Oversizing Drive due to Thermal Considerations

[einnh]

If a drive has to be oversized (125A -> 156A) due to the ambient temperature around it. Does the upstream components require the same oversizing?

I assumed the over-sizing requirement was so the components in the drive could operate at the 125A range at the desired temperature. From a mechanical standpoint I don't see how the system could use over 125A so why oversize the drive upstream components?

Does this even make sense?

 

[rQx]

The upstream component has its own max temperature to be considered, the VFD ( I suppose that is what we are talking about) is mostly the weakest point.

 

[einnh]

The upstream component has its own max temperature to be considered, the VFD ( I suppose that is what we are talking about) is mostly the weakest point.

You are correct, but assuming those upstream thermal limitations were accounted for I don't see a reason to up-size the conductor/reactor/contactor sizes to meet the new current rating of the drive.

Thank you.

 

[rupej]

Yes, the upstream components need to be sized higher because the drive is capable of drawing more current. This could be due to a motor change "Oh look, we can install a bigger pump because the drive is oversized!" or due to an overload situation.

Leaving it with an undersized feed is just poor design, unless the cost is a huge issue.

 

[Mark Snodgrass]

Is this an Allen Bradley? If so you may have to up-size. If you look in the Technical Data book for the drive series you are using there is a table with breaker and fuse sizing.

Just a quick look at the table for a PF700, the input amps rating goes from 117 to 131 when increasing from a 125A to a 156A both driving a 100hp motor.

Your need to upgrade will depend on what is already installed. As I understand the NEC (and I am no expert), you can under size overcurrent protection as long as your wires to the device are protected. Now that may mean you get nuisance tripping because the load draws more current than the breaker setting. If you replaced a 15A breaker with a 5A on a receptacle circuit at your house, you are still protected but the vacuum will trip the breaker almost immediately, but a 120W lamp wouldn't.

So the answer is maybe, the up-sized drive may run fine with the same input size. But then again it will draw more current and may keep tripping your fuse or breaker.

 

[GTUnit]

Always look at your application from a "big picture" standpoint and not "just what I'm doing now".
Rupei has it exactly right.
What if 5 yrs from now someone fixes the cooling and puts a bigger motor on the drive.
They will ASSUME the power feeds are sized accordingly for the drive (as they should be and code requires) and they will run the bigger motor.
The conductors can then overheat and cause a fire.
Power feeds and conductors should be sized to the MAX FLA of the drive.
You then handle overload protection with the circuit protection device.

 

[rQx]

Always look at your application from a "big picture" standpoint and not "just what I'm doing now".
Rupei has it exactly right.
What if 5 yrs from now someone fixes the cooling and puts a bigger motor on the drive.
They will ASSUME the power feeds are sized accordingly for the drive (as they should be and code requires) and they will run the bigger motor.
The conductors can then overheat and cause a fire.
Power feeds and conductors should be sized to the MAX FLA of the drive.
You then handle overload protection with the circuit protection device.

Maybe it's not my area since this applies to the american market but if I understand you correct I both agree and disagree with you.

The wires after the fuse and before the drive is sized according to the fuse and not the drive. If you then put a larger motor on the drive, the wires cannot overheat since they are protected by the upstream fuse.

I agree that it can be a good idea to size upstream components according to the larger drive since it will make a future motor "upgrade" easier. But then it would have to go all the say back to the power source witch also demands larger cables to the machine etc etc.

 

[jraef]

The NEC will REQUIRE that the conductors be sized for the max input amps of the VFD, regardless of the size of the motor it is connected to. So on a 156A rated PowerFlex drive, the maximum input amps is 146.5 (it is lower than the output because the output amps includes power factor, input does not). Article 430.122 (if you are on the 2011 code still) then says your conductors must be rated for 184A, making them; 3/0 in must cases and that's as far as you need to take it.

If however your area has adopted the 2014 code, then a new addition is Article 430.128, which dictates that the DISCONNECTING device must also be sized based upon the VFD max amp rating, not the motor. Then 430.130 goes on to say that the OCPD can be sized per the motor, or per the VFD mfr requirements as stated. If you are using a Circuit Breaker and both the OCPD and the disconnect, you have to pay attention to the frame rating of the breaker, not just the trip rating. Generally you will end up there anyway, just pay attention to it.

So bottom line if we assume this is a 100HP 460V motor with a chart FLA of 124A (because you said "125A"), the NEC per 430.52 would allow a CB to be as high as 300A for the MOTOR. The VFD is rated for 156A, so (again assuming A-B), the MAXIMUM breaker size per the instruction manual is 450A for the VFD, so a 300A CB would be no problem and since the input amps for the 156A drive is 147A, 115% of that is only 169A so again, no problem on the 430.128 disconnect sizing either.

If you use fuses though, it's a little different. The max. time delay fuse size for the motor per 430.52 is 217A (round up to next size so 225A). But for the 156A VFD, the MINIMUM fuse size is 200A, so I would use the 200A fuse in a 200A disconnect to avoid having to go with a 400A disconnect to hold 225A fuses.

