Here's a table of wire ampacity...the max amps for any gauge and type of
wire (cu, al, THHN, UF, etc) before it would get too hot. Heat in the wire
is proportional to the (square of ) the current [amps] in the wire. A
larger diameter wire, with lower resistance, will carry more current before
it gets "too hot". [ BTW, the "HH" in "THHN" is the temperature rating of
the insulation; "HH" means HOT HOT (very technical), but translates to 90
degrees C, IIRC]. Ampacity doesn't change with length.
http://www.armstrongssupply.com/wire_chart.htm
Voltage drops due to the resistance of the wire as well, but the resistance
is additive for length, as well as being constant for wire diameter [gauge];
it is inversely proportional, meaning that large diameter wire [smaller
gauge number...don't you love the American measuring systems???] has lower
resistance, and hence a small drop in voltage per foot of wire. The drop
adds up as you go further. Think water pressure in a 200 ft long garden
hose.
for a given application, heat [amps] or voltage drop [length] will be the
dominant factor to choosing gauge.
---
you could use 8 ga Cu THHN for 55 amps; although 10 ga wire would not have
over 3% voltage drop, it isn't rated for over 40 Amps, meaning it would get
too hot if you used it, even though the voltage drop would be OK
HTH a little in understanding why both limits need to be considered.
PM
<<<Subject: Re: [Shop-talk] Gauge of wire
I agree with Mark. When I was teaching house wiring in a HS vocational
setting, we used #10 for 30 Amp driers.
I do not remember what size we used for the range. It was either #6 or
#8.>>>
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