Wire Amp Rating - Which chart is correct?

[CJ8Dave]

I'm not real knowledgeable about 12 volt wiring. In doing some research on wire size and AMP ratings I find several different charts. They rate things very differently.

Just for example sake, I'm using 100 amps.

These charts state that you can run 100 amps with 10 gauge wire at 10' length.
http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm

http://www.tessco.com/yts/industry/products/itm/automotive/get_wired.html

However, this chart states that you need a 4 gauge wire at 10' long to get 100 amps.
http://bestboatwire.com/catalog/includes/languages/english/wire_gauge_chart.html

Finally... this chart states that that you can run 100 amps with 6 gauge wire up to 23 feet in length.
http://bestboatwire.com/catalog/includes/languages/english/wire_gauge_chart.html

Very confusing for me. Which is correct (if any)? Thanks for any assistance you can provide.

 

[DaveInDenver]

There are two factors mainly to be concerned with on wire sizing, safety and ratings. Safety is top one of course. Wire is made of a conductor, typically copper, that is not perfect. It has slight resistance that is a function of the diameter of the wire.

The larger the diameter (or cross section), the lower its resistance. Stranded wire might confuse you, but this is several smaller wires in parallel, which lowers the equivalent resistance. If you Google parallel resistors you'll see the formula.

Since a wire has a resistance, a given current will produce heat. This is a derivative of Ohm's law, power = current ^2 * resistance. So small currents produce less heat even with a higher resistance. All wire heats up a little, what you are concerned with is the wire itself heating to the point of failure (e.g. a fuse), but more so the temperature where the insulation melts and fails. This will be a lower temperature than the wire fusing temp and is the cause of shorts and arcing, which starts fires.

Safe handling charts will be functions of the insulation type (various plastics, teflon, silicon, etc) for one and conductor alloy or purity of the copper, e.g. the actual resistance. Two wire gauges could have two different ratings, so you have to read into the chart to know for sure.

So let's assume you are within the safe zone of the wire itself, the other concern is the capacity of the conductor. That same resistance causes a voltage drop at a certain current. That is directly Ohm's Law, V = I *R. At high current this can be significant. Say a wire is carrying 100A and has a resistance of 0.05 ohms (e.g. 50 milliohms), this produces a 5V drop, which means you're feeding 14V on one end and getting 9V on the other. Going to a larger wire with, say, 0.015 ohms of resistance reduces this to 1.5V and might be the difference between a radio working or a fridge not cutting out.

The minimum wire size for this will almost always be much larger than a safety chart because starting with 12V or even 24V means you have a lot less margin for error at the load end.

So you have to determine what the chart is telling you.

 

[CJ8Dave]

Thank you DaveinDenver. Your reply was informative and educational and probably explains why the charts are so different. I'm looking for a accurate and safe reference chart that can be used as a "rule of thumb" when planning & installing 12v. electrical items in my jeep. Items like: Air compressor - draws 35 amps, an ARB or Engel Fridge - .7 to 3 amps, etc....

Distance from my Battery to rear of Jeep is 10'.

 

[redneck44]

Try this
http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

Ideally you want a max of 3% voltage drop, 10% on non critical components.
Forget the amp ratings on cables, it's very misleading.
Don't forget to calculate for the entire cable run, positive and negative.

 

[CJ8Dave]

Found another chart that seems to address points made by DaveinDenver.

http://www.altestore.com/store/Sola.../Solar/descfiles/lorentz/4-wiresizechart1.jpg

 

[CJ8Dave]

Thanks !

 

[dwh]

pa

How much current any given wire can carry depends on A) the wire type (copper, aluminum, tinned, bare, stranded, etc.), B) the insulation's temperature rating, and C) how you use it.

For instance, a wire that is rated to safely carry 100a in open air might only be safe at a max of 55a if it's used inside an air conditioner or a microwave (called chassis wiring), and that same wire might only be rated to be safe carrying 30a inside of a pipe (conduit) or bundled with other wires.

So a wire has to be sized to safely carry whatever X load is at Y temperature in Z situation.


The second factor is voltage drop. As mentioned by Dave, a certain size wire will have a certain voltage drop over a certain distance at a certain load, and the lower the voltage you are working with, the more the voltage drop percentage matters.

So a wire which might safely carry a certain load of amps, might also (absolutely will in those first two charts in the OP) have too much voltage drop when carrying the max current.

For example #10 wire with a 100a load on it (which bloody well better be all alone in open air), over 10' distance (5' one way) is gonna have about a 17% voltage drop which is about a 2v drop. So 12v is now 10v. If that load is an inverter, then forget it - in the 12v battery world, 10.5v is considered a dead battery, and most inverters auto-shutoff if the voltage drops to 10.5v.


Those first two linked charts (in the OP) are silly. Yea, theoretically you might be able find some wire somewhere that is rated to carry those kinds of amps in open air - but actually doing it is gonna kill you on the voltage drop so why bother. The third chart is right. (And read the notes at the bottom of that page!) The 4th link goes to the same page/chart as the third link.

 

[4x4junkie]

Good info above. 3% is generally max loss you want for most voltage-sensitive items (fridge, radio equipment, etc.), though unless I'm doing something wrong, that calculator redneck posted seems quite adept at suggesting some rather massively huge cables (for the OP's requirement it simply outputs: "Too large" lol)

I use this one myself, and for 100 amps to be carried a distance of 10 feet (not exceeding 3% drop), #2 awg cables is what it comes up with (2.7% loss @25°C).

 

[redneck44]

Good info above. 3% is generally max loss you want for most voltage-sensitive items (fridge, radio equipment, etc.), though unless I'm doing something wrong, that calculator redneck posted seems quite adept at suggesting some rather massively huge cables (for the OP's requirement it simply outputs: "Too large" lol)

I use this one myself, and for 100 amps to be carried a distance of 10 feet (not exceeding 3% drop), #2 awg cables is what it comes up with (2.7% loss @25°C).

I hope you put your lengths in metres.

 

[DaveInDenver]

I would be remiss in adding that the NEC is generally the source for wire carrying capacity, at least for most usual insulation types. They are only concerned with the safe number and leave it to the designer to interpret what is appropriate.

There is a maximum voltage rating for an insulation, above which you can no longer safely assume it will not break down and conduct. For mobile use this is going to be of no worry, typical values are 300V, 600V or something similar.

So the code is only worried that at some given ambient temperature at max voltage and current the wire will not become unsafe. So you can see that at 300V a 5V drop might not be a problem, well below 3%. That is why all these other charts are developed to use.

 

[4x4junkie]

I hope you put your lengths in metres.

That was it lol.

Same #2 AWG (for 3% drop)

 

[CJ8Dave]

Thanks to everybody for the input and education. It is much appreciated.

Dave