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Thread: 2 Motors with VFDs (2) on 1 breaker

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    2 Motors with VFDs (2) on 1 breaker

    On a current remodeling project our client wants to put all motors on VFDs. We discovered two 5 Hp, 17.5 amp, 208 volt, 3 phase motors on one 50 amp breaker. Our VFD manufacturer indicates that a 5 Hp VFD has an input of 16.7 amps and requires a 30 amp breaker. The panelboard with the 50 amp breaker is full. No spare space. The wire appears to be #8 TW rated at 40 amps and can easily be replaced if need be. My question is, is the 50 amp breaker adequate or should it be increased to a 60 and replace the wire with #6 THHN, 75 degree C rated at 65 amps. If the 50 amp breaker is adequate, is the existing #8 TW wire adequate.?

    Also, as a side question, the motor is rated at 17.5 amps and the input to the drive is 16.7 amps. How do you ever get the full horsepower out of the motor?

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    Quote Originally Posted by bnbowen@uiuc.edu View Post
    On a current remodeling project our client wants to put all motors on VFDs. We discovered two 5 Hp, 17.5 amp, 208 volt, 3 phase motors on one 50 amp breaker. Our VFD manufacturer indicates that a 5 Hp VFD has an input of 16.7 amps and requires a 30 amp breaker. The panelboard with the 50 amp breaker is full. No spare space. The wire appears to be #8 TW rated at 40 amps and can easily be replaced if need be. My question is, is the 50 amp breaker adequate or should it be increased to a 60 and replace the wire with #6 THHN, 75 degree C rated at 65 amps. If the 50 amp breaker is adequate, is the existing #8 TW wire adequate.?

    Also, as a side question, the motor is rated at 17.5 amps and the input to the drive is 16.7 amps. How do you ever get the full horsepower out of the motor?
    How to get two circuits into a panel with open space for only 1 circuit - install a feeder and supply either another panel or taps for two separate circuits.

    How do you get full power when the drive input amps is less than motor amps - consider the nameplate amps of the motor includes whatever power factor the motor operates at when fully loaded. The drive input likely never sees this power factor, but the output likely does.

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    Quote Originally Posted by bnbowen@uiuc.edu View Post
    On a current remodeling project our client wants to put all motors on VFDs. We discovered two 5 Hp, 17.5 amp, 208 volt, 3 phase motors on one 50 amp breaker. Our VFD manufacturer indicates that a 5 Hp VFD has an input of 16.7 amps and requires a 30 amp breaker. The panelboard with the 50 amp breaker is full. No spare space. The wire appears to be #8 TW rated at 40 amps and can easily be replaced if need be. My question is, is the 50 amp breaker adequate or should it be increased to a 60 and replace the wire with #6 THHN, 75 degree C rated at 65 amps. If the 50 amp breaker is adequate, is the existing #8 TW wire adequate.?
    This is a feeder. The ampacity required is 16.7 + 1.25*16.7 or about 38 amps. The 50A breaker is probably OK for the feeder. I did not do the calcs but I would guess off hand it is allowed.

    Edited to add: Now that I think about it, you might be forced to use the FLA of the motor and not the VFD input current to determine the ampacity required. If that is the case, the FLA used has to come from the tables. Off the top of my head I don't know what it says for a 5HP 208V 3 phase motor. If it exceeds 17.7A you are out of luck.

    You would need to put some smaller OCPD downstream in front of each VFD if the maximum OCPD allowed for the VFD is 30A. A couple of 3 pole class CC fuse blocks should work fine.

    Quote Originally Posted by bnbowen@uiuc.edu View Post
    Also, as a side question, the motor is rated at 17.5 amps and the input to the drive is 16.7 amps. How do you ever get the full horsepower out of the motor?
    Power factor. The VFD input PF is close to 1 while the motor PF is not, so the motor current will always exceed the input amps of the drive.
    Last edited by petersonra; 02-02-14 at 04:32 PM.
    Bob

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    Quote Originally Posted by petersonra View Post
    Power factor. The VFD input PF is close to 1 while the motor PF is not, so the motor current will always exceed the input amps of the drive.
    Could be single phase in and 3-phase out.
    And you have to take into account distortion PF as well as displacement PF.

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    Quote Originally Posted by petersonra View Post
    This is a feeder. The ampacity required is 16.7 + 1.25*16.7 or about 38 amps. The 50A breaker is probably OK for the feeder. I did not do the calcs but I would guess off hand it is allowed.

    Edited to add: Now that I think about it, you might be forced to use the FLA of the motor and not the VFD input current to determine the ampacity required. If that is the case, the FLA used has to come from the tables. Off the top of my head I don't know what it says for a 5HP 208V 3 phase motor. If it exceeds 17.7A you are out of luck.

    You would need to put some smaller OCPD downstream in front of each VFD if the maximum OCPD allowed for the VFD is 30A. A couple of 3 pole class CC fuse blocks should work fine.

    Power factor. The VFD input PF is close to 1 while the motor PF is not, so the motor current will always exceed the input amps of the drive.
    It is not a feeder, or at very least is improper installation of feeders/branch circuits. If the 50 amp breaker is supplying the drives with no overcurrent devices in between then it is a branch circuit, though it probably is supposed to somehow end up being two branch circuits with the appropriate protection device, there is more than one way to do that as well making it so there is no single correct solution to the problem here.

    Quote Originally Posted by Besoeker View Post
    Could be single phase in and 3-phase out.
    And you have to take into account distortion PF as well as displacement PF.
    Single phase in and three phase out would easily give you different amp ratings on input/output of the drive, but would be higher input than output amps, OP has higher output amps (based on motor nameplate) than the input amps of his drive, but apparently both are rated same HP. About has to be differences in PF to come up with this for the same HP rating, or he has a motor that is more inefficient than the drive manufacturer ever thought possible.

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    Quote Originally Posted by kwired View Post
    It is not a feeder, or at very least is improper installation of feeders/branch circuits. If the 50 amp breaker is supplying the drives with no overcurrent devices in between then it is a branch circuit, though it probably is supposed to somehow end up being two branch circuits with the appropriate protection device, there is more than one way to do that as well making it so there is no single correct solution to the problem here.
    I suppose I agree with you. Sort of. It can't be a code compliant branch circuit or feeder circuit the way it is described.

    It can be made into a complaint feeder circuit fairly easily by adding fuses for each VFD.

    I think it could be made into a compliant BC by just changing the CB to a 30A unit. It might trip now and then but I think it would also be compliant. I would have to think about that one for a while.
    Bob

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    Quote Originally Posted by kwired View Post
    Single phase in and three phase out would easily give you different amp ratings on input/output of the drive, but would be higher input than output amps, OP has higher output amps (based on motor nameplate)
    Yes. Missed that.........

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    Quote Originally Posted by petersonra View Post
    It can't be a code compliant branch circuit or feeder circuit the way it is described.
    That is mostly what my point was. If you put separate overcurrent protection on each drive, the 50 amp protected portion is a feeder, and each drive is on a branch circuit. If you put both on a single 50 amp circuit, then it is a 50 amp branch circuit, but as is has other code issues.

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