Wire size and ocp for VFD

[fbhwt]

Trying to figure out the proper wire size and over current protection for this VFD on this air handler, the electrical contractor wired this with 3-#6awg and #12 ground by way of 50a breaker. The drawings for this building show this air handler (AHU-1) with 1 20hp motor but, there are 2 20hp motors. I am being told by an engineer that I need 3 #4awg and #6g, OCP at panel to be 110a. The picture is from the drive cabinet.

 

[iwire]

I see input power as 172 amps.

 

[fbhwt]

I see input power as 172 amps.

170.2 I noticed that also

 

[dkidd]

SCCR is only 5K. I'll bet that is also a problem.

 

[iwire]

SCCR is only 5K. I'll bet that is also a problem.

Great point.

I agree a 170+ amp 480 volt circuit will likely exceed 5K.

 

[fbhwt]

Great point.

I agree a 170+ amp 480 volt circuit will likely exceed 5K.

Would you mind explaining what this means?

 

[iwire]

Would you mind explaining what this means?

The HVAC unit is only tested and listed for use on circuits that don't exceed 5,000 amps of fault current. There is a good chance the circuit required for the unit could exceed 5,000 amps of fault current.

Keep in mind that no breaker or fuse opens immediately, during the time between the fault happening and the breaker or fuse opening the current jumps well above the breaker or fuse rating only limited by the impedance of the circuit.

 

[iwire]

Here is a recent thread about this very problem.

It boils down to the manufacturer being cheap and passing the problem to the EC or engineer

http://forums.mikeholt.com/showthread.php?t=175987

 

[fbhwt]

The HVAC unit is only tested and listed for use on circuits that don't exceed 5,000 amps of fault current. There is a good chance the circuit required for the unit could exceed 5,000 amps of fault current.

Keep in mind that no breaker or fuse opens immediately, during the time between the fault happening and the breaker or fuse opening the current jumps well above the breaker or fuse rating only limited by the impedance of the circuit.

Maybe this would explain why the drive burnt up?

 

[infinity]

Conductors are sized based on the 125% of the input current of the drive.

430.122 Conductors — Minimum Size and Ampacity.
(A) Branch/Feeder Circuit Conductors. Circuit conductors supplying power conversion equipment included as
part of an adjustable-speed drive system shall have an ampacity not less than 125 percent of the rated input current to
the power conversion equipment.

 

[petersonra]

Here is a recent thread about this very problem.

It boils down to the manufacturer being cheap and passing the problem to the EC or engineer

http://forums.mikeholt.com/showthread.php?t=175987

If the manufacturer was not "cheap" they would go out of business. The HVAC market is almost completely price driven, and most of the time the people buying HVAC equipment are clueless about SCCR issues so they buy the cheapest thing they can find.

 

[iwire]

And if I can't wire it they won't sell any.

The thing that has changed is enforcement

 

[LMAO]

So if someone is stuck with this HVAC how would day get around this problem? Current limiting fuses? What is the practical solution?

 

[Jraef]

Trying to figure out the proper wire size and over current protection for this VFD on this air handler, the electrical contractor wired this with 3-#6awg and #12 ground by way of 50a breaker. The drawings for this building show this air handler (AHU-1) with 1 20hp motor but, there are 2 20hp motors. I am being told by an engineer that I need 3 #4awg and #6g, OCP at panel to be 110a. The picture is from the drive cabinet.

So I'm a bit confused here about what you are doing. Are you splitting out just the VFD or feeding the entire Air Handler? The AHU is a unit, which clearly states that the total FLA is 170.2A. But looking at the breakdown, it does appear as though there is a motor rated 27A and something else, likely a heater coil. There may be more than one motor as well, but it may be that it never runs more than one at a time. We just don't know. That nameplate gives you some fuse sizes, 100A Class J for one circuit, 40A for the other, but references a drawing that we can't see. You need to study that drawing to understand what's going on. But really, a 60A feeder seems too small for the stated total load.

 

[fbhwt]

So I'm a bit confused here about what you are doing. Are you splitting out just the VFD or feeding the entire Air Handler? The AHU is a unit, which clearly states that the total FLA is 170.2A. But looking at the breakdown, it does appear as though there is a motor rated 27A and something else, likely a heater coil. There may be more than one motor as well, but it may be that it never runs more than one at a time. We just don't know. That nameplate gives you some fuse sizes, 100A Class J for one circuit, 40A for the other, but references a drawing that we can't see. You need to study that drawing to understand what's going on. But really, a 60A feeder seems too small for the stated total load.

The VFD drive 2 20hp motors, the engineer explained it to me like this: The drive is rated 50 HP. The Load for a 50 HP is 65 AMPS. The overcurrent protection for a 50 PH motor across the line at 460 is typically 2.5 * LOAD=162.5 amps. So; we are about the rating for the unit which is 170.2. The breaker in this case will be 175A.

You can buy the drive with a By-Pass Panel. The motor can start across the line at 2.5 times the load instead of using the VDF to start at 1.25 times the load.

Let me know if you have any more questions.

In our case. The unit has 2-20HP units. The across the line starting of those units is much lower than the 50 HP. The typical load for a 20HP is 27A. The across the line starting for one motor is 2.5*27=67.5A. If you add the other motor load that's 67.2+27=94.5A. So the breaker in this case is 100A. The manufacturer allows us to use a 110A breaker.

 

[Jraef]

So are you or are you not going to utilize the bypass option? If not, then the across-the-line issues become irrelevant. The drawing you posted doesn't show a bypass, it only appears to be using the VFD to start and control the motors. So all you need to do is size the circuit to feed the VFD based on the NEC rule for that.

If there is a plan to use the bypass, then it is no different from any other system. But there are no separate contactors shown for those two motors. If the bypass system just bypasses the VFD, both motors will start at the same time and that's going to be how you size everything.

 

[fbhwt]

So are you or are you not going to utilize the bypass option? If not, then the across-the-line issues become irrelevant. The drawing you posted doesn't show a bypass, it only appears to be using the VFD to start and control the motors. So all you need to do is size the circuit to feed the VFD based on the NEC rule for that.

If there is a plan to use the bypass, then it is no different from any other system. But there are no separate contactors shown for those two motors. If the bypass system just bypasses the VFD, both motors will start at the same time and that's going to be how you size everything.

No bypass, looks like 430.122(A) is this correct?

 

[Jraef]

No bypass, looks like 430.122(A) is this correct?

Yes, with no bypass, you use 430.122. But that just covers the conductor sizing. The OCPD for the drive should be chosen from the Drive mfr's literature. Some are NOT UL lusted to be protected by Vircuit Breakers, you MUST use fuses. It varies from mfr to mfr, so RTFM.

 

[fbhwt]

Yes, with no bypass, you use 430.122. But that just covers the conductor sizing. The OCPD for the drive should be chosen from the Drive mfr's literature. Some are NOT UL lusted to be protected by Vircuit Breakers, you MUST use fuses. It varies from mfr to mfr, so RTFM.

Ok thanks, JJS-90 fuses ahead of drive, I'm drawing a blank; RTFM ?

 

[Jraef]

Ok thanks, JJS-90 fuses ahead of drive, I'm drawing a blank; RTFM ?

Read The "Factory" Manual.

(Generally, the "F" stands for something other than Factory though...)