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I'm making a sequenced rocket launcher. I want to apply 5 amps for 3 seconds through a 12 inch run of 24 gauge wire.

Since the launcher has four slots, I figured I could use cat5 ethernet cable. This bundles the necessary 4 pairs of wires nicely and is color-coded and conveniently available. Since each ignition will have its own circuit, each 24 gauge pair will only see the surge once.

Is the 5 amp surge going to be too much for the 24 gauge wire in the cat5 cable? From examining tables like these (http://www.powerstream.com/Wire_Size.htm) I know that my current requirement is about 10 times the limit for 24 gauge wire. Max current is further degraded by the wires being bundled. However, I can't find good data on acceptable surge limits.

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  • \$\begingroup\$ How many time you allow before second fire? \$\endgroup\$ – GR Tech Jan 11 '15 at 21:16
  • \$\begingroup\$ Why not try a test? \$\endgroup\$ – George Herold Jan 12 '15 at 2:08
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The [Wikipedia article][1] on American wire gauge suggests that the fusing (melting) current for 24-gauge copper is 29 amps. That would put you within the acceptable range.

I would normally worry about voltage drop with such a current in a small gauge wire. From the same article, 24 gauge is about 25 milliohms per foot, you'll have two feet of wire, so that's 50 milliohms, and at 5 amps you'll drop about 0.25 volts. That won't have a measureable effect on the igniters. Based on that voltage and the current, you can calculate the wattage and, based on the heat capacity of copper and the mass of the copper in the wire, figure out how much hotter it's going to get. If you have a limit, it's much more likely to be the temperature tolerance of the insulation rather than the copper conductor.

If I were thinking about this, I'd just give it a try and see how hot the wire got.

The big drawback of cat5 in this application - I assume you're using it to connect to an igniter for the engine - is that the individual wires are solid (and not very flexible) and the flexibility is even worse with multiple wires. You will also run the risk of breaking wires as they flex over time. I've used this sort of cable to do connections in the past and hated my choice.

If I were building this, I'd look for something in the 12-14 gauge stranded - something like speaker wire would be fine. If I needed the pairs to be color coded, I'd do it with a Sharpie.

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  • \$\begingroup\$ You're right: the voltage drop information is what was really worrying me. I thought cat5 was offered in both solid and stranded core. My project has a weight requirement, so I can't quite go as large as 4 pairs of 12 gauge. In any case, you got me the info I needed. \$\endgroup\$ – RocketLauncher Jan 12 '15 at 5:02

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