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Wiring Info/Help/Understanding

Discussion in 'Audio & Electronics' started by GordDavey, Sep 18, 2019.

  1. GordDavey

    GordDavey Senior Member

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    chrisbh17,
    That's exactly what went thru my mind. I see the power and ground that came with the USB are 14ga or maybe even 12ga, yet they are supposed to be fused at 10 amps. So looking at the power outlet with 18ga and 20 amps, your mind takes a large step backwards and goes... this can't be right. Maybe I shouldn't use those wires for this USB. But as I look around trying to sort it out, I see that depending on the drop % (I'm guessing it means percentage voltage drop), the charts we typically see are 1 or 2% drop and those show 12 or 14 ga for 20amp circuits. But I saw a reference that auto makers use a 5% or higher drop... and in those charts an 18ga can hold 20 amps of current, just not for the length of wire I estimate to be... but close. I don't understand when to use that sort of drop... perhaps it's the type of wire itself, because I do see different wire type references in the charts too and maybe the wire type determines how much voltage will be lost over x amount of distance. I guess I'm hoping to "get schooled" a bit, as I've got tons of mechanical and computer background, but almost no electrical knowledge.
     
  2. KC Cryptkeeper

    KC Cryptkeeper Senior Member

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    The fuse is to protect the wire not the device. No wire in your truck is the 60 or 75 degree wire. Also take in account that to achieve a voltage drop of over 3% in any circuit the length will have to be well over 100 foot per conductor even at 12 volts. Fuses are allowed to be rated at 125% of the load and if not available you move to the next size larger fuse. So if the your load is 16 amps at 125% the correct fuse that is made is 20 amps. Quit reading to much into the whole spectrum of this, it is a truck nothing branch circuit wise in your struck runs wiring about a #8, I'm sure every wire in our looms have been calculated by the company to maximize profit and reduce costs while maintaining proper specs.

    It's entertaining that your first post says your not an electrical guy but then you tell me that the National Electrical Code is wrong, I didn't make up the chart it is the code standard. The code has other charts for special applications for low voltage and communication wiring, I posted the chart for basic everyday sizes. The chart is a guideline that should be enough for our trucks as well.
     
    Last edited: Sep 19, 2019
  3. pacofortacos

    pacofortacos Senior Member

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    Your USB only can pull a maximum of 36 watts (maybe a tad higher if any power is wasted in the 12V to 5 V conversion). 18 ga. wire is more than sufficient.

    That chart is a bit different than most, I personally would not rec. 14 ga. wire for 20 A., 15A yes but not 20A. That is 12 ga. territory. Though it does depend on the number of conductors and other factors.

    For reference, the National Electrical Code (NEC) notes the following ampacity for copper wire at 30 Celsius: 14 AWG - maximum of 20 Amps in free air, maximum of 15 Amps as part of a 3 conductor cable

    https://www.stayonline.com/product-resources/reference-circuit-ampacity.asp
     
    Last edited: Sep 20, 2019
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  4. KC Cryptkeeper

    KC Cryptkeeper Senior Member

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    Agreed with the code numbers and the derating factors giving 14ga on a 20 amp rating and 12ga on a 25 amp rating. Those numbers changed in either the 1999 or 2002 code. Prior codes were 15 and 20 amps respectively. These numbers were changed in order to keep conduit fill from affecting wire size. Anything in our trucks would be considered free air, the cable conductor rating is on a cable with a protective sheath or in a conduit. The chart was to give him a reference point on which to start with not just a guessing game because someone on the internet said so.
     
  5. pacofortacos

    pacofortacos Senior Member

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    I was wondering why the change.

    I have always found pulling close to 20A made 14 ga. wire warmer than I like, but then again I don't want my wires warm at all :) That and it has usually been using 2/3 wire cables, not single wire free air.

    Thanks for the update info!
     
  6. GordDavey

    GordDavey Senior Member

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    KC Cryptkeeper,

    I'm not tying to go against what your telling me at all, nor the national wiring code. I looked at the chart for 60 and 75 degree because they are the lowest values on the chart in the pdf your URL referenced, all else was higher. I really am just trying to reconcile how FCA can use 18ga, when the chart you reference says 14ga for a 20 amp circuit. There must be a reason they can select that wire size.

    All I would like to be able to do, is to figure out what size wire and fuse I need for a given situation and if I can tap off an existing circuit. Perhaps I'll do some bed lighting in the future with some LED's, I'd like to understand if I can simply tap off an existing wire or if I need to run a new one of some particular size. (I know LED uses next to nothing in terms of power... maybe a bad example, if I were to run an air compressor at the back then I need to understand how to choose the right size of wire and fuse for it)

    pacofortacos said the USB would be about 36 watts and that 18ga would be sufficient... how did he come to that conclusion? Should I be figuring out the max watts and run that thru some math to get to amps required then multiply that at 125% to get total amps, then selecting the wire size from the chart that can handle at least that many amps?

    You sound very knowledgeable, but your first reply was wire's wire, here's a chart, stay at 80%, which doesn't mean much to me. I've heard "the fuse is there to protect the wire and that's it" so many times over the years and I believe it. I'd like to understand so I don't have to ask each time I want to do something, so I don't end up being a pest... which I know I might already be becoming for asking this in the first place. But I'm sure there are a lot of guys in my situation trying to understand how to pick a wire size and a fuse size for a project and not be afraid of damaging things.
     
