coiled cable

[Desie]

When does it become potentially dangerous for a 240v live strand to be coiled?
i.e. how many turns of the coil would render the cable to dangerous overheating?

In a consumer unit some advice given is to coil the cables over once to enable excess for future amendments or alterations. Is it dangerous coiled twice or three times?

Does ohms law come into play here? What would be the calculation or factor to take into account if it did?

 

[Spark123]

It becomes dangerous when it is coiled as such the cable can't dissipate enough heat and exceeds its temperature rating. This will also depend on the amount of current passing through and the size of the cable. I suppose you could also end up with a current transformer if you did something really daft.

 

[Desie]

Thanks Spark I understand the theory of coiled cables becoming heat generators but would like to put some practicality on it.

Using a domestic example when would it become dangerous for a 13A 2.5mm cable pulling 3Kw. i.e. At what point would there be a risk of fire through overheating?

Is there a calculation? I seem to remember something called the left hand and right hand rules in Physics at school many moons ago. Is this heading in the right direction? Or am I barking up the wrong tree? (Or am I just barking....!!)

Cheers

 

[Spark123]

Flemings left hand rule? iirc isn't it something to do with field, motion and current i.e. how motors work? Isn't the right hand rule for generators?

edit: http://en.wikipedia.org/wiki/Left_hand_rule

 

[Desie]

Maybe I am just barking then!!

I had a feeling it was something to do with motors but didn't want to make myself look totally stupid. Failed again there then!!!!
Well motors get warm don't they!!

Okay okay stop prattling Desie........

If anyone can help with any indications or calculations it would be welcome.

I am just curious if at 13A would a 2.5mm cable for example get excessively warm with for example 5 tight coils or 10 or a 100m roll of the stuff?

 

[JohnD]

I would have thought you could treat it as "bunched cables" and see what the de-rated current-carrying capacity would be, and then see if you were overloading it.

Since the phase and the neutral conductors are running close to each other I can't see that there will be much of a magnetic field.

 

[Spark123]

I was thinking of grouping factors, I'm thinking you could possibly apply it to a single insulated 70deg c cable rating??

 

[Steve]

i once visited my uncle, he was in his shed, with a reel of 2.5mm² on the bench, connected at each end. I sez to him "wot ya usin that for?" he sez "its a transformer" he was powering it from the secondary of a 12V transformer thank god think he was charging a 6V leisure battery with a 12V charger

Oh, and his shed was built rather strangely. It was built next to the last pole of an 11,000V overhead line, so the steel anchor wire from this end pole went through his shed roof and wall If he had built it 2 feet further back . . .

 

[Adam_151]

i once visited my uncle, he was in his shed, with a reel of 2.5mm² on the bench, connected at each end. I sez to him "wot ya usin that for?" he sez "its a transformer" he was powering it from the secondary of a 12V transformer thank god think he was charging a 6V leisure battery with a 12V charger

Wouldn't work too well considering batteries are charged with DC...

 

[Steve]

i once visited my uncle, he was in his shed, with a reel of 2.5mm² on the bench, connected at each end. I sez to him "wot ya usin that for?" he sez "its a transformer" he was powering it from the secondary of a 12V transformer thank god think he was charging a 6V leisure battery with a 12V charger

Wouldn't work too well considering batteries are charged with DC...

no, he meant "resistor", rather than transformer.

 

[Jaymack]

When does it become potentially dangerous for a 240v live strand to be coiled?
i.e. how many turns of the coil would render the cable to dangerous overheating?

In a consumer unit some advice given is to coil the cables over once to enable excess for future amendments or alterations. Is it dangerous coiled twice or three times?

Does ohms law come into play here? What would be the calculation or factor to take into account if it did?

If this is a single core cable on alternating current, coiling it would create an inductance coil and impedance into the circuit; but this is minimal for a few turns. The resistance would virtually remain the same, therefore there would be no increase in the temperature.

Jaymack

 

[plugwash]

The resistance would virtually remain the same, therefore there would be no increase in the temperature.

wrong wrong wrong, temperature depends on heat output and heat loss. packing many current carrying conductors together will reduce the effectiveness of thier heat loss and hence increase thier temperature.

 

[Jaymack]

The resistance would virtually remain the same, therefore there would be no increase in the temperature.

wrong wrong wrong, temperature depends on heat output and heat loss. packing many current carrying conductors together will reduce the effectiveness of thier heat loss and hence increase thier temperature.

The OP referred to the heating effects on coiling of cables: - “When does it become potentially dangerous for a 240v live strand to be coiled? i.e. how many turns of the coil would render the cable to dangerous overheating?” What you are referring to is the cumulative effect of bunched cables, entirely different.

Jaymack

 

[The Jeep]

Thanks Spark I understand the theory of coiled cables becoming heat generators but would like to put some practicality on it.

Using a domestic example when would it become dangerous for a 13A 2.5mm cable pulling 3Kw. i.e. At what point would there be a risk of fire through overheating?

Is there a calculation? I seem to remember something called the left hand and right hand rules in Physics at school many moons ago. Is this heading in the right direction? Or am I barking up the wrong tree? (Or am I just barking....!!)

Cheers

I'm not aware of any law of physics that states that current flowing in a coil produces any more heat than it would if it was straight. It does create a strong magnetic field, but the heat effect is, as already stated, only due to the proximity of the wires and lack of heat dissipation that results.

 

[JohnD]

But the magnetic effect when the P&N conductors are in close proximity, is slight, because each core generates a field in the opposite direction to the core next to it, which cancels each other out.

This is why, with metal enclosures, we have to put all phases and neutral through the same hole or conduit, so the magnetic effect and eddy current are net nil.