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Radio Wiring Harness - Wire Gauage

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  • Radio Wiring Harness - Wire Gauage

    Hi guys,

    I own a PAC C2R-VW2 for my MK5:

    I believe all the wires are of 18awg. My question is that, is 18awg enough for the power wires? Looking at those ampacity charts it looks like the maximum load of 18awg is 16amp for chassis wiring (well I am not even sure it falls into the chassis wiring category since air ventilation back in the dash isn't very good), but I would guess running at half of the maximum is what we would usually do?

    I tried but failed to find the gauge number of other similar wiring harness. My amp is just a TDA7850, but my NUC can also draw as much as 65W. Will I run any risk if I just use the 18awg power wires? Or I am being too paranoid as many people install aftermarket radios with these wiring harness?

    Will be very grateful if you can answer my questions.


  • #2
    Maybe you can suck it and see. If the wire(s) prove to warm, either replace or strip insulation near the ends and solder a parallel wire.

    But FYI, the following was my original ramble....

    The TDA can do 4x85W = 340W into 2R (Ohm) speakers or 4x50W = 200W into 4R, so that's ~34A or 20A using the usual divide by 10 rule.
    If you add a 65W NUC then add 6.5A.
    Hence an expected max of ~40A else ~26A.

    Unfortunately I never use ampacity charts other than f.ex Powerstream's AWG table for sanity checking. That table reckons 18AWG = 16A for chassis wiring or 2.3A for transmission (which I take to mean still-air suspended) so hence I'd suggest that 18AWG is too thin.
    But if the length is short enough it may be ok - ie, fatter end conductors or connectors conduct enough heat from the cable. I'd say that PAC cable is too long, but a test and feel (for heat) might be worth a try.

    I design by acceptable total voltage drop. EG - if a 2m run from the battery to the device and I allow a 1V drop, that's 0.5V for (say) 40A per meter.
    V=IR hence R=V/I = 0.5V/40A = (max) 0.0125R per meter = 12.5R/km and hence 15G (10.44R/km) => 14G, or 16G (13.2R/km) if a bit more than 1V is acceptable.

    Actually that's strange - normally my method never comes close to exceeding ampacity charts yet 14G & 16G are rated for 32A & 22A chassis wiring respectively so I reckon I've mad a mistake somewhere...
    Coincidentally substituting a total 2V drop above @ 40A means 18G.
    Maybe my error is that I usually deal with smaller voltage drops & bigger currents, and I allow for the GND wiring as well. (I usually only allow a 0.5V total drop for large loads like headlights and bigger...)

    Of course tables are at a certain temperature and as temp increase so does resistance... And I've forgotten if those AWG tables are at 25C or 65C or 85C etc...


    • #3
      Hi OldSpark,

      Thanks very much for your detailed information. Now I just learned the divide by 10 rule.

      I think my stock speakers are 4ohm. And so the max should be ~26A - not the best value for those 18AWG wires. Fortunately there are 2 wires crimped to +12V (one to CAN interface and one to the radio), and 3 wires crimped to GND (one to CAN, one to steering control and one to radio). Since I don't think the CAN circuit will draw much current I can fork and merge those wires and have 18AWG * 2 for the power line.

      Or to avoid the mess, does anybody know a similar wire harness with thicker wires?

      Anyway I am not a fan of loud music, and after I got a shunt I will probably measure the actual current the amp will draw for my usage. Maybe the wires won't really get too hot and 18AWG will do.



      • #4
        Considering you won't be at full power, the 18G may well be ok. Heat is one of the best tests for excessive current - or rather, too small a wire, too dirty connector or switch contacts, etc. (Fuses can normally run quite hot, but they depend on melting (fusing) to operate.)
        Of course 1/2 volume means far less than 1/2 power. (Is it 1/10th? I forget the dB relationship, but others here should know. Else a 2 minute google.)

