Questions for the electrical engineers

[nick_the_fish]

The leisure battery wiring in my 105 has been added piecemeal over the years as requirements have changed and is therefore a bit of a mishmash. I want to rip out all of the leisure battery wiring this spring and rewire it with a more considered design.

The basic design is to have 3 fuse boxes – 1 in under the bonnet, one in the cab and one in the boot. Each circuit will be fused straight off the battery terminal so there will be no unfused wiring. I know the max current draw for each of component on each circuit, and taken into account possible upgrades/additions. These have then been used to calculate max wire sizes and fuses. All cables in the system will be fused below the capacity of the cable so the fuse will blow before the cable does. I have also taken into account voltage drops using a useful calculator on 12voltplanet. Each cable will be protected in a sheath to prevent rubbing on the cable.

My question are:

1. Should each fuse box sit on a dedicated circuit from the battery? Or should the cab fuse box be feed from a spur on the bonnet fuse box, and the boot fuse box from a spur in the cab fuse box? I’d rather do it the second way as there will be less wiring to protect, especially at the pinch points.

2. What sort of safety factor should be added when calculating cable sizes and fuse sizes? I have had a look on the internet but there seems to be some disagreements as to whether it is needed and what it should be.

3. Is there any advantage to running an earth back to the leisure battery rather than earthing via the chassis?


Thanks in advance

 

[StarCruiser]

1. Post up a quick sketch of what you want to do Nick.
2. Fuses should protect cables and cables should be large enough for the fuses to blow (as resistance is involved) and of course minimise volt drop. If you use large enough cable (starter cable) you don’t necessarily need to fuse it as long as the battery will be depleted before there is damage to the cable or it’s surroundings.
3. No point, use the body and make a good connection with big enough cable both ends.

 

[Chris]

Rich can you clarify please. I value your electrical knowledge. If I have a vehicle where I am tapping into the main battery and that battery is earthed to the chassis then clearly you can make all system earths to the chassis. But if I build a system where there is a second battery in play which at times is separated electrically from the vehicle battery (say a split charge) is there any advantage at all to keeping all the appliance earths back directly to the second battery? I read that this was the best thing to do in such a system due to eliminating any chance of parasitic drain anywhere. I am not arguing that this is right, but I have wired my three batteries in this way so that the rear battery only has connections from the CTEK feed and return and all earths return to the same battery. It's worked brilliantly so I am guessing it's not the WRONG way to do it, but there may be no advantage at all. Running additional earths for everything in the leisure set up is a but of extra work but not that much.

 

[moggy1968]

My intention is that my domestic battery, which is in the back, will be earthed to the main battery. All the domestic appliances will then be earthed to the domestic battery.

 

[nick_the_fish]

Thanks Rich,

1. (please excuse the amateur hour sketch)

Option 1 - 3 separate circuits one for each fusebox

Option 2 - 1 circuit with fuse box's being fed from each other.

i'd rather go with option 2 in order to save cable, but it will mean that the first cable run from the battery to the first fusebox is going to need to be some pretty massive cable. For info here are my initial loading calculations

2. ok great. So there is no rule of thumb along the lines of , for example, max load +10% to allow for surges?

3. Great.

Thanks again Rich much appreciated

 

[nick_the_fish]

Moggy, yep my leisure battery is also earthed to the main battery as per the installation instructions of the split charge system. presumable from an earthing point of view they are then the same battery??

 

[StarCruiser]

The thing is, most battery chargers (Intervolt included) deal with only the + side of things with the - side connected together. This needs to be done (obviously) with a conductor. This can either be a cable or the vehicle body. So why not use the vehicle body? It’s a far better conductor, cannot move or get connected to + (provided the + is properly protected) and is likely to be connected anyway.

BUT just make sure ALL the connections are of sufficient size. EG the small connection from the 80 battery to the inner wing. If you were using the leisure battery to jump start, this cable would be in circuit and would take full starting current and would quickly let the smoke out.

