Building a wiring harness - Am I choosing the right wire?

[montyjohn]

I’m looking to build my own wiring loom for a kit car, and want to achieve it on a budget.

I’ve never done this before so I want to check I’m buying the right wire before I hit the trigger and buy a load of the wrong stuff.

I don’t know yet what headlight bulbs I will be using, radio power requirements etc so I want to build the loom up one system at a time probably over several months before wrapping it all up and calling it a day.

For this reason, universal or custom paid for looms aren’t of interest as I don’t have the experience to plan that far ahead.

I started pricing up all the various colours I might use, and various wire gauges and the bill got silly really quickly and I’d end up with a load of wire I’d never use.

So I’m hoping to get away with two sizes of wire (ignoring feeds to fuse-boxes and battery terminals of course).

Relay rigger wire: (see link)
20 awg – 0.5mm2 rated for 11 amps (use a 5 amp fuse)

Everything else (unless it draws less than say 3 amps): (see link)
14 awg – 2mm2 rated for 25 amps (use up to 20 amp use, is that too close?)

Will the 2mm2 be ok for everything else over 5 amps? Quick check:

• Headlights, 60W each, therefore 5x2 amps draw. 15 amp fuse. 2mm2 wire should be good.
• Injection fuel pump, 10 amps I believe, so all good.
• Blowers (hmm, maybe this will be the exception, will have to wait and see what unit I buy)

Maybe there’ll be something else that uses a heck of a lot more that I haven’t thought of, but in general, can anyone see a problem with my plan to buy a load in the two sizes above?

I’ve already sorted the battery terminals with 50mm2 cable so happy with that.

Regarding the fuse-boxes, there’ll be one at the back and the front to reduce the amount large diameter wire needed. I’ll use some old thick cable I have for now, and buy the fuse box feeder wires later when I know what my total peak draw will be.

Build page here is anyone is interested

Edited by montyjohn on Tuesday 27th July 16:09

Edited by montyjohn on Tuesday 27th July 16:26

[Penelope Stopit]

3 Years ago Escort MKII cable was 64 Pounds and 50 Pence, that's the total price for a full loom apart from tape, battery cable, fuse-boxes, relays, plugs and terminals

What sort of budget are you on?

Those cable sizes aren't close to what you should be using

Would you like me to post some information about sizes and methods used?

Can also link you to good priced cable

[montyjohn]

If I can get all the wire for £60 I'll be more than happy with that.
I don't have a budget as such, but like to know I'm getting good value and don't want to throw money at the problem to make it go away.
The problem solving is part of the fun.

If you have any info that can explain where I have gone wrong by all means send it through.
I thought that by keeping the wire rating circa twice the peak load that would be it.

I assume the 0.5mm2 is fine for the trigger wires? I believe relay could typically draw under an amp, but not tested any yet.

Also, any links to your favorite suppliers would be much appreciated. Thanks.

EDIT:
Forgot to mention.
I'm using a factory loom for the engine, so it's the body loom I need to build. And there wont be any things fancy that you would get on a 1980s Ford.

Edited by montyjohn on Wednesday 28th July 09:17

[Penelope Stopit]

The jobs all about weight and thickness of the finished loom

Thin wall cable is the best and it does look like that's what you've chosen to use

See https://www.workshopwarehouse.co.uk/advicecentre/E...

The relays you use will very likely be of 80 ohm coil resistance (0.15 Amp), you can use the thinnest thin wall cable on the market for activating the relays

Choosing cable as 2 x load will very likely be going too big for some circuits

montyjohn said:

• Headlights, 60W each, therefore 5x2 amps draw. 15 amp fuse. 2mm2 wire should be good.

Tells me that you're planning on using the same cable to power both 60 watt main beam filaments, not a good method

All the lighting circuits will be far better if fusing is carried out for LH & RH, a LH light will work even when a RH light has shorted and blown its fuse, plus a single headlamp bulb can then be fused much closer to its current draw rating - 5 Amps fused at 7.5 Amps

It's not possible to calculate cable size without knowing the cable length and the current draw through that length

What you seem to have overlooked is the volt-drop along the length of cable, I could be wrong though and what you've done is simply go real big for most circuits

Use volt-drop calculator halfway down this page for cable sizing https://www.12voltplanet.co.uk/cable-sizing-select...

NOTE ABOUT ABOVE CALCULATOR - It's calculating for insulated return rather than one way cable length for earth return vehicles, half the cable length for true result or half the result of the calculation

Some cable suppliers, you may have already viewed them

https://www.autoelectricsupplies.co.uk/product/70/...

https://www.autosparks.co.uk/electrical-components...

https://www.asap-supplies.com/electrical/cables/pv...

[montyjohn]

Awesome, Thanks for all the information.
When I read this in your original post:

Penelope Stopit said:

Those cable sizes aren't close to what you should be using

I thought you meant I’m using wires that are too small. I was trying to work out how that could possible be. Makes sense now.
I’ll be honest, as I’ve never done this before, weight and size hadn’t even occurred to me to be an issue. So my plan was to go big and go simple.

So appreciate the warning.

I was following this chart which seems to suggest that 1.5mm2 and 2.5mm2 are the smallest that would be used and thought I was being naughty by going lower on the trigger wires.



You're approach makes a lot more sense however and usign the calculator get figures more comparable with the ratings form suppliers which is more comforting.

Penelope Stopit said:

The relays you use will very likely be of 80 ohm coil resistance (0.15 Amp)

Bang on, I checked them this morning and they all came out at 80(ish) ohms.

