18 replies to this topic

[Jaybraham]

Good Afternoon All,

 

I'm sure somewhere in the archive a similar question has been asked in a different way but I haven't been able to locate such information so please humour me.

 

I'm planning on building a custom wiring loom for my mini for a number of reasons mostly because the loom currently fitted has seen better days, had cables replaced so the loom colours do not match that of any wiring diagram completely and also because the car doesn't have many electrical components even by a mini standard so spending £500+ on a new loom where half of it would be disconnected seems rather large waste of money.

 

Now I get building a custom loom isn't as simple as writing it down, and I understand there will be comments about different current draws, volt drops, measurements and everything else however with this in mind I'm still going to attempt it. I have a good understanding of circuitry in general and electrical diagrams so I am content it is within my skillset. I also have close friends who are radar and/or avionic engineers by trade so electrical understandings and cabling is kind of covered from their end but its not hand in had with automotive and we had a discussion I was hoping someone here might be able to clarify.

 

So here is my question:

 

The below part of a wiring diagram ripped from the forum was most helpful but also led to how the battery recharged and how the system doesn't burn itself out. Now to better understand how to set up my loom I first wanted to clarify why exactly the smaller rated cables connected from the starter solenoid do not overheat and combust. 

 

The main battery cable connects to the solenoid which in turn connects to the Alternator, Ignition Switch, Starter Motor & the No charge warning lamp. When the engine is up and running the alternator should be providing 45Amps of power at around 13.5-14V thus keeping the battery charged. Some of this power will be drawn by things plugged in so will likely be less but how is it that the cables connected to the solenoid at a common connection, which are a small gauge wire not designed for such current not burn out or get damaged?

 

Is it that the electricity generated will take the path of least resistance which is back down the battery cable to recharge and the smaller draw required on cables leading to either fuses or ignition only draw the required and the excess its travelling to the battery?

 

 

 

I feel I have waffled a little trying to explain my question however that's how my mechanical brain works. Any information provided will be of great help and value among a spirited debate of friends and the designing of a wiring loom intended to not combust in use.

 

Thanks in advanced 

[sonscar]

Simply put the wires carry the current demanded by the user,sidelights may draw 3a headlamps 10a etc.the alternator and battery does not force power down the wires.The starter may draw 100a and the battery require lots of amps to charge it so the wires are bigger.there are more scientific nuances to this.Steve..

[MiniMadRacer]

I agree with the above, in the simplest of terms the size of the wire is decided by the amps the thing powered at the end of the wire demands. So if you have a wire connected to a 12V car battery the gauge of the wire is decided by whats on the other end, not the battery end. So a 3 amp bulb will get away with a thinner wire than a 100 amp bulb would. As I understand it, apart from cost, there is no reason not to go "over spec" on the wires, assuming of course that the thickness of the wire can still fit into the appliance you are trying to power up.

 

On a separate not but relevant is also the resistance of the wire, the thinner the wire, and the longer the wire, the higher the resistance, so usually longer wire runs would have a thicker wire to compensate for the increased resistance. Increased resistance in laymans terms means a voltage drop.

 

I hope that kind of made some sense.

[stuart bowes]

amps aren't 'forced' down the wiring to places where they're not being used

 

they are pulled as necessary by whatever is at the other end, that's what decides the wire gauge requirement

[Jaybraham]

Thanks for the quick replies up to this point. So basic understanding of the replies the smaller cables are "protected" from overheating and damage by the fact that the end products do not pull more than the cables/ fuses are rated?

[stuart bowes]

correct

 

in the same way that you can use 1.5mm^2 cabling for 6a lighting and 2.5mm^2 wiring for 13a sockets but they come from the same 230v incoming supply, your light wiring doesn't melt just because you plug a hoover in

 

that wasn't meant to sound sarcastic or anything it's just an example lol  (not the best example because that's AC and goes through a dist. board into separate circuits via separate breakers.. but you get the general idea)

Edited by stuart bowes, 18 February 2025 - 04:54 PM.

[MiniMadRacer]

Well yes and no. The cables have no protection bar the fuses really, so if you put the wrong thickness of cable (ie too thin) and the wrong fuse (too low a rating) to power an ancillary that requires more than the cable can withstand then the fuse or the cable will "blow / melt".

 

The rule of thinmb really is use the cable that the powered unit requires, and use a fuse that will blow before the cable would melt.

