CRAZY STEVE is it okay to comment or would you rather we just read what you have to tell us? I'm subscribed too.
Planning Stage. What you are going to need and where to place it, so that you can run wire to and from these components.
Ampacity, or what size wire do I really need? This is apparently one of the least understood parts of designing a harness, judging by the amount of misinformation I see out there. Search the web for automotive wire amp ratings and you'll get results all over the map. I've seen some that are reasonably accurate for most applications, to some that are off so wildly that using them will guarantee a fire. So what's the answer? I'm going to explain all the factors that are considered by the engineers when they design a harness and at the end give you a set of recommended values and wire sizes for specific places. But it's important to understand why I'm using the numbers I am so you have a clear idea when selecting wire sizes. The first thing to know is any wire can have any amperage applied to it. But apply too much, and the wire will overheat; go high enough and it will melt. Everybody knows this, but the trick to know is how much is too much? A wiring system is designed to allow you to control various electrical devices. The actual construction of it (materials and methods) determines how well it delivers the needed voltage/current to those devices and manages the generated heat caused by the current (amperage) used. This is the core issue for sizing wiring (actually, almost all components need to be sized); using wire large enough to keep temperatures down to safe levels that won't damage components or the wire. Now, when the OEM engineers do this, they have all needed information to make informed decisions, but you won't; it's possible to get that information, but this will be far too complicated for pretty much any home builder, and probably most vendors as well. The second thing you need to know is any wire will heat up when any amount of current is applied to it. If the amount of current is small, the heating will be small also. More current, more heat. So when you see amperage charts that don't tell you the operating temperature, the amp rating is pretty much worthless. And even if the temperature rating is given, what does it mean? This a fairly complicated subject, and I'll try to clarify it. One readily available source of wire ampacity ratings with temperature factors included is the NEC (National Electrical Code). Now, I hear some saying (again) that this isn't for cars and can't be applied. Sorry, but it can; amps are amps, no matter where they are. But unless you know how to read these charts and particularly the notes and 'other' charts that go with them, you can easily get the wrong idea and information. Here's a link to the 'main' ampacity chart and this also includes de-rating info for ambient temperature as well as multiple wires in a common raceway or cable. Scroll down to page two... http://www.cooperindustries.com/con...r_Derating_Ambient_Temperature_Correction.pdf Let's start with what the numbers mean for the #8 wire (I'll get back to the smaller wires further on). There's three temp columns, one at 60C, one at 75C, and one at 90C. These numbers represent two things; first, the temp rating of the insulation on the wire. So a 60C wire can operate at up to 60C (140F) before thermal breakdown of the insulation starts to occur. Second, you have different amp ratings in each column; these represent the maximum current that can be carried without the wire exceeding it's temp rating. But note that this assumes an ambient temp of 30C (86F), so what this is really saying is if you apply those maximum amps to the wire, it will heat up to those temperatures from the current. So if you jump over to the 90C column and apply 55 amps, the wire will now heat to 194F. Now, this would imply that if the wire insulation is rated for the temperature, it's safe to use those numbers; this is only partially true but as a practical matter no, for other reasons I'll get into later. But these 'higher' numbers will come in handy, as I'll explain when I get to system design. Now, go back to the link and look at the secondary charts. You've got a temperature correction chart, as well as a derating chart for more than three conductors in a raceway or cable. The temperature correction chart really needs to be looked at, as underhood temps can easily reach the higher numbers. Same thing goes for the 'more than three conductors' chart; while it's unlikely anyone will be using conduit or cabling for their harness, if you 'bundle' the wires together you will see a similar effect due to transmitted heat from one wire to another. I'll note here the actual ampacity ratings for 10, 12, and 14 wire. Don't use the numbers in the chart, but note the maximum allowable overcurrent