430.22, 430.24 Branch Circuit and Feeder Circuit sizing

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mikek2

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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.
 
Re: 430.22, 430.24 Branch Circuit and Feeder Circuit sizing

A few quick comments (I am not near my NEC so, I can't quote sections).

VFDs are not 100% effcient so they will always draw more than the motor alone.

Fuse protection for the VFD must take into account the short circuit rating as well as the startup current of the electronics. You usually need to use the fuse sizes recommended by the VFD manufacturer to prevent nusiance tripping on startup. These "standard" fuses typically only prevent catastophic failure of the VFD (special high speed electronic fuses are not normally used with small VFDs). These fuses satisfy the NEC requirement for the motor short circuit protection.
 
Re: 430.22, 430.24 Branch Circuit and Feeder Circuit sizing

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.

Not what I wanted to hear but it does remove any question.

The VFD manufacturer does recommend some "unusual" fusing for their VFDs. They are typically quick acting rather than time delay and sized larger, ampere wise, than the code or Bussman recommendations. These can be used but we have consulted with the rep and we have been using RK1 type Bussman fuses instead. They have been satisfactory in protecting the drive and meeting code requirements.

Appears to me that the way the code is written you are wasting a lot of equipment capacity. Your distribution equipment has "imaginary" load on it that is committed for something that will never happen. Guess I need to turn in a request for change in the 2008 code. :roll:
 
Re: 430.22, 430.24 Branch Circuit and Feeder Circuit sizing

I think the reason for 430.22 exception #2 is that motors on VFD's may be upsized without changing the VFD feeder. Imagine your 1HP motor on a 5HP VFD. The motor burns out once, and someone decides to just replace it with a 2HP. It is unlikely they would replace the wiring feeding the VFD, so the code requires the wiring to the VFD to be sized for the maximum motor the VFD can run.

But 430.22 only deals with a single motor. I don't see any requirement in the NEC to size the feeder for a group of motors for the sum of the VF input ratings. It is unlikely that all the motors out of one panel would be upsized from 1HP to 5HP. One or two, maybe, but probably not all. I would size the feeder for one 5HP motor and all the rest 1HP motors. Then I might upsize it somewhat depending on how much I thought motors might be upsized.

STeve
 
Re: 430.22, 430.24 Branch Circuit and Feeder Circuit sizing

There is no imaginary load involved. However the code recognizes that you might at some point decide to upsize the motor (a pretty common situation with VFDs) so requires that the feed to the VFD accomodate the maximum the VFD might end up using.

For the VFD sizes you are using this should not cause you a whole lot of grief.
 
You're wanting to bend the NEC to fit your odd design

You're wanting to bend the NEC to fit your odd design

Don't take that personally about the odd design. I'm just saying that, for whatever reasons, you and/or your customer have decided to install 5 HP VFD's when you only have 1 HP motors. You can't really expect the NEC to bend around such a decision.

The fact is that the nameplate input amperage for all VFD's is considerably higher that the NEC table amperage for a motor with the same nominal horsepower. So even though I don't think the code writers did a very good job stating it, I believe that the intent in 430.122(A) is clear and that logic says you should also use the drive nameplate rating when sizing a feeder for a multi motor circuit.

Although in this particular case, I can understand how "unfair" that seems since you are only putting 1/5 of the drives' capability on line, that is a very unusual situation. In general it makes perfect sense to use the higher load rating of the drive rather than the NEC table ratings.

Has anybody had a clear answer from the NFPA on which value you should use when calculating feeder ampacity for a mutiple motor feeder? As MikeK2 says, the section on calculating feeder ampacity for multimotor loads does not mention VFDs. While 430.122 (A) does say "Feeder", the text of it does not seem to address multimotor feeders.

I've seen plenty of threads here where the advice is "stick to what the code says, not what you think it might mean", but in this case, it looks like the code fails to say anything.
 
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This thread is almost 2 years old.


11-23-2004
 
iwire said:
This thread is almost 2 years old.


11-23-2004
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Thanks for the wakeup call Bob. I thought that while reading this I was having some sort of weird deja vu.
 
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