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Thread: oversized circuit breaker question

  1. #1
    DIY Junior Member Kyackr's Avatar
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    Question oversized circuit breaker question

    I am going to wire an electric heater in the semi finished upper level/attic of my home. My panel (100 amp Cutler-Hammer) has an existing unused 40 amp breaker from a former electric range that was replaced with a gas range prior to my buying the house. The electric heater I plan to buy is a 5,000 watt, 240 volt unit rated 21 amps. I assume a 30 amp breaker with 10 gauge wire would be typical for this heater but wonder if I can use this existing 40 amp breaker and still have the safety aspect of the breaker function? I am assuming I may want to used 8 gauge wire for the heater if using a 40 amp breaker.. I know you can't use an undersized breaker but was not sure of problems with an over sized breaker? Thanks

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    Electrical Contractor Jim Port's Avatar
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    If you install wiring rated for 30 amps with a 40 amp breaker the wire could overheat before the breaker would ever trip. This would be a fire hazard and a code violation.

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    DIY Junior Member Kyackr's Avatar
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    Quote Originally Posted by Jim Port View Post
    If you install wiring rated for 30 amps with a 40 amp breaker the wire could overheat before the breaker would ever trip. This would be a fire hazard and a code violation.
    Thanks for your reply Jim.. What about if I use proper size (8 gauge) wire for the 40 amp breaker? Is the safety problem due to the breaker exceeding the amp rating on the heater... 21 amps? From what I understand I multiply this by 1.25 which is 26.25 amps..so a 30 amp breaker would be the correct size.. and 10 gauge wire adequate. Seems like it would be OK.. and up to code as long as I use 8 gauge wire from heating unit (which will be direct wired) too the 40 amp breaker ?

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    Electrical Contractor/Instructor jwelectric's Avatar
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    The NEC is a minimum safety standard. To do anything less than what it requires is an unsafe installation.

    In 210.20(C) we are told;
    210.20 Overcurrent Protection. Branch-circuit conductors and equipment shall be protected by overcurrent protective devices that have a rating or setting that complies with 210.20(A) through (D).
    (C) Equipment. The rating or setting of the overcurrent protective device shall not exceed that specified in the applicable articles referenced in Table 240.3 for equipment.

    All this is saying is #10 and 30 amp breaker is what is called for.

    You are mistaken when you make this statement;
    Quote Originally Posted by Kyackr View Post
    I know you can't use an undersized breaker but was not sure of problems with an over sized breaker? Thanks
    If the breaker is undersized all that will happen is the breaker trip. If the breaker is oversized or in other words to large then most likely the fire department will do their best to save what they can. That is if they get there in time which doesn’t happen too often.

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    DIY Junior Member Kyackr's Avatar
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    Quote Originally Posted by jwelectric View Post
    The NEC is a minimum safety standard. To do anything less than what it requires is an unsafe installation.

    In 210.20(C) we are told;
    210.20 Overcurrent Protection. Branch-circuit conductors and equipment shall be protected by overcurrent protective devices that have a rating or setting that complies with 210.20(A) through (D).
    (C) Equipment. The rating or setting of the overcurrent protective device shall not exceed that specified in the applicable articles referenced in Table 240.3 for equipment.

    All this is saying is #10 and 30 amp breaker is what is called for.

    You are mistaken when you make this statement;

    If the breaker is undersized all that will happen is the breaker trip. If the breaker is oversized or in other words to large then most likely the fire department will do their best to save what they can. That is if they get there in time which doesn’t happen too often.
    Thanks JWElectric. yeah... well i did know that overloading a circuit would just trip the breaker but I had no idea underloading a breaker was a problem? I mean if I have a 15 amp single pole circuit that never reaches even a quarter of the load that would be needed to trip the breaker.. all is fine yet if I connect this 5,000 watt 240 volt 21 amp heater to a dedicated 40 amp with 8 gauge it is some how a problem? seems as long as the wiring is correct for the breaker it's ok? do you understand my confusion ? Why no problem when I have other circuits that never exceed the wiring and breaker load. I have many 15 and 20 amp circuits that never reach the breaker trip limit. why is this over sized wiring and breaker a problem for this heater but i can have a single 100 watt lightbulb on a 20 amp circuit ? Is it the heater that might burn up from too much juice?

