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#90474 - 11/22/04 08:23 AM 430.22, 430.24 Sizing branch circuits and feeders for VFDs
MikeK Offline
Junior Member

Registered: 04/14/04
Posts: 3
Loc: Richmond, Va, USA
A question has come up on a design for some panels with multiple Variable Frequency Drives and I was hoping you could provide a clarification. I am using the 2002 code as a reference. 1999 is still in effect in our area but they are basically the same. I have not gotten the 2005 version yet.

430.22 deals with sizing branch circuit conductors that supply a single motor. Normally the size of the conductor is based upon 125% of the full load current rating of the motor. Exception No. 2 states that when you are using and adjustable speed drive system such as a VFD you shall use 125% of the rated input to the power conversion equipment rather than the full load current of the motor.

I do not fully understand why they use the maximum rating of the drive rather than the actual load which is the motor but I can live with this section without problem. The issue is an interpretation of 430.24 which deals with several motors or motors and other loads. This would be the guidelines for sizing the feeder.

430.24 states that you use 125% of the full load current rating of the highest rated motor plus the sun of the full load current ratings of all the other motors in the group. It refers back to 430.6 A. Neither 430-24 or 430.6A make any mention of power conversion devices or adjustable frequency drives. (Although they are covered in 430.6C)

Here is where I get confused.

In order to minimize inventory we have standardized on 5 HP Variable Frequency Drives to be used for all motors from 1 HP to 5 HP. The rated input amps at 480 VAC for this drive is 9 amps. Lets take a worse case example. I have a panel with 10 VFDs all serving 1 HP motors. If I look at the full load amps of the motors I need to size the feeder for approximately 19 amps (10.25 x 1.8). If I need to be concerned with the input power to the conversion equipment then I need to size the feeder for roughly 92 amps (10.25 x 9). This is a HUGE difference.

My interpretation was that 430.24 which determines the feeder wiring does not mention power conversion devices so I did not need to use that information in sizing the feeder although I would need to use it in sizing the branch circuit. A design firm I have working for me feels that the feeder needs to be sized using the rated input power to the VFD. This results in much larger equipment. In this case we have existing motors utilizing contactors that are being replaced with VFDs. I would need to repull the feeder to the panels, replace the main disconnect on the panel, replace the feed to the panel in the MCC, etc. even though the actual load at the equipment has not changed at all.

The next step would deal with the overcurrent protection. I have been sizing the overcurrent protection for the motor and VFD based upon the FLAs of the motor and table 430.52. This has worked fine for years. There is a section 430.52-5 which allows you to use different fuses for power electronic devices. Sometimes some manufacturers recommend a quicker fuse to protect the electronics rather than the dual element time delay. Using the Bussman SPD Motor Circuit Protection Tables the optimal branch circuit protection for that 1 HP motor would be a 3 2/10 LPJ_SP fuse. This would never allow the 9 amps for the rated input power of the VFD to happen.

Finally, what I need is an answer in sizing the feeders. Do I use the FLA of the motor or the rated input power of the VFD? If it is the input power of the VFD, could you explain the logic behind this reasoning.

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#90475 - 11/22/04 12:41 PM Re: 430.22, 430.24 Sizing branch circuits and feeders for VFDs
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Note the FPN to this section.

 Quote:
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.

The disconnecting means shall be permitted to be in the incoming line to the conversion equipment and shall have a rating not less than 115 percent of the rated input current of the conversion unit.

FPN:Electrical resonance can result from the interaction of the nonsinusoidal currents from this type of load with power factor correction capacitors.


That is from the 2002 NEC and now the 2002 NEC Handbook commentary which is opinion.

 Quote:
The operating characteristics of the adjustable-speed drive systems may create harmonic currents that could excite a resonance condition in the circuit if capacitors are improperly applied for power factor correction. Power factor correction with harmonic filtering can safely be applied to this type of load through proper design. For specific details of the harmonic effects related to capacitors, the capacitor manufacturer should be consulted.


You may want to rethink using oversize VFDs due to these code sections.