 

[moggy1968]

The thing is, most battery chargers (Intervolt included) deal with only the + side of things with the - side connected together. This needs to be done (obviously) with a conductor. This can either be a cable or the vehicle body. So why not use the vehicle body? It’s a far better conductor, cannot move or get connected to + (provided the + is properly protected) and is likely to be connected anyway.

BUT just make sure ALL the connections are of sufficient size. EG the small connection from the 80 battery to the inner wing. If you were using the leisure battery to jump start, this cable would be in circuit and would take full starting current and would quickly let the smoke out.

It’s a good point, I just have to be happy in my own mind I’m getting a good earth point, which I know the vehicle has. It also gives me the option of working it in to the same external battery isolator that the main batteries have. I asked intervolt if I could bridge the 24v and 12v earths together so I only had to run 1 cable back but they said no. I don’t have space to run another cable so at least 1 cable will need to be run to a local earth so guess I might as well do both and save the hassle. Got the 6mm stranded cable btw, just need to install it now!

 

[Towpack]

I’d also go with using the chassis/body for earth, the only exeption being to feed some sensitive radio equipment where you would run a separate Earth back to the battery and fit chokes in both wires to help avoid/ eliminate interference.

 

[chapel gate]

There are a hell of a lot of factory earth points dotted about the 80.
iirc a return earth to the battery counts towards the voltage drop as well

 

[StarCruiser]

It’s a good point, I just have to be happy in my own mind I’m getting a good earth point, which I know the vehicle has. It also gives me the option of working it in to the same external battery isolator that the main batteries have. I asked intervolt if I could bridge the 24v and 12v earths together so I only had to run 1 cable back but they said no. I don’t have space to run another cable so at least 1 cable will need to be run to a local earth so guess I might as well do both and save the hassle. Got the 6mm stranded cable btw, just need to install it now!

With your setup you have the fully isolated 24v to 12v converter which is why they say to isolate completely. With the 12v DCC the earth is common to both systems.

 

[chapel gate]

..and dotted about the 100 series..

 

[StarCruiser]

A couple of points worth making here.

The vehicle ‘earth’ has little or nothing to do with the Earth we stand on.

In electrical practice the negative of a DC (battery) system and the neutral of an AC (mains) system is often (not always) ‘grounded’ or ‘earthed’. This is done for cheapness so that protective devices only need to be placed in one conductor, the positive + in a DC system and the Live or Phase conductor of an AC system.

Concentrating on vehicle electrics, connecting one pole to the body was also done to help reduce corrosion through electrolytic action though this isn’t something we really need to concern ourselves with here. The main thing is that using the body as a huge all over terminal for one side of the battery is very convenient and limits the possibility of wires becoming shorted to it without making themselves known (by blowing the fuse). There’s nothing wrong with running negative wires back to a common terminal as long as they are unlikely to come into contact with larger section positive terminals and short circuit to them as there will then only be the fusing for the larger cable which will probably not blow. Again, using the body at the nearest attachment point or even creating a new one avoids this potential for fire very neatly. Using the body as earth also vastly simplifies the wiring. Toyota do a combination by bringing multiple earths from say lampholders, back to one earth point in the body.

When sizing wires as Nick has done (very neatly I must say Nick) there’s many factors to take into account that all stem from resistance and it’s presence, along with a load on those wires, which will always cause an amount of heat in the cables. This heat has to be able to get away under normal conditions or if it cannot get away (cool) fast enough then the cable size may need to be increased to compensate for this or be ‘de-rated’ as is the term.

In certain situations there are no rights and wrongs, there are many factors which if met will make a situation safe, these are not precluded. However there are a set of rules that if followed make for a straightforward installation that is safe.

