Logic Behind Over Sized Fuses

<....must size....conductors based on the 150A potential current....>

The fuses are there to protect the wiring.

The locked-rotor current will be the same, regardless of whether the motor were operated DOL or VFD. So what's wrong with that?

The overload is there to protect the motor.

Just set the FLC in the VFD parameter list. What's wrong with that?

And what does ABB have to say on the topic?

Personally, I would have fused at 80A slow blow and used #4AWG, not 150A, which requires at least #1AWG. Conduit size goes from 1" to 1.5" also. Don't know where you got the idea, but there is no overload - only the fuses and the VFD setting for FLA. Again - what would be the logic behind choosing quick blow 150A instead of slow blow 80A?

If there is a scope for heat to trap inside the fuse body then an overrated size fuse (by AWG size ) may not be overrated. It may blow at lower current in a little bit more time and one can call it a crude slow blow fuse.

As quick as VFD's usually react to a problem, I am a bit surprised they need special fusing, but that is the most logical reason. Thank you.

In the NEC, motor wiring is sized at 1.25 x FLA, unless required to be larger on account of voltage drop. Fuses are sized 1.75 x FLA, or next higher standard size. Circuit breakers can be 2.25 x FLA. I am not 100% sure of those exact multipliers, but neither of them requires the wire to be sized above the original 1.25 x FLA. Bear in mind that the motor is further protected by an overload device, which also protects the wires. The higher fuse or CB rating allow for starting current, which is brief.

(This is for normal motors. There are different special rules for fire pump motors, which do not use overload protection.)

"In the NEC, motor wiring is sized at 1.25 x FLA, unless required to be larger on account of voltage drop."

But also in the NEC, VFD input wiring MUST be sized to 125% of the VFD Maximum Input Current regardless of the motor that is connected to it.

Bear in mind that the motor is further protected by an overload device, which also protects the wires.

Uh -no, unless you count the VFD as an overload. Customer European designed circuit has no overloads in it. We have fuses, VFD, contactor (w/o overloads) wired to kill output in an emergency stop, and wires to the motor. The VFD is the motor protection.

The higher fuse or CB rating allow for starting current, which is brief.

So does a 80A time delay fuse. Surprisingly, the cost is about the same, so why not? So, if I still size at 125%, guess I have no complaints.

the US we are required to size (and protect) the feeding conductors at 125% of the VFD maximum input current, there is no consideration for the motor FLA as far as the input wiring goes.

Therein lies my beef with the design. (note again I did not build this panel. The fusing and conductors into the VFD are by the customer's usual panel builder) The VFD is designed for anything from 50 to 100HP, so the potential input is a bit on the large size. But with a 50HP motor and the ability to program the VFD to that level of output and protection, why size the input protection and feeders so large?

Jraef think couldn't explain better. You just ignore (regarding before VFD wire) what's after VFD. It takes care of itself. All IGBTs are shortcirquit rated, cirquit logic is faster than needed and software takes care of pretty much everything. The only thing one could do to lower before VFD wiring cost, would be to use smaller fusing according to expected load. But that's something only VFD manufacturer could say, because he can warrant if smaller fuse could handle the inrush current, So let it be. Hope it's not a mile long wiring.

The Fuses are correct these are to allow 3 x the motor full load current on starting.

I think what you are concerned about is the cable size from the VSD to the Motor? If that is the case the cable required from the VSD to the motor is the cable required to supply full load current to the motor IE 60 amps. But if you are concerned about the cable supply to the VSD Panel, I would suggest the following: For protection of the supply cable to the VSD Panel Install you slow blow fuses at a lower factor, maybe 80-100amps the use a suitable supply cable for the fuses you are using, remember that these fuses are only protecting the supply cable.

The fast blow fuses that are incorporated in the VSD control panel are there only to protect the semiconductors within the VSD from failure due to a fault within the motor or motor supply cable.

Conclusion: Size your cables as you have always done, install slow blow fuses on the supply to the VSD Panel, do not change the VSD Panel Fast Acting Fuses, by doing this you have protected the intergerty of the supply cable and the VSD.

Cheers

Joe

Thank you. I can not change the arrangement ahead of the VFD as that was done by the customer's panel builder, and that is how it must stay. I have sized the conductors to the motor a bit over requirements to avoid a problem in the field. The end user is not the customer. The customer re-sells these process skids to other industrial manufacturers. The E-Stop contacor the panel builder installed after the VFD has the same wires into it that the panel builder ran into the VFD. (sized for 150A). They are huge compared to a #4AWG THHN (90 degrees) that 125% times 59.6A requires. I ran #2AWG THHN so that the end customer doesn't have a lot of silly questions about the difference in size of conductors feeding the motor. They are still smaller, but it is not as obvious.