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#90476 - 11/22/04 02:49 PM Re: 430.22, 430.24 Sizing branch circuits and feeders for VFDs
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
This particular discussion hits close to home for me, since I've been buried in the inner workings of VFDs for 5 of the past 10 or so years.

The problem with the section is that in many ways, a VFD is like a transformer rather than like utilization equipment. If you reduce the load connected to the output side of the VFD, then you will also end up reducing the current carried on the input side of the VFD.

On top of this, the VFD will eliminate _all_ of the 'inrush' current associated with starting a motor in the first place.

There are some loss terms that will scale with the size of the VFD; even if the load stays small, but I don't see this as a large factor.

If the drive includes programmable protection for the motor, and is programmed to protect a 1 hp motor, then it is not going to consume significantly more than 1hp of input power, even if the drive is _capable_ of running a 10hp machine.

-Jon

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#90477 - 11/22/04 03:12 PM Re: 430.22, 430.24 Sizing branch circuits and feeders for VFDs
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Jon I certainly could not argue with you on the workings of a VFD.

That said the NEC seems black and white on this issue.

I am sure the CMP had a reason for this requirement.

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#90478 - 11/22/04 03:33 PM Re: 430.22, 430.24 Sizing branch circuits and feeders for VFDs
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Joe Tedesco must have been watching as he was kind enough to send me an email with some new 2005 NEC sections.

My Thanks to Joe T.


2005 NEC
 Quote:
X. Adjustable-Speed Drive Systems

430.120 General.

The installation provisions of Part I through Part IX are applicable unless modified or supplemented by Part X.

FPN: Electrical resonance can result from the interaction of the nonsinusoidal currents from this type of load with power factor correction capacitors.

430.122 Conductors — Minimum Size and Ampacity.

(A) Branch/Feeder Circuit Conductors. Circuit conductors supplying power conversion equipment included as part of an adjustable-speed drive system shall have an ampacity not less than 125 percent of the rated input to the power conversion equipment.

(B) Bypass Device. For an adjustable speed drive system that utilizes a bypass device, the conductor ampacity shall not be less than required by 430.6. The ampacity of circuit conductors supplying power conversion equipment included as part of an adjustable-speed drive system that utilizes a bypass device shall be the larger of either of the following:

(1) 125 percent of the rated input to the power conversion equipment

(2) 125 percent of the motor full-load current rating as determined by 430.6

430.124 Overload Protection.

Overload protection of the motor shall be provided.

(A) Included in Power Conversion Equipment. Where the power conversion equipment is marked to indicate that motor overload protection is included, additional overload protection shall not be required.

(B) Bypass Circuits. For adjustable speed drive systems that utilize a bypass device to allow motor operation at rated full load speed, motor overload protection as described in Article 430, Part III, shall be provided in the bypass circuit.

(C) Multiple Motor Applications. For multiple motor application, individual motor overload protection shall be provided in accordance with Article 430, Part III.

430.126 Motor Overtemperature Protection.

(A) General. Adjustable speed drive systems shall protect against motor overtemperature conditions. Overtemperature protection is in addition to the conductor protection required in 430.32. Protection shall be provided by one of the following means.

(1) Motor thermal protector in accordance with 430.32

(2) Adjustable speed drive controller with load and speed-sensitive overload protection and thermal memory retention upon shutdown or power loss

(3) Overtemperature protection relay utilizing thermal sensors embedded in the motor and meeting the requirements of 430.32(A)(2) or (B)(2)

(4) Thermal sensor embedded in the motor that is received and acted upon by an adjustable speed drive

(B) Motors with Cooling Systems. Motors that utilize external forced air or liquid cooling systems shall be provided with protection that shall be continuously enabled or enabled automatically if the cooling system fails.

FPN: Protection against cooling system failure can take many forms. Some examples of protection against inoperative or failed cooling systems are direct sensing of the motor temperature as described in 430.32(A)(1), (A)(3), and (A)(4) or sensing of the presence or absence of the cooling media (flow or pressure sensing).

(C) Multiple Motor Applications. For multiple motor application, individual motor overtemperature protection shall be provided.

