Feeder/branch protection of a multiple vfd control panel

[testsubject]

Hi guys!

I have been spending quite a bit of time yesterday and today trying to get a definitive answer to this question regarding protection an enclosure.

I have combed through the NFPA 79, UL-508a, NFPA70, and other documents and have not come up with anything concrete.

Here is an example of the question.

I have a control panel with (4) Powerflex525s: 480VAC, 3P, 0.5hp. Each VFD will have (1) 0.125hp (0.4A) motor attached.
The PF525s max input rating is 1.9A.
The wiring and line breakers installed for the branch are rated for 1.9A.

The question comes up when calculating the feeder FLA and protection.

At a two previous employers, the philosophy was to add up all the input ratings of the drives and call that the FLA and use this times 1.25 for the disconnect fusing. (This would mean 1.9*4=7.6 and I would use 10A fuses)

At my current employment, they add up all the motor FLAs*150% and call that the FLA and fuse appropriately. (This would mean 0.4*4*1.5=2.4 and I would round to 3.)

I do not think this is adequate but need to have documentation to back me up.

We are a UL shop (as were the two previous employers) and UL never mentioned that there is a problem with this assessment at any of these places.

I am aware of Article 430-122 but that only applies to the branch.

I hope this was clear enough. I am trying to find documentation to point to one way or another (or a completely different way I am unaware of….)

Thanks in advance for your input.

 

[jraef]

430.122 actually only refers to the feeder CONDUCTORS, not the OCPDs.

As to the main OCPD and feeder for the panel, UL does not require one so there are no rules for sizing it. That has to do with the installation, which is the purview of the NEC and is spelled out in article 409.21. For the feeder protection (either IN the panel as a "Main" or in the external feeder for it) you use the sum of the LARGEST OCPD in the panel x 1.25, plus any resistance heating loads x 1.25, plus the combined FLA of all remaining loads that can be operating at the same time. That is the maximum amount, no "rounding up" to the next nearest. One thing that UL does add here however is that for the purpose of sizing the FLAs of inverter controlled loads, you use the INPUT current rating of the inverter.

This basic rule is repeated in 430.24 for the conductors themselves, but there are more exceptions listed in that section.

 

[testsubject]

Jreaf,

Thanks for responding.

So, to use my example from above:
The MCP for the PF525s would be 2.5A (which is the lowest rating of the adjustable MCP we use)
2.5A *1.25(the first VFD) + 3*0.4A (the other 3 motor FLAs)=4.95A and this would round down to 4A.

Am I interpreting this correctly?

Although I have a copy of the NEC it is dated 2011 and does not have the same verbiage you displayed.

 

[califflash]

+1 on Jraef’s response. This is the way to do it.

You’re slightly off OP. The input current comes from the max rated input current of the drive. You don’t use the rating of the MCP for sizing the main.

Example:
Panel includes
25b-V2P5N104 (x2)
25B-V6P0N104 (x1)
10Amps of resistance loads.

Max load is the 25B-V6P0N104 so that (24A x 1.25)
Sum of all resistance loads plus the engineering factor (10A x 1.25)
The sum of all remaining loads (9.6 x 2)

Finally add it all up together and that is your max rating for the panel.

You will need to size your conductors and fuses accordingly and if the main circuit protection exceeds the max rating for any one component you will need to add supplemental protection to those devices (almost always).

For motor control panels I always added the safety factor from 430 for conductor sizing just to be on the safe side. The math above provides an absolute max on the panel infeed. All motors circuits below the main infeed can use the 430 sizing.

You can view the current NEC version on NFPA’s website for free.

 

[califflash]

+1 on Jraef’s response. This is the way to do it.

You’re slightly off OP. The input current comes from the max rated input current of the drive. You don’t use the rating of the MCP for sizing the main.

Example:
Panel includes
25b-V2P5N104 (x2)
25B-V6P0N104 (x1)
10Amps of resistance loads.

Max load is the 25B-V6P0N104 so that (24A x 1.25)
Sum of all resistance loads plus the engineering factor (10A x 1.25)
The sum of all remaining loads (9.6 x 2)

Finally add it all up together and that is your max rating for the panel.

You will need to size your conductors and fuses accordingly and if the main circuit protection exceeds the max rating for any one component you will need to add supplemental protection to those devices (almost always).

For motor control panels I always added the safety factor from 430 for conductor sizing just to be on the safe side. The math above provides an absolute max on the panel infeed. All motors circuits below the main infeed can use the 430 sizing fuses and conductors, or in the case of VFD’s the max fuse size and recommended type to maintain the SCCR.

You can see the current version of the NEC on NFPA’s website free of charge.

 

[testsubject]

Califflash,

This is where I keep getting confused.

What is represented by the sum of the FLA of all other loads? Is this supposed to be the max rating of the VFDs or the FLA of the motors attached to the VFDs? If it is the max rating, then I am grossly undersizing the fusing and wiring for the feeder per code and grossly oversizing the incoming FLA for the control panel.

Thanks for the tip of being able to look at the NEC for free on the NFPA website!

 

[testsubject]

Snip

You will need to size your conductors and fuses accordingly and if the main circuit protection exceeds the max rating for any one component you will need to add supplemental protection to those devices (almost always).

Shouldn't the Main Circuit Protection (feeder) exceed the max rating of any individual device (branch) in a multi-device panel?

 

[califflash]

The sum of all other loads is anything other than the resistive loads and is where you get into some grey area. With VFD’s, since they’re technically motor loads, you could use the FLA rating from chapter 430 or the max input rating. I typically use the max input rating of the drive since motors have a tendency to get changed based on what’s available in plant inventory.

Good catch on my comment around the max fusing. What I mean is that on motor control panels the main load may not require a supplemental protector since the main circuit protection can Provide both primary and branch protection. The smaller loads will likely require supplemental protection. Think of a 500hp soft starter control panel, with a 1hp supplemental pump, and control power transformer. The main indeed could protect the big motor and the smaller loads will require supplemental protectors. The smaller loads likely fit well within the engineering factor of the panel so wouldn’t necessarily impact the sizing of the main protector at all.

 

[jraef]

Shouldn't the Main Circuit Protection (feeder) exceed the max rating of any individual device (branch) in a multi-device panel?

It will. What he was referring to is if you DON’T have additional OCPDs for your other loads, you MIGHT be able to use the Main as the only OCPD if the conditions are just right. In your case they will not be. VFDs have very low SCCR ratings by themselves, but usually have decent ratings when used with specific OCPDs. In the case of PF525s, they can be used up to 65kA if protected by A-B Bulletin 140 protectors, or specific fuses (I never use fuses so I’d have to look up which ones). So those will be the OCPDs in your panel.

The reason it’s often advantageous to use the INPUT current of the VFDs is because it is usually lower than the output current and/or motor FLC. That’s because a VFD presents a high Power Factor (.95) to the line because of its internal capacitors, but the output current rating has to include the reactive current for the motor. In your case because everything is so small it’s not likely to make any difference, but it can when dealing with larger motors and drives.

 

[califflash]

Very well articulated Jraef! I’m obviously out of practice!

 

[testsubject]

Thanks for your responses.

I think I have enough now to develop configurator.