What size breaker for miller 255 (Millermatic)
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  1. #1
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    What size breaker for miller 255 (Millermatic)

    I'm going to up grade from the 180 to a 255. It looks like it pulls 44 amps. Is that correct? So a 50 amp breaker?

  2. #2
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    Re: What size breaker for miller 255 (Millermatic)

    Quote Originally Posted by Reebz View Post
    I'm going to up grade from the 180 to a 255. It looks like it pulls 44 amps. Is that correct? So a 50 amp breaker?
    I have maxed out Millmeratic 200, 250, 250X and 251 on a 40 amp breaker with 8ga wire and didn't pop the breaker, so you will be fine with 50. I think the 251 and 250X are 42 amp max, but I know the 250 was 44 amps like the MM 255....was testing that one this past weekend.
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  3. #3
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    Re: What size breaker for miller 255 (Millermatic)

    The breaker protects your wiring, not the machine, so you need to know what size wire you have and how long a run it is before you change breakers.

  4. #4
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    Re: What size breaker for miller 255 (Millermatic)

    Quote Originally Posted by walker View Post
    The breaker protects your wiring, not the machine, so you need to know what size wire you have and how long a run it is before you change breakers.
    No, in this type of circuit the breaker provides short circuit protection and very rarely protects the wire, only on general use circuits with multiple outlets. What protects the wire is the applied load. A 255 calls for 8 wire in pipe or 6 in cable. (if its the same as a 251) In reality it wont overheat a 10 wire with 035 and c25. The machine comes with 50 end then it uses a 50 breaker.
    Some units like the stick buzzers allow 12 wire and 50 breaker depending on wire method.

  5. #5
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    Re: What size breaker for miller 255 (Millermatic)

    The manuals are confusing at best and the newer machines are even worse. I havnt sit down and studied all the details yet, some of it applies to industrial where there may be a possibility of more than one machine to a circuit. The problem with some of that is that using a common recept leaves it open to use on other machines,,, your 180 is legal on 50A if the wire is 12 or larger. The minimum wire sizes are for single circuit in pipe most cases. Up a size if cable is used, again not a big deal in home shop, wouldn't have a problem with 8 cable on 50 for them.
    I run my 250 with 10 cord and it doesn't warm no matter how hard I run it with short circuit.

  6. #6
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    Re: What size breaker for miller 255 (Millermatic)

    Thanks guys. Mild up grade to the breaker and I'm ordering a 255!!!

  7. #7
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    Re: What size breaker for miller 255 (Millermatic)

    "It looks like it pulls 44 amps. Is that correct? So a 50 amp breaker?"

    Max power from the machine is 74 amps.

    RATED power of 260amps is 41 amps.

    The new MM255 is a 350amp machine.


    Specs call for an 80amp CB and #8 wire minimum iffin' ya wanna run it wide open
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  8. #8
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    Re: What size breaker for miller 255 (Millermatic)

    Brocoli1. I'm confused (which is easy to do), Do you mean 50 amp breaker or 80?

  9. #9
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    Re: What size breaker for miller 255 (Millermatic)

    Ed, I am curious as to how they do this with 50 ends? I think some of that is allowed for hard wire. Does the machine come 50 end? Or is it field wired?

  10. #10
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    Re: What size breaker for miller 255 (Millermatic)

    I think they make a 60 recept, that would prevent a smaller machinefrom being plugged in to a hopped up circuit but my bet is a guy that has been making it with a 180 will make it on a 50 circuit to a 255 too. Wire it 8 with a 50 and go from there.

  11. #11
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    Re: What size breaker for miller 255 (Millermatic)

    https://www.millerwelds.com/files/ow...79299C_MIL.pdf


    Looks like it comes with a 6-50p.
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  12. #12
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    Re: What size breaker for miller 255 (Millermatic)

    but the machine is rated at 260 welding amps with only 41 input amps so it still has quite a bit of power even if running off a 50amp CB.
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  13. #13
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    Re: What size breaker for miller 255 (Millermatic)

    Quote Originally Posted by Sberry View Post
    No, in this type of circuit the breaker provides short circuit protection and very rarely protects the wire, only on general use circuits with multiple outlets. What protects the wire is the applied load. A 255 calls for 8 wire in pipe or 6 in cable. (if its the same as a 251) In reality it wont overheat a 10 wire with 035 and c25. The machine comes with 50 end then it uses a 50 breaker.
    Some units like the stick buzzers allow 12 wire and 50 breaker depending on wire method.
    I respectfully would disagree.

    It is my assertion that a breakers only job is to protect the wire from overheating and starting a fire.

    The down-sizing in wire for welders vs. a normal circuit is allowed because of duty cycle.
    Since a welder will not put a sustained load on a circuit, like an electric motor or heater can, the wire is allowed to be downsized based on the duty cycle of the welder.
    This smaller wire is specified in the electrical code and is for welder only dedicated circuits.
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  14. #14
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    Re: What size breaker for miller 255 (Millermatic)

    Minnesota Dave
    That makes sense to me

  15. #15
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    Re: What size breaker for miller 255 (Millermatic)

    It is my assertion that a breakers only job is to protect the wire from overheating and starting a fire.
    Since this is basic I am going to generalize a little. Motor circuits can also use breakers smaller than "normal" rating. They can do this when hard wired, the welder exception is the one that allows it to a receptical. Look at the directions for AC units, minimum wire, max breaker.
    Of course its for duty cycle for a welder but the only place its really for thermal is for generals with multi recepts on the same wire. Thje breaker is in place in most of other circuits to provide short circuit interruption. Its a calculated load from mfg that protects it from thermal, the breaker protects from faults either within the machine or leading up to further thermal protection.
    This is why stuff comes 120V is generally limited to 20A, the internals are not designed to go on larger circuits. 30 at 120 is specialized or has additional ocpd like a travel trailer.
    These types of circuits the wire is sized to the load not the breaker. Putting a recept in the line changes it as the operator can simply plug and unplug different equipment. Hard wire is different, cooking equipment for example, 14 wire allowed to be connected to 30A, 12 to 50, the electric range is designed with these principles and is fundamental to circuit design regarding overcurrent protection.
    About the only time the breaker protects the wire from thermal,,, or overheating is in case of someone plugging too many appliances in to a general.
    Last edited by Sberry; 12-05-2019 at 06:29 PM.

