When is a relay needed?

[Bitsilly]

I would like to wire in my rear screen demister, but also a few things like heated front w/s and a two speed wiper motor, reverse light, cigar lighter etc.
Is there a rule of thumb for when a relay is needed?

I just bought a load of 12v 16 amp wire and it looks really thin to me! I don't want any meltings!
So are relays the answer or just thicker wire?

Additionally, how do you specify 'normal' auto wire?

Cheers

[murty]

Relays are used when you need to feed a large current using a small amp rated control (e.g. switch).
For example, if the switch is rated at 1A, but you need it to feed power to something that uses 10A, then a relay is used with the switch feeding the coil of the relay, and the power delivered through the switch side of the relay. The wire itself has nothing to do with relay or no relay, but has to be sized to handle the current with margin.

[Spider]

As well as the good information murty has offered, Relays can be useful in very low voltage systems, like in a car, where you have a High Current Circuit and you don't want it to suffer from too much voltage drop while trying to keep the wires a sensible size, eg, headlights. If you fit relays in the front of the engine bay, you can keep the heavy current wires quite short, especially when compared to the original wiring route. It's a bit surprising how much better the standard sealed beam headlights are just doing this. for every 1% of voltage lost, it drops the lighting brightness by 4%. It can be the same for other circuits that can be voltage sensitive.

[winabbey]

Additionally, how do you specify 'normal' auto wire?

This chart appears on the BMC Australia engineering drawing for the 1970 era Mini saloon main harness. You can see that the wire size varies by application and is identified by number of strands.

Thought it may help you as you plan and make extensions to your harnesses.

AYA9221 Main Harness Mini Wire Spec.jpg

[Bitsilly]

That is very helpful.

Above and beyond as usual.

Many thanks

[Bitsilly]

ps ST'NDS mean strands I guess?

If so does that way of specifying still work?

[Bitsilly]

Forget that!
I had a check on Ebay, 44 strand seems to be used but not so much the others.

[Spider]

ps ST'NDS mean strands I guess?

If so does that way of specifying still work?

It's very kind of Doug to post that factory listing and while it shows the number of stands, sadly it's missing the size of each strand, there is no standard size for a strand.

[Bitsilly]

True that, but it still gives a basis of comparison.

Were there any relays on a mk1?

[Bitsilly]

PS Spider, really good point on the voltage drop, it was something I had not considered at all.

[Spider]

True that, but it still gives a basis of comparison.

Yes. If one has an old harness to measure a strand, then you're home and hosed - almost.

I don't know the date of that drawing, but there was a change in 73 or 74 from Imperial to Metric. The company who made the harnesses here for BMC / Leyland, Cable Maker's Australia, changed to metric cable sizes in 1973.

The other think here is I'm pretty sure the UK cars used a slightly smaller wire gauge than our locally made cars, certainly so in later years.

Were there any relays on a mk1?

No.

[Bitsilly]

If in doubt, make it stout, of something you know all about!

3 years in college and that was all I learned!

[winabbey]

I don't know the date of that drawing, but there was a change in 73 or 74 from Imperial to Metric. The company who made the harnesses here for BMC / Leyland, Cable Maker's Australia, changed to metric cable sizes in 1973.

The initial drawing is dated 1970 so for the round-nose Mini models.

Later Clubman/Leyland era harnesses were made by Sphinx instead of CMAPL. These were no doubt metric measurements, at least for wire lengths.

Drawing AYG9121 Wiring Harness Specification and Symbols was the reference to the individual electrical harness drawings and contained detail of connectors, taping, soldering, crimping, relevant Standards, etc.

Below is an extract from that drawing which says wiring strands were 0.012" unless specified otherwise.

AYG9121 Wiring Harness Specification and Symbols Subset.jpg

[Exminiman]

This might help also, old thread on TMF https://www.theminiforum.co.uk/forums/t ... ire-guide/

[Spider]

Thanks Doug for that missing info !

It seems there's only a few different sizes

9 strand - 2.5 sq mm
14 strand - 3.9 sq mm
16 strand - 4.5 sqmm
28 strand - 7.8 sq mm
44 strand - 12.5 sq mm

If in doubt, make it stout, of something you know all about!

In regards to automotive wiring, while tempting to agree, one also needs to be mindful of loom density (or how bulky the loom will get). Depending on the changes and additions you're contemplating, the bulk of the loom can be an issue and one that really needs consideration at an early stage of a build to make sure the loom will actually fit where you want it too, gromet holes are adequate (though usually it's the plugs that can dictate the size of these).

Going to a 'stouter' (bigger) wire for one or two wires won;t make a difference in the loom density, but when you get 15 - 20 or more wires together, it does make quite a difference.

Another possible thing to consider here is if you are going to LED Lamps all round, you can go way down in wire size as say for one example, the Brake Lights originally drew 3.5 amps but going to LED, they'll draw less than 0.5 amps, so you can go to a smaller wire and thus reduce loom density. Another fact here to possibly consider - if you are getting marginal on wire sizes - is mutual heating, ie, if you have a heavy current circuit that operates for extended periods (headlights for example), these wires will warm up and in doing so, reduce the current capacity of all other wires that are bundled to them. Sensibly, this ONLY comes in to play if you are getting close to the wire (hehehe) in sizes. However, and it may depend on the connectors you choose to use, I don't go any smaller than 1.0 sqmm and preferably 1.5 sqmm in a car, not for current carrying capacity, but for mechanical strength.

[Bitsilly]

Gents,
this is amazing, thanks so much.
It is all a big help.

[mk1]

If you are using a switch that is working within its normal range, i.e. Using a 20A switch to switch a heated screen you don't need a relay.

I agree with you on the current rating V size of cable. Rated cable looks terribly thin to me. As a rule of thumb, any ancillary wiring I put in is usually double the rating that I need. i.e. I have just wired front & rear heated screens in my current project & used cable rated at 40A. Not necessary, but just gives me a bit of extra confidence.

[finchy533]

This is a good explanation of some of the factors here:

https://www.12voltplanet.co.uk/cable-si ... ction.html

In particular, it has a really handy voltage drop calculator - if you play around with the figures you can easily see why you might want well over the rated current for longer/higher current runs.

It also makes the point about modern thin wall PVC cable - I think this is a common reason why modern cable might seem under-specified at a glance when you're used to older cables. But if you look at the cross-section it becomes obvious the difference is in the insulation, not the conductor.

[Bitsilly]

Yep, the wire I have bought is modern and thin walled.
The old loom has the core, then the wall insulation, then the braided cover, then the loom wrap, but I guess the core and the quality of insulation is what counts.

[Jack of all trades]

As we're on the subject of wire i have made a list a while ago that could be of use to someone.

stnds A AWG mm2 Use
9 5.75 22 0,5 side, tail lamps General wiring
14 8 18 0,75
28 17.5 14 2,5 Horns, Headlamps Major feeds
44 25.5 12 4 Generator, controlbox, Am.meter