800 HP AC Drive Wiring

I have a few questions on wiring 800 hop ac drives

1.0 Should the cables be vfd cable or will standard 500 mcm wire be fine? Why not?

2.0 Should each phase go to the motor in it's own conduit or all in one? Why?

3.0 Would there be a problem runing 2 phases in one conduit and the third phase in another? Why?

4.0 if one 2 phases are done in 500 mcm wire and the third in 2 250 mcm wire ( which makes a 500) be a problem

This drive will be running at 90 hz base.

I think there are problems with this installation but i don't know the answers to these questions to argue the fact.

1. Before VFD, standard cable, from VFD to motor shielded cable. VFD is source of interference.

2. You are not using one cable with 3*phase and 1 ground?

3. ^^

4. I recall here was discussion about your (USA) regulations deny use of two wires to get bigger crossarea.

Kid
All current carrying conductors should be in same conduit to ensure magnetic fields cancel to avoid inductive heating of conduit - at least that is what I was taught.

RE "special VFD" cable from VFD to motor -- read the VFD and motor manuals and follow their guidance. There is a distance issue also which you have not mentioned.

I assume you are running motor on 480 which gives about 722 amp (PF not included). You do not say which conductor type you are using but it looks to me 500 mcm is not large enough.

This reminds me of wrestling with submarine shore power cable two 400 amp 3 conductor. 12 pound per foot supposedly.

I would consider going up to 4160 V motor - takes you into other issues I know.

If you are having problems running pipe large enough for the conductors then you can do paralleled conductors to take advantage of smaller easier to handle pipe. Run all 3 phases neut and ground in one pipe and run a duplicate set in the other. Do NOT skimp on pipe size - one size larger than required makes it much easier to pull - use lots of wire lube.

Cable tray may be worthy of consideration also.

Dan Bentler

The run is about 50 feet

It will be 2 500 mcm cables for each phase.

It has 3 3" pipes Conduit.

I have to seperate the phases into each conduit there is no other way to do it with the space we have.

Does this really cause problems in the field or is ot more of a rule of thumb/best theory.

The Plc Kid said:

The run is about 50 feet

It will be 2 500 mcm cables for each phase.

It has 3 3" pipes Conduit.

I have to seperate the phases into each conduit there is no other way to do it with the space we have.

Does this really cause problems in the field or is ot more of a rule of thumb/best theory.

Click to expand...

It really does cause problems. I would suggest getting motor lead wire, it is rated differently. It is normally physically smaller in diameter for a given ampacity than regular wire and is much more flexible.

The Plc Kid said:

The run is about 50 feet

It will be 2 500 mcm cables for each phase.

It has 3 3" pipes Conduit.

I have to seperate the phases into each conduit there is no other way to do it with the space we have.

Does this really cause problems in the field or is ot more of a rule of thumb/best theory.

Click to expand...

YES!!! it will cause problems when using metallic pipe or going through an opening that is metal. What you will create is a current transformer that will circulate a current in the pipe and can potentially get thermally hot. Don't do it. What kind of insulation is on the cable? What size ground are you running? Who did the design on this? The engineer who did this needs to get this right. If you need to use VFD cable, then you should know it is a shielded cable and MUST be in one conduit - you can not separate them out. I seriously doubt that you will get one of those (a vfd cable) in a 3" conduit.

Power equipment is nothing to do half-a--ed. It either needs to be done correctly or not at all. If you do not have the authority, then notify your management.

I would not let this start up.

Kid

The run is about 50 feet
REPLY well at least there is some good news

It will be 2 500 mcm cables for each phase.
REPLY should be enough ampacity


It has 3 3" pipes Conduit.

I have to seperate the phases into each conduit there is no other way to do it with the space we have.

Does this really cause problems in the field or is ot more of a rule of thumb/best theory.
REPLY
NEC Art 300 3 B says all conductors of same circuit shall be in same pipe. 300.3 allows for paralleled conductors like you WANT to do but not as you intend to do.

It also allows to run ground outside of the pipe. READ THIS CAREFULLY lots of catches.

Another thought since you are doing 3 runs of pipe use 3 conductors per phase. I would tend to go 4" pipe (if you can get there) and maybe (?) tripled 400 (300?) mcm to give sufficient ampacity and to give ease of pull.

Cable tray is another option that may be worthy of consideration. If you run special VFD cable this may be your only option.

I sure hope DickDV looks at this.

Are you the only one on this project? I think this is a lot to expect from someone as young as you. I admire your ambition and willingness to take on new and learn but this is not something to learn the hard way on. Get yourself a copy of NEC, American Electrician Handbook, NFPA 70 etc if they will not give you the engineering support you damn well need and should have.

ONE GUY CANNOT DO IT ALL

Dan Bentler

4" is the largest conduit i can get correct?

The Plc Kid said:

4" is the largest conduit i can get correct?

Click to expand...

No, you can get up to 6" in both rigid metallic or intermediate metal.

looking at a 500 hp siemens drive we have they have 2 4/0 parrallel and 2 2" pipes with one run of each pahes in each pipe so 4/0 phase a,b,c in pipe 1 and second 4/0 phase a,b,c in pipe 2 this also about a 50 foot run but we have never had any problems with it in production about 3 years.

If i can get vfd cable who makes the best?

Just me on the project our EE quit last year and they never replaced him "because of the economy"

Bruce

Have any info on some reading material for what you described?

The Plc Kid said:

Just me on the project our EE quit last year and they never replaced him "because of the economy"

Click to expand...

Kid

This project is over your head at your current stage of learning. Ten years from now no problem.

I gotta go will respond later today.

Dan Bentler

The Plc Kid said:

Bruce

Have any info on some reading material for what you described?

Click to expand...

This is from a book that I had in college when I took power engineering 20 years ago. I found it on google. If you are going to be involved in this type of stuff, you need to start learning what you can. You should really tell your boss that this is over your head and they need to hire an engineer to look over the design. Hopefully you will get one that will share his/her knowledge with you. There are many other things to think of when dealing with these large motors (overcurrent protection, arc flash rules, breaker coordination, etc). If you approve this, then you might be legally responsible if someone gets injured or the plant burns to the ground.

As for the 500HP motor, you can run all 3 phases in one conduit and then have a parallel run of the same 3 phases. You should run these so the lengths are close to the same.

As for VFD cable, I've used Belden and Lutz (because of the smaller OD for conduit installation). Most of the VFD cables are tray rated.

One last bit of advice: Don't trust everything (or everybody) that is on the internet. Verify it for yourself.

Kid, At only 50 feet motor lead length, I would not be too concerned about using special inverter motor lead wire. Your choice of twin 500mcm's is fine.

But you need to take seriously the precautions about how the leads are grouped together in the conduits. You must run all three phases in the same metallic pipe. In this case, that would mean two pipes each big enough for three 500mcm conductors. You can run the ground outside the pipe and this sounds like a good case for doing that. Those would be copper conductor grounds, not the pipe!