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Old 04-07-2021, 05:53 PM   #1
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1968 28' Ambassador
Portland , Oregon
Join Date: Feb 2020
Posts: 81
Calculating wire run length for wire sizing

Hello friends,

I'm starting to delve into the 12v systems for my 28ft 68 Ambassador. I'm planning the wire runs and circuits.

One thing that has me severely confused is that there seems to be some disagreement, or different standards on how to correctly size the length of a circuit. My friend who has just finished a beautiful Argosy build, rebuilt his entire system, and also has a lot of electrical engineering in his background tells me to measure the wire run by the load wire only for calculations.

His advise completely negates the info I've come across on these helpful websites such as this website for van builds in which they ask for the total run of the positive and negative for calculations. When I calculate by that standard then most of my wire run lengths become incredibly long. For instance, my circuit for the lights in the back bedroom measured from the fuse box up front is at least a 40 foot run when you consider the length it takes to run up the wall and down to the light switch. I have 5 LEDs on that circuit and the draw would be around 1.75 amps max. With their calculator my wire run is 80feet and its telling me I need to run 10awg wire. Ive never herd of anybody running that heavy of a gage wire to a few leds. Seems off?

Do you think these van people double their wire length as a safety standard? I guess you could get away with it in a sprinter, those wire runs aren't too long. It's like this with most online wire gage calculators because most are from van build blogs.
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Old 04-16-2021, 04:36 PM   #2
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1987 31' Excella
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I can't imagine needing 10awg wire for that load.
The factory wiring, from before LEDs were a thing, wasn't that big, so why would it need to be with dramatically lower loads.

I'm about to strip my '87. So far the 12v wiring I'm seeing isn't bigger than 14awg save the battery cables.
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Old 04-17-2021, 11:54 AM   #3
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1976 Argosy 24
1961 28' Ambassador
1968 26' Overlander
Lakewood , Colorado
Join Date: Jun 2014
Posts: 115
Hi, remembering back to taking my '68 apart, The famous "control panel" about middle of the trailer streetside (good idea), Battery all the way in rear curbside (not so sure about that) but I think they got it right with what had to be an 8awg wire from the battery to the panel, it was close to 20 feet of wire. Putting mine back together I paid particular care to sizing the solar, inverter, and battery wire, lots of 4,6 and 00 gauge stuff. LED light only runs were 14 ga.. 12 volt runs for other things like fantastic fans are 12 ga. Wire to compressor fridge is 10 ga. but only 6 ft long. Batteries and 12 box are right at the front of SS wheel well. 120v is all 12 ga. Shore power to breaker box is 8 ga. 10ga would probably be ok assuming 30 amp service. Made use of the wiring books that VTS has sold, be happy to send them to you.
Regards, Mark D
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Old 04-20-2021, 05:56 PM   #4
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2017 25' International
Austin , Texas
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I like this chart

https://www.bluesea.com/resources/1437

It has directions for measuring the run length and seems to suggest 12 ga for that run but just barely past 14 ga.
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