[MiniMadRacer]

Crossed posts, I agree with Stuart above

[stuart bowes]

well yes assuming the 'smaller cables' are correctly rated for the application of course

[Jaybraham]

Perfect thank you. Don't worry as a mechanical engineer anytime I ask electrical question I usually get a sarcastic response from the electrical friends of mine so whether meant or not I've come to expect it haha.

Between the replies it's cleared up alot to help with my understanding and designs in mind for how I gauge my wire correctly and fuse it also. In most instances I've found thin walled cable designed for automotive uses where the amperage rating is slightly higher than the "correct fuse" for the circuit. 6a cable for 5a fuse, 11.6a wire for 10a fuse etc etc.

Again thanks and highly appreciated for the information of the forum members

[stuart bowes]

no harm in over-rating the wiring at all, I guess the only downside there is trying to keep the loom thinner for ease of fitting I suppose, and arguably weight

 

negligible differences in most cases but at certain points there are a lot of wires in one place, particularly the point where it goes through the bulkhead, less so for the section that goes back to the boot but trying to pull cables through narrow gaps can cause damage to the wiring and potential shorts to ground

 

one person recently pointed out that exact problem when pulling cables into the boot but I think that was partly because of the self-tapper screws holding the rear window catches in IIRC

Edited by stuart bowes, 18 February 2025 - 04:32 PM.

[KTS]

plenty of very good reference material in the FAQ's section

 

https://www.theminif...253-electrical/

 

e.g Current draw per circuit

 

https://www.theminif...trical-circuit/

 

if overhauling the loom i would recommend incorporating relays into circuits to reduce loads through the switchgear as well as dedicated earth cabling 

[Jaybraham]

plenty of very good reference material in the FAQ's section

https://www.theminif...253-electrical/

e.g Current draw per circuit

https://www.theminif...trical-circuit/

if overhauling the loom i would recommend incorporating relays into circuits to reduce loads through the switchgear as well as dedicated earth cabling

Yeah all this info is/has been taken into account for a custom loom. Alot of the information was used for planning a better fusebox layout and what wire and fuses will be needed for the system

[alpder]

Using relays, instead of running big amps through the aging switchgear, is definitely a good plan. My own bespoke-wired Mk2 has relays on just about everything. There are handy automotive relays which incorporate a fuse in the switched circuit, saving complexity when wiring the fuse-board. I put reverse diodes across the coils of all the relays: important if there are sensitive electronics in the car though likely overkill otherwise.

 

It's also worth remembering that the maximum amps capacity of a wire isn't the only factor in the choice of wire size: consider also voltage-drop along the wire, particularly with things like headlamps that you would prefer to get the maximum possible glimmer from.

 

While you're rewiring, worth thinking about adding some USB sockets on the dash, for satnav and phone charging. And an electric wash and an intermittent wiper are lovely to have. Adding hazard lights to older cars can only be a plus.

 

Important things - like ignition - should be wired on their own separate fuse and circuit. So that a short-circuit in, say, the heater motor doesn't bring you to a miserable halt in the fast lane.

 

For the front-to-rear cable, I used an 8-core trailer cable which does a neat job and is robust. 12-core is readily available if you wanted to put accessories like 12V sockets in the boot for the cool [beer] box.

 

Later cars will require separate circuits for left and right sidelights and headlamps, so that the failure of any one wire (or any one fuse or relay) can't kill both heads and both sides at the same time. It's the law... and your insurers will love to have the excuse not to pay out, if you haven't adhered to it.

 

The trickiest wiring to understand is, weirdly, the wiper. The wiring for two-speed and parking is not intuitive. And the trickiest wiring to create is the mahoosive busbar that you need to distribute the 12V around the relays. It's worth calculating just how many amps you might draw all-at-once through this, if you turn on everything at once: heater, wipers, dip headlamps, flashing the main-beam, sounding the horn, braking, foglamp, and any lamps on a trailer... a domestic electrician would say "diversity" lets you ignore the worst-case but on a cold wet foggy night when you swerve to avoid a crash and hit the brakes, the lights, and the horn... all the electrics going phutt due to overload is the last thing you need.

 

And declaring all the diy electrical mods on my insurance didn't even raise the premium. Having a few extra toys (USB, intermittent wash/wipe, hazards, fog wiring to the towbar) maybe even helped with the agreed value.

[Spider]

I know this will likely be the type of reply you've asked not to read, however just to say this all the same.

Before going too far down this rabbit hole, I'd suggest pricing out all the materials - the wires, plugs, sockets and other consumables as you might be in for a rude surprise as to what it will actually cost. I can get these parts at trade pricing, for middle of the road to better quality parts and wires, I can't make one for less than I can buy one ready to fit.