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# What formula(s) do I need to know what gauge wire I need?

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#### trend

##### Member level 5
I have had to run a lot of wire lately.. and I am tired of the "use the thickest gauge you have on hand.." idea.. because I have wired a machine with 20gauge wire for push buttons.. and now I see that this just adds a lot of clutter..

So basically I just would like a tutorial or formula or something that would explain what gauge wire I need for situations that have the following variables:
differenet voltage requirements
differenent mA requirements
differenet cable length requirements..
and if I could understand wire interference, that would be great! because I seem to have a good bit of interference in my current project.

man.. that was offly wordy, for something that didn't need to be

thanks-Lee

#### House_Cat

Re: What formula(s) do I need to know what gauge wire I need

First - The voltage rating of a wire has nothing to do with the gauge. The voltage rating is based on the insulation.

Second - The interference is a function of the signal levels on the wire. You can get interference from adjacent current carrying conductors if the magnetic field surrounding those conductors can cut through your signal conductor. Likewise, you will cause interferrence with other nearby conductors if the magnetic field surrounding your conductor can cut through them. There are two ways to reduce interferrece in conducting paths - 1. The strength of a magnetic field is inversely proportional to the distance from the field. Space your wires away from other wires. 2. Shield your signal from other conductors. Perhaps the easiest way is to use a twisted pair of wires for each signal and its return line. This is the way telephone lines and network lines are "shielded". The twist causes field cancellations between the two wires. Another way is to use coaxial cable. More can be learned by searching the web.

Finally - To decide what gauge you need, you need to decide how much voltage drop from wire resistance your application can handle. The resistance of the wire will also determine how much heat your wire will generate while the current is flowing (I squared times R = watts ). The heat will become important if you have a long run in an enclosed area such as a conduit or a wall, and it will determine what insulation the wire must have - insulation varies in its ability to handle heat, and voltage breakdown of insulation is a function of heat. A couple of useful web sites are listed below. Do a web search using Google for "wire gauge" and you will find many more.

http://www.powerstream.com/Wire_Size.htm
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/resis.html#c2

### trend

Points: 2

#### trend

##### Member level 5
Re: What formula(s) do I need to know what gauge wire I need

thanks guys!

Honestly, I got
https://www.powerstream.com/Wire_Size.htm
after a quick google.com search, but the calculator made only a little since.

thanks house_cat for the great explination!

#### Borber

Re: What formula(s) do I need to know what gauge wire I need

One aspect of selecting wire diameter is allowed voltage drop. But sometimes more important is to select wire diameter on basis of maximal current that will flow through wire.

#### trend

##### Member level 5
Re: What formula(s) do I need to know what gauge wire I need

Got another question though...

in this example:

(6volts output over 30awg with 6feet cable with 1 amp load)

It says I will be dropping 2.1volts..
I will get 3.9volts at the end of the circuit
35percent drop

--
That means, I will be inputting 6v 1amp into the wire, and outputing 3.9v 1amp at the end of the wire (loosing into space 2.1volts.. 0amps)

Is this correct?

So basically you don't loose amps, just voltage.. and "by loosing" what happens to this voltage?(just turns into heat?)

thanks guys!
Lee

borber, that is a good point.. somehow I looked over in the last couple of minutes.. but the only rule of thumb I was using...

The calculator I was using on:
https://www.powerstream.com/Wire_Size.htm

let me input these factors... 30awg at 480volts 3-phase@15amps, cable 60feet long.. and it outputed some figures.. that made this wiring look feasible..

I guess my other question is.. How do you know what wire to pick based wire diameter? The chart on powerstream.com only tells you the max amps per gauge.. not the max voltage.

thanks again guys!
Lee

#### Borber

Re: What formula(s) do I need to know what gauge wire I need

The calculation is correct but you will disipate 2.1W on wire heating. For 30 AWG max. allowed current for chassis wiring is .86A and .142A for power transmission. As I remember for small transformer the wire diameter is calculated on basis that allowed current density is 4A/mm^2.

#### trend

##### Member level 5
Re: What formula(s) do I need to know what gauge wire I need

so.. amps do not get disipated? just volts?

#### House_Cat

Re: What formula(s) do I need to know what gauge wire I need

so.. amps do not get disipated? just volts?

Heat is dissipated, or rather electrical energy is converted to heat. The current is electron motion through the wire, like fluid through a pipe. The voltage is the potential energy difference measured from one end of the wire to the other, like the pressure of a fluid moving through a pipe. The heating of the wire is due to the resistance to current flow, like friction opposing the flow of fluid through a pipe.

Amps, or current, flows through the wire. Voltage, or potential energy, appears across the wire (end to end). The wire is a resistor, and Ohm's law applies (E=IR, or Voltage equals the current times the resistance).

### trend

Points: 2

#### IanP

Re: What formula(s) do I need to know what gauge wire I need

This maybe usefull:
Rough calculation of resistance of a copper wire:

R=ρ*l/s

where:ρ=0.017 (constant for copper) [Ω*mm²/m], =0.016 (silver) and =0.023 (gold)
l=length in m
s=crossection in mm²

### trend

Points: 2

#### trend

##### Member level 5
Re: What formula(s) do I need to know what gauge wire I need

Great!!

thanks guys, I believe I know enough to get by now (in this area)

thanks-Lee

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