  16. #16
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    Re: What size breaker for miller 255 (Millermatic)

    The fact that the wire is sized at 125% of the fla is what protects it from thermal, the breaker is for short circuit ground fault or line to line fault. Breaker may be substantially upsized under some circumstances. Up to 50A for 12 wire for example. similar to wire within common fixtures, current limited by the style of the lamp base and large enough for short circuit of a 20A device, normally 16 wire, like an extension cord.
    Our system of plug and cord says,,, if its all wired correctly the plug design is to insure adequate supply but also to limit short circuit. In hard wire we size the wire to protect from overheat and can size the breaker for short circuit.
    Last edited by Sberry; 12-05-2019 at 07:01 PM.

  17. #17
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    Re: What size breaker for miller 255 (Millermatic)

    I agree with the Broc, 260 is a lot of poop. run 045 short circuit easy. That's only 41A,, at 48 it would be close to 300A

  18. #18
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    Re: What size breaker for miller 255 (Millermatic)

    Although not welder specific, an example of what I was saying: https://c03.apogee.net/mvc/home/hes/...pc=foe&id=4682
    I am not an electrician, so I refer to the words of the professionals.

    "Equipment Protection
    Fuses and breakers are also used to protect electrical appliances and equipment from damage or complete burn out due to overload. The fuse or breaker at the breaker box is sized to protect the wire, but is not necessarily sensitive enough to protect a small-use device plugged in on the circuit. In this instance, a fuse or breaker is generally built into the appliance or electrical equipment to protect it from overload.

    Electric motor for example, electric motors draw large in-rush currents when they are starting. A typical motor will draw five times as much current while starting compared to its normal running current. Therefore, a motor that draws 3 amps at full-rated load while running, will draw 15 amps while it is starting. A 12 gauge copper wire used to supply the motor with electricity would normally be protected by a 20 amp fuse or circuit breaker. If the motor locks up and draws 15 amps, this current flow will burn out the motor winding very quickly but not cause a 20 amp fuse or breaker to shut off power to the motor. A second fuse or breaker to protect the motor from overloading is clearly necessary in a situations like this.

    Note also that when individual equipment fuses or breakers are used, they are used in addition to the regular circuit protection. The fuse or breaker at the main service panel or sub-panel protects the circuit wire against dangerous overloads. The equipment fuse or breaker protects the individual piece of electrical equipment and adds safety to the system."
    Dave J.

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  19. #19
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    Re: What size breaker for miller 255 (Millermatic)

    Right, some are required to have additional thermal. The above explanation is very basic and aimed at beginners so to speak. They didn't include all the advanced theory and extrapolate which, when and where. It didn't include everything about circuit design.
    I am not at pc where it's easy to cut/paste in regards to individual aspects of that post, it doesn't go in to details like,,, how do they protect without additional fuses, that should have said,,, it may include.
    Pressure washer has many of these fundamentals in it. Only additional protection in the machine is reset on burner blower. Mine is 30A on 240, shares some parts with 120v models. The transformer uses 1/2 the current of the 120v but they increase the size of the leads to it. Small amount of wire increased in size in order to avoid additional fuses.
    Last edited by Sberry; 12-05-2019 at 11:34 PM.

  20. #20
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    Re: What size breaker for miller 255 (Millermatic)

    A good example of some of this is power strips, the reset or breaker actually protects the wire ahead of it. I have an air blower, has a duplex on it to plug in to but since it has 16 cord and multiple outlets there is a 10 breaker,, after the wire.
    I did look at the link you posted, I am not sure who that was aimed for but it is super general and correct in some cases but far from complete. I think it was written by a writer rather than an electrician or engineer.

  21. #21
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    Re: What size breaker for miller 255 (Millermatic)

    It is correct in that if there are internals that need thermal protection additional fuse may be needed, what it doesn't say is that internals may also be sized that it's not needed. The AC225 welder is a good example, comes with 12 cord,, no other internal protection, all sized for short circuit at 50A. Small wire feeders, 240v models, have 12 cord to switch followed by additional protection, got self resetting breaker.
    Another example is an electric range, got a 6 cord to a lug that all the rest is 12 wire with no additional thermal. It could run continious, possible depending on oven design but cord sized to carry the total load, 50A breaker to allow it but internal wire sized for each connected load and to survive a fault up to 50A breaker rating. Breaker is simple a switch and fault protection. All calculated load protected.
    In some cases even circuit entrance doesn't have main breaker, done by load calculator to a buss with 6 spaces or handles.
    Last edited by Sberry; 12-06-2019 at 12:08 AM.

  22. #22
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    Re: What size breaker for miller 255 (Millermatic)

    Modern Xmas lights have 2A fuse, wire is not heavy enough for fault protection on 20A. Old designs used way more power but also used heavy enough wire as not to need additional fuse. They were essentially fixture wires.

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