Line Induction

[EMan9999]

Hello,

I'm planning to run a 500 Amp 60Hz AC line, secured to the ground by short 2" steel clamps spaced at certain intervals. If the clamps I used went around the whole cable and were closed metallically via a bolt, would there be any significant induced currents flowing in the clamp, such that the clamp itself would heat up?

Thanks!

EMan

 

[peebee]

Not so long as all circuit conductors pass through each clamp. Do that and you should be OK for heating (I'm not so sure that "secured to the ground" meets code, but that does not pertain to your question).

If you use a separate clamp for each circuit conductor (or any other similar installation such that all conductors don't pass through the clamp), the net current through the clamp will not be zero. That will heat the clamp.

 

[EMan9999]

Hmmm ... I was actually intending to clamp a single phase. Would you consider the clamp to be acting as a ferromagnetic core or as a short-circuited secondary winding to the main cable? Either way, would insulating part of the clamp (ie. using a non-metallic bolt) eliminate heating almost completely? Thanks again.

Regards,

EMan

 

[dpc]

Yes, you will induce current in a metal clamp that encloses a single conductor. Insulating part of the current path will certainly help. A non-metallic clamp would be better.

 

[peebee]

Per NEC 300.20(B):

"(B) Individual Conductors. Where a single conductor carrying alternating current passes through metal with magnetic properties, the inductive effect shall be minimized by (1) cutting slots in the metal between the individual holes through which the individual conductors pass or (2) passing all the conductors in the circuit through an insulating wall sufficiently large for all of the conductors of the circuit. "

This is written more to address something like a steel escutcheon plate, but you'll get a similar effect with your clamps. BTW -- there's many half-circle shaped clamps available for electrical work with a tab for the fastener, you might consider something like that in lieu of your full-circle clamp. Alternately, perhaps you could use nylon wire ties to attach your cabling to a fastener.

I'm still concerned about you running wiring on the floor -- what's up with that? How are you protecting wiring fastened to the floor?

 

[EMan9999]

The clamps in question are Unistrut's Cush-A-Clamps (as in the above pic), and ideally, we would like metal for strength in our application.

As a background, the current design calls for putting two of these side-by-side (but not touching). A circuit phase will go through each of these, and the third conductor would go in the central space on top between the clamps; all three cables will then be tie wrapped together.

Regarding the connection to the ground, I meant to simplify the issue by saying that the clamps would be mounted on the floor, but in actuality, their layout will span across an elevated, grounded tray.

I am wondering how big of a difference a semi-circular clamp would make, as I take it from the NEC quote that the generated currents are due to magnetization.

 

[RAMConsult]

EMan, The generated currents are due to induction and apply to any conductive material; magnetization would only affect ferrous materials.

 

[EMan9999]

I read that time-varying magnetic fields also cause power dissipation in the form of eddy currents (induction currents flowing in nearby ferrous material) and hysteresis losses (magnetization). I'm now thinking that the losses due to eddy currents and magnetization would occur whether or not the (ferrous) steel circular clamp is metallically closed or open.

 

[whipit]

If you have to use metal clamps. What about using alumunum clamps instead then you don't have to worry about induction.

 

[peebee]

If you're installing these in a tray I'd still recommend either cable ties or lacing. That's exactly what those products are there for, and they've served admirably in critical installations for a long time. UV-resistant cable ties are required for sunlight resistance where installed outdoors.

P-clamps might be more suitable for your application than U-bolts.

Also, have you considered installing all three conductors within a single clamp?

Why do you feel that cable ties are OK for 1/3 of your conductors? Why wouldn't you provide metal clamps for all or none of the cables?

See

http://www.cooperbline.com/pdf/catalogs/CT02MAN.pdf

for more info on ties & clamps.

 

[jbartos]

Suggestion: It depends on the single phase cable construction. If it shielded and shield grounded, it would not matter. However, if the cable is unshielded, then the conductive metal straps that are grounded may pose a problem. Also, any other conductive metal on which the cable is installed will pose a problem to the unshielded cable.