Wire sizing

[Colt45ws]

Going to run a power line to the trunk in my car. Its approximately 10-12 foot plus a foot to two to the frame for ground and another foot or two back to the engine(alternator). Max load is about 100A, Fused 125A. Im thinking 2ga should be good, but Im getting conflicting information. Nearly every chart Ive found is different. Some say 4 is okay and others say I will need 2/0.
Thats a pretty good range. I dont mind buying step more than minimum for safety, but I need to figure out the minimum. The price range for the gauge range posted is $1.63/ft for 4 and $4.23/ft for 2/0 battery cable from my source. A hefty difference.

 

[mechtech2]

You could also run parallel lines using the smaller gauge. A decent option.

 

[Colt45ws]

Thats...interesting. If I ran two 4ga lines it would be essentially a 1ga. I have to buy 25ft anyway, so it would work.. Hmm. I will have to measure the EXACT distance to determine if I can get away with one 25ft length or if I will need another. If I need more than the first 25ft then it would be more than just buying the minimum of 1ga.

 

[eljefino]

It's not really the safety, as a huge wire like that won't overheat-- the chassis it's bolted to will conduct the heat away. The guys who want you to use welding cable just want that last little bit of performance/ lack of voltage drop.

Believe a miata (battery in the trunk, starter in the front) only uses around a 4 gauge cable.

 

[Gotch]

Originally Posted By: mechtech2
You could also run parallel lines using the smaller gauge. A decent option.


...so long as what you are hooking up is not sensitive to voltage drop. Extra connections are less efficient at transfering power. Miniscule yes, but still there. Also a lot more area for corrosion to set up.

 

[chad8]

I am an electrician. Electrical code says #2 aluminum or copper minimum will be good for 100 amps at max of 90 degrees centigrade ( wire is rated different according to ambient temperature the casing can handle). that is where all the confusion is. do not parallel wires. Too much to connect correctly . Go on craigslist and see if you can find someone selling audio wire , welding cable , machine wire(called S-O ) or other flexible casing wire rated at 90c degrees or higher.
Kids buy their hot new audio system and smash the car all the time. I have found it very cheap this way. Then it has the ring connectors , fuse blocks ,tap splices and shut offs included for about what you would pay for the wire.
Check fleabay too.
good luck

 

[supton]

I like the notion of using parallel lines. That way you can always decrease the resistance if you find the drop is too much, without over spending / too much weight. Rule of thumb is that you half the resistance every 3 gauges. I'm looking at a chart, and 40g is 3240ohm/km, while 38g is 2190ohm/km.

10g is 0.1ohm/100ft
8g is 0.067ohm/100ft
6g is 0.047ohm/100ft
4g is 0.024ohm/100ft
2g is 0.015ohm/100ft
1g is 0.012ohm/100ft
0g is 0.0096ohm/100ft

Something I would contemplate is adding a GND return, if voltage drop is of utmost importance. Steel is not as conductive as copper. You could bond various panels together, and to chassis; but run of 0g from battery to point of load will also help drop. This I would size to take full starter load though, regardless of the size of the positive wire size--apparently it has happened in the past that, when the battery to starter connection fails, other returns to the battery can be "forced" to take the starter current, which can melt undersized wires.

 

[eljefino]

What's OP doing, a stereo?

Chart here says you can do 7.2 feet of #2 cable with an inconsequential 2% loss in voltage at 100 amps: http://www3.telus.net/public/dmmackay/voltage_drop.htm

Your stereo can probably handle a 5-10% voltage drop from 14.0.


If your wiring permits, you could set your alternator's voltage sense wire to sense from back in the trunk, so whatever you lose in transmission, you get back in alt output.

 

[NateDN10]

There are some handy calculators that will give you voltage drop vs. current and wire gauge. (here, for example; scroll 3/4 of the way down the page)

In your scenario, using 4AWG wire, the wiring would drop 0.6V. For 2AWG, that's 0.4V, and for 2/0 it's 0.2V.

 

[Gotch]

The other problem going parallel is if one conductor/connection fails the whole load is on the remaining...can you say roman candle?

 

[Donald]

My guess is the OP wants to power a winch or supply power for a tailgate party.

 

[supton]

Originally Posted By: Gotch
The other problem going parallel is if one conductor/connection fails the whole load is on the remaining...can you say roman candle?


Not quite. If 0.6V drop was too much, using a second parallel wire means 0.3V (for example). Both are well within normal drops of most wiring, and would not cause fires. However, the equipment in question may have issues--it may fail to deliver full power, give other issues, or whatever. Which would then be the indication to find the faulty wire.

