Electrical question/opinions

Sorry you took that as a jab at you, it was not intended that way. There was a lot of confusion over the first chart that was posted, so I thought I'd supply some useful info for others that would read the post.

Unfortunately, asking the max current for a 12AWG wire is like asking the tire size for a 15" rim. That 31x10.50-15 tire may or may not be right. Circuit length, which is what sets up the voltage drop, matters.

(Ever see a high inrush current load trip breakers and fail to run only because the wire size was too small - even when it was OK according to the NEC tables? Things like compressors will do that. I know that isn't car wiring but it is a good example.)

 

I liked the one where these guys were out hunting, on the ride home they had a blown fuse and used a .22 bullet to replace the fuse.

Bob

 

In industry everything comes down to $$, so they specify the smallest wire possible for any application with a margin of safety thrown in.

Get into aircraft wiring and everything becomes payload. Any mil spec for wire will have how many lbs per foot in it. Where I work, they have elaborate tables for the current capacity of wire that figures in the length.

For cars out of the factory, the same deal applies. Pennies are pinched everywhere. Anywhere they can save, they do.

Bob

 

Momentary draws are not going to be much of a consideration. Yes, they can happen, but usually they are in rush, and only happen for a second or so. Even a short circuit can go 100's of times more than the fuse for a fraction of a second. I've seen tests in trade magazines where a #12 wire took 1200 amps to melt in two, so it's not going to melt if it got 50 amps for a few seconds.
The graph looks pretty good to me. I don't normally go over the std. wire gauge ratings, regardless of length of wire, or insulation type. I know it can do it, I just don't do it.

 

Standard practice is to use 30 amp fuse on 10 gage, 20 amp on 12 gage, 15 amp on 14 gage, 10 amp on 16 gage. That's really all you want to know.


There is your answer.

Funny how persons who don't know how do it, have to overthink the situation at hand. If you follow the above advice and have a good clean ground, you won't have any problems.

Some of the posters are confusing home wiring (A/C) with wiring a car (D/C). I was a journeyman electrician in a former life. Now I work on cars. The above standard practice will suffice for 99.9% of your wiring needs.

 

One other thing, now that we have beat this poor horse to death, is that fuses are made to accomodate the inrush current to start a motor. Look at your box of fuses, i bet it says 'slo-blo' or some variation of it. Modern cars have the same fans and they are fused at 20a or less.

 

None of my blade fuses has slo-blow written on them.

You have made my point. Let me guess, electrician by trade?

Probably one in ten of you electricians could wire a car in 4 days. Its a diifferent take on elecltricity. Samr thing. Only different,

As a side note, anyone ever remember Squirrel giving bad advise? Just saying.

 

A good rule to follow is not drawing more then 80% of your conductors amp rating. A 20 amp circuit should carry no more then a 16 amp load. A 30 amp wire and circuit would be better for your fan.
Dom

 

With what you and Squirell are saying, is it irrellevant on the length of the wire?
As in a 14 ga wire will easily hold 15 amps or less when the load is 5' from the fuse / power source, or 25' from the fuse / power source?
This is a sincere question.

 

When I was teaching I used to have an ancient (out of the 50's) Chrysler training film that explained wire size as being similar to a water hose. Too many amps trying to push through an under size wire was shown as being similar to too much water being pushed through an undersized hose. You ended up with a failure somewhere.

Non car but somewhat related as I have two 100 ft extension cords that I use a lot. I've got a little 2 hp 8 gal hf air compressor that I was having a hell of a time getting to work on one of the cords while it worked fine in the garage. I drug out the other cord one day and hooked it up and hooked compressor up out in the driveway to air up a tire and this time it kicked right on and worked great. My old faithful cord that I have had for probably 20 years has too small of gage of wire in it to carry the load that the compressor put on it.

 

Another option to a slo-blow fuse is to install a section of fusible link wire near the power source (but not in the interior)

Rule of thumb: Fusible link wire should be 4 gages smaller than the wire it protects.

