VSD supply cable sized to motor

[123MB]

Hi All

I have read several posts on here about sizing of VFD supply conductors... most of them deliberating the technical merits of the rule in the NEC stating the conductors need to be sized to the VFD, not the motor, with a 20% allowance (or similar)

I am in Australia and we follow IEC regulations, as far as I know there is no such requirement.

I am working for a customer who is a switchboard builder and is trying to save every $ they can. In addition, the designer has oversized the VFDs (in some cases, by 50%). So the switchboard builder does not want to size cables to the drive, they want to size to the motor (with an allowance for THD, drive and motor efficiencies, etc). This includes the line side protection as well as the cable.

So my question is, what is the technical feasibility of this approach. You guys have talked a lot about the NEC requirement, but I can't find where anyone takes the requirement apart and tries to determine WHY it exists.

All I can think of is the requirement to allow for charging the DC bus, but with soft charging circuits, this is not so much of a problem.

Can anyone think of why it would be technically incorrect to size line side cables and breakers to the motor size and not the drive. Assuming the motor size will never change.

Thanks.

 

[jraef]

This is a gross condensation of a few years worth of panel discussions within NFPA (our National Fire Protection Agency is who writes the National Electric Code) starting in the mid-90s, which culminated in the first iteration of that special conductor sizing rule being implemented in the 2002 NEC. As with most rules in the NEC, the nexus of them is usually investigations of the cause of fires, and was the case here. Why it exists here is for two reasons:

1) We require that ALL equipment branch conductors be sized at 125% of the equipment FLA rating as a minimum. The basis of that goes back decades but basically is because we have a conglomeration of different older voltage standards spread out over a wide geography, and legacy systems are not required to conform to current standards if already in existence. So I can have 480V in one town, and 440V down the road. The "fudge factor" allows for that, plus undetermined voltage drops and other things that tend to happen in real life.

2) If I have a 50HP VFD running a 30HP motor today, 4 years from now Joe Bubba electrician may come along and see the 50HP label on it and change out the motor to 50HP. Joe Bubba may have noticed that his motor leads were too small, but would ASSume, because of older motor starter wiring standards that were in place before VFDs existed, that a device rated for 50HP would be fed with conductors sized for 50HP. The NEC rule removes the possibility of that ASSumption being a problem by actually requiring it.




"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[dpc]

In NEC land, the circuit to the VFD is sized based on the VFD rating, for the reasons jraef stated. But what really happens is that the feeder breaker is sized, then the conductor is sized based on the breaker rating. But a typical MCCB can only carry 80% of its rating on a continuous basis (when in an enclosure). So breakers for loads considered continuous are sized at 125% of the maximum load current. The feeder conductors must be sized per the breaker rating, so they end up at 125% (roughly) of the load. Motor feeders (VFD to motor) are sized using a completely different method, although the end result is similar.

 

[iceworm]

Yes, occasionally I do spend time looking at the NEC trying to understand the physics behind the Code - in particular: What is it trying to protect us from? I always keep in mind: The NEC is the Law of Man. And, the Laws of Man do not trump the Laws of Physics.

Can anyone think of why it would be technically incorrect to size line side cables and breakers to the motor size and not the drive. Assuming the motor size will never change

Yes, there is one reason. The drives are not 100% efficient. NEC 430.122.A says to size the VSD incommer to 125% of the rated input current (VSD nameplate current). Even if the VSD were matched to the motor, the input current could easily be 5% higher that the motor FLA. Now, if one oversizes the VSD, what are you going to pick for a reasonable input value? Motor FLA by T430.250 + 5%? Motor FLA by motor nameplate +5%? Some other engineering judgement?

******
Lets take the case of a 480V, 40hp motor fed by a 60hp VSD. This meets your spec of the VSD oversized by 50%. For demonstration, I'll use a VSD rated input of T430.250 (for a 60hp) + 5% = 77A x 1.05 = 81A

The feeder CB is sized by 430.52, for a 40hp motor, TM cb, 2.5 x 52A = 130, next size up => 150A

The ampacity for the feeeder to the VSD is sized by rated input, 81A x 1.25 = 101, select #2CU => 115A

The ampacity for the feeeder from the VSD to the motor is sized per the motor FLA table for a 40hp motor, 52 x 1.25 = 62.5A, select #6CU => 65A

******
My personal opinion is this is not an NEC game of, "What IF??". Once the design goes out the door, there is nothing anybody, including the NFPA, can do to prevent non-compliant installation.

So, could you pick a smaller feeder to the VSD? Yes - you are not bound by the NEC. This isn't a physics question - it's a question based on the Laws of Man in your jurisdiction.

However, considering your customer is shaving the copper that close: Whatever you pick, I'd think you really want a well documented, defensible position.

ice

Harmless flakes working together can unleash an avalanche of destruction

 

[iceworm]

But what really happens is that the feeder breaker is sized, then the conductor is sized based on the breaker rating. But a typical MCCB can only carry 80% of its rating on a continuous basis (when in an enclosure). So breakers for loads considered continuous are sized at 125% of the maximum load current. The feeder conductors must be sized per the breaker rating, so they end up at 125% (roughly) of the load.

A different take on conductor/CB sizing in NEC land (as noted motor feeders are different):

Interestingly, "Ampacity" is defined as:

100.1 The maximum current in amperes that a conductor can carry continuously, under the conditions of use, without exceeding its temperature rating.​

Keep this in mind as I go forward.

Conductors are sized per NEC 210.19, Conductor Minimum Ampacity and Size (branch ckt - similar section for feeders). Paraphrased: 125% times the continuous load + 100% of the non-continuous load. (Hummm, so why is the continuous load added at 125% when the "Ampacity" is already defined as the "continuous" rating? Perhaps, as dpc said, "a typical MCCB can only carry 80% of its rating on a continuous basis (when in an enclosure).")

Overcurrent protection, CBs, are sized per 240.4, Protection of Conductors
Paraphrased: Conductors are protected in accordance with their ampacities. Next size up is often available.

So, one calculates the load. Selects a conductor such that the ampacity meets or exceeds the load - paying attention to continuous/non-continuous. Selects a CB such that the conductors are protected.

As for the CB 80% continuous load rating, and continuous/non-continuous load issues, one could look at UL489 as an example. For CBs 30A and smaller, in an enclosure, and at 40C ambient, the CB must never trip at 100%, and must trip within 1 hour at 135%. Consider this, the CB could trip in one second at 101%, or never trip at 134% - and meet spec.

My personal opinion is the NEC induced slop in the CB ratings/load calcs has to do mostly with the slop in the CB/fuse tolerances.

ice

Harmless flakes working together can unleash an avalanche of destruction