allowed; 30A, 20A, and 15A respectively and use these numbers. These were raised to allow derating without having to increase the wire size as these are the most common sizes used; basically a cost-saving measure. Hopefully, this has given an understanding of wire ampacity rating, or your eyes are glazed over or you have a headache....LOL. Bottom line, these are the maximum amp numbers you want to use for the various sizes of wire; all from the 60C column and the first numbers are AWG sizes (the number in parenthesis is the equivalent metric size if that's what you have available): 18 (1mm) - 6A, 16 (1.5mm) - 8A, 14 (2mm) - 15A, 12 (3mm) - 20A, 10 (6mm) - 30A, 8 (8.5mm) - 40A, 6 (16mm) - 55A, 4 (20mm) - 70A, 3 (25mm) - 85A, 2 (35mm) - 95A. If you're wondering where I got the values for the 16 and 18 wire, the NEC lists those separately under 'fixture wires' in another place. They're not used as circuit conductors except in lighting, and as low-current control wires. One more thing; go back to the link and look at the 'terminations' chart. Note that there's none rated for 90C, and the 75C ones are rare (and expensive). So if the terminals you're using are sourced from the electrical industry, it's almost a sure bet they're only rated for 60C. If they're a specialty automotive connector, they're probably unrated and I'd use 60C to be safe. The system will only be as good as it's weakest link. The NEC arrived at these numbers by using accepted engineering knowledge and extensive testing, so they are gospel. Is there 'wiggle room' in them? I have no doubt there is, as I'm sure they've included a safety margin of some sort. If you have an electrical engineering degree and access to all electrical data for your vehicle, you could calculate just how much 'wiggle' there is. But lacking that, these will work and be conservative enough that they should give a trouble-free installation. In the next installment, I'll get into overcurrent protection (fuses) and sizing control components (switches and relays) properly.
As far as questions, it'll probably be best if you hold off until I get all major parts posted as I'll probably address it at some point. And thanks for the interest!
Subscribed! Thanks for taking the time and making the effort to educate us. Looking forward to the next post.
Great info Crazy Steve but my ampacity must be very small........got to let this all sink in..........
This perfect timing, I'm getting ready to 're wire my 54. The previous owner wired the whole car with black wire and some "questionable" wires. Posted using the Full Custom H.A.M.B. App!
Thanks to Crazy Steve!! Just starting to rewire my 35 dodge pickup,It has been converted to 12 v neg ground at some time in the past still running generator, but due for new switches and wire.Larry in down town Tumwater.
So which post is part #1. If you say post #1, then the title does not say that. I think he made some changes over the last 24 hours. I like the idea of going through this information blog, but it will still not keep the questions from coming in from members. Interested in finding out how much information he lists about modern day computer based equipment.
Thhn wire insulation and conduit tables are different than automotive wire. Engine compartment temps are generally 90c / 194f Amps are amps this is true, but the insulation is what's governing the critical failure point and how many amps the automotive wire can carry. It's a good idea to ask that kit vendor what type of insulation they are using because cost of the wire goes up as the quality of the insulation goes up. Dropping the insulation rating would equate to higher profits for the manufacturing party. What types of insulation are on automotive wire? There are two main categories of automotive wire – PVC and Cross-Linked. The biggest difference between the two categories is temperature range. Cross-linked automotive wire can withstand much higher temperatures than PVC automotive wire. The three main types of PVC automotive wire are: GPT - used for general circuit wiring and rated to 80 °C TWP - lead-free, thin wall automotive wire rated to 105 °C HDT - heavy wall automotive wiring rated to 80 °C PVC is insulation is extruded, which is created by heating PVC and then extruding it through a die on the stranding. This insulation can be melted with a heat source, changing the form. The three most common types of cross-linked automotive wire are: GXL – thin wall, most common type, works with most standard automotive connectors, rated to 125 °C SXL – standard wall, rated to 125 °C TXL – extra thin wall, best for applications that require minimal size and weight, rated to 125 °C Cross-linked insulation is created by extruding the material through a tube, under heat and pressure, in order to 'cross-link' or change the molecules of the insulation to another state.
