Conduit Is Getting Hot

Sounds as if the each phase is ran in a seperate conduit, induction heating- Should have 1 lead from each phase installed in each conduit.

Can anyone say: "Edy Currents" ? Good advise guys....

Donald

If you want to write correctly, you should have written:-

Can anyone say: "Eddy Currents" ? Good advice guys....

not

Can anyone say: "Edy Currents" ? Good advise guys....

Spell checker will not help here, only correct knowledge of spelling will.....we need it for the non English speaking colleagues, are you by any chance one of them?

No I couldnt say "EDY" Current cause I had a brain fart and couldnt remember the darn phrase

Are you sure the conduit is well grounded?

Are you sure the conductors are within their Ampacities?

One of these factors heats up your conduit. (or it maybe simply the sun?)

Wangito.

If it is induction and you ground them, I am not actually sure that will make a difference...!

The Eddy currents will still flow..... Safer is a probable yes, but they should have already been fully grounded anyway.........

I have not seen this effect personally, but I feel that a really large conduit should have been used so that all 4 cables (3 phases plus a ground, I am pretty sure that neutral will not be a requirement here) can pass through....

I am interested in the final fix!!

It will.

Eddy currents are still energy with nowhere to go. If strong enough, and here we are lacking more input from the poster, it will drive the metal conduit to saturation, and the next step will be heat dissipation. I don't say it will go to zero because of grounding, but will organize mag-field to move it far enough from saturation. Safety is an important topic as well, but out of this thread's scope.

Wangito.

Amego's, stick all wires in same conduit, no problem.

James

Amigos, spells with "I"....

All cable ARE in the same conduit, At least that's what the poster told me in a direct massage.

Wangito.

What he wrote was:-

The system is 3 Phase motor 300 hp and is feed with 3 - 500 Kcmil copper wires in 3 separate 1 1/4" Rigid Metallic Conduits.

That is 3 separate conduits, if he only had one, he would not have the problem in the first place!!!

Also Automate has only posted once by my counting!!!!

Now I'm confused, are these conductors in one conduit or three?

3 or three or trois or Drei....seperate conduits. Read the original post!!!!

Duh... you think! Of course it says in the OP that these three conductors are in separate conduits.

However the statement in post 15, raises the question of how many conduits are being used for this motor.

Here are a few statements to help you further:-

The original poster has said clearly 3 conduits.

Post #15 is not the original poster.

The original poster has not corrected himself, so we must assume that post#15 is simply wrong.

The problem seen is only possible if there is 3 separate conduits or severe cable overheating. Overheating would have been noticeable where the cables go into or come out of the conduit, this was not noticed, so it is Eddy current heating, no more, no less....and to be expected in such a setup.....

Wow, you are a tiger.

James

I am not and elec engineer but can you run a smaller wire that carries no real load of each phase parallel to the main phase line in each pipe to reduce the inductive field of the mains?

Not sure if that come out right but I hope you get the gist.

They are right. The circuit conductors MUST be run in the same conduit. This is not up to the US code.(NEC) If the conduits (metal) are tight from panel to load, they can act as an acceptable grounding source. But, you combine the conductors so they cancel each other. How to correct?: $$$$$$$ NEC 300.20 - to avoid heating......

Good luck,

James

You have left a few items out that should be known. What is the Voltage of the motor for instance? If this is a 460 VAC motor then what type of cable are you using? Copper or Aluminum? A motor that size should be copper. Lets for an instance assume it is a 460 VAC motor with copper 500Kcmil and that the cable is rated for 75 C temp. The motor would be in the range of 361 FLA, the cable is rated at 380 amps. All conductors should be sized at 125% of the rated Full Load Amps which would put you at 451 Amps minimum current carrying capacity. That would put you in the 700Kcmil wire range, it would explain why your conduit is overheating because your cable is overheating as it is undersized. NEC 101 my friend. Hope this helps.

Well said...

What is happening here is what's known as a "circulating sheath current".

As other have mentioned, all three phase conductors should have been in one pipe. That way the vector sum of the induced current in the conduit equals to 0.

To fix it... Pull all three wire into one raceway or... ensure that each of the three metal conduits are only bonded to ground at the source end of the run and insulated from ground at all other points, including where they connect to the motor. Doing this breaks the circuit for the circulating current.

If contact with ground is reestablished anywhere along the run, the circuit is once again closed and and the circulating current will now flow again.

When you fix it... make sure to ensure, via a second conductor in a separate raceway, that the equipment bonding is maintained.

Rick...

Automate:

NEVER install conductors in separate conduits. From your answer I understand that you didn't. so far so good. Working temperature for same power cable, will differ if mounted in a closed conduit or open air. Go-ahead and measure the surface temperature of the cable's jacket add approx 50% to know ≈ conductor temp. go to the manufacturer's specs. if within, just don't worry about it anymore, but if out, replace ASAP.

Wangito.

Ohh... by the way, what is the system voltage and what is the FLA rating of the motor?

We have all been talking about induced currents in the conduit, what if the 500 kcmil conductors are too small. The way I see it, with any system voltage lower then 460, these conductors would be to small for a 300 hp motor.

Good point. I brought up these issues in post #10.

Thanks for all the help, "THE FORUM" has lead me to a solution.

THE SOLUTION:

I have decided to run a parallel feed to the motor. Should be able to run a parallel 3/0 with #3 ground, 90 Degree C, this will give me 450 amps of coverage. The motor FLA is 354 so 125 times that is 442.5 so coverage should be sufficient.

Again I Thank You for Your Response.

Nope... to small.

A parallel run of 3/0 R90 will give you 210 amps each conductor or 420 amps in total. At least it will in Canada and most of our ampacity ratings are the same as the NECs. Still to small, you need parallel 4/0 conductors, which will give you... 235a x 2 = 470 amps.

Rick...

Yes, make sure that when reading your ampacity tables you are looking at the MULTIPLE conductors in the same raceway table, the ampacity values are slightly lower.

Also, when you run the parallel conductors, make sure you still divide them up so that one of each phase is in each of the conduits.

I am curious, what did you do with the original installation?

James