Dude.... Thank you.... For people building their own harness, you have no clue how useful this information is.

-ono1

 

Thank you! Few Q's though:
-where does the miata 18-22ga wire in to the ecu/small wire side of the relay fall in place?
-16ga is a bit big for 4amps. All the 8 rolls of wire I bought, say otherwise. They have ratings on the back of the package, quite handy.

 

If the OEM uses smaller wire, it's legal to use that same size wire in retrofit (as per my provincial legislation). That was mentioned in my parent post.

Also this is really only about power wiring. There are many cases where it would not apply.

The "handy" chart on the spool of wire, although not strictly bogus, is not there to inform you. It's there to get you to pick up the spool of wire in the store, because when you handle a product by hand you are far more likely to buy it. Secondly it provides an impulse buy trigger … you see the current, see the wire, yep, that's what I have in my hand, so it instantly overcomes an objection. You are much more likely to buy a product you hold in your hand, the chart encourages you to do that, and then it overcomes an objection. Sales and Marketing 101.

Just in case you didn't know. Learned that stuff 35 years ago when I was selling product.

Also, chances are it's GPT rated Automotive Primary Wire. This is the wire commonly found in retail stores because it's the cheapest wire you can get for Auto use. This is not suitable wire for full-vehicle wiring (see below, post #7). GPT is a Society of Automotive Engineers (SAE) Grade wire. SAE Gauge is about 10% smaller than American Wire Gauge (AWG) wire, so GPT 16 Ga is not equivalent to AWG 16 Ga.

Also the control side of a relay uses very little power … under 100 mA (1/10th Amp) is typical. It is worth noting that Mazda chose a wire gauge (18~22) that would be considered overkill strictly based on the ability to carry 100 mA over a reasonably short distance. If you were wiring an ECU or some audio device internally, you might use even 30 GA for 100 mA over less than 12 inches. So, it "falls in place" on the side of a very conservative wire gauge selection, which is exactly what I suggest you do as well, in general.

 

16GA is not a particularly heavy or thick wire; even 14 GA is pretty reasonable. So I don't see a huge penalty in using 16GA, for example weight wise for the lengths we use in the car. The weight saving potential of going to a lighter gauge, going strictly by current demands only, is in the order of one or perhaps two pounds overall vehicle weight.

A 1000 feet of 16 GA wire weights maybe 8 pounds; 1000 feet of 22 GA weights maybe 2 pounds. So if you had nearly a quarter mile of wire in your Miata you would save 6 pounds by switching all 16 GA wiring to 22 GA.

You will save much more weight if you remove unnecessary wiring (the car at the factory will have much wiring for options in place regardless of whether the car is fitted with all available options) for a net reduction of vehicle weight.

 

The charts you see online and probably on your package of wire are based on room temperature (not a reasonable assumption in a car) and a single wire surrounded by air alone. When you bundle wire, you generate more heat. Anytime a wire carries current, it heats up. When you place wire in elevated temperatures, it's ability to shed heat falls. As wire heats up, it's resistance rises and it is capable of carrying less current. There are good reasons for the guidelines versus some chart using unrealistic assumptions.

Also, if you spend any amount of time poking around older cars you will see burnt wiring somewhere. It's true that in a perfectly working, no fraying, no broken or cut outer jacket, etc system maybe 16 GA is overkill for some low current applications, but that is with zero safety factor and with no margin for the factors that affect wire current carrying ability.

Cars are moving, vibrating, rained-on, worked on, maybe missing grommet type installations. If the connector at one end is not perfect, you don't actually have a 16GA connection there.

People also misjudge the effect of having an automotive battery in the circuit. A 12V car battery is capable of delivering over 200A to anything connected to it. A short, broken wire, etc will all result in extremely high current flowing through a wire. That is why we use fuses, but if you think fuses act instantly you are wrong. They take large fractions of a second to many seconds to blow, and that's when they are properly rated. A lot of damage can happen in the meantime.

If there is resistance in that circuit's power wiring, the excess current is wasted as heat, and with 200A amps available, that's a lot of heat that can easily create hundreds of degree temps on the wire. A larger gauge will withstand more heat before burning up the outside sheathing, and that will lead to potential shorts against the chassis or some other grounded object, and that leads to more heat, and so on.

So, in the real world, do not assume the charts you find elsewhere are the final word … they refer only to perfect wiring with no issues whatsoever. Your car is likely not to be that way, or if it is, to stay that way.

 

It's critical that anyone working with automotive wiring and circuits understands exactly how much current is in play with DC voltages.

An AC-rated switch that can happily withstand the arc currents in a 220V 15 Amp home circuit will burn up quickly if used to switch 12V DC at even 5A because in a DC system you are always dealing with the full current of the system whereas in AC there is zero voltage across the contacts 60 times a second and less than full current for almost all of the rest of the time. You need robust wiring, DC rated switches (which cost much more than AC-only rated switches … because they need to be much more robust) and proper Automotive grade wire.

