changing wire gauge

[Matt Warnock]

I have a question about wire gauges. I've seen a plethora of information
about gauges of wire and the current that should pass through them. What if
the wire thickness changes inline, such as for a very short time? I'm
thinking about what happens at connectors or fuse boxes where the cross
sectional area changes. Is it the smallest thickness that's the most
important or the overall average thickness because that affects the overall
resistance?

thanks for any incite!

-matt

 

[John Popelish]

I have a question about wire gauges. I've seen a plethora of information
about gauges of wire and the current that should pass through them. What if
the wire thickness changes inline, such as for a very short time? I'm
thinking about what happens at connectors or fuse boxes where the cross
sectional area changes. Is it the smallest thickness that's the most
important or the overall average thickness because that affects the overall
resistance?

thanks for any incite!

-matt

If you are only worried about total wiring voltage drop, a small
length of undersized wire often makes little difference. But if you
are worried about the temperature of the wire, a necked down section
will certainly get hotter than the larger wire, and with high enough
current may melt its insulation or even melt the wire, acting as a
fuse. You should calculate both effects to make sure you will not
have problems with either voltage drop or peak temperature.

 

[Matt Warnock]

how much "headroom" is there in the rules about wire thickness?

http://www.powerstream.com/Wire_Size.htm

has some rules. 6 gauge wire will run 101 amps in a chassis wiring
configuration. 8 gauge will do 73 amps and 10 gauge can do 55. When does
it really start redlining? The description says its a "conservative
rating".

 

[John Popelish]

how much "headroom" is there in the rules about wire thickness?

http://www.powerstream.com/Wire_Size.htm

has some rules. 6 gauge wire will run 101 amps in a chassis wiring
configuration. 8 gauge will do 73 amps and 10 gauge can do 55. When does
it really start redlining? The description says its a "conservative
rating".

These ratings are based on some packing factor. I don't know what
that is without a bit of digging, but a wire can carry a lot more
current without overheating when it is suspended in air than it can
when it is tightly bundled with lots of other wires that are also
carrying the same current density (amperes per cross sectional area).
More conservative ratings apply when the wire is wound tightly around
a hot magnetic core. The rating also implies some maximum temperature
based on the insulation temperature rating, so vinyl coated wire will
have a lower maximum ampacity than, say, teflon coated wire, all other
things being equal. You might also have to worry about other
materials against the wire that either melt, degrade or catch fire
easily.

None of this addresses the question of acceptable voltage drop, which
is a system question, not strictly a wire question.

The rule of thumb is, if you don't know how to calculate temperature
rise and voltage drop, you use conservative wire sizing rules to cover
your ass.