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    Test, Don't Guess! cacher_chick's Avatar
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    For me, the most baffling part of this conversation is why one would just not install the correct breaker and wire and be done with it.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by Kyackr View Post
    Thanks JWElectric. Is it the heater that might burn up from too much juice?
    See I knew you would do it, yep, you answered your own question.

    There is a big difference between the receptacles and lighting outlets throughout our homes and the circuits we install for fixed appliances. If something goes wrong in the heater then overcurrent device is what protects us from fire.
    Should we start installing overcurrent devices to fixed appliances that are oversized, how are we to stop a fire from starting?

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    DIY Junior Member Kyackr's Avatar
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    Quote Originally Posted by cacher_chick View Post
    For me, the most baffling part of this conversation is why one would just not install the correct breaker and wire and be done with it.
    understandable bafflement..
    if it was square d or a brand more common and available in my area .. at least from a *******, home depot etc i would just get a new 30 amp breaker .. but it's cutler-hammer and while i have found cutler-hammer brand online.. the double pole, 240v 30 amp breakers i've seen on the online catalogs do not look like the one for the dryer in my panel so using the existing 40 amp one is appealing.

    I probably will just end up searching for a cutler hammer 30 amp breaker but I sure would like someone to explain why this is a hazard when a single 100 watt light bulb on a 15 amp 120 circuit posses no hazard even though the circuit can handle many many more amps. ..why is this a problem assuming I match the correct gauge wire for the breaker amperage.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    There's nothing wrong with using that heavier wire, but to comply with the manufacturer's installation instructions, just change the breaker to a 30A one...no need to replace the wiring UNLESS you can't attach it to the device properly (i.e., say screw terminals that won't accept that larger wire. The breaker is there to protect the wiring in the wall, on a single circuit, it can also be used to protect the item being powered, but still, it's to protect the internal wiring. The heavier wire will ensure a lower voltage drop over its run, and is often done with longer runs.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  10. #10

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    Quote Originally Posted by Kyackr View Post
    What about if I use proper size (8 gauge) wire for the 40 amp breaker?
    A 30A breaker is a LOT cheaper then the cost of going to 8-ga wire.

    Breakers are cheap, fires expensive. Just do it right. If you can't bring yourself to do it correctly, step aside and let a pro do it.

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    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by Gary in NJ View Post
    A 30A breaker is a LOT cheaper then the cost of going to 8-ga wire.

    Breakers are cheap, fires expensive. Just do it right. If you can't bring yourself to do it correctly, step aside and let a pro do it.
    Completely agree.


    I cannot believe that you cannot find a 2p30 amp C-H breaker. They are as common as any other breaker.

  12. #12
    Moderator & Master Plumber hj's Avatar
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    The internal HEATER wiring for a 5500 watt element is #10 wire, so you should NOT use a 40 amp breaker.
    Licensed residential and commercial plumber

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    Licensed Electrical Contractor Speedy Petey's Avatar
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    Quote Originally Posted by hj View Post
    The internal HEATER wiring for a 5500 watt element is #10 wire, so you should NOT use a 40 amp breaker.
    The internal wiring of the unit has no bearing on the branch circuit conductor size.

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    DIY Junior Member Kyackr's Avatar
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    Thanks everyone. I'll just have to find me a Cutler-Hammer 2 pole 30 amp breaker.. probably have to look at other than a typical home improvement center etc..more like the places electricians get supplies at. It's all good

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    Electrical Contractor jbfan74's Avatar
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    Depending on the panel, you need a ch230, or a br230, both listed at lowes and HD.

    Good Luck.
    Yes I am A Pirate-Jimmy Buffett

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