Below is a diagram for my eng compartment electrical system as an example...
I have 3 fusible links right next to the Ford starter solenoid relay (mounted on the fenderwell). All the "always hot" wires have protection. BTW, the Ford relay is a good idea too



Sorry to hijack

 

I've been uncomfortable with the fusible link wire since it melts and might damage wiring bundled with it. That is probably needless worry, but I do that a lot. I used a Mega-Fuse in my charge lead from the alternator, just like Ford did through the '90s, my thought being that was the proper fuse for the alternator I was using on its original application so it should work fine now. Fuses are easier to swap.

 

Someone posted about wire being like a hose for water, and that's a good way to think of it. Voltage is the water pressure and current is the flow.

Smaller wire has higher resistance to flow and will cause voltage drop. The wire may be fine with 15A through it but the voltage at the end point may be less than the 13.8V you put in. With some things it doesn't matter. For headlights, fuel injection computers (!!!!), and ignition boxes it may matter.

1. Length of run (full loop, from power and back so the ground lead matters too).
2. Load current.
3. Wire size to properly feed the load.
4. Fuse to protect the wire.

All fuses have a time/current curve to "blow", where the higher the current the faster they blow. A typical fuse is rated to carry 80% of its value. A 20A fuse will fail if it has 20A through it all the time.

Now we can start discussing when we need a relay and watch the whole thread burst into flames!

 

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Funny how persons who think they know how to do it refuse to look at the facts. I would rather know that the circuit is designed correctly to handle all possible conditions than throw a dart based on some "Standard Practice" It's that .1% that can leave you in the side of the road in flames.

 

Once you go down that road, where do you stop? Sure you could design and build a circuit, then an electrical system, with multiple protective links, oversized wiring, auto reset breakers, all soldiered terminals,spring tensioned wires,etc. Then, you'd need fail safe brakes-maybe a quad master cylinder instead of dual, conduit encased lines, overbuilt fittings; then runflat tires, a backup power system in case the motor fails, full airbag safety system-on and on. Oh, and you'd have to get all the other gonzos off the highway,too. You'd end up spending $250K on a $20,000 car.

Everyone, from NASA to Hot Rod Harry always has to make decisions on what will be "good enough" at an acceptable cost. Most of us drive pretty safe cars. None of us drive fail safe cars.

You could have a fully protected 8 gauge wire circuit carrying 10 amps-and get hit by lightening-and we'd find you in flames by the side of the road.

 

I had a dead battery in my motorhome once and no jumper cables.
I connected the aux. generator battery to the main batt. with two 12 Ga. wires. (about 15')

Yes, the engine started... just as the wire burst in flames. I guess it will only handle 600 amps for a few seconds. (but I was on the road again!)

 

The point is that if you have the information and knowledge about how something should works then you can make an educated decision as to how to best do something based on your needs. NASA made many decisions based on what was "good enough" relitive to the risk factors. They could not have made those decisions unless they understood what the possible outcomes would be. Most things that we do when we build a hot rod involves trade offs but I think we all want to strive to make it as safe and reliable as possible.

 

No matter what, there is going to be some voltage drop through a wire, and that varies with current draw, wire size, and wire length. The question then becomes; how much voltage drop can I tolerate. The only way a chart showing that isn't helpful is if you don't know the current draw of the devices you want to run, or the chart is based on voltage drop different than what you want.

Because wire costs money, Japanese cars typically have wire that is sized as small as possible. In some cases the wire on Japanese cars is sized based not on voltage drop, but how much heat the insulation can tolerate! Adding anything to a circuit, even headlight bulbs with a little higher wattage, can overload the wiring in some of those cars.

 

The line is somewhere between getting carried away, or saying F-it, it's too much trouble to figure this out and do it right. All the time people get away with doing things they shouldn't. On the other hand, the more you ask for trouble, the more likely you are to get some.

 

I pity the fool that follows this chart and puts a 40A load on a 14 gauge wire even if it is for only 5 ft. That wire is gonna cook if it's a constant draw... 14 gauge should only be loaded to 15A. CTJ

 

wired hundreds of cars without a chart, squirrel is the closest to my system, never a problem..........oh yea there was a time the customer set the body on the tail light wire, blew the 10A fuse....