what wire size

joohn

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Can you recomend a wire gauge that I can use to power two sst-90 in parallel I think the load is 7.6volts @ 9Ah there are no runs longer than 3" (75mm) I would like to keep the size down if I can.
 

xul

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Can you recomend a wire gauge that I can use to power two sst-90 in parallel I think the load is 7.6volts @ 9Ah there are no runs longer than 3" (75mm) I would like to keep the size down if I can.
Figure the equivalent resistance of these two LEDs in parallel and size the wire to be less than 10% of that resistance.
 

VegasF6

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AH is a measurement of time.

sst-90 can be powered up to 9 amps. Typical voltage will be somewhere around 3.6 (don't have the datasheet to be more accurate.)

Parallel circuit would yield 3.6v at up to 18 amps. Series circuit 7.2v at 9 amps.
 

xul

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Parallel circuit would yield 3.6v at up to 18 amps. Series circuit 7.2v at 9 amps.
3.6v/18v = 0.2 ohms.
Total circuit resistance, including contact resistance, should be 10x less. This might prove challenging if not impossible.
 

VegasF6

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xul, I gotta admit I don't know what you are talking about. 3.6V/18A = .2 ohm resistance. I follow you up to then, but wire size should be 10 percent of that? 10 percent in what sort of measurement? Mils? AWG? Inches? MM?

The charts show to use 9 gauge or larger. If you wire a house, in the United States at least, you would pull 14 AWG Romex for 15A outlets, 12 AWG Romex for 20A outlets. For low voltage wiring you would have to worry about voltage drop so you would want to take wire impedance into account, but for a 3" run I would ignore that entirely. So, I would consider either 12 or 14 pretty much safe. That's only if the OP means a parallel circuit, which is open to debate from his specs.
 

xul

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10% of 0.2 ohms is 20 milliohms. This might be easy to achieve with short pieces of heavy wire [1' of #14 copper is about 2.6 milliohms at 20 celsius] but the contact impedance of switches and battery contacts in these kind of lights may gobble up a lot of this 20 milliohms.
A good relay contact at rated current shows 30 mV or less and a bad one shows 100 mV or more.

It's the same problem with these daisy chained 12v 50w undercabinet puck lights or a 12v battery cranking a 200A starter motor. If you want them to work, each connection has to be very clean and very tight. With 120v at reasonable currents the problem is almost nonexistent.

You would be surprised at the number of electricians and engineers who do not know about contact resistance or 4 terminal Kelvin measurements.
 
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joohn

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The reason I asked the question was to find out the gauge I cant follow the calculations myself my trade is a carpenter, so 10 gage is 2.6mm dia that seems awful thick to me?
 

LuxLuthor

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I agree with Xul that 16 AWG wire for a length of 3 inches. IMHO, based on experience...with 70 watts (7.6V x 9 Amps) over 3 inches, I think you would be fine with 18AWG. I know for a fact that I have setup a number of 100+ watt incandescent lights with 18AWG and had no significant voltage drop, or wire heating up.
 

xul

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Grainger shows 15A max for #12 and #14 extension cords, and 13A max for #16. There are tables on the 'net that extrapolate this to smaller sizes; the current squared controls the heating in the wire and the max wire temp depends on the type of wire insulation.
 
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