"... why size the input protection and feeders so large?"

Because it's the law! From what I've read, the change was made to the code because in situations exactly as you describe, people were sizing VFD input conductors for the connected load, but later on someone else would see that the VFD could be used on a larger motor and just crank it up, paying no attention to the input cable sizing. YOU may know what you are doing, but the Code cannot assume that you will always be there.

Never happen on this beasty - this is a set machine for seperation on a frame work with piping and control, and the only seperator that will ever fit on there is what is in place and the motor would be very difficult to up-size, not to mention dangerous as the seperator spins at 10000 RPM. To change to a larger motor would be asking to grenade the device and all literature indicates this. As I understand from tales of ones that failed, the rotating portion can go through concrete block walls 100 feet away if it grenades. In a general plant use one assumes someone might want to upscale, but on a pre-built machine with definite dimensions and piping locations, it would be very expensive to try to up-size this. Much cheaper and safer to sell the entire unit and buy a larger one. I am not too concerned about someone up-sizing the motor.

Again, YOU know that, and I don't have reason to doubt you, but you asked a "why" question, not permission. "The Code" cannot make distinctions. If it is going to be inspected and you can speak with the inspector, you can state your case and request a variance.

Good luck with that.

Thank you. Remember what I said earlier - this is an European design. The panel shop is tied into an European Corp. too and builds to the European design, so American codes probably don't concern them. Technically the wiring is inside an industrial machine, not a plant, and they can fall back on NFPA 79 Electrical Standard for Industrial Machinery and get away with some apparent violations of NEC. The inspector won't be digging too far into this skid, as it is packaged machinery, any more than he would try to inspect the inside of an air conditioner. It's still safe, but due to space constraints some weird wiring practices are allowed. I have never done it, but know I can use the framing for conduit if I have to. (I follow NEC on power wiring) Now, some customers do require the panels to be UL inspected, but never is the entire package inspected. (by the way, these panels were not inspected before we recieved them and never will be) Panels that are inspected, are inspected before installation on the skid. So, if these were to be inspected, the 150A fuses would be required.

I suspect the real reasoning for the 150A fuses lies somewhere between the fellow who said it was to protect the semiconductors, and what you said, but not to follow code, but to allow the same panel to be dropped into the next size up of this skid. The designers, who used to build these themselves, are real big into "modular".

As a final wrap up, thanks to all who responded. Today everything changed. The unit is now to go to Europe, so any device not CE rated must be changed, and the VFD/motor combination, that is for 480VAC 60HZ, will now be changed to 400VAC 50HZ. As it stands, the panel builder will get the completed skid unit and change the VFD and motor at his shop, so the wiring to the motor, whatever it becomes, is now his problem.

Noooo... Phys. You are too good an engineer (and a human being) to say ..heh heh, not my problem any more. Aren't you a wee bit still worried that someone somewhere could get hurt ?

So...what can we do together to mitigate that risk ???

No - no problem passing it back to whence it came in the first place. The panel builder started this situation, and now he has it back in his hands. When the motor, and I now understand the gearbox interface too, is changed the location of the motor connection point will no doubt change and the panel builder will have to run new conductors to the motor. At that point it is his problem, as my company is totally out of the loop. This panel builder is a sub-division of a major European electrical manufacturer and no doubt can handle the wiring quite nicely to the now required European standards.

Ah, that's different. i worked for ABB and have lot of respect for their technical ability. No doubt they will ensure that all European codes will be respected. No worry here.

I want to add one condition

Look the VFD is there to control & protect the motor .Now the motor as usual will have definite FLA, upto which it can consume maximum.So what we need is the protection of this overload current which the vfd might do.

Now condition comes on the input side of the VFD. VFD might have a suitable eff correspondingly whatever will be the output current that VFD can deliver at the output as per mentioned in VFD product , VFD can consume current like " OUT CURRENT/ EFF" which might be a little bit larger than output.

Protection at the input of VFD ie a fuse or circuit breaker , as said is based on VFD input rating , but as we can see input is higher than output,. In case when an overload happens, since inout fuse is sized for VFD input doesnt it affect the motor since the input is higher ?