Custom VFD Panel Advice

[Ramos617]

I am taking a role in which I have a large role in the design of a custom dual VFD panel and I'd like some sound advice before I proceed. So this is a 3-phase panel for controlling 8 fan motors at the load but that is beside the point. The feeder breaker of 150A splits to (2) 100A breakers, so I have to split the wires on the output side of the 150A breaker at some point before the 100A breakers. I have a distribution block which will accommodate #3/0 from the 150A breaker to the block but the output side has 4 outputs per phase but max out at #2AWG each. The #2AWG wire is on the verge of the ampacity rating of 100A for the slave breakers. Will this pass an UL inspection if #2AWG THHN wire is used from the block to the 100A breakers? THHN is rated at 90 deg so 130A correct??

Also an unrelated question: On a distribution block or breaker would it be ok to put multiple conductors into a sleeve that combine to the less than the max wire size for that terminal? For example; with a circular terminal of a max of 400mcm wire size, would it be ok to insert (2) #1AWG (84MCM each equiv.) wires into a sleeve and then insert it into the terminal?

 

[ptonsparky]

Would getting the proper distribution block alleviate some of the problems?

 

[paulengr]

I am taking a role in which I have a large role in the design of a custom dual VFD panel and I'd like some sound advice before I proceed. So this is a 3-phase panel for controlling 8 fan motors at the load but that is beside the point. The feeder breaker of 150A splits to (2) 100A breakers, so I have to split the wires on the output side of the 150A breaker at some point before the 100A breakers. I have a distribution block which will accommodate #3/0 from the 150A breaker to the block but the output side has 4 outputs per phase but max out at #2AWG each. The #2AWG wire is on the verge of the ampacity rating of 100A for the slave breakers. Will this pass an UL inspection if #2AWG THHN wire is used from the block to the 100A breakers? THHN is rated at 90 deg so 130A correct??

Also an unrelated question: On a distribution block or breaker would it be ok to put multiple conductors into a sleeve that combine to the less than the max wire size for that terminal? For example; with a circular terminal of a max of 400mcm wire size, would it be ok to insert (2) #1AWG (84MCM each equiv.) wires into a sleeve and then insert it into the terminal?

Get training or hire someone to do this design. Too many obvious rookie questions. Electrical work is highly Code driven and very detailed. It mixes electrical, thermal, and mechanical design all at once. If you don't have a good working knowledge of the Codes, you are in way over your head.

1. Get a copy of NEC. Follow it. Not very convenient but you can read online. If you're a panel builder get UL 508A instead. This is what you will be held to. These issues are clearly laid out in the Codes. UL 508A contains some limits and if you're building lots of these 508 might make more sense but that's more of a performance standard. 508A keeps you out of trouble. A 508A panel will pass 508 but instead of being performance tested, it's rule based.
2. Wrong distribution block. Get the right one or change design. If I did this myself with all 3 breakers so similar in size I would just run the same size wire to all three with either multiple hole lugs or studs and crimped on lugs on the breakers, all sized for the same ampacity. No confusion, one cable size for the builder, fewer mistakes, no Code issues, easy to do.
3. Also breakers, why? Is this allowed by the VFD instructions? You probably should be using fuses. Read the VFD manual. Follow the instructions on feeding it to the letter. If the customer uses a large transformer to feed it and you get a dead short, the VFD can not only burn up but eject parts. Siemens screwed this up on a big project in Australia and the parts punched through the cabinet. They believed their VFDs were so fast fuses were not necessary. It's a safety issue and not just because your VFD will be a smoldering mess.
4. Although the gutter rules apply at some point due to short lengths a sleeve is a wireway. Need to derate your already limited conductors to account for the fact that you are bundling so heat dissipation is an issue and that's what derating is for. Are you following gutter or tray rules in the panduit?
5. How many cables are you jamming under one terminal? You think the inspector will pass that? How many is the terminal rated for?
6. Split? You realize that tap rules apply here? If I as you say split the load side of a breaker to feed two loads, the breaker is there to protect the wiring on the load side. So if the wire ampacity is now half what it was, does the breaker still protect it? Nooe. A tap rule allows some cheating but there are several cases and exceptions so reading the NEC is easier. Suffice to say tap rules would apply if say you have a 150 A breaker feeding two 50 A breakers (50 + 50 is less than 150) but probably don't when 100 + 100 exceeds 150 A. Tap rules are an NEC thing though, not UL 508 A.


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[Jraef]

It does appear as though you might be in over your head here a little by virtue of the nature of your questions and assumptions. It would be best if you could learn at the side of someone more experienced before being tasked with this. But given that you have come here for help, I'm guessing that is not an option.

So first off, you don't use the 90C column on THHN unless you are de-rating for voltage drop or raceway fill. For something like this, you use the 75C column.

But even in the 75C column, 3/0 is fine for 150A, and #2 is good for 115A, so I don't see the problem here.

But MORE importantly, breakers are sized to protect the conductors, but the conductors feeding the VFD must be sized at 125% of the VFD input amps. You didn't say what size the VFDs are so we can't say if #2 is even the correct conductor size, nor do we know if a 100A breaker is appropriate without knowing the VFD rating.

 

[kwired]

#3 copper is good for 100 amps @75C. Your lugs are sufficient for the application unless you need to adjust ampacity for some reason.

