I installed a 3 phase motor for an AHU, with a Fla of 52A. I used #4awg conductor size, the O.L. is set on the vfd for 70A, and my circuit bkr is set for 200A. The customer's engineer is questioning this configuration, even thou I refer him to the section 430.52 of 2002Nec, he still does not accept the installation, because the bkr is not protecting the conductors. Did I misunderstood this section, or Is he right on his demand?
single polyphase motor circuit protection.
- Thread starter joelfrank
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bphgravity
Senior Member
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- Florida
Re: single polyphase motor circuit protection.
Both the OL protection and the ground-fault, short-circuit protection seems a little high? The OL setting at around 135% of the FLA and the breaker is near 400%. What factors did you use to make the setting determination?
I would think that #6 conductors, 60A overloads, and a 125A breaker should be sufficent.
Both the OL protection and the ground-fault, short-circuit protection seems a little high? The OL setting at around 135% of the FLA and the breaker is near 400%. What factors did you use to make the setting determination?
I would think that #6 conductors, 60A overloads, and a 125A breaker should be sufficent.
Re: single polyphase motor circuit protection.
I used 300% for bkr for 160A adjusted on the 200a bkr. sorry bout that one. I agree on the high OL, but isn't the factor for the bkr 300%? And he will still say that a 125A bkr is too high for a #6 Awg. According to my nec interpretation this combination is OK, because it is a single motor circuit and not a branch circuit.
I used 300% for bkr for 160A adjusted on the 200a bkr. sorry bout that one. I agree on the high OL, but isn't the factor for the bkr 300%? And he will still say that a 125A bkr is too high for a #6 Awg. According to my nec interpretation this combination is OK, because it is a single motor circuit and not a branch circuit.
petersonra
Senior Member
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- Northern illinois
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- Semi-retired engineer
Re: single polyphase motor circuit protection.
430.2 Adjustable-Speed Drive Systems. The incoming branch circuit or feeder to power conversion equipment included as a part of an adjustable-speed drive system shall be based on the rated input to the power conversion equipment. Where the power conversion equipment is marked to indicate that overload protection is included, additional overload protection shall not be required.
I am not all that sure what this means, but it would appear to me that where an adjustable speed drive is used, the normal motor wire and ocpd sizing goes out the window, at least on the feeder side.
Also, generally speaking, the manual that comes with the VFD will tell you what size to make the branch circuit ocpd feeding the VFD. Since this is part of its UL listing, you have to follow that advice. I think some even tell you what size wire to use.
[ June 09, 2005, 04:05 PM: Message edited by: petersonra ]
430.2 Adjustable-Speed Drive Systems. The incoming branch circuit or feeder to power conversion equipment included as a part of an adjustable-speed drive system shall be based on the rated input to the power conversion equipment. Where the power conversion equipment is marked to indicate that overload protection is included, additional overload protection shall not be required.
I am not all that sure what this means, but it would appear to me that where an adjustable speed drive is used, the normal motor wire and ocpd sizing goes out the window, at least on the feeder side.
Also, generally speaking, the manual that comes with the VFD will tell you what size to make the branch circuit ocpd feeding the VFD. Since this is part of its UL listing, you have to follow that advice. I think some even tell you what size wire to use.
[ June 09, 2005, 04:05 PM: Message edited by: petersonra ]
Re: single polyphase motor circuit protection.
joelfrank,
I think your installation is just fine,the lock rotor amps on similar motors is 232amps,also the #4 wire can't hurt anything. The VFD has it's own motor protection? That would make sense as the motor uses varying amounts of current at varying speeds.
On the point of short circuit protection,you are allowed by NEC to size the c.b. to hold the starting current(232amps),what size c.b. would he be happy with? Maybe you should put it in and have him see what happens.I don't think he fully understands that the feed wires for this circuit are protected.
frank
joelfrank,
I think your installation is just fine,the lock rotor amps on similar motors is 232amps,also the #4 wire can't hurt anything. The VFD has it's own motor protection? That would make sense as the motor uses varying amounts of current at varying speeds.
On the point of short circuit protection,you are allowed by NEC to size the c.b. to hold the starting current(232amps),what size c.b. would he be happy with? Maybe you should put it in and have him see what happens.I don't think he fully understands that the feed wires for this circuit are protected.
frank
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
Re: single polyphase motor circuit protection.
the VFD typically provide ol protection to the motor.
the incoming ocpd provides protection for the branch circuit and the vfd itself.
the startup current to the motor is typically not as high as with across the line starters, becasue the vfd uses a fixed v/hz output so the starting voltage is very low.
in any case, the branch circuit will not see a huge increase in current on startup like it does on an across the line starter.
the VFD typically provide ol protection to the motor.
the incoming ocpd provides protection for the branch circuit and the vfd itself.
the startup current to the motor is typically not as high as with across the line starters, becasue the vfd uses a fixed v/hz output so the starting voltage is very low.
in any case, the branch circuit will not see a huge increase in current on startup like it does on an across the line starter.
tony_psuee
Senior Member
- Location
- PA/MD
Re: single polyphase motor circuit protection.
petersonra,
Tony
Tony
petersonra,
You are right, normal rules go out the window. What this means is you have to size everything for the VFD rating, not the connected load. VFD's are current rated devices, although they are marketed with HP or kW ratings.
Tony
Tony
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