There are a few other things to consider in automotive wiring that will affect current ratings. Most of the car builders will bundle their looms, which will bring a dozen or more wires into a small taped group. When you bundle wires the current carrying rating is less, so ampacity of the wires is de-rated to a smaller amount. This usually isn't a big factor if all your current draws are less than the wire rating, but if you have circuits that are pushing the limits, then all the other wires wont hold their rated ampacity either. Another consideration is wires in free air vs. those running in something like a loom or rocker panel, etc. Free air ratings are actually higher than standard NEC ratings.
Excellent discussions... A question I have always had, and forgive me if its been answered, is this; I know running too small a gauge wire is bad, is there drawbacks to running larger wire (cost obviously) than is necessary??
Electrically, there is no downside; bigger is better. The only consideration is going too big can cause issues with matching the wire terminals to the device. But you can nearly always go up at least one size. Generally speaking, nearly all 'plug-in' devices will accept up to #10 wires, it's when you get above #8 that you start having problems.
Thanks for the answer Steve, and you perfectly described the issues I've had when trying to "upsize" and that is the fact that finding proper terminals is difficult-
A lot of good info here. As an A&P mechanic/avionics tech I agree with all. A good source to find the correct wire size is to use the FAA's "AC 43.13-1b", there is chart to determine wire size in relation to amp's, distance, voltage, and if it is in a bundle/conduit/free air. A pilots biggest fear is fire while in the air, so that is why the chart was created. It will keep everybody safe. Mark
Ampacity again.... After re-readng part three and the follow-up comments, I see there's still some confusion about wire ampacity that I didn't get clear enough (my fault..). Different temp ratings with different amps allowed, what's it all mean? I can hear some asking themselves, if you're only allowed to use the 60C ratings for max amps, why do the higher ratings exist? What good are they if I can't use them? Here's how you use them..... If you go to the link I posted... http://www.cooperindustries.com/con...r_Derating_Ambient_Temperature_Correction.pdf ... look again at the main chart. This gives various amp ratings based on the insulation rating. The key thing to note here is all these ratings are based on a ambient temperature of 30C (86F). In other words, if you have that wire laying on your bench and your shop is at 86F, when you apply the amps as listed in a particular column that wire will heat up to the maximum allowable temperature as shown in that column. Copper wire is copper wire, it doesn't know (or care) if it's in a car, house, or whatever. It's also important to understand that any wire of a particular size will heat up pretty much exactly the same; the different insulations are simply to allow this heating without the insulation breaking down under the heat. But what if the ambient temperature isn't 86F? Now look at the temperature correction chart in the lower left corner of page two of the link. These numbers are multipliers that you apply to the wire ampacity to correct for ambient temps. If ambient is under 86F, the wire can actually carry more amps. But if the temperature is higher, the maximum amps goes down; the higher the ambient, the less amps you can put into the wire. Again, these numbers are correct for any copper wire (unless you have some sort of exotic copper alloy) regardless of type. The different columns simply reflect the insulation ratings. Now here's where it gets tricky.... I've stated that you need to use the 60C amp values when sizing the wire for individual circuits and that's absolutely true BUT what do you do if you have wiring that is operating in a higher ambient temperature? If you de-rate from the 60C numbers, the wire quickly gets too small for the load and you're forced to use larger wire. This is where the higher amp ratings come in.... Let's do an example. Say you have a circuit that requires 32 amps; this is larger than the 60C rating for #10, so you need a #8 wire. Number 8 is good for 40 amps @ 60C, so you have plenty of capacity. But this wire goes into an area where the temp is higher than 86F, so you now need to de-rate it. Let's say the ambient is 140F. Now, if you're using 60C wire, you've already exceeded it's temp rating, so that's no good. But if you're using a 90C wire, you apply the .71 multiplier in that column to find the safe maximum amps, using the maximum amp rating at 90C which is 55 amps. So 55 times .71 (the multiplier at 140F ambient temperature) equals 39.05 amps, more than the 32 amps you need for the load. This is the reason these higher temp amp ratings really exist; not to allow you to use them when sizing the circuit, but to give you a 'cushion' when applying de-rating factors without having to increase the wire size. I will comment on the 31Vicky posted temperature ratings for common automotive wire. Being even higher ratings, this will allow even more amps to be applied to the wire. BUT, without knowing just what those maximum ratings are or having a engineered derating chart, any larger numbers a non-engineer would plug in would just be a dangerous guess (and I include myself). I would be very hesitant to try to extrapolate from the chart shown, and also note that as temp goes up, the derate factor increases faster than the temperature. Heating of the wire from amperage is not linear, but exponential. So while these higher-temp wires can carry more amps, it may not be as much more as you think; a 25% increase in temp rating does not necessarily translate into a 25% increase in amps. One critical thing to remember is not every electrical part of a circuit will have the same temperature rating; you must limit amperage as much as possible to the rating of the lowest-rated part to insure a safe, reliable system. This can be a lot to wrap your head around. I was asking that you hold questions to the end, but this is important enough that if some still have questions, I'll try to clarify this before moving on...
