cable overheating 3

[mel127]

Greetings,
We have a problem of overheating cable/conduit here. We have 3 sets of wire coming from the swithcgear on the control room. The 4" conduit that is penetrating the flooring is overheating. Cables are installed one conduit for each phase? Is the conduit acting as a CT that is it is overheating?Or there are other rason behind this.
Thanks.

 

[dpc]

Is conduit metal?

Is each phase in a separate conduit?

If answer to both of these question is yes, the conduit may be getting hot due to induced current from the magnetic field created by the cable. The solution is to put all three phases in each conduit to minimize the net flux.

 

[itsmoked]

mel127; Yes.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[TurbineGen]

Go back to your college electromagnetics class and you'll see that dpc hit this one on the nose. The current in the wire will incude a circulating current (eddy current) in the conduit. Running all three phases in one conduit cancels out most of those currents.

I would suggest using 9 smaller cables, 3 inside each of your 4" conduits with each conduit having all three phases inside. This should minimize the heating issue and you'll still be able to use your 4" conduit.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[amptramp]

Also, the NEC Article 300.20(A) requires conductors carrying alternating current installed in ferrous metal enclosures or raceways shall be arranged "so as to avoid heating the surrounding ferrous metal by induction."

I worked at a cotton mill once where the maintenance electrical staff wired a 1200A ILine panel without distributing the phases properly. The hot conduits were found when one of the electricians took a break by sitting on the 3" LB's fed from the main switchgear before they entered the ground approx 75' from the panel. On inspection of the ILine panel, the phase conductor insulation had melted away by about a foot. It was a "miracle" nothing shorted out.

If you hired an electrical contractor to perform the installation, I would recommend hiring a different contractor.

 

[waross]

I have seen this several times in Central America. It is prohibited by code in the US and Canada. As soon as you rearrange your wiring to put all three phases in each conduit the reduction in heating will be dramatic.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[xxjohnh]

Use aluminium conduit it will cure.

 

[mel127]

Guys thanks for all the replys. I appreciate logging on to this site to gain more knowledge.

 

[waross]

Although using aluminum conduit with each phase in its own conduit may almost eliminate the heating, it is not allowed by North American codes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[pittengineer]

Getting back to Turbogen's comment above about "Induced Eddy Currents".

Aren't these induced currents and eddy currents that you spoke of magnetic flux currents as opposed to actual electrical currents.

Wouldn't it be the same as if you wrapped a coil of wire around a core, with the current in the wire causing an electrical flux to circulate in the core?

Just trying to understand.

 

[dpc]

The time-varying magnetic flux caused by the current in the conductor induces a voltage and thus an electrical current in the enclosing conduit.

 

[amptramp]

Eddy currents (or Foucault currents) are "electrical currents" in the sense that they are circulating electrons in a conductor. The eddy currents are the ohmic result of electromotive forces induced within the conductor by changing (alternating) magnetic fluxes per Faraday's Law of Induction.

Proof the currents are "electrical currents" is the Joule heating generated by these currents.

Here is a link to a paper from TI on the subject:

http://focus.ti.com/lit/ml/slup197/slup197.pdf

Here is the Wiki on eddy currents (I didn't read it closely so the standard wiki warning applies):

http://en.wikipedia.org/wiki/Eddy_current

Finally, these eddy currents are so fundamental to electrical installations that the NEC includes an entry specifically related to this problem and states it is a problem associated with ferrous metal and induction. The solution to the problem is not rocket science - just distribute the phases per the NEC mandate.

 

[dpc]

Nice link to TI paper, Amptramp - thanks.

 

[boyax]

dpc is correct,no need to argue.If it can accomodate, pull these 3 cables and install it in one conduit.If not resize the cable say, 3x 3 run and utilized 3 conduit,but proper distribution of cable is needed if you are using multi conductor/cable.Please don't forget to consider the ampacity of your new cable.