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Author Topic: Maximum current in wires.  (Read 1162 times)
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Hi,

I just finished my project of controlling a LED Strip with my Arduino.
I like the result and plan to add more strips !

I'd like to control a 10m LED Strip of 144W.
This means that at 12V, there will be 12A going through the wires.
That seems to be a lot :/

Do I need special wires for that ? What would be the minimum diameter of the wires ?
I have some MOSFET that can handle up to 46A (that seems strange because the pins of the MOSFET looks very tight :/)

Thanks a lot for your clarifications smiley-razz

P.
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Do I need special wires for that ?

Yep.

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What would be the minimum diameter of the wires ?

Depends on the distance. Don't forget "distance" is the full loop, not just from the power supply to the LED. See this table:

http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

Good luck!


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12 A...... I will choose 14 gauge electrical wire, standard 120 V - house electrical wiring. Go to your local hardware store to check it out.
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Most "household extension cords" will be rated for 10 to 15A, so you can probably get away with one of those (and they'll be rated, somewhere, for amps...  http://www.homedepot.com/webapp/catalog/servlet/ContentView?pn=Extension_Cords_Reels )
In fact, such cords are frequently a convenient and cheap way to get wire in that size range...  (Just make sure it's difficult to accidentally plug your 12V LEDs into a 110V outlet!)
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With low voltage high power circuits you need not only to ensure the wire is thick enough to take the current without overheating, you need to check that its resistance is low enough not to use up too much voltage through its IR product.  Here you load is 1 ohm (12V / 12A).  You probably want you wire total resistance to be less than around 0.05 ohm to avoid dropping more than a half a volt or so (the LEDs will be in series with current limiting resistors such that the brightness will be quite sensitive to voltage).

So take the wires total length into account when choosing it - length times resistance-per-unit-length should be nice and small, as I say 0.05 ohms or less would be ideal.

Same goes for the MOSFET, ignore the current rating, that's only relevant on infinite heatsink, go by the Rds(on) value - that wants to be 0.01 ohm or less with a medium sized heatsink, 0.02 ohm or less with a cooling fan or large heatsink.  (Power dissipated in MOSFET is IxIxR, so R must be tiny to avoid excessive heating)
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take the wires total length into account when choosing it - length times resistance-per-unit-length should be nice and small, as I say 0.05 ohms or less would be ideal.

That'd be nice, but I'm not sure it's realistic.  16g wire (most extension cores) apparently has a resistance of about .004 ohms/foot, and 12g wire (getting pretty huge) is still up near .0016 ohms/foot.

http://www.cirris.com/testing/resistance/wire.html

It might be better to pump up the source voltage a bit to correct for losses in the wiring.  If the LED strips have constant-current drivers, it may not be necessary.
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I think you'll find you'll need to connect the 12V in multiple places to avoid burning out the wires in the LED strips themselves.
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Below is a useful link for sizing wire for maximum current capacity. Note that in most cases it's not the copper wire that is the limiting factor but the temperature rating of the insulation used on the wire to prevent melting due to heat rise from the (I x I) X R power being dissipated by the wire resistance.

http://www.interfacebus.com/Copper_Wire_AWG_SIze.html


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Their listed "current carrying capacity" seems VERY low compared to common practice. (<4A for 16g wire?  Nor do I have any 1000W hair driers with 12g cords...)
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Their listed "current carrying capacity" seems VERY low compared to common practice. (<4A for 16g wire?  Nor do I have any 1000W hair driers with 12g cords...)
If you read the notes it looks like they are referring to VERY long wires.  The notes also specify a different current rating based on length.  If you were running that 1000W hair drier 200 ft from the outlet on an extension cord you WOULD need a 12 or 10 awg cable.
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I forgot to mention. CrossRoads is correct.
I think you'll find you'll need to connect the 12V in multiple places to avoid burning out the wires in the LED strips themselves.

In order to get consistent light all the way across the strip at the least you will need to connect to both ends of the strip.  Other wise you end up with a bright end near the power and a dim end at the far end.
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Their listed "current carrying capacity" seems VERY low compared to common practice. (<4A for 16g wire?  Nor do I have any 1000W hair driers with 12g cords...)

Normally cable manufacturers quote the resistance on either 1Km lengths or 1000ft,
The reason they have quoted the table like that, is that once a cable gets warm its resistance INCREASES.
http://www.allaboutcircuits.com/vol_1/chpt_12/6.html

Also the resistance depends on where your cable is from.
Most of the 'good' crap out of China is low quality copper, the real copper cable crap out of China, often has aluminum or steel strands that have been chemically treated to look like copper that is mixed in with the real strands.

Sometimes it pays to get 4 core 16A then parallel two of the cores to give 50% of the single core resistance and double the current, and not worry about temperature rise.

« Last Edit: February 21, 2012, 07:51:13 pm by hardcore » Logged

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