        The CAN should take negligible current - not that I'm that knowledgeable on CANs etc. But comms (communications) usually takes negligible power - far less than the circuitry that transmits & uses it


        • #5
          18 gauge wire SHOULD be fine for all deck power. MOST decks over rate their stated output and factory is generally 18g wire. You can always splice in 16 gauge wire if you are worried but if you are using the factory wiring you will see that it is 18 or 20 gauge in most cases so upgrading the wire won't do you any good unless you run 16g wire all the way to the speakers.

          OS has simplified the amperage calculation.

          When looking at the OHM rating of the speaker it really only relates to how much wattage will come from the amplifier to the speaker. You will want to look at the rated output of the amplifier for what OHM rating you have.

          In MOST cases assuming an unregulated amplifier most amplifiers I have seen are rated for 4ohm output. If your speaker is 2ohm then it will be pulling twice the power, 8ohm would be half the power. In practice the amplifier may or may not do this. Because of how they are regulated it can vary greatly from amp to amp and you can not assume an automatic doubling or half. Also realize that in most cases an amplifier works harder to produce power for 2ohm than it does for 4ohm and with most amplifiers I have had experience with the 4ohm speaker will get cleaner sound from the amplifier since it wont be working as hard. Higher quality amplifiers may be so close you can't tell the difference and not all amplifiers will work as well at a lower ohm.

          Beyond all of that when you look at the amperage you need to use this formula:

          V(olts) * A(mps) = W(attage) which can be converted to A = W/V.

          So if you ever actually run your vehicle at 10 volts then you can divide the wattage by 10 to get the amperage but in reality with the key off you are running ~12 volts so you should divide by 12 minimum. MOST running vehicles will be ~13.5-14 volts which means then you divide by as much as 14volts. This is a more realistic view of the capacity of your wire. Also realize that your amplifier will likely work better with the vehicle running than with the key off and may produce more power. You need to check your manual to see what wattage your amplifier pushes out at what voltage.

          Also note that if you bridge an amplifier you are halving its OHM rating. In other words an amplifier hooked to a 4ohm speaker running in normal stereo mode would see the speaker as a 4ohm load. If the amplifier was running in bridged mode so both channels are running the speaker it would see the speaker as a 2ohm load.

          As an example I have an audio Art professional amplifier called an MS 100. This amplifier is rated at 2 x 50 watts at 12 volts. This amplifier was tested at a friend of mines shop that had the equipment to record the output of this amplifier and he measured 2x50 at 12 volts as the manual stated but at 14 volts the amplifier put out 2x75 and these are all at 4ohm. If I run this amplifier bridged at 14 volts it was measured at 375 watts. This amplifier was a "cheater" amplifier designed to beat the rating system for competitions.

          IASCA used to carry a chart that discussed what sized wire you should use depending on the power running through the wire. If you look on their web site you can download their manuals. I believe what you are looking for would be in the competitor handbook but I have not seen a recent one. Mine had a chart in it.


          • #6
            Of course then there are responses from experienced people that have taken amp output from the specs (hence 2R output is not double 4R output)...
            And those that understand what the DIV10 rule relates to and why div12 is NOT used.


            • #7
              Hi redheadedrod,

              Thanks very much for your information. So I also need to measure the actual current the amp draws at *different voltage*.

              A bad news to me - I found out that those wires are more like 20AWG than 18AWG, I compared them with some 18AWG side by side, used a caliper, fitted them into wire stripers, etc.

              Fortunately like I said before, I have pairs of parallel wires for the power lines, thanks to the need of power from the CAN circuit:

              Click image for larger version

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              and all of the other wires I use to build the system will be 16AWG, I still have a maximum of rough 20A.



              • #8
                Originally posted by OldSpark View Post
                Of course then there are responses from experienced people that have taken amp output from the specs (hence 2R output is not double 4R output)...
                And those that understand what the DIV10 rule relates to and why div12 is NOT used.
                Whatever OS... Don't use engineering principals or what is taught in schools or what OHMS law states.. Just make up your own rules and maybe you will be "close enough"...
                So I will use your tactic.. Show me documentation that shows that you should use Div10 from a reputable source. Not from a blog or some guy talking about it but show me engineering specifications or from an instructional guide for professionals where it states you do NOT use OHMS law and just figure div10... That or drop it.. OHMs law IS the law of electricity... And it is NOT guess work...

