #13
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    The switch (L211N here) takes two plug fuses, sorry if that wasn't clear. It made sense to me that they were each 125V (at any AMP rating) since the wires are separate. Is that incorrect?

    Thanks again, for the input! -Rick

  14. #14
    Rick,

    There is no problem putting the larger fuses in since you already have the breaker sized to protect the wire. I would not put 20A fuses in. In most cases a fuse will blow much quicker than a circuit breaker will trip. That is not always a true statement, but with residential grade breakers, it almost always is. If you don't want to worry about the fuses either leave them bigger or put shorting links in. Since you have the larger fuses, keep it that way. The set up you described is perfectly safe and fully compliant to the NEC.

  15. #15
    There is nothing wrong w/ the OP plan, the wiring is protected at 20A upstream & if wanted to use a 200A disco w/ 200A fuses protected upstream at 20A, still the only harm would be lugs not rated for the conductor size. That is a extreme example but there is no harm in the OP's plan.

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