Dealing with cable spare/surplus length

[Keeper of the Keys]

This topic may be more appropriate for the electric forum I am not sure, I guess the mods will move it if they feel so...

 

So basically a long time ago I was taught to deal with my surplus 16A/230V/1p+n+e cables by doubling the cable and then rolling it up.

This is based on the fact that for every winding in the spool you then have a winding aimed in the other direction.

 

Recently I wanted to apply this 'trick'/method to a 32A/380V/3p+n+e cable but a colleague of mine claimed that for this 'scale' of cables this does not work.

I am having a hard time seeing why it would not work, the physics of the solution didn't change, the only reason I can see is that small differences in the windings add up much more when dealing with this 'scale' as opposed to a 'regular' cable...

 

The other solution I know for extra length of these types of cable (other then finding a spot where no one is walking during the show and just spreading the cable out on the floor like my colleague preferred) is 'stacking' the cable in a figure 8, I know this is common practise by some of the biggest European rental outfits where the long 3-phase cables are in a flightcase 'stacked' in a figure 8 and the surplus length is just left inside the case.

 

So I was hoping for some insights and comments on the mentioned methods, maybe someone can confirm my theory on why it not good to do the folded double 'stacking'/rolling.

 

Oh and I hope I'll pick up the correct English terms while we're at it... thanks.

[Simon Lewis]

Many mistakenly think that it is inductance that gives rise to heating effects - and therefore try to store the cable in a way that minimises this supposed effect. In fact, it is cable resistance that generates the heat, and the way in which the cable is stored / deployed will determine how well this heat is dissipated. Stacking cables on top of each other will not aid this process!

 

There's a "technical answer" in this month's L&SI that answers this very query, and gives a better answer that I can ;-)

 

Simon

[Keeper of the Keys]

You were refering to this:

Q: I recall that in my younger days as a theatre sparks, I was told by my elders and betters never to coil up cables which are in use because of the risk of over-heating due to induction. For the same reason, I was told not to hang them over or wrap them round metal objects such as cleat hooks or scaffolding tubes. I have seen this advice repeated frequently in the context of electrical health and safety since then, as Google will testify. More recently I was told that the problem with coiled cables is an issue when they are coiled on a drum and therefore not able to radiate heat from the inside. Reviving my hazy engineering knowledge, considering that as the outgoing and return conductors run together within a cable, I would have thought that any induction effects would be cancelled out. Because of this, I am wondering if the whole induction rationale may have been something of an urban myth.

 

A: Your supposition is right, it's not inductive heating. As the neutral and phases will carry ostensibly equal current there is no imbalance and thus no eddy heating. There would be inductive effects if you wrapped lots of turns of a single wire around a ferrous object and then wrapped more turns of another cable around the same ferrous object separate (mechanically) from the original - you'd be making a transformer. This is unlikely in practice as all cables in entertainment use are generally multicore unless installed in conduit or trunking - in both cases phases are run together. Having said the currents are equal, this is not always the case where there is a high neutral harmonic current, for example, but this is unlikely to be a problem in this context.

The reason you shouldn't coil cables is because they warm up and the less airflow present the hotter they get. The heat comes not from induction, but from the resistance of the cables: as current flows through, power is dissipated in the form of heat as a result of the resistance of the wire itself. Socapex cables, for example, typically have 1.5mm CSA conductors and the length they are run to often puts them at the limit (or in excess of) their current carrying capacity for the required voltage drop. As such, they get warm because the excess resistance is just acting as a heating element. If radiated heat is an issue, then try and keep as much airflow as possible around the cables. An example is running excess socapex cable in large loops near the dimmers on a show. A large coil of cable hung from a bar would be okay but if thrown onto a floor with other cables piled on top would be less so. A quick glance at the tables in the IEE Wiring Regulations will show what a difference the 'fixing' method makes to the current carrying capacity of a cable - for example, a 1.5mm CSA single-phase cable enclosed in trunking is rated at 18.5A and in free air such as on cable tray at 26A - a big difference of 1.725kW!

 

What I infer from this is that as long as the airflow is good enough the actual method of stacking is less relevant... (ad I'm feeling stupid that I forgot that the cable itself carries the countering loop)

[Seano]

So basically a long time ago I was taught to deal with my surplus 16A/230V/1p+n+e cables by doubling the cable and then rolling it up.

This is based on the fact that for every winding in the spool you then have a winding aimed in the other direction.

 

Many mistakenly think that it is inductance that gives rise to heating effects - and therefore try to store the cable in a way that minimises this supposed effect. In fact, it is cable resistance that generates the heat, and the way in which the cable is stored / deployed will determine how well this heat is dissipated.

 

Quite apart from which, its complete nonsense. The cable already has an internal 'return path' - every amp flowing one way along the live is coming back the other way in the neutral. Doubling the cable makes no difference at all, even in theory let alone the real world.

 

edit: Oops. That last post was added while I was writing this. Soz, didn't mean to labour the point.

[Brian]

And before anyone says 'ah, but there is no parallel return paths if you are running singles so inductance will be a problem' - trust me it isn't. If you do the calculations to work out the inductance it is so small as to be negligible.

[Dave]

And before anyone says 'ah, but there is no parallel return paths if you are running singles so inductance will be a problem' - trust me it isn't. If you do the calculations to work out the inductance it is so small as to be negligible.

I'm not 100% sure what situation you're referring to, but inductive voltage drops do become an issue with large cables. BS7671 tabulates both resistive and reactive voltage drops for sizes above 16mm, for both singles and multicore cables (multicore in this context means 2-4 core SWA for example). With singles, the reactive (inductive) component becomes significant around 120mm and dominates from about 240mm.

 

The inductive drops depend on the physical arrangement of the cables and are highest for spaced layouts, as the return path is further away and so the resultant magnetic field around each cable is greater.

 

Of course, apart from the largest powerlock feeders, that size conductor is unlikely to be encountered in the production world.