Hot End Wires!!!

Hi, I'm just half way through my first Prusa Mendel build and a bit confused about the whole Hot End wiring thing and just wondered if anyone could set me straight! I've browsed around and not been able to find much information on the subject!

Right to start with I've purchased the Mk 2 J Head Nozzle [reprap.org] from [thingfarm.org].

Now this uses a 5.6ohm, 5W resistor, which should draw just short of 1A. Though am I right in thinking I would be running this over capacity for use in the Hot End?

I read somewhere that the resistor needs to be at around 20W to heat the Hot End which would draw just short of 2A!

So what current rating wire should be used?

Should I use:-
7/0.2 = 1.4A Max (adequate for normal use).
16/0.2 = 3A Max (adequate for running at 20W).
24/0.2 = 6A Max (just to be on the safe side! lol).

Also would the 7/0.2 equipment wire be ok for wiring up the thermistor?

If it helps I'm planning on using the Gen7 Electronics for my build.

Thanks in advance for any advice!

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[indieflows.blogspot.com]
[www.emakershop.com]

5.6R will draw 12/5.6 = 2.14A. You should use at least 16/0.2. Any wire is fine for the thermistor.

The resistor is driven way over its rating but the rating is for it in free air whereas we are attaching to something that acts as a heat sink and we limit the temperature to about 240C with the thermistor. As long as there is good thermal contact between the resistor and the block I have not had one fail.

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[www.hydraraptor.blogspot.com]

Ah right I see now! So basically the resistor will be running at around 25.7W (V2/R = P, 12x12/5.6)! 5x times over capacity to heat the Hot End.

Now just to confirm would this wire be ok [www.maplin.co.uk] temperature wise?

It has a PVC insulation (rated to around 95 degrees) though will be around 50mm away from the resistor (resistor leads) plus covered in heat shrink tubing!

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[indieflows.blogspot.com]
[www.emakershop.com]

I find PVC wires go hard and brittle over time when used for the resistor.

I prefer PTFE insulated wire and use PTFE sleeving over the resistor and thermistor leads with heat shrink over the join to the wire. Some people use heat shrink all the way to the resistor and that seems to work although I doubt it is rated for 240C.

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[www.hydraraptor.blogspot.com]

Must admit I never considered the insulation going brittle, which obviously isn't very good for a wire which needs to move!

So does this sleeving look ok? [uk.farnell.com] max operating temp 250 degrees!

Also does this wire [uk.farnell.com] fit the bill? 6A max temp 190! (though highest I can find for this diameter!).

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[indieflows.blogspot.com]
[www.emakershop.com]

Yes they look fine.

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[www.hydraraptor.blogspot.com]

Champion, cheers once again Nophead!

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[indieflows.blogspot.com]
[www.emakershop.com]