Current and Wire Guage

[NightShift]

I have 12 or 10 AWG wire running from relays to my headlights on my car. I am looking for H4 connectors/wiring harnesses and I found one being sold with 14 AWG wire attached to it.

Now I don't want to lose any current going from the 10 AWG wire down to the 14 AWG wire...kinda like connecting a firehose to a garden hose. I know this will be affected if the the 10AWG hooks up to a long piece of 14 AWG, but if it's a really really short piece, like an inch or less, will it carry the current easily? Or would I need 10 AWG right up to the contacts?

[Zelandeth]

I'd say that power loses aren't really the problem here, but overheating could well be. If I were you'd I'd have the thicker, lower resistance wire right to the contacts, as there could be a risk that the short length of thinner wire could still overheat, and in worst case cause a fire. I'm not familar with the wire guages being mentioned here, but if the wire ona circuit I'm working with has a heavy duty cable, I make sure it has that cable right to its termination connections at source and load.

[Brock]

14 should be fine, it is rated to 15 amps or 15ax12v=180w So unless the lights are over 180 watts it won't start a fire, but yes you will loose a bit in line loss. I ran almost all my headlights on #10, then connected them to the actual lamps with the 14, so I think you in the same boat, it should be more then safe, just make sure you have a 15 amp fuse on the beginning of the feed.

[Albany Tom]

No worries - 14ga copper wire will handle about 20 amps, or 240 watts at 12V without overheating. You don't loose current, the effect you're thinking of is the voltage drop as the current flows through the wire. This is directly proportional to the length of the wire, so a few inches won't make any difference.

What *may* make a difference is the quality of the connections. If the connectors get much warmer than the wire when it's running, then you've probably got a bad connection.

[NightShift]

Ok, cool, I forgot that I downgraded my bulbs from 90/100w to 60/55w. But my high beams are still 100's, but thats H1 bulbs. If I ever do want to go back to the higher ones that will be approaching the 180/200w limit.

[INRETECH]

It is not just the diameter of the wire that matters, but the length as well; figure out what is going to be your loss

Loss = Length (Ft) X
Value (Below) X
Current (Amps)

Thus, if you had 10ft of #14 and ran 5 amps that would be .0297 Ohms, or .14v loss

Wire
AWG Ohms
--- ----
4 .000292
6 .000465
8 .000739
10 .00118
12 .00187
14 .00297
16 .00473
18 .00751
20 .0119
22 .0190
24 .0302
26 .0480
28 .0764

Table from:

http://www.epanorama.net/documents/w...esistance.html

[lessing]

Wire diameter does not mean squat if the contacts in the plug connector can not handle it. Just because it is the correct style connector, does not mean some company did not cheap out on the copper for the contacts. As a rule, your limits on wire runs are dictated by two factors, voltage loss over distance (which is not a problem in a car) and ampacity of the smallest wire in the run. It does not matter if there is 50 feet of #2 with a 14 gauge 1 inch piece on the end, the whole run is 14 gauge. If the connector only has the circular mils equivelant of 18 gauge, you just dropped the whole run to 18 gauge.

Find the spec of that particular connector, then pick the lowest rated value. A short run of smaller wire is equivelant to a large fuse rated at the ampacity of the wire.

[Brock]

Lessing, that isn't quite true. In 12v renewable energy systems we regularly run #2 for 200 feet or so, then go to #10 to get in to the charge controller. If you ran 10 the whole way the line loss would eat you alive. So the thicker wire for 99% of the run helps a lot. The connections at each end are a vital link in the chain though. But in a car the short distances don’t matter as much, unless your running high amperage.