  7. KC Cryptkeeper

    KC Cryptkeeper Senior Member

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    The chart actually says it is 20 amps only for derating purposes, sorry maybe not on the pdf but in the actual code. The true rating for 14ga is 15 amps. I realize that the chart throws people off especially when you don't get to see all of the context. Don't worry FCA and all manufacturing companies know exactly what they need size wise according to engineers that do it for a living.

    the USB calculation for the 36 watt would be: 36/12 = 3amp. watts/voltage =amps. the rating then is 125% for "starting" current to not blow the fuse. This is mainly in a motor or heating type circuit but is permissible overall. so say your heated seating you want to install is 1000watts. 1000/12 = 8.33 amps times 125% is 10.41 amps. After a 10 amp fuse in most cases the next size larger fuse would be a 15 amp. but technically since the heat is only 8.33 amps the wire size can 18ga.

    I'm not dogging you for wanting to ask or learn for sure otherwise I wouldn't have even begin to get involved, like I said in other posts the chart is a guide for you to have a baseline on sizing. For me at work it is the gospel when doing my job in a new construction or remodel. The reason that the wire is the protected by the fuse is in my world it it the weakest link and causes the most damage when it fails, just like you worried about from the beginning.
     
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  8. GordDavey

    GordDavey Senior Member

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    KC Cryptkeeper,

    Thank you, the light bulb is coming on so to speak. I'll need to google "derating purposes". Do you have a reference amp capacity chart that goes up to 22 ga ? That's the smallest size wire I've seen in our truck's wiring diagrams.

    If you don't mind, I'd like to ask about something a bit more "real world"...

    1) Let's say I wanted to install 2 of these 36W output USB devices using 1 wire. Each of these USB devices comes with a bit of wire that has an inline 10 amp fuse. That would protect the wire but is it required to protect the device?

    So, 3 amps for one, means to me 6 amps for 2, I would think it would require a bit more power in than you get out thanks to converting down to 5v, so 6 amps min, even though there is no startup load, would it still be a good idea to use the 125% as a safety net... especially when the input is unknown? If so, then 6 becomes 7.5 amps, looking at your chart reference and pacofortacos, it looks like I could still use the same 18ga wire. Can I eliminate the 2 inline fuses and just connect it to an open location in the fuse box that can then have a 10amp fuse on it?

    2) let's suppose for a moment that I have 2 sets of these 1000 watt heating pads at 8.3 amps each, and they both come with an inline 10 amp fuse (which they do). Above it was said 1 set of these could be run on an 18ga. But if I run one wire that can power both, then I would look for a wire that can handle 8.3 x 2 (16.6 amps) which on both charts looks like I would need a 14 ga wire. Would I put a 20 amp fuse at the beginning of the 14ga wire, then use 2 18ga wires off of it to go to each set of heating pads, would I leave the 10 amp inline fuses in each of the 18ga wires ?

    Please correct me if I'm wrong anywhere.

    I understand that if I don't connect the wires to a "spare" or unused fuse in the fuse box that is pre-built to have a RUN or RUN/ACC relay on it, then I should get a relay and run the main circuit power from the battery thru the relay.
     
  9. pacofortacos

    pacofortacos Senior Member

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    "pacofortacos said the USB would be about 36 watts and that 18ga would be sufficient... how did he come to that conclusion? Should I be figuring out the max watts and run that thru some math to get to amps required then multiply that at 125% to get total amps, then selecting the wire size from the chart that can handle at least that many amps?"

    Bingo!

    Power in = power out + losses.

    The USB is good for ([email protected] ) X 2 plus any internal losses when converting from 12V to 5V.
    So each USB port is good for a maximum of 15 watts and there are 2 of them rated at 3A each.
    2 x 15 = 30 watts plus I threw in 6 watts loss which is probably on the high side.

    As a general rule, I see most power ports (plastic cigarette lighters) melt way before they blow a fuse under a heavy load. A short yeah the fuse pops, but run a heavy load and they get hot and often melt.
    Of course most of the heat is made at the poor connection inside the port where whatever you are plugging into the socket meets the power pin.
     
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  10. pacofortacos

    pacofortacos Senior Member

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    "1000 watt heating pads at 8.3 amps each"

    Not possible on 12 volts. P=VxA so 8.3A x 14 v = 116.2 watts
    Are those being fed 120V or are they rated for 120V?

    "So, 3 amps for one, means to me 6 amps for 2, I would think it would require a bit more power in than you get out thanks to converting down to 5v, so 6 amps min, even though there is no startup load, would it still be a good idea to use the 125% as a safety net... especially when the input is unknown? If so, then 6 becomes 7.5 amps"

    Are you going to run 2 of the dual usb adapters? Or are you just considering the one dual unit?
    They are rated at an output of 3A @ 5 V EACH port, but the input isn't 6A due to the input being 12/14V. That why you go for the wattage and convert from there.
    So if you run 2 separate adapters @ 36 watts (still a guess that there is 6 watt loss) that will equal 72 watts of power needed at 12-14V. P=V*I so P/V=I so 72/12=6A input just like you figured :) I would 10A fuse it and call it a day.

    Many things will actually have the input voltage and current needed as well as the output voltage and current when converting. On most things it is printed on them if not in the specs.
     
    Last edited: Sep 21, 2019

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