Morris Minor Owners Club

Hi all, can somebody please confirm that 17amp cable would be suitable to use for the wire between my heated rear screen and the dash connection? Thanks Matt

Most likely yes, but it depends on the rating of your rear screen heater. Is it a saloon or traveller, stick on or built into the glass? Does it have a rating on the packet or supplier's website?

Hi simmitc. It’s a saloon one from esm as pictured. Not sure of rating ?

Do you already have it?

Measure the resistance...

Use V= IR to give the current it will take

Current = Volts (I would use 14.6 as a maximum) divided by Resistance.

Then make sure you have a lot of head room on the cable size, and the fuse is rated at more than calculated, and less than the cable is rated at!

Can I suggest the obvious, that you use the 'ask a question' facility available on the website and ask ESM what the thing is rated at.
it's seems a bit shoddy of them not to put such basic and essential specification details on the listing.

Matt ,
Once you've identified the rating of the Heated rear windscreen it will help you use the correct rated / size of
wiring. In turn , given it's likely electrical consumption , you can choose an appropriate relay switch.
There are a number options , a straightforward relay , a fused relay , a Timer relay or a programable Timer relay.
The cost rises in line with the complexity but in every case ensuring the relay can cope with the electrical demand
is paramount.
Some examples are linked ( other suppliers are available , I'm just a satisfied customer )
https://www.autoelectricsupplies.co.uk/ ... ategory/36
https://www.autoelectricsupplies.co.uk/product/836
The auto switch off , after 10 minutes ensures that if you forget to switch off the rear screen .... one cold and frosty morning... it's not draining the battery.
Bob

I have been in touch with esm and they can’t help with the rating, just that’s it’s suitable for 12v minor wiring ?

Thanks for the info Bob much appreciated

mattyguk1 wrote: ↑Mon Sep 14, 2020 5:27 pm I have been in touch with esm and they can’t help with the rating, just that’s it’s suitable for 12v minor wiring ?

That's terrible. Whoever makes if for them must know how much juice it takes!
To say it's suitable for 'Minor Wiring' is ridiculous, since if you're fitting it after market, there's no standard wiring to use. Did latter models ever even have a standard heated window?

I'd wait until it arrives and using the info provided already by Matt and Bob, design the circuit accordingly. I'd wire it to an ignition controlled supply, via a timed relay.

Edited to correct my typos...

Yeah not good! Matt I’ve got a multi meter but not great with it, could you please run me by the procedure to find the resistance and current needed?

mattyguk1 wrote: ↑Mon Sep 14, 2020 6:50 pm Yeah not good! Matt I’ve got a multi meter but not great with it, could you please run me by the procedure to find the resistance and current needed?

Basically, use the Multimeter set on Resistance/Ohms to get the reading between the 2 terminals on the screen.

Lets say it's 2 Ohms.
Go to a website like https://ohmslawcalculator.com and put some numbers in:
Voltage 14
Resistance 2 (ohms)
and it gives you the answer 7 Amps

Do it again with .5 Ohm and you get 28 Amps...
So you see that hopefully your reading between the terminals will be around the 1.5 to 2 Ohm region.

The internet really makes things like this easy.

Great thanks, I’ll give it a go tomorrow and report back

BTW all the Fords I have had, with heated screens, have had a 30A fuse...

Matt wrote: ↑Tue Sep 15, 2020 8:52 am BTW all the Fords I have had, with heated screens, have had a 30A fuse...

My Ford has a heated front screen, with separate 40A fuse each side, but everything is relative to the individual installation. My windscreen is huge and has hundreds of elements through it.

However, this relic of the 60s/70s, the stick on heater, only has a 5 amp fuse....
https://www.holden.co.uk/p/screen_heater_12_volt

But I'm sure mattyguk1 will have the details soon.

Oh yes, absolutely - but it gives an idea of the order of magnitude.

If he calculates a 1A fuse or a 100A fuse is needed, its probably an error ;)

Test done , I’ve got 1.6 ohms across the screen so I’ve done the test and get 9.125 amps. Does this mean 17amp cable is fine with a 15amp fuse?

Yes, that's then combination I would use

Good result. The system works.
1.6 Ohm was within the range I expected.

You might want to try a 10 Amp Fuse to start with.
Good quality equipment Fuses (rather than car fuses) have different characteristics, some are 'Slow Blow' in order to cope with an initial spike in current when switching equipment on; while others can be 'Quick Blow' to increase protection on sensitive equipment.

I've no idea what the manufacturing tolerances are on a typical car fuse, but you've just calculated that at 14.6 v you'll be drawing 9.125A.
So using the basic numbers, you've got a margin of 0.875A or 8.75% of a 10A fuse rating.
I'd go with that before jumping to the 15A which would be near a 40% increase in expected current before the fuse ruptured.

The fuse is to protect the cable rather than the device... but nothing wrong with using a 10A one. Its just not got enough "head room". Remember the current will vary with temperature - not just of the screen but at ambient before its turned on!

You could stick a 10A in, and if it blows change to 15A knowing you have the headroom in the system to accommodate it?

Matt wrote: ↑Wed Sep 16, 2020 3:29 pm The fuse is to protect the cable rather than the device... but nothing wrong with using a 10A one. Its just not got enough "head room". Remember the current will vary with temperature - not just of the screen but at ambient before its turned on!

You could stick a 10A in, and if it blows change to 15A knowing you have the headroom in the system to accommodate it?

I'm looking at it from both aspects. protection of the 'device' and of the wiring.
The 10A fuse is admittedly not a great deal of leeway over the expected current consumption (though 8+% isn't bad), but the 15A is equally not a great deal of leeway compared to the rating of the cable. Ideally, 12.5 A fuse would be good, but I know of no such beast in the automotive world.
Hence giving it a try with a 10 first and see if it survives. I doubt manufacturing tolerances are particularly accurate on the fuses. You might find the 15A fuse not rupturing until nearer 17 A .