can someone point me to a web site that has an on-line wire size calculator,

i'm building a motorhome, and need to work out hundereds of wire runs, some
will have .5 amp loads on them, some 250 amp loads, some 1 foot long, and
some 40 foot long and so on,

most of the voltage drop calculators i've found are aimed at household
wiring, and want you to put in the wire size your using, and it'll tell you
the volt loss,, a pain to do for every load in my van,

what i want to do is:

in one box enter the system voltage.. this will be wither 12 volts or 24
volts DC,
next box enter the current the loads on the wire will pull in amps,
next box enter the length of the wire in feet,

maybe have an option for the percent volt drop allowed.. for most systems 2
to 3% max is allowed, but for the charging system 1% is the most volt drop i
want.

then press calculate, and it spits out the wire size i need to keep the volt
drop below the set limit at the end of the run for the load it'll have on
it,

i.e::

12 volts,
25 amps,
30 foot length of wire,
2% volt loss allowed,

'calculate'

result: you need to use xx gauge wire, or xx amp rated wire,

i'm in england so the AWG system isnt used that much it seems, especialy not
for automotive wires where they measure the insulation with the conductor to
give the gauge.. so if wire A has 2mm thick insulation, and wire B has 4mm
thick insulation, both have the same conductor core size, wire B will be
sold as a lower AWG figure than wire A, indicating wire B carries more
current when i convert the AWG to the current capacity of the wire,

so idealy something that gives out the current capacity of the wire needed
to do the job would be a hell of a lot easier to use,

i.e to run 8 amps down a 18 foot wire, getting no more than 2% volt drop i'd
need to use 17.5 amp wire (i thin that one's correct)

Anyone help please, this is driving me nuts,

there are too many wires to be used to just use welder cable for everything
and be totaly safe so most circuits will need looms with the correct size
wire in them, with the relevent safety margins to reduce volt drop and wire
heating.