FPN: The relationship between motor current and motor temperature changes when the motor is operated by an adjustable speed drive. When operated at reduced speed, overheating of motors may occur at current levels less than or equal to a motor’s rated full load current. This is the result of reduced motor cooling when its shaft-mounted fan is operating less than rated nameplate RPM.

(D) Automatic Restarting and Orderly Shutdown. The provisions of 430.43 and 430.44 shall apply to the motor overtemperature protection means.

430.128 Disconnecting Means.

The disconnecting means shall be permitted to be in the incoming line to the conversion equipment and shall have a rating not less than 115 percent of the rated input current of the conversion unit.


The NEC in 2005 will still require the feed to a VFD to be sized to the input rating.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#90479 - 11/22/04 03:40 PM Re: 430.22, 430.24 Sizing branch circuits and feeders for VFDs
Joe Tedesco Offline
Member

Registered: 10/07/00
Posts: 3325
Loc: Boston, Massachusetts USA
 Quote:
430.2 Definitions.

Adjustable Speed Drive. A combination of the power converter, motor, and motor mounted auxiliary devices such as encoders, tachometers, thermal switches and detectors, air blowers, heaters, and vibration sensors.

Adjustable-Speed Drive System. An interconnected combination of equipment that provides a means of adjusting the speed of a mechanical load coupled to a motor. A drive system typically consists of an adjustable speed drive and auxiliary electrical apparatus.
_________________________
Joe Tedesco, NEC Consultant

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#90480 - 11/23/04 04:30 AM Re: 430.22, 430.24 Sizing branch circuits and feeders for VFDs
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
Bob,

I certainly agree that the NEC is _very_ clear on the position that it takes. I am curious as to _why_ the CMP takes this position.

'The rated input to the power conversion equipment' is a somewhat fuzzy number, kind of like the 'current rating' of a bit of wire. It is related to things such as how you define overload, how you define ambient conditions, and what you expect the life and reliability of the equipment to be. Take the same piece of hardware, but change the label to require operation in a 20C air-conditioned room rather than a 40C room, and the electronics will be able to tolerate higher operating currents.

The answer probably lies with UL requirements for how the power conversion equipment gets rated, and also with the fact that the manufacturer probably rates the drive under maximum load only, not under reduced load. In other words, the 'problem' created by MikeK's installs may not be a real safety issue, but instead a lack of data issue.

It may be the case that if the VFD drive manufacturer is willing to supply test data and a label that says:

with load XX the input current rating is YY
with load XXX the input current rating is YYY
with load XXXX the input current rating is YYYY
the maximum load served is XXXXX with an input current rating of YYYYY,

then using an oversized inverter with non-oversized wiring would be acceptable _under the current rules_. However I have never seen such a label, and the drive datasheets that I have looked at all specify maximum allowable currents.

Okay, here is a funny on the issue: If I have a VFD rated at 50A _at sea level_, and I install it in a mountain location at 9000 feet, and the manual specifies that the maximum allowable current is to be reduced by 10% for each 3000 feet, then what is the rated current number that I should use for the purpose of this code section? (Note: I cannot recall a drive manual that specifies this, however as I stated above the current handling capacity of the power electronics depends upon cooling, and I have seen manuals for systems that require derating with altitude because of reduced cooling. So this is a doubly hypothetical situation.)

Something that I've never done, but which really is the correct way to find the answer to this question: how do I trace back the history of a code section, including the CMP discussions?

-Jon

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#90481 - 11/23/04 04:36 AM Re: 430.22, 430.24 Sizing branch circuits and feeders for VFDs
MikeK Offline
Junior Member

Registered: 04/14/04
Posts: 3
Loc: Richmond, Va, USA
Thanks for the responses. I found a draft copy of the 2005 NEC (I have not purchased it yet, since we still are using the 99 code). I did find article 430-122 and it now refers to feeder/branch circuits.

I wish I knew their reasoning. In contrast we use minimum size 1 starters. They are good for 10 HP. The code does not require me to size for a 10 HP motor. I really do not see much difference with a VFD.

It appears that we end up wasting a lot of electrical resources by sizing this way. Looks like I get to submit a request for change for the 2008 code. Don't think it will do much good, none of my other ones have been successful but I can't complain if I don't at least try.

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