When you're ready to answer some questions: Relays - When and why? Is it just mfgs trying to save on wire size and bulk? Solenoids - When and why? How to tell wire quality and what type? Why fusable links and not just big fuses? How to determine quality switches? What's the difference between household 20A 120 V elect switch and auto elect switch? I know the answers to some of these but would like a better way of explaining it.
I'll answer a few of these now, I'll get to the rest when I get to system design.... 'Solenoid' is a term that has gotten used rather sloppily. Generally speaking, a solenoid performs a mechanical function, but may also operate a switch as a secondary function (GM starter solenoids as a prime example). If only an electrical function is done, it's a relay but 'solenoid' has come to mean 'large relay' in common parlance. The typical 60s-80s Ford 'starter solenoid' is really just a big relay.... The fusible link is a fuse, just a different type. Every fuse type is rated and/or designed to clear a fault or overload in a specified amount of time, generally measured in milliseconds. A fusible link is a 'slow blow' type, able to withstand overloads for a time but designed to melt if the overload goes on for too long or you have a major fault (short circuit) to prevent catastrophic damage. A fuse is too easy to replace, if a fusible link goes you need to identify the problem and being harder to replace, that will tend to make most people have it done by a pro. As far as the difference between a household switch and an automotive switch, there isn't much in most cases. As long as you don't exceed the switch rating and mount it properly, no reason it won't work....
Crazy Steve, when your ready to answer more questions in the near future, please clarify for me which wires can be run in bundles & the best locations for the bundles within the interior of the car. Do specific bundles or single wires need to be separated by a certain distance? Thank you, excellent info & appreciate you taking the time to write & share this information. Posted from the international space station & powered by the Full Custom H.A.M.B. App!
Here's a video that is helpful: http://www.powerblocktv.com/episode...ring-basics-gauges-schematics-troubleshooting
The difference between household and automotive switches is the voltage rating. Household switches typically are rated at 120V and 15amps. This means that the switch can handle 1800 watts. Now take the typical automotive switch rated at 32V @ 15 amps. This switch is only capable of 480 watts. I think this might want to be pointed out.
That's why switch ratings are rarely if ever given in watts.... Voltage ratings for switches are given to describe the arc-suppression capabilities. The amp ratings describe current-carrying capabilities and are quite independent of voltage ratings. Reduce the voltage and the 'watts' rating goes down on either type switch; at 12 volts both switches would only be good for 180 watts.
The chart is in chapter 11 of AC 43.13-2b, it is about page 30. the chart that i use to teach students is a little out dated, but it is still very good. The only reason i can find that it is out dated is because it is a lttle complicated. Steve I am sorry at this time, I am having a hard time remembering how to post pictures. (if somebody can help me out I will post the chart). I will try tomorrow while I am at work.
The Jalopy Journal
