Electrical 101

**AndyG** · 19th August 2014, 07:46 AM

Some questions/thoughts from an electrical novice.

1. Despite what logical drawings show, ideally you should not run wires from the battery?
a. All +ve from a fuse box
b. All –ve from a bus bar or stud.

2. Should you/do you need a fuse or circuit breaker between a battery and a fuse box, if yes, which?

3. Solder vs Crimp, A average crimp is better than a poor solder ? (talking novice here), some boating sites say crimps are more vibration resistant than solder joints. Personally i lean to crimps with a heat shrink sleeve.

4. Tinned copper, any advantage in a MV?

5. What size fuse.
a. For example 2 * 75w lights = 150 /12 = 12.5 amp, Would you use a 15 amp fuse or go up the next size.

6. Cable sizes, first the assumption, 6 block fuse block supporting Driving lights, etc under the bonnet, say 6 * 15 = 90 amp worst case scenario.
a. Being fundamentally lazy i found:
b. Handy Hints – REDARC Electronics
c. And using the assumptions of 2 meters, 30 C, 90 amp load & 1 volt drop, i need 3.1mm or 13 B&S
d. And using the assumptions of 2 meters, 30 C, 90 amp load & 0.5 volt drop, i need 6.2 mm or 10 B&S
e. With a 90 amp load highly unlikely, i guess 6 mm should be fine.
f. But raises a new question, what is acceptable voltage drop ?

7. And before you disconnect batteries, most modern cars, even DEfenders, need you to go through a process so the ECU? does not get upset.

I should have left the defer std, and never started !

**Dorian** · 25th August 2014, 01:06 PM

So these are my humble opinions (Slow day at work)
1.
a. Depends on what you want, if it's for a 2nd battery then directly from the main is probably the best place, if it's for a UHF then run from the accessories supply.
b. yes from a stud on the frame, g/box is best.
2. No fuse or breaker needed but the feed wire should be bigger than the total load draw on the fuse box. The feed wire from the battery should be double insulated, or run in plastic conduit.
3. An average crimp will always be better than a good solder as the solder can melt. Over time the metallurgy of the tin changes in the solder as this happens the joint becomes brittle and the resistance of the joint increases.
4. Tinned will offer a small advantage as the tinning is slightly more corrosion resistant than copper.
5. Fuses protect the wire not the device. This prevents fires in harnesses and melted solder joints So if you are drawing 12.5 amps then you should use a 1.5mm^2 or 2mm^2 wire and a 15 amp fuse.
6. The Redarc link is quite good, although it sizes the wire slightly higher than I would. Remember the temperature is the ambient conditions of the wire. So anything running near the engine bay should be up around 40C to 50C.
e. At a minimum you should be using 1mm^2 per 10 amps plus 0.25 mm^2 per 10 amps after the first 10 amps. so 11.25mm^2
f. Remember the voltage drop is realized as heat generated in the wire. A voltage drop of 1 volt at 90 amps gives you 90 watts of heat dissipation in the wire. that is a lot of heat, (touch a 45 watt light bulb after 30mins) if you are running a winch for short bursts then this might be OK as the wire has time to cool in between. If you are running lights all night then you probably might not be running nights all night. For continuous use, i'd be keeping it down 0.1 to 0.2 volts. If you are designing it to be used in the middle of the day in the desert in summer then 0.1v might be too much but if's it's to be used in knee deep snow then 0.5 might be OK. Similarly if the wire is run by itself then it has opportunity to get rid of it's heat, but if it's bunched up with a whole heap of others then it's ability to dissipate the heat and not melt is reduced.

7. Whats the process? Maybe that's some of my problems with the red light coming on

"I should have left the defer std, and never started !" - Did nobody tell you when you bought a defender that it was a kit car!

Cheers Glen

**AndyG** · 25th August 2014, 01:21 PM

Thanks Glen,
I seem to remember reading, User Manual, Maintenance Manual that you had to go through a key sequence to put the ECU to sleep, otherwise if you disconnect the Battery it thinks its been molested, stolen etc.

Cant be more specific, it was idle reading while waiting for the Deefer on the water.

And thanks for the advice, appreciated, i think i am largely on the right path. 8B&S seems like the size to start with for a small remote fuse box. As 40 amp is prob more realistic & put a 40 amp fuse at the battery end.

N.B it's amazing how the cable size changes when you play with the allowable voltage drop in the Redarc tool.

Andrew

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