# maximum length of wire?

Discussion in 'Electronic Basics' started by [email protected], Nov 15, 2008.

1. ### Guest

I have some video equipment i need to power off directly from my car's
battery. I need a wire that is not too big, because i need to solder
on a special quick-release connector that will be used to easily
disconnect the cable from my video equipment. The cable requires 2
wires inside, one for positive and one for negative on the battery.
The biggest cable i found with 2 conductors, has 2x 22AWG wires
inside. Some tell me that it is too small and some say it will work no
problem. I don't want the wires to start melting and cause a fire! The
video gear will draw no more than 1 amp total, directly off the car's
12V battery. My question is, what is the maximum length of cable i can

2. ### whit3rdGuest

The fuse limit for typical insulation on 22AWG wires is well over
2 amps, so PROVIDED WITH A SUITABLE FUSE it's safe to
connect it to the 12V battery. Your battery does NOT connect
directly to this wire, it connects THROUGH A FUSE (1.5A
would be a suitable size).

The important part here, is that the fuse prevents the wire
from burning up under ANY fault condition, including higher
current than your intended device (the video gear) uses.

3. ### Rich WebbGuest

For some back o' the envelope approximations...

battery will really be about 13 V when not loaded but we'll swag the
equivalent load resistance at 12 ohms.

AWG 22 cable is about 16 ohms per 1000 feet, so using a 100 ft run, you
battery and you'll have a bit more than 10 V to work with at your load.
Your equipment may not be happy with that.

If you follow Tim's recommendation and use heavier gauge cable with
short transition pieces to the necessary wire size for the connector,
things look a bit better.

With regular 14 AWG zip cord, you're now running only about 2.5 ohms per
1000 feet. That 100 foot run now only adds 1/4 ohm to each leg, so your
total draw (with that same 13 V battery) is pretty close to 1 A, with
only about 0.5 V lost in the cabling. Happy equipment!

As whit3rd mentioned, fuse it. Preferably run from a polarized aux
connector to a spare fuse on the vehicle's fuse box, rather than hook
directly to the battery. (Nobody would EVER hook black to positive, red
to ground, would they?)

If you ever intend to run from an operating vehicle, be aware that the
equipment will see some nasty transients. If the stuff is designed to
run from a car's electrical system, you're good. Otherwise, be aware
that the magic smoke may come out. See the link for a description of
some issues http://www.industrologic.com/autotransients.pdf

4. ### EeyoreGuest

Good thing he's in N. America (Canada) or he wouldn't have an earthly clue
what an AWG is. Does Canada use AWG or have they fully metricated now ?

Graham

5. ### Rich WebbGuest

One may note that the OP mentioned that the "cable i found with 2
conductors, has 2x 22AWG wires" and infer from that statement that he
indeed has some clue (earthly or not, to be determined).

6. ### Fred AbseGuest

The OP wrote: "I need a wire ...2x 22AWG wires"

Says it all.

7. ### krwGuest

Ability to use and ability to think are two different things.

8. ### gazzGuest

just a quick note, is the equipment designed to run directly off a car
battery? or are you powering an item that has a 12 volt DC input from a

just that video equipment will be pretty sensative to noise and spikes, and
you get a lot of them on a car's electrical system, also the voltage at the
battery terminals will vary from 11 volts to 14.8 volts, depending on if the
engine is running or not.

i've run my hard drive camcorder fixed to my dashboard whilst driving in my
motorhome last year, it too wants a 12 volt DC supply at 1 amp max, but no
way i'd feed it with the power directly from the battery/alternator.

i was going to get one of the proper 12 volt camera power supplies to run
it, but it was easier for me to run an extension cable from the living area
of the motorhome to the dash connected to the main inverter, part of that
reason was the 12 volt Dc power supply was about 50 quid, the extension
cable was 2 quid, and i use the mains adaptor and that has the neccisary
filters to take out any noise on the power.

9. ### neon

1,325
0
Oct 21, 2006
The melting point of AWG GAUGE #22 can be calculated as I=Kd raised to the power of 3/2.
K is a constant depending on material copper is 10,244 and d=0.0253 even these are aproximation. But the resistance per 1000' is 41.17 ohms. In general the smaller the wire the better clearance of a short. A car battery can supply 500 to 1000 amps into a short if it does not clear you got problems. I sugest #14 stranded house wireing easy to get and easy to inplement. You can add two inline fuses to clear a fault that is a good idea and put the fuses at the battery end not at the other end.