Penelope Stopit said:

All the lighting circuits will be far better if fusing is carried out for LH & RH

Seems obvious now you mention it. Good tip. Thanks

Penelope Stopit said:

What you seem to have overlooked is the volt-drop along the length of cable, I could be wrong though and what you've done is simply go real big for most circuits

I’m aware of voltage drop, but by ignoring weight and size and over-sizing the wire, I felt I could ignore voltage drops. Now I know that weight and size is an issue, I guess I can’t be lazy anymore.

However, back to my old tricks, since I'll have a fuse boxes at the front and the back, I'll be surprised if I have a return length of more than 2m so according to that useful calculator I can go with 0.35mm2 for anything up to 3amps @ 13.5V which I think will be most circuits (I don't think I can get less than 0.35mm2).

Then, for bigger loads, again, assuming 2m return fpor now, 0.5mm2 will do up to 6 amps which then only leaves a hand full of additional circuits that can probably be almost fully covered by 1mm2 with 14 amps rating subject to length.

Since my body is GRP, I may add some oversized (sorry I wont go too big, promise, but I have to fitire proof it right?) earth busses for things like headlamps etc to keep my wiring to a minimum.

If the above all souds sensible then I guess I mainly need a load of 0.35mm2 followed by 0.5mm2 and just a hand full of meters of 1mm2 unless I come across any special cases. And then decide which wire on a case by case subject to length.

Thanks again. You've saved me from over spending which is always a good thing.

[Penelope Stopit]

montyjohn said:

Regarding the fuse-boxes, there’ll be one at the back and the front to reduce the amount large diameter wire needed. I’ll use some old thick cable I have for now, and buy the fuse box feeder wires later when I know what my total peak draw will be.

Stop Lights = 42 Watts

Tail Lights & Number Plate Lights = 20 Watts

Reverse Lights = 42 Watts

Rear Fog Lights = 42 Watts

LH Indicator = 21 Watts

RH Indicator = 21 Watts

Total is less than 16 Amps if all are on at the same time which they can't be

Cables are still needed to supply the rear fuse-box

Can't see much if any cable saving

Also you would need to fuse the cables running from the dashboard area to the rear fuse-box, it's not a good idea to run unnecessary un-fused supplies from front to back

Heated Rear Screen if fitted and Fuel Pump are the only consumers that will draw a relatively high current

If the battery is fitted in the boot, ignore all the above

[Penelope Stopit]

Crossed posts, will read your update now

[Penelope Stopit]

Ok, we were both posting at the same time and I hadn't seen your latest post

Very good then, it was worth you asking and me replying about cable sizes

Good to know that you were going to be going too big at times, we all do it

You will be surprised at the weight and diameter of the loom once it's all taped up, thin wall is amazing cable

The below image shows how little there is in the way of cables to get the basics up and running, ignore the German colour codes

Enjoy the job

[Penelope Stopit]

montyjohn said:

I'll be surprised if I have a return length of more than 2m so according to that useful calculator I can go with 0.35mm2 for anything up to 3amps @ 13.5V which I think will be most circuits (I don't think I can get less than 0.35mm2).

You may find thinner multi-strand cable used for telecoms and others....., this is ideal for relay activation but the cable won't be very strong and there could be a big problem in finding crimp terminals to match the cars components/plugs for such small diameter cables

[montyjohn]

Thanks again for your help.
I should have mentioned the battery is in the rear so I have a convenient power supply point for a front and rear fusebox.

Looks like I have a plan then.

I probably won't go lower than the 0.35mm2 for trigger wire. Having wires pulling out of crimps would worry me and I guess at some point the current rating would be so low for the wire that finding a small enough fuse could be a problem.

It's weird I generally don't see lower than 5 amp fuses in factory fuse boxes. If they are using 0.35mm2 trigger wire I imagine that wire could get a little toastie before the fuse goes. I've seen you can get 3 amp fuses which I'd be more comfortable with.

Thanks again.

[Penelope Stopit]

You're welcome

From a previous link

Thin Wall will operate down to -40°C and up to 105°C

12/0.20, 0.35mm², 7.0A - cable OD 1.4mm thin wall cable will be cool when under a load of 7 Amps

Blade fuses are fast blow

If a relay went short circuit or the relay activation cable shorted to earth, the 0.35mm², 7.0A will momentarily conduct a much higher current and blow a blade fuse rated at 15 Amps or greater, the maximum current depends on the length of the cable between its supply point and the point of short circuit to earth

Many moons ago, a single cable of a vehicle rear loom burnt out from front to back, cable was rated at 11 Amps and originally fused at 15 Amps, a 20 Amp fuse had been fitted by mistake

The fitting of a 20 Amp fuse was thought to have been the cause of the cable burning out when it shorted directly to earth at the rear

An identical car was found and the same 15 Amp fuse was replaced with a 20 Amp fuse, same cable was deliberately shorted to earth at the rear and the 20 Amp fuse blew as good as instantly

Keep the faith, you don't need to be too concerned about the relay activation cable fuse rating, 7.5 Amp or 5 Amp fuses are perfect for the job

The smaller the blade fuse the more delicate it is and the more chance it has of failure due to vibration or it being knocked

Edited by Penelope Stopit on Thursday 29th July 12:14

[slybunda]

iv used this 13 core cable to do some wiring. makes it far easier than bundling up single wires together:
https://www.ebay.co.uk/itm/133221290728

Edited by slybunda on Wednesday 4th August 23:20

[GreenV8S]

I know somebody who did something similar with ribbon cable. It was a maintenance nightmare, with joints everywhere wires needed to join or leave the loom and no colour coding anywhere. I suggest you just get a supply of wires with the appropriate current rating following as close to colour coding standards as you can, and loom them up in the conventional way. I've never done a whole vehicle but I've done substantial parts and it's not especially difficult - just needs some care, time and materials.