Date: Tue, 18 Jan 2022 10:13:44 -0500
Reply-To: Derek Drew <derekdrew@DEREKMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Derek Drew <derekdrew@DEREKMAIL.COM>
Subject: Best Wire Selector -- Voltage Drop Charts Mayhem
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Since both Dennis and David B are sometimes on this list, this question
has a chance of being answered here.<br><br>
Basically, what I have discovered is that the various charts that are
online to help you determine voltage drop and to help you pick wire size
(and fuse size) for a device or for a main trunk line are all over the
map with different values being presented. <br><br>
My question is what the default charts (or tools) that the 12 volt
experts on this list actually use to plan 12 volt runs in their vans.
<br><br>
Some advice I have seen is to keep main trunk lines voltage drop down to
3% and to keep accessory lines to a voltage drop of something like 10%.
<br><br>
I was hoping to collect the information in one big chart so that if I
knew that I had a given load (example: I have 30-amp cigarette lighter
power outlets) then I would know to pick a 10 gauge wire for that outlet
and to fuse that outlet accordingly. <br><br>
Please let's forget about the wisdom of running 30 amps through any
automotive cigarette lighter for the moment as I am sure we all have
nightmares of pulling a male plug out of one of these and finding out
that the end of that plug is red hot for one reason or the other [such as
not being pushed in with enough force and positional stability under
vibration].) I also do not want to start any discussion of what type of
power outlet is best in a vehicle. Flosser (the one with the little green
light-up rings) wrote back to me to say that their outlet is rated for
something absolutely stupid, like only 7 amps or something, whereas CASCO
USA-style cigarette lighters are rated for 30 amps. Many wisely are
converting to twist lock style outlets, but these are rated less....
something like 15 or 20 amps. <br><br>
Anyway, it sounds like if you have a device rated for 30 amps then you
use a slightly higher-rated fuse, such as a 35 amp fuse, for example.
<br><br>
The steps seem to be: <br><br>
1) determine max continuous amp draw of the device<br>
2) pick a wire that will power that device with a voltage drop of
only..... how about 5%?<br>
3) determine the maximum power that your wires can handle with a voltage
drop of, OK, say, 10% not to over-engineer things. <br>
4) pick the lower of the two values provided in 1) device draw or 2) wire
capacity <br>
5) pick a fuse that is a little bigger than this value<br><br>
The trouble with this is that voltage drop tables found online (and wire
size selection charts) generally do not agree with each other. Charts for
boaters differ from charts for automotive use and these differ from
charts for other uses. The trouble is compounded because most charts have
steps in them that makes it hard to look under the hood and see what is
really happening. They will say what wire size to use for 35 amps and
what wire size to use for 40 amps and so forth, but they don't specify
what wire size to use for 37 amps. Moreover, the information is hard to
read because most charts only report information by the major awg steps
(e.g., 18, 16, 14, 12, 10, etc.) which makes things a bit opaque to see
what is actually going on. The confusion is compounded because, for us,
there are double the number of wire sizes to keep track of since we have
the european sizes 1, 1.5, 2.5 and so forth. Forget for the moment that
American *automotive* SAE AWG sizes are apparently slightly smaller than
regular American AWG sizes and can carry slightly less current in a given
size. For my own wires, I came to suspect that to keep corrosion problems
at bay long term we should choose Marine pre-tinned wires such as those
sold by Ancor instead of non-pre-tinned wire commonly available for
general or automotive use. <br><br>
To really understand your wire sizes, apparently, it makes the most sense
to convert your brain (or your formulas) to think in terms of circular
mills. <br><br>
I have to think that one of our 12 volt experts on here has a better
go-to source for decisionmaking than I have settled on so far. <br><br>
I did get as far as making an Excel calculator to figure out what wire
size to use based on the formula: <br><br>
Circular Mills = (k=10.75 * AMPS * # of Feet) / voltage drop<br><br>
.... where k is a constant value magic number of 10.75<br>
.... where "# of feet" includes the entire length of wire in
the circuit, both positive side and also negative side<br>
.... where "voltage drop" is entered in Excel as a
percentage.... e.g. "5%"<br>
.... where the required circular mills is calculated and then selected
from our AWG choices by referencing this chart and picking a wire that
has at least the circular mills asked for as a result of the formula.
<br><br>
Circular Mills Table<x-tab> </x-tab><br>
Wire Size<x-tab> </x-tab>CM<br>
18<x-tab> </x-tab>1537<br>
16<x-tab> </x-tab>2336<br>
14<x-tab> </x-tab>3702<br>
12<x-tab> </x-tab>5833<br>
10<x-tab> </x-tab>9343<br>
8<x-tab> </x-tab>14810<br>
6<x-tab> </x-tab>25910<br>
4<x-tab> </x-tab>37360<br>
2<x-tab> </x-tab>62450<br>
1<x-tab> </x-tab>77790<br>
0<x-tab> </x-tab>98980<br>
"00"<x-tab> </x-tab>125100<br>
"000"<x-tab> </x-tab>158600<br>
"0000"<x-tab> </x-tab>205500<br><br>
All of this begs the question as to what voltage drop guidelines we
should be following.... 3% seems a little overengineered and 10% sounds
like too much. 5% sounds like a happy medium, but I noticed that not too
many engineers are providing 5% tables so this does not seem to be a
popular choice. <br><br>
<br>
SOURCES (Do not agree with each other)<br>
<a href="http://circuitwizard.bluesea.com/" eudora="autourl">
http://circuitwizard.bluesea.com/</a><br>
<a href="http://www.bcae1.com/fuses.htm" eudora="autourl">
http://www.bcae1.com/fuses.htm</a><br>
<a href="https://law.resource.org/pub/us/cfr/ibr/001/abyc.E-09.1990.pdf" eudora="autourl">
https://law.resource.org/pub/us/cfr/ibr/001/abyc.E-09.1990.pdf</a><br>
<a href="http://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/" eudora="autourl">
http://www.offroaders.com/technical/12-volt-wiring-tech-gauge-to-amps/</a>
<br>
<a href="https://www.bluesea.com/support/articles/Circuit_Protection/1441/Part_2:_Select_a_Fuse_and_Fuse_Holder_For_Your_DC_Product_Installation" eudora="autourl">
https://www.bluesea.com/support/articles/Circuit_Protection/1441/Part_2%3A_Select_a_Fuse_and_Fuse_Holder_For_Your_DC_Product_Installation<br>
<br>
</a><x-sigsep><p></x-sigsep>
____________________________<br>
<b>Derek
Drew</b>
<br>
Sharon CT<br>
derekdrew@derekmail.com<br>
Email is best normally but...<br>
Mobile: 703-408-1532 (voice and texts) </body>
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