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Thread: VFD Wire Size

  1. #21
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    Quote Originally Posted by kwired View Post
    or they may put a 40 Hp on it. When that time comes whoever does so must determine if what is there is sufficient for what they want to do. If driven machine is rated 75 HP one would think it would already have a 75 HP motor on it.

    Maybe they derated the drive for some reason? Lots of starts, breaking, accelerating, reversing?
    Maybe the OP could provide more detailed information ?
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    Quote Originally Posted by Besoeker View Post
    Maybe the OP could provide more detailed information ?
    I think in post 17, he admits to ordering a larger drive than he needed and had he thought about it, the smaller drive would have been adequate.
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    Quote Originally Posted by ptonsparky View Post
    I think in post 17, he admits to ordering a larger drive than he needed and had he thought about it, the smaller drive would have been adequate.
    In OP it says replacing 75 HP drive with same size.
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    Quote Originally Posted by Besoeker View Post
    Maybe the thinking is that someone might put a 75 hp motor on that 75 hp VFD at some point in the future.
    In my experience that happens quite often actually. The issue is one of original intent vs the 2nd or 3rd or 4th guy down the road coming across the drive rating and getting pressure to get more out of a machine or process. Should they look at thconductor size? Sure, but under pressure things happen. That’s the entire rational for 430.122 from what I heard.
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    Quote Originally Posted by Jraef View Post
    In my experience that happens quite often actually. The issue is one of original intent vs the 2nd or 3rd or 4th guy down the road coming across the drive rating and getting pressure to get more out of a machine or process. Should they look at thconductor size? Sure, but under pressure things happen. That’s the entire rational for 430.122 from what I heard.
    I had place that had a 250 HP drive sitting around collecting dust, but a need for using a drive on a 50 HP motor. Worked fine with input conductors and overcurrent protection already selected for 50 HP. Was not going to overheat anything because if the motor were overloaded the overload protection was set for the 50 HP motor FLA.

    Later came a need for drive on a 100 HP motor. We purchased a 50 HP drive and moved the 250 HP drive to the 100 HP application.

    I really see it not a whole lot different than using a Size 4 contactor in a place where a size 3 would work. Or even with size 4 you can use it on 60, 75 or 100 HP, but will need different minimum conductor and overcurrent device for each one.
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    Nobody said it wouldn’t work, it will. The issue is whether it is code compliant. It isn’t.
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    Quote Originally Posted by Jraef View Post
    Nobody said it wouldn’t work, it will. The issue is whether it is code compliant. It isn’t.
    I know. Another case of NEC making itself a design manual IMO. What is significant enough risk here to call a threat to life or property?
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  8. #28
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    Quote Originally Posted by kwired View Post
    I know. Another case of NEC making itself a design manual IMO. What is significant enough risk here to call a threat to life or property?
    In my opinion, it’s because of the inherent flexibility of what an inverter drive brings to the table. Unlike a simple contactor that is just on or off, a power converter is more akin to a generator in that it produces power on the output side while at the same time consumes it on the input side; input power is the “fuel” for that generator. As such, it can, in theory and in practice, consume current at a different rate than the output side uses it (note that I said current, not power). For example all VFDs are inherently capable of consuming power from a single phase input and producing 3 phase on the output. In that process, the current on the input side will, compared to the motor current, increase by the square root of 3, so 1.732x the output current. If you had sized the input conductors based solely on the motor nameplate HP, they would in fact be a fire / safety risk. So rather that rely on the installer to understand all of this, the NEC simply requires that the input conductors be sized to the input current rating of the VFD. This effectively reduces the risk of misunderstanding the technology.

    I teach classes in VFD technology and believe me when I say that brobably 50% or more of the electricians and technicians that attend my classes come in not knowing that one issue, and that is just one issue that can mess people up. These are quite often very smart experienced electricians and technicians, but if they have never been taught something, it’s not as intuitive as sizing a switch.
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  9. #29
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    Quote Originally Posted by Jraef View Post
    In my opinion, it’s because of the inherent flexibility of what an inverter drive brings to the table. Unlike a simple contactor that is just on or off, a power converter is more akin to a generator in that it produces power on the output side while at the same time consumes it on the input side; input power is the “fuel” for that generator. As such, it can, in theory and in practice, consume current at a different rate than the output side uses it (note that I said current, not power). For example all VFDs are inherently capable of consuming power from a single phase input and producing 3 phase on the output. In that process, the current on the input side will, compared to the motor current, increase by the square root of 3, so 1.732x the output current. If you had sized the input conductors based solely on the motor nameplate HP, they would in fact be a fire / safety risk. So rather that rely on the installer to understand all of this, the NEC simply requires that the input conductors be sized to the input current rating of the VFD. This effectively reduces the risk of misunderstanding the technology.

    I teach classes in VFD technology and believe me when I say that brobably 50% or more of the electricians and technicians that attend my classes come in not knowing that one issue, and that is just one issue that can mess people up. These are quite often very smart experienced electricians and technicians, but if they have never been taught something, it’s not as intuitive as sizing a switch.
    Can say some of the same things for a transformer. I once needed to set up 120/240 temp power for construction where only voltage available was 480/277. They had a 25 kVA transformer sitting around, but if one would have needed purchased would only have gotten 10kVA. Connected it to an existing 30 amp fused disconnect to see if 30 amp fuse would hold when energizing - it did so wired it with 10 AWG and 30 amp fuses.
    I live for today, I'm just a day behind.

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