Date: Tue, 18 Jan 2022 22:16:15 +0000
Reply-To: "ddbjorkman@verizon.net" <ddbjorkman@VERIZON.NET>
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From: "ddbjorkman@verizon.net" <ddbjorkman@VERIZON.NET>
Subject: Re: Best Wire Selector -- Voltage Drop Charts Mayhem
In-Reply-To: <CACARJKq58Lo+eNtMRYGwkE+S=0nUPWdyVXFpzeRnC2XF4ovF7g@mail.gmail.com>
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Hey Alex;That is a bunch of great info.
The things this list come up with continues to amaze me. We are a very eclectic group. Not only that, we have a bunch of folks from different backgrounds and interests. And we all love to share. And we have cool rides.
Dave B.the Boston one
-----Original Message-----
From: Alex MeVay <alex@MEVAY.ORG>
To: vanagon@GERRY.VANAGON.COM
Sent: Tue, Jan 18, 2022 3:13 pm
Subject: Re: Best Wire Selector -- Voltage Drop Charts Mayhem
Hi Derek,
You're on the right track here. A few comments:
Your step 3 I think should instead be to see what wire size can handle
the required current without overheating.
My go-to resource has been this link:
https://www.powerstream.com/Wire_Size.htm
The "maximum amps for chassis wiring" is the current the wire can
handle without overheating (though this will depend on bundling,
proximity to other heat sources, etc.).
There is a current-for-voltage-drop column, but I don't use this.
Instead there is a milliohms/ft. (or Ohms/1000ft) resistance number.
Say you have an 8 amp load on a 10' round trip run of 12AWG, V=I*R, so
your voltage drop would be 8*10*1.588m, or 127mV, or about 1% on a 12V
system.
Generally, fuses should be chosen for at least 125% of the maximum
load. In 12V systems, though, the wire size is often determined by
voltage drop (your step 2) and not current handling (your step 3). As
long as the fuse is small enough in value to protect the wiring (i.e,
less than the "maximum amps for chassis wiring"), don't be afraid if
it ends up substantially larger than the expected load current. Don't
try to over-optimize and subject yourself to nuisance blowing.
As far as how much voltage drop to aim for, 3% is probably good for
"smart" loads like inverters and car stereos which have low-battery
cutoffs and will crap out with too much drop. For dumber loads, like
lighting, fans, USB chargers, etc. 5% or more would probably be fine.
Regarding tinned wire, I agree...use it whenever possible. With
properly terminated connections, it's no more reliable than bare
copper; the difference is in maintenance. If you ever need to crimp
on a new terminal, or splice something in, you may find untinned
copper covered with black oxide that is very difficult to remove.
Hope this helps,
Alex
