Magnetic Field - Stage Cable

[cbq9911]

In the theater/entertainment lighting world we use lots of 12/14 TPR cable, feeding six circuits, frequently loaded to near 20 amps each. My question is about coiling excess in-use cable. Apparently it is standard practice to figure eight the coil to reduce heat & the magnetic field produced by a fully loaded cable (as apposed to a circle coil). I could see this being helpful with dispersing heat (shorter stack, more air space) but don?t understand any magnetic field issues. Each circuit has its own neutral and would normally consist of 120 volts using all three phases or possibly just one phase (based on the configuration of the dimmer racks) but always with their own neutral. Thanks for any and all input!

Link to product mentioned:
http://www.lexproducts.com/cs/Entertainment_product_line?market=Entertainment&productLineId=17

 

[charlie b]

I don't know anything about the particular types of cables you are describing, nor anything about the practices of your industry. But if any given cable has a conductor bringing current to a load and a neutral wire bring current back from the load, then the net magnetic field, as seen outside the cable, is essentially non-existent. You are right about that.

 

[iwire]

I thought the figure 8 was more about not tangling the heavy cables for easy break down, I imagine the extra air space would also dissipate the heat.

 

[LLSolutions]

I've done alot of stage work and this argument comes up often, The figure 8 is for the No tangle issue and ease of load out. The magnetic field stuff on a multiconductor cable is hearsay, most canceling should be done in the cable assembly. Same way you coil a mic cable surely you're not worried about magnetic fields there. As for coiling single feeder cables; 1 per phase I could believe it may help. I thought that 6 circuit stuff was 12/19 hot/neutral/ground per circuit and one for the case?

 

[iwire]

As for coiling single feeder cables; 1 per phase I could believe it may help.

As long as all the single conductors are run parallel with each other it will act just like a cable. :smile:

 

[quogueelectric]

The heating may be coming from the dimmers clipping your output wave. Anything resembling a square wave tends to cause heating from harmonics in the system. If a square wave goes from flat line 0 hz to straight line upwards theoretically infinite frequency. It has to definitely hit the resonant frequency of the circuit at each square junction. This could explain the cable heating.

 

[LLSolutions]

As long as all the single conductors are run parallel with each other it will act just like a cable. :smile:

I agree with that practice 100%, but I've seen the run come to the disconnect and you have an extra 50' x 4 cables of 4/0 and they end up getting coiled individually, in that case would the coiling technique come into play? I've done it when load bank testing gens and ups's and the debate always comes up.

 

[zbang]

With multicables, current "out" must equal current "back", tus the fields will cancel pretty much inside the jacket. The manner of coiling don't make a difference in the overall magnetic field. OTOH, doing a figure-8 or flip-coil will make it much easier to take the whole thing out when you're done. On yet another hand, loading up all the circuits in a multicable to 20a (for 12g) is a really bad idea unless it's really in free air, and I wouldn't do it even then.

When dealing with single conductor feeders, I will always figure-8 them, and if space is tight, they're stacked on top of each other. This will also cause the fields to cancel fairly well. Since I'm pretty picky about cable dress, it's easy to take the feeders out.

I'm also of the opinion that straight coiling of flexable cables should be reserved for anything under about 12' long. Longer than that and it's a flip-coil. There's a pretty good tutorial on this at http://www.techno-fandom.org/~hobbit/flipcoil/howto.html.