My home builder pre-pulled a length of 18-8 solid copper wire (18 AWG, 8 insulated conductors inside single jacket) into the ceiling of a room where I want to use some LED lighting with a remote power supply (that is, the power supply is at one end of the 18-8 and the LED lighting is at the other end). I can easily calculate the needed current / wattage for the lighting, but I'm having a hard time determining the various limits of the wire (current, voltage drop) to be safe and effective.
If I'm reading the charts correctly, this wire should be about 6.4 milliohms per foot and have an ampacity of 10 amps at 20degreeC insulation temperature and 60degreeC conductor temperature (not sure how that's possible or exactly how they measure that).
My LED lighting to be connected would draw no more than 52 watts, or about 4.3 amps at 12 volts or 2.2 amps at 24 volts.
My proposed power supply will happily provide 150 watts at 12 volts before its own overcurrent protection gets triggered. Therefore, I believe it would be advisable for me to fuse the output connections before the long wire runs to the ceiling at some lower value to prevent the wire from becoming an ignition source in the case where the LED strip itself shorts out.
So my questions are:
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How long a run of the 18-8 wire can I used before I run into problems? Since the wire is already there and I'm loathe to tear out my ceiling, what are alternative solutions if my voltage drop is too high? Can I pair up conductors in the 18-8 to have instead of 4 pairs, have 2 pairs of paired conductors?
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What current value fuse would make sense? I'm thinking 80% of the 10 amp ampicity, or 8 amps might be a safe value.
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Have I read the charts and gotten correct values for resistance and ampacity? Do my numbers sound right?
I'm an amateur at this — my college EE courses are ancient memories. I probably know only enough to be dangerous, as the saying goes. Feel free to set me straight or advise me in anyway appropriate. I'm not math averse, so if pointing me at the proper equations or references is appropriate, do that. I can do the calculations, but as with anything engineered, there's rules of thumb, acquired wisdom and outside factors to consider beyond something as simple as E=IR.
Best Answer
Lets simplify this a WHOLE LOT.
As long as you are within 50' of the source, 18 gauge wire can safely carry 10 amps.
That's 1200 watts you can supply with a 50' run. As long as you don't plan on exceeding that power, you have nothing to worry about.