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Old 03-29-2015, 09:04 PM   #11
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Go as big a guage as you can manage for the wire pull. A 6 or 4 will provide enough capacity to handle as much amperage as you might need. You said you had a 100amp block. While you will never need 100amps up there, it would be nice to be able to up the fuse size and increase your power if you decided to add some large DC component .

My .02

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Old 03-29-2015, 09:04 PM   #12
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Mustang: I assume you got your voltage drops based on published resistance numbers or did you use the tables you posted? (I couldn't make them out.) If not they are in the tables of the NEC (I can get them for you if you'd like). Assuming your calculations are based on the correct resistance values (I don't have my code book handy right now) you have neglected to consider the voltage drop across the return path to the negative terminals of the battery - unless your tables take that into account. You have to consider that the voltage drop from the battery to the load, then across the load, then back to the negative battery terminal is going to be 12V. So 12V less the drop to the load and less the drop back to the negative terminal is what you have left across the load which you hope is something close to 12V. Single phase AC and DC voltage drops are calculated basically the same way i.e. distance x 2.
IMHO I would add an inline fuse at the source sized for the expected load so the wire size could be minimized. I wouldn't think LED lighting would amount to much current?

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Old 03-29-2015, 09:21 PM   #13
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Also consider that you are starting with higher than 12 volts, so the drop should still keep voltage + 12
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Old 03-29-2015, 09:55 PM   #14
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Originally Posted by andrewjsmith View Post
Coming off the battery now is a 12/2 and 10 awg wire that looks like it feeds the fuse panel now. That is run about halfway down the trailer. If they can run something that small, I should be fine with 8/2 wire to only power up 15 amps max
Yes, and yes.

You said storage compartment and LED lighting. How much more load could you possibly ever tag on? Another light maybe?

As a simple practical matter...

Install your #8 AWG wire and fuse it at 30 amps (a very conservative protection given your intended use). IF you ever overload that 30 amp fuse to failure, then you should be concerned and take the time to do more calculations.


P.S. - That 30 amp fuse will never blow in my lifetime.
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Old 03-30-2015, 05:39 AM   #15
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I just went through my old numbers and the latest calculations and they changed the formula from automatically including the return run in the initial calculation. Now you need to x 2 the numbers in the calculation. Updated my notes.

Rechecked my TT's numbers and I am OK. I originally calculated mine when it was still included in the calculation.

Thanks for the info,

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Old 03-30-2015, 05:53 AM   #16
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Didn't even think about this before but I could run the fuse block into the front compartment and only need about 8 feet. Then I can run all my LED lights from there. Better that way? I'd still have voltage drop from the panel to the lights, but I could manage with smaller gauge feeder wires.
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Old 03-30-2015, 06:44 AM   #17
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This should go without saying, but since the word fuse and breaker are used generically, and, there is a class for auto-reset breakers, I'll say it.

Use actual fuses. Never use auto-reset breakers for anything. I have no idea why they exist.
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Old 03-30-2015, 10:53 AM   #18
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Originally Posted by Mustang65 View Post
You should run both a positive and a negative cable directly from the batteries to the rear storage compartment. snip

Just curious. Why should a negative conductor be strung all the way from the battery? A connection to the trailer frame would provide the negative side of the circuit with a much shorter length of wire. That's how my main converter panel is connected.
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Old 03-30-2015, 03:15 PM   #19
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Andrew Based on your projected load , have a safety factor and lessen the voltage drop you should use 8/2 with ground (fused at 50A at both ends) nm cable or use a 8/3 sj cable. Nm or romex as it is called has solid conductors where as SJ cable has stranded conductors which are easier to work with. If you are going run under the tt you should use nm, if you are going inside through cabinets etc Sj cord will be ok if kept out of harms way. Larry
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Old 03-30-2015, 03:52 PM   #20
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Determining Cable Size - Low Voltage Landscape Lighting Design Help

This may have some relevance.

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