# Impedance/Voltage vs Gauge

Hello,

Two copper cables have the same Impedance and Voltage but different Gauge. Will they act differently?

Application: copper cable assemblies used for internet. Both with 100 Ohms impedance and 3.3 Volts but they have different Gauges (24, 26, 28 and 30).

Related Electrical Engineering News on Phys.org
berkeman
Mentor
Hello,

Two copper cables have the same Impedance and Voltage but different Gauge. Will they act differently?

Application: copper cable assemblies used for internet. Both with 100 Ohms impedance and 3.3 Volts but they have different Gauges (24, 26, 28 and 30).

What is the context of your question? Is this for schoolwork?

What other characteristics are listed on the datasheet of Cat-5 type data grade cable? Can you see some other characteristics that might change with wire guage?

berkeman
Mentor
Not school work.
I'm trying to set up communication between multiple Snippet Blades in a computer rack.
I'm comparing different vendors and I was wondering if gauge had an impact when all other properties are identical.
http://www.molex.com/molex/products...EMBLIES.xml&channel=Products&langPref=english
I would think that the attenuation would be higher with the smaller wires -- do you see that listed on comparative datasheets? As long as they are holding the Zo the same, attenuation is all that I can think of at the moment...

sophiecentaur
Gold Member
The impedance you refer to is, I think, the 'Characteristic Impedance' (Z0) of the transmission line. If a line is terminated with a resistor equal to Z0 there will be no reflection of signals. Google it.
Z0 is a function of the dimensions of the line (ratio of inner diameter to outer diameter or ratio of conductor diameter and spacing). It isn't a resistance value of the actual wire that is used - in fact it is often assumed that there is no 'copper loss' on transmission lines.
You can have a whole range of gauges of wire for the same Z0. What you choose to use would often relate to the Power or the frequency of signal that it is needed to carry. What affects the losses in most lines is the dielectric material used as a spacer between conductors.

berkeman
Mentor
The impedance you refer to is, I think, the 'Characteristic Impedance' (Z0) of the transmission line. If a line is terminated with a resistor equal to Z0 there will be no reflection of signals. Google it.
Z0 is a function of the dimensions of the line (ratio of inner diameter to outer diameter or ratio of conductor diameter and spacing). It isn't a resistance value of the actual wire that is used - in fact it is often assumed that there is no 'copper loss' on transmission lines.
You can have a whole range of gauges of wire for the same Z0. What you choose to use would often relate to the Power or the frequency of signal that it is needed to carry. What affects the losses in most lines is the dielectric material used as a spacer between conductors.
Not sure I understand what you're saying, but there are definetly losses in TLs -- I deal with them all the time. The dielectric losses are part of it, but the ohmic losses are part of it as well.

http://en.wikipedia.org/wiki/Characteristic_impedance

.

dlgoff
Gold Member
This probably doesn't need to be considered by the OP, but I think one would have to consider the environment. Temperature changes can effect the impedance due to mechanical expansions?

Notice the "?". The amount of influence due to temperature may be negligible.

berkeman
Mentor
This probably doesn't need to be considered by the OP, but I think one would have to consider the environment. Temperature changes can effect the impedance due to mechanical expansions?

Notice the "?". The amount of influence due to temperature may be negligible.
That's a good point. Temperature definitely affects the Zo. Enough so that in our twisted pair networks, we have to limit the "average" temperature of the network wiring to 55C, despite the fact that any part of the wiring can be at the extremes of the -40C to +85C Industrial temperature range. I think that variations in the dielectric with temperature dominate, but I'm having trouble finding that old information... Not sure if it would be different for the different AWGs that the OP is asking about, though.

sophiecentaur
Gold Member
Not sure I understand what you're saying, but there are definetly losses in TLs -- I deal with them all the time. The dielectric losses are part of it, but the ohmic losses are part of it as well.

http://en.wikipedia.org/wiki/Characteristic_impedance

.
True but I was firstly pointing out that the "100 Ohms" isn't 100 Ohms of series or shunt resistance, which is what the OP seemed to be suggesting. Also, in examples like UHF feeder, it is largely the dielectric that accounts for the loss. The 'low loss' feeder you can buy looks little different in construction from the bog standard apart from the foam or spiral dielectric.
If you want to run 100m of feeder from a transmitter to an antenna then, of course, the odd few dBs from Ohmic loss is highly relevant.

It's pretty simple really.

As noted by others the line impedance is standard for cat5 which I believe we are talking about.

However the line resistance and current capacity per metre is not. This is important when using POE, particularly over long runs, where you will lose power to resistive attenuation.

I am not familiar with american gauge so you will have to look this up or perhaps others might help here?

berkeman
Mentor
Not school work.
I'm trying to set up communication between multiple Snippet Blades in a computer rack.
I'm comparing different vendors and I was wondering if gauge had an impact when all other properties are identical.
http://www.molex.com/molex/products...EMBLIES.xml&channel=Products&langPref=english
So I'm thinking now that the answer to the OP is "no difference" for his application. Short runs within a single rack of equipment, so the distance effects like loss won't really come into play.

There may also be mechanical reliability problems if you insert the wrong gauge wire into a punchdown termination, designed for a particular gauge.