Wiring - relay and fuse ratings

[RichardHyde]

Hi everyone,

I'm planning to improve my electrics as part of installing the CA18DET engine.

What are your thoughts on the using using a 20A fuse and 25A relay for each of the following, wired to the battery at the main starter motor power feed :-

- BMW E36 headlights with 55W (about 5A) bulbs
- BMW E36 main beam with 55W bulbs
- 12 inch electric cooling fan 1
- 12 inch electric cooling fan 2
- horn

The battery, Bosch 044 and Facet type fuel pump will be located in the boot. I'll use a 30A fuse and relay to feed both pumps.

Are there general rules for power consumption, fuse rating, cable rating and relay rating ? For example, the 55W / 4.58A what cable, fuse and relay should I use ?

Should I consider add more relays / fuses for anything else while I'm at it ?

thanks, Richard.

[yorkshire_spam]

Hi everyone,

I'm planning to improve my electrics as part of installing the CA18DET engine.

What are your thoughts on the using using a 20A fuse and 25A relay for each of the following, wired to the battery at the main starter motor power feed :-

- BMW E36 headlights with 55W (about 5A) bulbs
- BMW E36 main beam with 55W bulbs
- 12 inch electric cooling fan 1
- 12 inch electric cooling fan 2
- horn

The battery, Bosch 044 and Facet type fuel pump will be located in the boot. I'll use a 30A fuse and relay to feed both pumps.

Are there general rules for power consumption, fuse rating, cable rating and relay rating ? For example, the 55W / 4.58A what cable, fuse and relay should I use ?

Should I consider add more relays / fuses for anything else while I'm at it ?

thanks, Richard.

P=IV, therefore I= P/V
So 2x55W lamps =110W, current = 110/12 = 9.2A
So a 20A fuse and a 25A relay seems to give plenty margin, I'd consider 1.0mm², 16.5A cable as a minimum
For the rest it "sounds" ok, but without any indication of the power rating it's hard to say.

My general rule of thumb for cable is the higher the better above the calculated load. At 9.2A I wouldn't be happy with an 11A rated cable, 16A sounds good though. But maybe I'm over cautious?
Generally a fuse close to the cable rating is good - if something goes wrong we want the fuse to blow before the cable burns?

[sprint95m]

BMW E36?
Are the same lights used on the 5 series of that vintage?
I have been in two BMWs fairly recently (a 90s five series and a much younger three series)
and both had poor lights, I suggest even worse than sealed beams.
Of course, it is entirely possible that they lights had dulled reflectors and/or dirty lenses.


Regarding relays, pretty much everything I guess.
That is my approach.
Definitely the heated back window.



Ian.

[Carledo]

You've SEEN my solution Richard!

22 fuses and now 14 relays!

Bear in mind that a fuse has 2 ratings, a blow rate (the one marked on it) and a continuous load rate which is generally around half the blow rate. The old fashioned Lucas glass fuses (if you find the real thing) actually show this on the little paper label within, 35a blow, 17a continuous. So if you are running, say, a single dipped beam headlight of 55w (4.6a) then choose a 10a fuse. For main beam each side is 65w+75w=140w (11.7a) so you need a 25a fuse for EACH side!

This is how moderns do it, which I have copied, with 4 headlamp fuses AFTER the relays, 1 each for o/s dip, n/s dip, os main and n/s main.

I've also segregated out all the ignition live circuitry so that each function that was originally supplied from a single fuse now has one fuse per function, 7 in all! Now, if the indicators or brake lights blow a fuse, everything else keeps working, which I consider a big improvement! I've done the same for permanent live (3 fuses) auxilliary ign live (2 fuses) and sidelight live (2 fuses) then the aforementioned 4 headlamp fuses plus 1 each for spotlights, horn and fan.

Steve

PS, I run a single 16a thinwall cable to each FILAMENT in the headlights, main or dip. It's a bit overkill, but better to have it and not need it......

[MIG Wielder]

I think there are 2 things to consider when specifying a cable.

* Cable current rating.
* Voltage drop in use.

Note that most PVC insulated cables are limited to 70 deg C so don't run them near cylinders blocks or exhausts.

There is a good guide here on cable types and colours.

Cable ratings : https://www.autoelectricsupplies.co.uk/ ... ategory/11

So for 2 x 55W headlights at 13.5V = 8.1 A so a 16A cable should be O.K. for 2 units.

(010103 28/0.30, 2.0mm², 17.5A - cable OD 3.3mm )

But do note the peak surge current will be a LOT higher.

Now for voltage drop. Lets choose a good figure of 250mV ( 1/4 V at full load)

Lets consider a theoretical cable that runs the length of the car from front to back. Say 15 feet or 5.0m maximum.

From the formula R ( ohms) = ( Sigma x Length )/ CSA
CSA = cross sectional area.
Sigma for pure copper cable = 1.68 x 10 ^-08 ohm.m
CSA = 2 x10^-6 mm^2

So for 250mV drop at 8.1 A the maximum resistance is 0.25 V / 8.1 A = 0.031 ohms
or 31 milli-ohms.

So for the 2.0mm ^2 cable a 5.0 meter length will have a resistance of …
(1.68 x 10^-08 x 5.0m )/2.0 x 10^-06

= 4.2 x 10^-3 ohms. or 42 milliOhms
So just over the 31 millOhms requirement.

So a good pragmatic solution.

Don't forget that the earthing wiring from the headlight / accessory wiring will also need uprating at the same time.

Tony.

[RichardHyde]

Thanks for your input everybody.

Yep I remember your fuse box Steve....it’s enormous !!

I’ve gone for 4m of 240A cable and I’ll use the same spec for the earth.

The Bosch 044 fuel pump will be in the boot and draws quite a lot of current. I’ll wire this up directly to the battery with a separate relay & fuse....one less component for the main battery to starter cable.

I’ll also connect the HRW to the battery with a relay & fuse to reduce the load.

[Jod Clark]

It is also worth considering that the fuse is also to protect the cable, not just the load, especially in harsh environments where heat and vibration will occur in stranded cables. Also pay attention to de-rating of cable when bundled / wrapped / or sleeved. If I was installing or modifying a circuit where a 10 amp fuse was appropriate for the anticipated load I would be using a cable rated at 20A if run as a single and 25A if part of a sleeved or taped loom.

[Carledo]

It is also worth considering that the fuse is also to protect the cable, not just the load, especially in harsh environments where heat and vibration will occur in stranded cables. Also pay attention to de-rating of cable when bundled / wrapped / or sleeved. If I was installing or modifying a circuit where a 10 amp fuse was appropriate for the anticipated load I would be using a cable rated at 20A if run as a single and 25A if part of a sleeved or taped loom.

That's a bit overkill Jod! A 10 amp fuse is used to protect a (for example) single dipped filament drawing less than 5a because of the 50% difference in blow and continuous load fuse rates. I concede that de-rating is an issue, (though perhaps not a 20% issue) but don't see the cable needs to be rated at 5 times the expected load even so! As far as I can see, if the cable rating exceeds the blow rate of the fuse, (even allowing for de-rating from looming) then the cable is protected. So the 3+ to 1 advantage I employ with 16a cable on a 5a load with a 10a fuse is more than adequate.

Steve