OCPD Sizing for Many VFDs

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drewstg

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I have been reviewing every bit of the NEC code possible to determine how to minimize the OCPD (Over Current Protection Device) size for an electrical panel containing many 2 HP motors controlled via VFDs (Variable Frequency Drive).

Typically over 50 3 HP VFDs will be used to control 1 to 2 HP motors on a 480 VAC 3-phase system. Both NEC and NFPA 79 state that the OCPD should be sized to be no smaller than the total FLA calculated + 125% FLA of largest load.

Assume a 3-HP VFD is rated at 5.6 FLA input while only driving a 1-HP motor at 1.85 FLA. Using the VFD rating for 50 motors plus a small 120 transformer, the OCPD required is larger than 350 Amps however, using real world continuous running loads the draw is never more than 130 Amps. Is there a way we can get an exception to NEC or NFPA79 based on historical data and use a smaller OCPD?

Thanks for your help,
Drew
 
Anyone can submit a proposal for a rule change for the next NEC. I think the cutoff date has passed for the 2008, though.

Why can't you just use 1 and 2 HP VFD's?

Steve
 
drewstg said:
Both NEC and NFPA 79 state that the OCPD should be sized to be no smaller than the total FLA calculated + 125% FLA of largest load.
Drew
Click to expand...
What your are addressing is 430.24 which says the CONDUCTORS shall have an ampacity of 125% of the largest motor plus the sum of the FLA of the other motors. Table 430.52 addresses the breaker size for the motors.
There is no minimum listed in the NEC. 430.6.C addresses VFD motors. As
long as you meet the conductor requirements, I believe you can reduce the breaker rating as you see fit.
 
430.6(C) References motor or control nameplate, or both.

Use the motor nameplate.

Treat it as a multimotor application.

Set the OC protection on each drive to the motor nameplate.

Size individual motor/drive conductors per 430.122(A).

Provide individual O/C protection for each VFD *** Forgot this

Size conductors for the multimotor feeder per 430.24

Size the main OC breaker per 430.62


Have I missed something?
 
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I'm probably wrong here or misunderstanding. But is the ocp for the VFD? In other words, on the line side of the VFD? If it is it needs to be sized according to manufacturer specs. If it's downstream then it will be differant.
 
First off everything in the system is either protected individually or by using group motor fusing guidlines...and of course the VFD output overload is sized for the motor nameplate rating as not to burn up the motor. That aside...

I think Bob may be on to something by following the NEC minimum conductor ampacity requirement to prevent overheating of the wires but simply installing a lower amperage UL 508 circuit breaker disconnect for the panel. Most of the modular MCCBs we use as the main OCPD can have a frame of lets say 600 Amps with just about any smaller size trip unit installed.
 
I just discovered another thread posted with the same question in November of 2004.

Here is a quote from that post...
mikek2 said:
The losses in the VFD are no where near enough to justify sizing it on input power. I found a copy of the 2005 code and it clarifies some issues. It states in article 430-122 that branch and feeders be sized based upon 125% of the input current rating of the power conversion device.
Click to expand...

It sounds like this addressed the requirement for the power feed. It really is unfortunate though that NEC is requiring existing systems to have larger power feeds simply by adding VFDs or systems using 3 HP VFDs controlling 1HP motors to have unused power. I don't think that NEC understands the application of multiple VFDs. This is costing companies big bucks. Hopefully someone has placed a request for a change in 2008 NEC.
 
You can make your fuses smaller, as long as they don't trip spuriously. The wires have to be as fat as they say to handle locked rotor. With the "smarter" drives, motors are less likely to sit there and cook themselves. The disconnect also has to open, so it has to be bigger.

The same guy who complains about the extra cost will sue you if his factory burns down if you acquiesce...

Matt
 
drewstg said:
I have been reviewing every bit of the NEC code possible to determine how to minimize the OCPD (Over Current Protection Device) size for an electrical panel containing many 2 HP motors controlled via VFDs (Variable Frequency Drive).

Typically over 50 3 HP VFDs will be used to control 1 to 2 HP motors on a 480 VAC 3-phase system. Both NEC and NFPA 79 state that the OCPD should be sized to be no smaller than the total FLA calculated + 125% FLA of largest load.

Assume a 3-HP VFD is rated at 5.6 FLA input while only driving a 1-HP motor at 1.85 FLA. Using the VFD rating for 50 motors plus a small 120 transformer, the OCPD required is larger than 350 Amps however, using real world continuous running loads the draw is never more than 130 Amps. Is there a way we can get an exception to NEC or NFPA79 based on historical data and use a smaller OCPD?

Thanks for your help,
Drew
Click to expand...
I thought that the 2005 NEC changed this to where the VFD's were treaded more like resistive loads instead of motors. Like you said, you have to use the nameplate of the VFD instead of the motors served. If you use 5 HP VFD's for all the motors (because of reduced parts inventory or because the cost the same as a 2 HP but don't bog down as easy), then I think you've bought yourself a higher (calculated) conductor load.

Please correct me if I'm wrong.

Sorry, I don't have my 2005 NEC handy. That would make me a geek! . . . Unlike staying up till 11pm on a Friday night, posting to an NEC forum. . .:eek:
 
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