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Thread: #10 conductor on a 50amp breaker 220/240VAC

  1. #11
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    It could also be that the 50A CB is feeding just a single motor load and there is another Thermal Over Load device down stream of the breaker so that it is protecting the conductors for running current and the 50A CB is there just for Short Circuit protection, per 430.52.
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    AC verses Breaker verses conductor.

    The way I read this in the NEC is a breaker must be the weakest link in the chain. Should a current drain occur on the conductor, regardless of what connects to the conductor, the breaker is suppose to protect the device and the conductor.

    The breaker size is suppose to match the name plate on the ac condenser. The conductor is suppose to match the breaker.

    It shouldn't matter device is on the conductor?

    I understand that there are various insulations on the conduct and heat variations that change the ampacity of the conductor. However, when I go to the site, I can not tell what type of insulation is on the wire. Its cut off, and I don't know about anyone else but I can't tell unless I read what is written on the conductor jacket.

    Thanks guys and if you have any more thoughts I would appreciate to hear them.

  3. #13
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    Question

    Quote Originally Posted by augie47 View Post
    It, as Dennis states, is possibly with Code based on 240.4(D) &440 Part III.
    Where 240.4(D)((7) would normally limit the overcurrent device on a #10 to 30 amps, 240.4(G) allows instances where that may not be the "rule"
    Art 440 is one of those areas listed in 240.4)(G) and 440.21 Part III sets forth the rules for selecting the circuit conductor ampcity and GFSC rating. Those are normally listed on manufacturers nameplate.
    The rationale, I believe, is that the AC factory overload will protect the #10 and the inrush requires a larger OCP which will still provide short-circuit protection.

    If I read this correct, the breaker is not there to protect the conductor from over heating? What is suppose to protect the conductor?

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    Quote Originally Posted by knightstar View Post
    If I read this correct, the breaker is not there to protect the conductor from over heating? What is suppose to protect the conductor?
    The overload device in the A/C unit.

    It might help to think of it as you might think of a "tap". If you have a 100 amp switch tapped from a 400 amp conductor protected by a breaker, the 100 amp switch feed is allowed to be rated at 100 amps, not 400.
    The fuse protects the tap from overload just as the HVAC overload protection device protects your #10. If the wire were to have a fault ahead of the overload (or fuse) the current flow will be large enough for the breaker to open.
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    Quote Originally Posted by knightstar View Post
    The way I read this in the NEC is a breaker must be the weakest link in the chain. Should a current drain occur on the conductor, regardless of what connects to the conductor, the breaker is suppose to protect the device and the conductor.

    The breaker size is suppose to match the name plate on the ac condenser. The conductor is suppose to match the breaker.

    It shouldn't matter device is on the conductor?
    The part that you have wrong is that in this installation is that the OCPD is not protecting the conductor from overload, it only is protecting the circuit from a ground fault or short circuit condition.
    Rob

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    All responses based on the 2014 NEC unless otherwise noted

  6. #16
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    Look at article 440.22. You may go 175% to 225% of the motor compressor rated current or the branch cir. selection current- whichever is greater.

    Not sure how this all plays out but my understanding would be that in some cases the OCPD can be greater than the nameplate.
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  7. #17
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    Quote Originally Posted by knightstar View Post
    If I read this correct, the breaker is not there to protect the conductor from over heating? What is suppose to protect the conductor?
    I think you editted this post after you posted, :0... Ok, I was to say something else but anyways; 9/10th of the time one is only protecting the circuit. It's when you get into a specific disconnect that your protecting that application, forward from this point. Your equipment is cutoff via the breaker/disconnect making it the protection to this device. The circuit to this service is still protected itself by some kind of breaker.

    Also keep in mind the math and ratio balancing that within electrical equations, generally if the voltage goes up the amps usuage goes down. In industrial it's not unusall to see tons of 20 AMP /480 motors, most only draw 5-7 continually (mostly less) well below when used to apply any required summary equation. MCA is the way to go if present and a good reminder.

    The NEC tells us to address the requirements of each type of circuit service, with-in are the specific's as presented
    with exceptions, use them.
    Last edited by cadpoint; 02-26-12 at 09:55 AM.
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    The only time I've ever seen something like this in the field has been #14 on a 30amp 3 phase breaker, the overloads of the motor starter rated at 7amp

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    Last edited by stickboy1375; 02-26-12 at 10:33 AM.
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    Stick has provided a good graphic. I will note one thing about this one it's based on the 2008 and earlier version of the NEC. This would no longer be applicable under the 2011.

    Rob

    Moderator

    All responses based on the 2014 NEC unless otherwise noted

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