RTU voltage drop

[wire twister]

I've inherited a roof top unit that the vfd is constantly going down on under-voltage. It is a 208v 3 phase unit with a MCA of 335A. Right now it is being fed by a 200' parallel run of 1/0's in the same conduit. I already know that the 1/0's are not large enough for the load and need to be replaced. My problem is the buildings voltage is low to begin with, 197 volts on the lowest leg, and the power company is being less than helpful since we are right at the 5% mark. Doing the voltage drop calculations using the 3 phase average at the MDP, 199.33v, I can only afford to loose 1.73 volts max to keep the VFD from tripping, VFD is set at 5% and they say it can't be changed. I have come up with running parallel 500's for a voltage drop of 1.5v.

Now on to my questions

Parallel 500's seem excessive to me. My calculations were based on the MCA of the unit. Is this correct?

Would the VFD trip on phase to phase under-voltage or average 3 phase?

Any help would be great I'd hate to do the install using flawed logic and be no further ahead than when I started.

 

[topgone]

You have just mentioned that the voltage at the MDP is rather low @ 197V average! Why not solve that item first? If it is served by a transformer, why not schedule a change tap to raise the secondary voltage a bit? If it is not possible to do that, how about a 24-hour recording of the service parameters (kVA load, voltage, amps) to see if some things can be done and avoid re-wiring the 200-feet 1/0 cables?

As for VFDs with undervoltage cut-offs, you will have to live with it - supply the correct voltage to the unit/s. Other VFDs have computer-like interface and can be adjusted to fit the supply voltage on the fly, IMO.

 

[kwired]

Is POCO voltage measured while loaded or with light or moderate load? If it is low with light loads it likely will only get worse when more load is added. Maybe transformer is undersized to start with. Good luck convincing them this.

 

[wire twister]

The parallel 1/0 is too small to use. The ampacity of parallel 1/0 is 340A. Because they are parralled in the same conduit I have 6 current carriers so I have to derate. 340*.80=272 smaller than the 335A MCA requirement of the unit. Correct?

Phase average is 199.33v, the 197v is the lowest phase to phase, I'm assuming that the VFD is looking at the average because it will run, just trips on the dip in voltage. So if I did the math correct I need the parallel 500 to compensate. The power co is using the not my problem excuse.
I guess my main question is do I base the voltage drop on the MCA or some other value and am I correct that the VFD would be looking at the 3 phase average and not phase to phase voltage.

 

[wire twister]

Is POCO voltage measured while loaded or with light or moderate load? If it is low with light loads it likely will only get worse when more load is added. Maybe transformer is undersized to start with. Good luck convincing them this.

Measured with the building loaded. All the readings were taken during a service call with the building operational. 120/208v 3 phase service feeding a grocery store if that matters.

You're right the poco doesn't want to hear it. They can be within 5% of nominal and are right on the edge so they don't care.

 

[augie47]

Depending on the specifics of the conduit run a new or added conduit might be less expensive, but it that's a nightmare for some reason you might consider a transformer at each end of the run...step it up to 480 then back down. The transformer taps would also allow you to more closely meet the drive specs.

 

[mark32]

RTU = Roof top unit?

 

[MEP_PM]

RTU = Roof top unit?

You are correct.


wire twister: What type of drive? The low voltage alarm set point may not be available to the user but may be adjustable by a factory rep.

 

[steve66]

Wiretwister:

I agree with you about the 1/0 being too small. That's not enough capacity for a unit with a 335 MCA no matter what you do about the voltage drop. THe wire should have a capacity of 335 amps after any derating.

Still, with only 200V to start with, I wouldn't make any promises that increasing the wire size will fix the VFD faulting out.

 

[wire twister]

You are correct.


wire twister: What type of drive? The low voltage alarm set point may not be available to the user but may be adjustable by a factory rep.

Hvac tech can't or won't adjust it & the factory rep refuses to adjust it. As far as what type of drive I didn't pay much attention because it wasn't my equipment. I try to let the original installer (Hvac tech) make adjustments to their own equipment. I always thought that motors could tolerate +/- 10% but I guess this equipment either cannot or the factory will not allow it.

 

[wire twister]

I think the solution for now would be for me to lean on the power company to correct the low voltage condition. While I'm waiting I'll connect our meter so I can put together some facts to present to the power company incase they won't budge. Then after the poco corrects the low voltage I'll replace the 1/0's with a set of 500's and correct the other issues I found with the install.

Thanks for all the help and ideas.

One last question. When calculating voltage drop to a roof top unit should you use the nameplate minimum circuit ampacity or is there another value that should be used?

 

[iMuse97]

One last question. When calculating voltage drop to a roof top unit should you use the nameplate minimum circuit ampacity or is there another value that should be used?

IMO, that is what I would use.

 

[MEP_PM]

Hvac tech can't or won't adjust it & the factory rep refuses to adjust it. As far as what type of drive I didn't pay much attention because it wasn't my equipment. I try to let the original installer (Hvac tech) make adjustments to their own equipment. I always thought that motors could tolerate +/- 10% but I guess this equipment either cannot or the factory will not allow it.

I agree the HVAC tech should make any adjustments to their equipment. But it is common for me to hear something "can't" be done when it really can be. Either the person doesn't want to do it or doesn't know how. Also if the factory rep is for the RTU and not the VFD, they may not be an expert on the VFD.

The Danfoss VLT spec I looked at indicated it's rated +/-10%. The Allen Bradley Powerflex 700 indicated the shutoff/fault point was 153V and the trip point was 120V. If you can get the actual drive info, you should be able to confirm the actual requirements on the VFD companies website.

Are these problems happening when there are actual power outages/blinks or generator test? The drive could be picking it up as an under voltage. There may be drive setting that can change certain alarms from latching/non latching.

 

[kwired]

Are these problems happening when there are actual power outages/blinks or generator test? The drive could be picking it up as an under voltage. There may be drive setting that can change certain alarms from latching/non latching.

I think this is very likely. I have seen this many times when the supply voltage was not as questionable as it is in this case.

You also need to remember that most likely the alarm is triggered by the DC bus voltage and not the actual AC input voltage.

 

[tesi1]

voltage drop calc's

voltage drop calc's

it may not make much of a difference, but the voltage drop should be calculated from the total sum of the amprege loads from the nameplate of the equipment. if
you are calcaulating it from the mca you will be including the additional 25% of the largest motor or load and therefore increasing your wire size more than you may need.

 

[iMuse97]

it may not make much of a difference, but the voltage drop should be calculated from the total sum of the amprege loads from the nameplate of the equipment. if
you are calcaulating it from the mca you will be including the additional 25% of the largest motor or load and therefore increasing your wire size more than you may need.

In my experience, the MCA is calculated with the 125% of the largest motor load included in the number.