In the OP's case he is after not "safely" sizing the wire, but rather minimizing voltage drop.

 

[JHZR2]

Are you looking at ac vs dc tables? 12V DC tables?

 

[Gotch]

Here's an extreme example, it has nothing to do with Voltage drop and everything to do with current and why running parallel is NOT a good idea.

100amp load - fused 125amps

I cheap out and get a deal on 10 lengths of #16 wire. I connect them in parallel. They all fail (not a short) but one, now we are running 100amps though a #16 wire...it will light up like a roman candle. Remember, we are fused for 125amps so the fuse won't blow.

 

[HerrStig]

Originally Posted By: supton
I like the notion of using parallel lines. That way you can always decrease the resistance if you find the drop is too much, without over spending / too much weight. Rule of thumb is that you half the resistance every 3 gauges. I'm looking at a chart, and 40g is 3240ohm/km, while 38g is 2190ohm/km.

10g is 0.1ohm/100ft
8g is 0.067ohm/100ft
6g is 0.047ohm/100ft
4g is 0.024ohm/100ft
2g is 0.015ohm/100ft
1g is 0.012ohm/100ft
0g is 0.0096ohm/100ft

Something I would contemplate is adding a GND return, if voltage drop is of utmost importance. Steel is not as conductive as copper. You could bond various panels together, and to chassis; but run of 0g from battery to point of load will also help drop. This I would size to take full starter load though, regardless of the size of the positive wire size--apparently it has happened in the past that, when the battery to starter connection fails, other returns to the battery can be "forced" to take the starter current, which can melt undersized wires.
The ground return is a clever idea. You're running the wire anyway. In theory, the unibody is welded together and has a large mass, but a good meter could be used to measure the path loss through the body.

 

[HerrStig]

Originally Posted By: Colt45ws
Going to run a power line to the trunk in my car. Its approximately 10-12 foot plus a foot to two to the frame for ground and another foot or two back to the engine(alternator). Max load is about 100A, Fused 125A. Im thinking 2ga should be good, but Im getting conflicting information. Nearly every chart Ive found is different. Some say 4 is okay and others say I will need 2/0.
Thats a pretty good range. I dont mind buying step more than minimum for safety, but I need to figure out the minimum. The price range for the gauge range posted is $1.63/ft for 4 and $4.23/ft for 2/0 battery cable from my source. A hefty difference.
BTW the Vic has a "cable trough" running to the trunk for the radios which trunk mount in the police version.

 

[supton]

I'll concede that your "extreme" example is true if you'll concede that using two runs won't result in a Roman candle.

I suppose that, as long as any given wire is properly sized to take the whole load, then paralleling wires so as to minimize voltage drop is perfectly acceptable. Perhaps that is the best way of putting it?

Originally Posted By: Gotch
Here's an extreme example, it has nothing to do with Voltage drop and everything to do with current and why running parallel is NOT a good idea.

100amp load - fused 125amps

I cheap out and get a deal on 10 lengths of #16 wire. I connect them in parallel. They all fail (not a short) but one, now we are running 100amps though a #16 wire...it will light up like a roman candle. Remember, we are fused for 125amps so the fuse won't blow.

 

[Colt45ws]

Oh yeah, I found the cable conduit.

Im planning for two amps, atm. One for my speakers, 62.5Wx4, which wants a 50A fuse so Im going to guess nominal max load is around 40A peak. Then another one for my sub, 1x150W, which is fused 30A. This is well under 100A, but I want spare capacity for Inverter or Air compressor, whatever.

 

[mechtech2]

Originally Posted By: Gotch
Here's an extreme example, it has nothing to do with Voltage drop and everything to do with current and why running parallel is NOT a good idea.

100amp load - fused 125amps

I cheap out and get a deal on 10 lengths of #16 wire. I connect them in parallel. They all fail (not a short) but one, now we are running 100amps though a #16 wire...it will light up like a roman candle. Remember, we are fused for 125amps so the fuse won't blow.


The cables won't fail.
If you assume failure, why not assume failure in all systems??
This is not a house.

 

[eljefino]

Originally Posted By: Colt45ws
Oh yeah, I found the cable conduit.

Im planning for two amps, atm. One for my speakers, 62.5Wx4, which wants a 50A fuse so Im going to guess nominal max load is around 40A peak. Then another one for my sub, 1x150W, which is fused 30A. This is well under 100A, but I want spare capacity for Inverter or Air compressor, whatever.


In that case the #4 ish sized cables might make sense, one to each amp, but don't connect them in the trunk.