 

[Mark Snodgrass]

RQX, that is what I was getting at. As long as the upstream current protective device (fuse or breaker) is sized correctly for the wire going to the drive, it doesnt matter what size the drive is after. You could put a 1000A drive on there and all it would do is trip the protective device, the wires would still be protected. The only reason he may need larger wire and breaker/fuse on the input is that the larger drive has a higher parasitic load on it. The motor load is still the same.

Allen Bradley specs the max breaker size for a given drive, not a minimum. In most cases if the max size was used originally then you could increase the drive by one size and still be able to keep the same input configuration. I have done that for a drive I had that kept failing due to high cycling and needed to go to the higher size to handle it.

 

[Mark Snodgrass]

jraef, I will be honest and say I have never actually read that part of the code before. I am not even sure how long it has been in there, I never have taken a code class or had a contractors license as I have always worked for a manufacturing plant and I did not require one.

 

[NetNathan]

The NEC will REQUIRE that the conductors be sized for the max input amps of the VFD, regardless of the size of the motor it is connected to. So on a 156A rated PowerFlex drive, the maximum input amps is 146.5 (it is lower than the output because the output amps includes power factor, input does not). Article 430.122 (if you are on the 2011 code still) then says your conductors must be rated for 184A, making them; 3/0 in must cases and that's as far as you need to take it.

If however your area has adopted the 2014 code, then a new addition is Article 430.128, which dictates that the DISCONNECTING device must also be sized based upon the VFD max amp rating, not the motor. Then 430.130 goes on to say that the OCPD can be sized per the motor, or per the VFD mfr requirements as stated. If you are using a Circuit Breaker and both the OCPD and the disconnect, you have to pay attention to the frame rating of the breaker, not just the trip rating. Generally you will end up there anyway, just pay attention to it.

So bottom line if we assume this is a 100HP 460V motor with a chart FLA of 124A (because you said "125A"), the NEC per 430.52 would allow a CB to be as high as 300A for the MOTOR. The VFD is rated for 156A, so (again assuming A-B), the MAXIMUM breaker size per the instruction manual is 450A for the VFD, so a 300A CB would be no problem and since the input amps for the 156A drive is 147A, 115% of that is only 169A so again, no problem on the 430.128 disconnect sizing either.

If you use fuses though, it's a little different. The max. time delay fuse size for the motor per 430.52 is 217A (round up to next size so 225A). But for the 156A VFD, the MINIMUM fuse size is 200A, so I would use the 200A fuse in a 200A disconnect to avoid having to go with a 400A disconnect to hold 225A fuses.

I don't think the interpretation of NEC here is correct.
If I use a 10HP VFD on a 7.5HP motor, I would size all conductors and protection for protecting the 7.5HP motor.
In the VFD setup, all settings are now made for the 7.5HP motor voltage and current ratings, so now the drive is a 7.5HP drive.
The drive is now actually "de-rated".

an example...
If I use an over sized contactor to start a motor....am I supposed to protect the contactor or the motor? Obviously the motor and because the contactor is over sized, it will be easily protected also.

 

[GTUnit]

I don't think the interpretation of NEC here is correct.
If I use a 10HP VFD on a 7.5HP motor, I would size all conductors and protection for protecting the 7.5HP motor.
In the VFD setup, all settings are now made for the 7.5HP motor voltage and current ratings, so now the drive is a 7.5HP drive.
The drive is now actually "de-rated".

an example...
If I use an over sized contactor to start a motor....am I supposed to protect the contactor or the motor? Obviously the motor and because the contactor is over sized, it will be easily protected also.

jraef is correct. There is no interpretation of the NEC code here. It is clear.
If you did it this way it would be a code violation.
The logic is flawed. The motor is not the load. The drive is the load and the protection devices are protecting it and not the motor.

 

[NetNathan]

jraef is correct. There is no interpretation of the NEC code here. It is clear.
If you did it this way it would be a code violation.
The logic is flawed. The motor is not the load. The drive is the load and the protection devices are protecting it and not the motor.

I sort of follow your logic.

What would it hurt to put 10A of protection on a 15HP drive that has been de-rated to only pull 10 amps? I do not see a danger here.

 

[GTUnit]

I sort of follow your logic.

What would it hurt to put 10A of protection on a 15HP drive that has been de-rated to only pull 10 amps? I do not see a danger here.

I know what you are saying but keep in mind that NEC code does have a problem with doing it that way. There are a lot of things I would like to do my way. In this case, since there are legal issues involved and Building Code to follow, its best to give advice following the "correct" /legal way of doing things.
May save someone a lawsuit later.

BTW Mike Holtz forums are a great resource on NEC and NFPA code/wiring questions. Those guys hash this stuff out daily.

 

[NetNathan]

NEC really gets on my nerve sometimes. I understand safety but sometimes the rules makes no sense or are overkill.

I use a cooling motor in a vacuum furnace.
The motor runs in a pure argon atmosphere enviroment.
Because it is in Argon it must be rated at 230V by NFPA 86. )I believe this is related to the fact that argon gas can be "charged" to carry current, so higher voltages MAY cause the windings to "flash over".)
Now NFPA is fire protection code......in a pure argon environment, how is a motor going to catch fire?
I have several jobs overseas that are running 415V motors for years with no issues..... in an argon enviroment.