 

[drktmplr12]

Tommy: Chicken wings! Let's think about this for a sec, Ted, why would somebody put a guarantee on a box? Hmmm, very interesting.
Ted Nelson: Go on, I'm listening.
Tommy: Here's the way I see it, Ted. Guy puts a fancy guarantee on a box 'cause he wants you to fell all warm and toasty inside.
Ted Nelson: Yeah, makes a man feel good.
Tommy: 'Course it does. Why shouldn't it? Ya figure you put that little box under your pillow at night, the Guarantee Fairy might come by and leave a quarter, am I right, Ted?
Ted Nelson: What's your point?
Tommy: The point is, how do you know the fairy isn't a crazy glue sniffer? "Building model airplanes" says the little fairy, well, we're not buying it. He sneaks into your house once, that's all it takes. The next thing you know, there's money missing off the dresser and your daughter's knocked up, I seen it a hundred times.
Ted Nelson: But why do they put a guarantee on the box?
Tommy: Because they know all they sold ya was a guaranteed piece of crap. That's all it is, isn't it? Hey, if you want me to take a dump in a box and mark it guaranteed, I will. I got spare time. But for now, for your customer's sake, for your daughter's sake, ya might wanna think about buying a quality product from me.
Ted Nelson: Okay, I'll buy from you.
Tommy: Well, that's... What?

a panel doesn't "pass" UL inspection. the inspection is by the panel builder at the factory. non-compliant panels leave shops every day. safe panels without UL labels are installed by property owners every day as well. the point is that no employee from UL will look at it and "certify it". the responsibility lies on the designer and the builder to know what is required and implement. they will then place a sticker with a serial # that building inspectors will view as a quality mark, but that doesn't mean the panel is necessarily safe. in most cases (99%), it is.

you need to purchase a copy of UL508A and ensure the design complies. there will be a steep learning curve, especially for a panel such as that you described. something basic like a simplex pump controller would be a better introduction. this is why members experienced in panel building, myself included, are saying to spend some time either consulting with an expect, face to face, or start educating your self before your deadline gets too close.

 

[mtbiker844]

what does NEC say about NOT running VFD outputs in the same conduit?

what does NEC say about NOT running VFD outputs in the same conduit?

hi Holt folks,

My back ground is not in NEC code and construction, just Controls and Maintenance and I could use some help as I could not find something in the NEC on this.

The question is regarding what the NEC says about NOT running VFD output cables in the same conduit?
We repeatedly have jobs where Electricians run one conduit for multiple VFD's output to motors. Or one VFD for multiple motors, when the O/L's are in the VFD enclosure so they run each set of cables in one conduit to the motors.

This results in issues Even if one VFD is running several motors as we have found there are problems with a VFD having nuisance faults etc.

This is also an unsafe practice as the power from one VFD [or even an energized motor controller] can be induced into the other cable in the same conduit. Someone may tag out one device and if the other one turns on they can get shocked while working on the tagged out circuit due to Capacitive inductance.

Some Electricians say they are within the NEC to do this. However they are not following the OEM instructions given to them and often are not aware of the why as the Construction Electricians are rarely trained on VFD's. All VFD OEM's require that the output power cables from each VFD be run in its own respective metal conduit to each motor [or motors]. This requirement is to avoid EMF [aka noise and cross talk] between VFD outputs.

Would you please help me with what sections of code to use when we spec out a job so they we can avoid confusion in the future?

 

[kwired]

hi Holt folks,

My back ground is not in NEC code and construction, just Controls and Maintenance and I could use some help as I could not find something in the NEC on this.

The question is regarding what the NEC says about NOT running VFD output cables in the same conduit?
We repeatedly have jobs where Electricians run one conduit for multiple VFD's output to motors. Or one VFD for multiple motors, when the O/L's are in the VFD enclosure so they run each set of cables in one conduit to the motors.

This results in issues Even if one VFD is running several motors as we have found there are problems with a VFD having nuisance faults etc.

This is also an unsafe practice as the power from one VFD [or even an energized motor controller] can be induced into the other cable in the same conduit. Someone may tag out one device and if the other one turns on they can get shocked while working on the tagged out circuit due to Capacitive inductance.

Some Electricians say they are within the NEC to do this. However they are not following the OEM instructions given to them and often are not aware of the why as the Construction Electricians are rarely trained on VFD's. All VFD OEM's require that the output power cables from each VFD be run in its own respective metal conduit to each motor [or motors]. This requirement is to avoid EMF [aka noise and cross talk] between VFD outputs.

Would you please help me with what sections of code to use when we spec out a job so they we can avoid confusion in the future?

Nothing in NEC on this, not really a safety issue. That don't mean keeping them separate isn't a good design practice though.

Capacitive coupling between conductors happens even when there isn't a VFD involved. The "capacitor" is weak enough there normally isn't enough available energy there to be hazardous to a person.

 

[Jraef]

Nothing in NEC on this, not really a safety issue. That don't mean keeping them separate isn't a good design practice though.

Capacitive coupling between conductors happens even when there isn't a VFD involved. The "capacitor" is weak enough there normally isn't enough available energy there to be hazardous to a person.

Yes, but the added problems with VFD outputs have to do with the fact that the output sine waves are no longer the same, so adjacent magnetic fields that would normally cancel each other out, will not see induced voltages superimposed on the conductors. Thne long distances involved in those conduit runs result in significant increase in the standing wave voltages that exist in PWM circuits, leading to problems in the VFD and the motor. This is not covered in the NEC explicitly, but the NEC DOES say that you must follow the installation instructions of the VFD, and as far as I know, EVERY VFD mfr will tell you that you must have separate shielded circuits for your VFD. If using steel conduit and individual conductors, that means separate conduits.

That requirement CAN be met by using the shielded VFD cable however.

 

[drktmplr12]

ill take this opportunity to suggest using XHHW if you aren't planning to use shielded VFD cable.

THHN is not appropriate because it has tiny air bubbles in the thermo-setting, making it inadequate insulation for the voltage spikes present on the VFD output conductors. Nothing in the NEC tells you to do this, but many of this forum would tell you the marginal increased cost of the XHHW is well worth it.