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Old 02-17-2008, 09:17 AM   #15
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Originally Posted by 68 Overlander
Are we talking about AC or DC? I was refering to the original post and am clearly off topic if it changed from DC to AC. Household and DC have not so much in common.
You are totally right, as Mr. Edison's fight with Westinghouse [and Tesla] demonstrated. But for the purpose of carrying current, they are exactly the same [at frequencies below radio--high freqs tend to put the current in the skin of the wire]--one amp of DC in a wire loses the same energy as one amp of AC in the same wire. Power loss is directly related to current--voltage in and of itself has no contribution to power loss. Therefore, the practical difference between the two [for the purposes of this discussion] is in power transmission--AC can transmit power at high voltage and low current, thereby being more efficient (a transformer steps the voltage down at the user) and as a result can transmit power over long distances. DC could only transmit power on the order of a mile or so, and the wires had to be huge due to its [relatively] low voltage and high current.

You can demonstrate to yourself the equivalence of the two power sources at the load--a 12V incandescent bulb will work exactly the same on 12 volt AC or DC. This is true for any type of resistive load. We use DC in RVs because it's supplied directly from batteries--converting battery power to AC requires an inverter and only recently (I say recently as if the solid state revolution was "recent") have inverters been cheap on acceptably efficient. An additional problem with inverters is they are always "on" and taking a little power from the battery even when there's no demand at the load. Which puts us in a situation like the train tracks--they are the width they are because that's the width the Roman chariots wheels were. I think [in vehicles] we're stuck with DC for a long time to come.

Just a note: In the work environment, particularly above 50 volts, DC is far more hazardous than AC. If you touch live DC you tend to be stuck to it, whereas AC makes your muscles twitch, not clinch, and you tend to be "thrown" off the contact. If you survive touching high voltage DC you typically have a very bad burn, but touching similar AC often has no lasting damage.


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Old 02-17-2008, 10:39 AM   #16
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OK, enough physics. I'm sure we all agree on that!

For practical info on wire sizes and maximum currents in the tow vehicle to trailer interface (7-way connector), see the bottom diagram in this post:


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Old 02-17-2008, 10:57 AM   #17
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These are the official wire size-fuse rules:

18 gauge - 6 amps (stranded only)
16 gauge - 8 amps (stranded only)
14 gauge - 15 amps
12 gauge - 20 amps
10 gauge - 30 amps
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Old 02-17-2008, 12:19 PM   #18
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I was refering to the voltage drop when I said they are not so much common. DC voltage drop is significant.
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Old 02-17-2008, 03:47 PM   #19
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I hope we have helped this poor guy. I am afraid he is going to run away. All he wanted to do was replace a

I guess we are just thinking too much about this.

Overlander is right in saying that 6 gage stranded would be fine to run to the converter. I doubt you will ever see enough amps running thru that wire at one time to do any damage. Just as a precaution, please fuse that wire!!!

I am posting a wire size chart and the formula in the electrical section of our site. Feel free to add anything you want there.

I did my chart in a spreadsheet and now can't get it on the post. I don't write code and it just dumped the format when I pasted. I will get the chart up asap.

Genuis has limitations, stupidity has none.
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