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Threading wires behind wood panels


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Dear All,

I was just about to wreck beautiful plywood lining to thread wires behind and then spend Sunday afternoon putting it all back together, when I struck upon the idea of using a tape-measure - the metal type that rolls up in its metal case. Simply attach the wire to the end or the meaure using masking tape, push the tape up behind the wood to the point you want the wire. Easy, it even goes round corners.

Gordon

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Dear All,

I was just about to wreck beautiful plywood lining to thread wires behind and then spend Sunday afternoon putting it all back together, when I struck upon the idea of using a tape-measure - the metal type that rolls up in its metal case. Simply attach the wire to the end or the meaure using masking tape, push the tape up behind the wood to the point you want the wire. Easy, it even goes round corners.

Gordon

 

Mmmm... Sounds good. Providing there are no batons blocking the way it should work fine.

 

However I was also under the impression that all cables were meant to be supported every 300mm (or is it 500mm?). I guess in practice this means all cables that aren't hidden behind linings have to be supported.

Edited by blackrose
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Electricians draw tape are ideal for this.

 

David

 

 

It was called a mouse in my day, the tale was that electricians would tie a length of string to the rodent and position a piece of cheese at the required destination. The modern hi tech innovation of the steel strip came along later. I never believed a word of it.

Edited by John Orentas
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It was called a mouse in my day, the tale was that electricians would tie a length of string to the rodent and position a piece of cheese at the required destination. The modern hi tech innovation of the steel strip came along later. I never believed a word of it.

Of course any sef-respecting mouse would chew up any existing cabling between it and cheese.

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:) The chaps in the laying cable accross the road trade sometimes use a mole. Its a kind of air powered digging machine the burroughs its way under the tarmac. This is to avoid digging up the road of course. Problem is it goes through anything it meets along the way, such as gas, and sewage pipes and the road has to be dug up, only more so, anyway.
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Electricians draw tape are ideal for this.

 

David

 

And I use an old liner from the MIG welder.....

 

 

It was called a mouse in my day, the tale was that electricians would tie a length of string to the rodent and position a piece of cheese at the required destination. ........ I never believed a word of it.

 

I've seen trained ferrets used for this purpose..... :rolleyes:

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Mmmm... Sounds good. Providing there are no batons blocking the way it should work fine.

 

However I was also under the impression that all cables were meant to be supported every 300mm (or is it 500mm?). I guess in practice this means all cables that aren't hidden behind linings have to be supported.

 

Just don't let them all bunch together at the bottom of a panel or they'll overheat.

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The lid off that white plastic trunking is ideal for fishing wires. You can tape extra lengths together if needed, it is flexible but at the same time rigid enough to be pushed.

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Though they'd be ok bunched together in a conduit...... :rolleyes:

 

 

If the conductors are sized to avoid excessive volt drop, overheating of wiring should never be a problem. Much better though to have the wiring properly organised passing through dedicated paths, behind a pelmet or below gunnels is good.

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If wires are bunched together they have to be derated, whether its in conduit, in a cavity or what. To get full current rating they should be at least 1 diameter apart, and mounted in air, not insulation. The figures for current rating assume the cable will be running at 600C, which is pretty warm.

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If wires are bunched together they have to be derated, whether its in conduit, in a cavity or what. To get full current rating they should be at least 1 diameter apart, and mounted in air, not insulation. The figures for current rating assume the cable will be running at 600C, which is pretty warm.

 

 

 

Steve, your comment would only be valid for the wiring of mains level voltages, for example 1.5mm conductors would commonly be used to carry up to currents of 15 - 20 amps. In low voltage DC installations no one would dream of using such a combination of cable and current. The volt drop calculations which are regularly discussed on this forum would preclude all such problems.

 

The 'current ratings' which are often seen for cables refer more to the temperature limits of the insulation than anything else.

Edited by John Orentas
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Steve, your comment would only be valid for the wiring of mains level voltages, for example 1.5mm conductors would commonly be used to carry up to currents of 15 - 20 amps. In low voltage DC installations no one would dream of using such a combination of cable and current. The volt drop calculations which are regularly discussed on this forum would preclude all such problems.

 

The 'current ratings' which are often seen for cables refer more to the temperature limits of the insulation than anything else.

 

Why wouldn't you dream of running 15-20 amps at 12v through a 1.5mm2 cable? Sounds reasonable to me. You'd have to watch the length as the volts drop as a % could easily be excessive.

You're right about the insulation being the limiting factor in conductor temperature, mineral insulated cables can run much hotter, and therefore carry more current, but then the volts drop will screw everything.

I would maintain that the voltage is immaterial, current is all. It doesn't matter what the voltage is, the volts drop and power dissipated by the cable is entirely dependent on the cable resistance and the current flowing. Also because the voltage is low, you are much more likely to be running a high current to get the power, therefore the cable is much more likely to be running at its limit so it becomes very important not to bundle cables, have them in insulation or in conduit, without derating them. Below is a table of correction factors.

 

These factors are additive, ( there are other tables for ambient temperature for example) and apply even if only a part of the cable run is bunched, ie cables going through a bulkhead

<DIV align=center><B><SPAN class=Booktext>Table 4.4 - Correction factors for groups of cables

 

Edit I pasted a table in here but it hasn't come out right, constructive thoughts on my ramblings appreciated.

 

Steve

Edited by Big Steve
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Why wouldn't you dream of running 15-20 amps at 12v through a 1.5mm2 cable? Sounds reasonable to me. You'd have to watch the length as the volts drop as a % could easily be excessive.

 

 

 

 

Hi Steve.

 

What I was getting at. If you were to wire a pump which draws only 10 amps at 12 volts midway up a 60 ft boat (40 mtr of wire) you would suffer a volt drop of approx 4.5 volts which you would no doubt find unacceptable, the cable however is rated at double that load so even here conductor temperature is not a significant factor.

 

In fact these days people would probably opt for a cable size of perhaps 6.0mm giving a drop of a more acceptable 1.17 volts. (even this close to 10%) A note on that cable reel will tell you it is good for 50+ amps.

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You're spot on John, I should engage brain before mouth. I'm so used to 240v I never thought about the huge volts drop, so as you say, cables will generally be running at far less than required to cause cable heating. I take back all I said about you :rolleyes:

 

Steve

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