Wiring Info/Help/Understanding

Hi All, I'm not an electrical guy although I am learning fast as I've done a bunch of work on my Ram 1500. But there are some things I do not know and need to understand better about wiring, such as guage, drop %, amp and length of wire to understand what is the correct stuff to use and where.

I have two things I'd like to add to my truck. #1 Rear Seat Heaters and #2 A dual USB quick charge outlet.

I'm trying to understand what size of wiring I need and what I can / should / shouldn't do so I don't melt wires or end up with a non working item... or start a fire. I have a ton of left over automotive wire from rebuilding my truck from salvage so I have all kinds of wire from extra body harnesses, engine harness etc I can use, or I can use wire from the auto store. As I understand it automotive wire is better and more capable than the plain old auto store wire (GPT).

#1 - Rear Seat Heaters. I purchased an aftermarket heater kit. 4 pads, 2 switches. It's the same kit many people buy with the relays and switches built in, each relay requires 10amps for the main wire and a 5amp for the switch wire.

I think I can do this one of two ways:
A) Run a 12ga wire from the battery to the rear of the center console and connect both main wires from the relay, then tap off a power outlet to power the switches. Would the 12ga be too big/small ? Does taping into an outlet for the relay end up meaning that I shouldn't put something with a large current draw in the outlet from then on?

B) Looking at the wiring diagram for the truck (bought from Tech Auth) the OEM rear seat heaters ran off the same fuse as the heated steering wheel. Could I just tap off the 16ga PK/YL wire in pin 7 of the yellow connector to supply the 20 amps to the seat heater pads and just run a new wire from the fuse box to power the heater switches? Is this a terrible idea? I'd much rather come out of the fuse box with current that only flows when in Acc/Run or Run Only than a direct connect to the battery. Less chance of battery drain I would think.

#2 - Switching the power outlet on the back of the center console to a dual USB which requires a 10amp circuit. The power outlet says it has a 20amp fuse and uses 18ga wire from the fuse box. I can see the wire steps down to 20ga (I believe) just when it makes it to the power outlet. Since I'm replacing the power outlet can I simply use the power and ground from the outlet for the 10amp USB device and swap the fuse in the fuse box from 20amp to a 10amp one? The wires that come with this thing must be 14ga or 12ga, and the wire at the power outlet is so small in comparison so that I truly wonder if this is a good idea or not. Advice on what I should do/wire size would be really helpful.

In doing research on wiring and trying to learn and seeing different wire type and gauge to amp and length charts, they often seem to contradict each other, and the wire gauge that FCA uses for things just doesn't make much sense when looking at the charts I've found. Like the power outlet saying its an 18ga 20amp from the fuse box, from what I read it should be a much larger gauge to support 20amps at the length I estimate the wire to be. Any help or URL's you guys have to help me understand how to choose the correct wire size for this sort of stuff would really be appreciated. I understand there are drop percentages involved and the only way I can see some of the wire gauge sizing match is if the drop percentage is quite large.

Thanks for any insight.

Ive done the rear seat heater thing. I too went over the wiring diagrams, again and again and again. Its almost like they are combining power for the heaters between the heated wheel circuit AND the seat heater switch circuit.

Far easier to run a wire back to the battery. Fuse that one inline. Then put a relay under the front seat, where you can grab ground (black) and a triggered power (pink/yellow) from the seat side of the harnesses (seat side because then you dont muck with chassis side wiring that is much harder to replace if something goes wrong). Then from the relay to each seat heater switch. I personally got a dual output relay so I could have one output to each seat heater, with a fuse inline between the relay and the seat heater switch. I grounded each seat heater switch to the rear seat mounting bolts (I installed the switches into the side trim panels next to each seat)

12ga from the battery to the relay is fine. I went 14ga from the relay to each seat heater...considering the RoadWire seat heaters were using 16ga (or smaller!) I figure 14ga is just fine for the run from relay to switch.

I have not done the USB port thing. I dont have a crew cab so I dont have an outlet back there....is yours 12V, 120V (inverted) or a single USB?

chrisbh17, the dual usb's are 12V (with QC3.0) good for auto and marine. I saw them used by another member and figured it would be much better than the standard power outlet, as everything the kids use back there is usb powered.
Found Here: https://www.amazon.ca/Charger-CHGee...GCTBG2&qid=1568827170&s=merchant-items&sr=1-4

GordDavey said:

chrisbh17, the dual usb's are 12V (with QC3.0) good for auto and marine. I saw them used by another member and figured it would be much better than the standard power outlet, as everything the kids use back there is usb powered.
Found Here: https://www.amazon.ca/Charger-CHGee...GCTBG2&qid=1568827170&s=merchant-items&sr=1-4

Click to expand...

I was asking what is in the truck currently.....if its a round 12V outlet, etc.

On the other hand, you could increase the wire gauge coming from the battery to your seat heaters, then tap off that for the USB outlet. I think 8ga would cover the seat heaters + that USB outlet. Then use a second relay triggered from the same source as the seat heaters (if you want) and only output from that relay to the USB port.

chrisbh17, currently the back of the center console has a stock 12V power outlet (Fuse is 20amp)

So you think it is better to not try to re-use the existing wiring from the existing 12V 20amp power outlet to run the 10amp dual USB. I am removing the existing power outlet and completely replacing it with this USB outlet.

GordDavey said:

chrisbh17, currently the back of the center console has a stock 12V power outlet (Fuse is 20amp)

So you think it is better to not try to re-use the existing wiring from the existing 12V 20amp power outlet to run the 10amp dual USB. I am removing the existing power outlet and completely replacing it with this USB outlet.