                Using div10 will give you some breathing room but it is NOT an accurate rating of the actual need of the wire. But then again the length of the wire also matters. The longer the wire the larger the gauge required.

                If you actually read my message I stated that each amplifier is different and you can't assume that 2ohm will pull twice the power of 4ohm. In theory it should but in reality it is much different. I also stated you need to look at the manual for the stated values.

                Beyond all that...

                Look for a number on the wire. Many times wiring will have the rating right on the wire or the gauge of the wire. You want to make sure the wire you use in a vehicle is rated against oil and gasoline if it will be used outside or UV protected if it will be exposed to the sun. If you run 16 gauge wire you will want to cut the wires as short as possible from your wire harness. If you can replace the actual plug or socket in your plug this would be preferable but not necessary. If you are using an automotive harness from a reputable name brand you can expect the type of wire is sufficient.

                Again, download the IASCA manual and there SHOULD be a quick reference chart to determine the gauge of wire you need dependent on the length of wire and the power it will carry. I haven't looked at the actual chart in years so I do not remember if it is rated in amps or wattage but using the formula above you can convert. Do NOT use the div10 method OS mentioned or you will be installing a larger wire than you need. The IASCA table considers an automotive environment. For wattage again, look in your amplifier manual to see what the rated value is for the OHM of the speaker you will be using. I am assuming the downloadable manuals from IASCA are free but I did not try to download any of them.

                One other note... You can NOT figure the gauge of the wire by looking at it. The quality of the material inside the wire will determine how big the wire is. Aluminum has more resistance than copper and very pure copper will be less resistive than standard copper. The more resistive the bigger the wire has to be to carry the same power. The Gauge of the wire is related to the amperage it is designed to carry at a set voltage range. If you compare an aluminum, low quality 1/0 wire to a high quality very pure copper 1/0 wire there will be a very significant difference. Just making the point you can NOT determine the size of the wire solely by the size. And I HAVE seen aftermarket harnesses with aluminum wiring... But in general if you are using just standard stranded copper wire you can guess and probably get close. I have some 1/0 high quality cable for an install I did that was something like $20 a foot 20 years ago. You can buy low quality 4gauge wire that looks the same if you just compare the size...


                • #9
                  Hi redheadedrod,

                  As I mentioned before, my wire harness is from PAC. I don't know whether it's a reputable brand or not. I am pretty sure it's stranded copper wire. There isn't any rating or marking on the wires like most of the other wires I have seen, and that's why I need to measure and guess. Also looking at the molded plastic housing of the connector and the terminals, I am not really fond of the quality of it.

                  So back to one of my original questions: do you guys recommend some brands for radio wire harness?



                  • #10
                    PAC is one of the better names out there from what I have seen...

                    But as I mentioned before if you are using deck power it should be fine.

                    The worst that will happen is the wire will get warm as mentioned by OS.

                    But if you are nervous about it then the powered speaker wires can be upgraded. Just cut the wire as close to the connector as possible and use 16 gauge wire and you should be fine. The short run of 18 gauge wire shouldn't be a big deal. (IF you cut it close and have like an inch or so this is very short and should be fine.) But as I also mentioned your factory wiring is likely similar in size to the PAC harness so if you don't like the smaller wire you will want to run wires all the way to the speakers. If you run it to the door you may want to install an easy to disconnect wire socket/plug somewhere near where the door is so you don't have to cut your wire if you ever have to remove your door.



                    • #11
                      The wires are actually closer to 20AWG than 18AWG.

                      But I just measured the current the amp draws at 12.5V, with only 2 x 4ohm speakers: <= 200mA. If it goes over 200mA, the music will become pretty loud that I think people outside the garage will know what song I am listening to. I am a bit surprised the amp draws so little current. Of course I will need 4 speakers at the end, but I start to think the current it draws will be well below 1A for my usage.

                      Next step I will measure the average current my NUC needs. With both cores at 100% it's about 1.8A, but I will need to wait until I get a 4G dongle and test again.

                      Last edited by ivanwyc; 03-27-2014, 02:17 AM.