A lot of what is sold is not true Automotive grade, and can't be expected to last the next 20 years. Also, and this is common in wiring "kits" sold for car stereo installations, for example … wire gauge is sometimes fudged because copper is expensive. Two cables (wire and sheath) of the same advertised gauge can have vastly different internal structure, with one having twice as much actual copper wire than another, the difference made up by using a thicker sheathing. Be careful buying wire and make sure you get what you paid for.

Naturally it's the copper gauge that matters. If the wire you're looking at can't provide complete specifications … number of strands and AWgauge of each solid strand, number of bundles of those strands, total aggregate gauge of bundles, and the type of cable … don't buy it.

Automotive Grade wire should be market on the wire itself, or graded on the spool. It must be one of the following five SAE grades:
GXL … for outside the interior, in the engine compartment, etc. -51C to 125C This is what OEMs use.
TXL … for areas that are not exposed to heat or the elements, basically car interior only. TXL has extra-thin sheathing; just another reminder that the wire gauge of the actual copper conductors can't be measured by looking at the OD of the wire. -51C to 125C
SXL … is a premium automotive wiring with thicker sheathing … used when abrasion or rough service is expected. -51C to 125C
SGXL … very thick premium wire. Generally this is what is recommended for the large gauge battery wiring, e.g. from battery to starter. -51C to 125C

All of the above use Cross-Linked Polyethelene (XLPE) insulation. XLPE is next best to Teflon (PTFE) for electrical properties. The GPT (below) uses PVC insulation.

GPT … is approved under SAE J1128 for vehicle chassis wiring … such as wiring a trailer light socket. This is the wire most commonly sold as "Primary Wire" in retail stores. Under SAE J1128 it is not to be used in engine compartments or for direct connection to a battery. In other words, only after a fuse and not near exhaust or engine. -40C to 85C

All of the above are SAE Grade wire for Automotive use. SAE Gauge is about 10% smaller than equivalent gauge AWG.

If you're looking for a "one spool solution" wire useful for under hood and elsewhere, GXL is the best option. With careful routing it can work almost anywhere in the car. Plus it's what all the OEMs use on the production line.

 

And finally, the Elephant in the Room … what happens when your car is involved in an accident? Is the wiring robust enough to withstand some shock loads without shorting, and causing a fire? When you "cleaned up" your wiring under the hood, did you move current-carrying bundles outside of the inner fender, where they are more likely to be cut / abraded in the even of an accident, and lead to a fire? Maybe you want heavier wire there if you want to "clean up" the OEM wiring?

You can buy wire that exceeds Automotive grade for very little money. Silver coated copper stranded Teflon dielectric wire is widely used in Aerospace and Aircraft construction. It is electrically the best dielectric on Earth and is extremely temperature resistant.

Regulations severely limit or prohibit outright the splicing of wire in these installations. Also, as a quirk in the manufacturing process, full spools are allowed to have splices. As a result, end-rolls of up to a few hundred feet are routinely sold as surplus at a fraction of the cost of acquisition. Boeing routinely sells wire to the surplus market, for example.

I generally pay about $5 per 25' for this wire, and that's without trying too hard to find it. That's about the same price as new retail GXL wire. It's definitely available for less if you're willing to do some digging. If you buy it new, for the same $5 you could buy maybe two feet. Also US residents can buy directly from Boeing's surplus store.

It's available in literally 100's of unique colours and identification stripes, rather than being limited to the 20 or so colour / stripe variants in Automotive grade. So, building harnesses with good identification anywhere in the bundle is easy. Generally speaking the PTFE jacket is thin in diameter so the overall diameter is quite small; comparing favourably to the lightest SAE grade wire.

Although the Silver coating on the copper strands is often mis-understood as some kind of high-performance purpose, the real purpose is Silver is non-corrosive ( or strictly speaking, the corroded form, silver oxide, is protective; it does not allow corrosion to penetrate to the copper substrate, and is equal in conductivity to pure silver) and prevents the green copper corrosion you often see migrating up beneath the outside sheathing whenever you have an installation that is exposed to water and air. About the only bad thing you can say about it is it's a little stiff and the Teflon jacket is hard to strip.

Because it's MIL-spec wire in AWG sizing, it contains about 10% more copper than equivalent SAE Ga wire. Typically rated -51C to 200C.

It's ideal for under hood use. If it burns, it's toxic but then again you need a real fire to get it to burn. All SAE grade XLPE wire would have started to burn 75C sooner. But if you want to be careful, you could limit it to under hood use and use less toxic SAE grade XLPE jacket wire for interior use.

You want an experiment? Take a mason jar, fill about half full with warm water, and mix in table salt until it's basically saturated. Take a 12" length of automotive primary wire, strip both ends. Place one end in the salt water solution, place the other hanging over something like a drink coaster, saucer, etc. You don't want this end to touch anything, just hang there.

Wait 24 hours. Check out all the salt crystals that have migrated through the wire to your coaster. Then sit back with a beer and think about corrosion in your auto wiring.

Cars do burn down. I prefer that mine doesn't.