Click to expand...

If its already 12V, then yes I would attempt to use it. Im not sure how it connects to the back of the stock 12V port, though....looks like you need quick connects for the one on Amazon, I doubt the stock connector is that simple

So it might involve cutting the stock wiring, which I am never a fan of.

chrisbh17, Thanks. I may just solder a wire on the power and ground with the quick connects and tape over the stock outlet. As the new USB is larger in diameter than the original power outlet, I have to drill the whole bigger, so there's no going back anyways.

https://media.distributordatasoluti...msOcal20(USA)oc_1_g_nec31016pdfClickHerea.pdf

Wire is wire it doesn't care if it is in a truck or your house. As far as voltage drop goes nothing inside a truck is going to drop more than the 3% to raise wire size. My recommendation is stay at 80% of the rated circuit to wire ratio and melting will not be an issue.

KC Cryptkeeper said:

https://media.distributordatasoluti...msOcal20(USA)oc_1_g_nec31016pdfClickHerea.pdf

Wire is wire it doesn't care if it is in a truck or your house. As far as voltage drop goes nothing inside a truck is going to drop more than the 3% to raise wire size. My recommendation is stay at 80% of the rated circuit to wire ratio and melting will not be an issue.

Click to expand...

KC Cryptkeeper. The chart you show contradicts what is actually in our trucks. For example the power outlet on the back of the center console has a 20amp fuse. The length of the wire must be around 10' as it has to go from the fuse box thru the firewall down to the floor, across to the middle of the driver seat door sill then under the seat, into the center console and to the back of the center console. Then the length of the ground is supposed to matter as well... maybe a couple of feet to a ground lug. So, 10 to 12' of wire total at a rough guess. Fuse 90/91 is what it is on, and it's a 20amp fuse which tells me whatever is plugged in can draw up to 20 amps before blowing the fuse, and the wire is safe as long as the draw is less than 20amps. The OEM wire in the truck is 18ga for that entire distance.

Your chart shows a min of 14ga for 20amps (60 DegC or 75 DegC wire), and doesn't account for the length of the wire, which other charts say is very important. I'm trying to figure out how they can get away with such a small wire, when everything I read says it should be much bigger. There's some sort of "art" to this science of wires that I don't understand, and I'd like to, it'll certainly help when figuring out what size wire is truly needed for any add-on mods.

GordDavey said:

KC Cryptkeeper. The chart you show contradicts what is actually in our trucks. For example the power outlet on the back of the center console has a 20amp fuse. The length of the wire must be around 10' as it has to go from the fuse box thru the firewall down to the floor, across to the middle of the driver seat door sill then under the seat, into the center console and to the back of the center console. Then the length of the ground is supposed to matter as well... maybe a couple of feet to a ground lug. So, 10 to 12' of wire total at a rough guess. Fuse 90/91 is what it is on, and it's a 20amp fuse which tells me whatever is plugged in can draw up to 20 amps before blowing the fuse, and the wire is safe as long as the draw is less than 20amps. The OEM wire in the truck is 18ga for that entire distance.

Your chart shows a min of 14ga for 20amps (60 DegC or 75 DegC wire), and doesn't account for the length of the wire, which other charts say is very important. I'm trying to figure out how they can get away with such a small wire, when everything I read says it should be much bigger. There's some sort of "art" to this science of wires that I don't understand, and I'd like to, it'll certainly help when figuring out what size wire is truly needed for any add-on mods.

Click to expand...

Exactly what I ran into. I've done plenty of 12v stuff in my years but what i see in the truck vs what I think I should see is sometimes very different.

I could understand if they stepped the wire size down once inside the cab but I'm seeing the same gauge through the whole circuit, all the while wondering how they get away with it.

The only thing I can come up with is that the tipm is used across various vehicles, so the stock fuses are kind of a lowest common denominator.... Our trucks might not use 20a on 90/91 but a 3500 or a minivan or a hellcat might. It's a stretch of an idea though.

That is the biggest reason I decided to run my heated seats back to the battery. That way I know for sure the size of the wire is correct all the way through. The dinky wires under the seat serve fine for a trigger but once I saw them any plan to use them for power went out the window

Sent from my Nexus 6P using Tapatalk

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.

GordDavey said:

KC Cryptkeeper. The chart you show contradicts what is actually in our trucks. For example the power outlet on the back of the center console has a 20amp fuse. The length of the wire must be around 10' as it has to go from the fuse box thru the firewall down to the floor, across to the middle of the driver seat door sill then under the seat, into the center console and to the back of the center console. Then the length of the ground is supposed to matter as well... maybe a couple of feet to a ground lug. So, 10 to 12' of wire total at a rough guess. Fuse 90/91 is what it is on, and it's a 20amp fuse which tells me whatever is plugged in can draw up to 20 amps before blowing the fuse, and the wire is safe as long as the draw is less than 20amps. The OEM wire in the truck is 18ga for that entire distance.

Your chart shows a min of 14ga for 20amps (60 DegC or 75 DegC wire), and doesn't account for the length of the wire, which other charts say is very important. I'm trying to figure out how they can get away with such a small wire, when everything I read says it should be much bigger. There's some sort of "art" to this science of wires that I don't understand, and I'd like to, it'll certainly help when figuring out what size wire is truly needed for any add-on mods.

Click to expand...

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.

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

pacofortacos said:

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

Click to expand...

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.

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!

KC Cryptkeeper said:

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.

Click to expand...

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.

GordDavey said:

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.

Click to expand...

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 ******* 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.

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.

"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 (3A@5V ) 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.

"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.