1. Fuse to protect the cable under overload and short circuit conditions wherever the size decreases.*
2. Protect the wires and cables from abrasion where necessary with sleeving
3. Tie the wiring down to avoid movement that could cause insulation damage.
4. Size the wire to easily feed the load. If in doubt use a size larger. If going a long way consider voltage drop
5. On small wires, consider their routing and mechanical strength. A half amp load may only require a small wire but choosing one slightly larger or even standardising on set sizes will prevent those wires breaking.
6. Always use flexible multi stranded wire in vehicles.
7. Give consideration to the heat generated and retained by making up looms. Again, reason to de-rate the cable and go up a size or two.
8. * Heavy cables that can take the full current of a battery or batteries in a short circuit situation need not be fused PROVIDED they will not be damaged before the battery is depleted (or exceptionally can do no harm if they are damaged).
9. Protect terminals from accidental contact by using terminal boots or placing out of harms way where nothing conductive can fall across the terminals.

 

[StarCruiser]

Thanks Rich,

1. (please excuse the amateur hour sketch)

Option 1 - 3 separate circuits one for each fusebox
View attachment 142232

Option 2 - 1 circuit with fuse box's being fed from each other.
View attachment 142233

i'd rather go with option 2 in order to save cable, but it will mean that the first cable run from the battery to the first fusebox is going to need to be some pretty massive cable. For info here are my initial loading calculations
View attachment 142234

2. ok great. So there is no rule of thumb along the lines of , for example, max load +10% to allow for surges?

3. Great.

Thanks again Rich much appreciated

Both circuits in your drawings are allowable if fused and constructed correctly. Option 1 allows each fuse box to have its own fuse which allows it to be disconnected individually. You cannot do this with option 2. Option 1 also has nothing more than the loading of each fusebox on each feed cable. It also means if you get a fault and the fuse blows feeding fusebox 1 you haven’t lost 2 and 3.

With option 2 you would need a cable to carry the entire loading to box 1 of fuse boxes 1 to 3, a cable to carry the load of boxes 2 and 3 between fusebox 1 and 2 and be capable of having the fuse blow at the battery in the event of a short circuit or overload at box 2 and a cable capable of carrying the load of fusebox 3 between fuseboxes 2 and 3, again protected against overload or short circuit at the terminal of box 3. Some serious calculations there, or just use a cable the same size all the way, correctly fused and able to blow that fuse if a short circuit occurs at the end of the line (box 3).

You can probably see why it’s easier to opt for option 1 at this point.

Your voltage drops are assuming full load on the fusebox feed cable I assume? In which case you should have no more than 6% for lighting or 8% for other uses (keep it to 6% max to keep it simple). You also have to consider motor starting loads like your water pump and air compressor which will be transient and allowed further volt drop as long as the voltage variation is within limits of those devices. Again, I would say if in doubt go up a size, or if you are unlikely to have all loads running at the same time you can assume less load on the fusebox. This usually needs to be definitely switched such as a changeover for two large loads for example.

Your calculations all come in under 6% which is fine. Assuming wiring as option 1.

If you are intending bunching some or all of these cables or running them under carpet or through thermal insulation you should consider increasing their size if necessary in order to keep any heat to acceptable levels.

 

[moggy1968]

With your setup you have the fully isolated 24v to 12v converter which is why they say to isolate completely. With the 12v DCC the earth is common to both systems.

I couldn’t really understand it though because if I take the 12v earth back the vehicle body, it’s earthed in tandem with the 24v anyway isn’t it?

 

[StarCruiser]

I couldn’t really understand it though because if I take the 12v earth back the vehicle body, it’s earthed in tandem with the 24v anyway isn’t it?

Depends on how your converter is configured Andy. If it has separate output terminals for both + and - on the 12v side then you need to use those. I didn’t look at their drawing, perhaps you could post it up or pm it to me and I’ll take a look.

 

[nick_the_fish]

Thanks again Rich @StarCruiser .

I've said it before but i'll say it again I really do appreciate the time and effort you take to respond to my questions.

I think from your comments it sounds like Option 1 is the way forward. I think its a really good point that if they are separate circuits then if 1 fuse box fuse blows I only lose one circuit rather than them all. I had been aiming for a max voltage drop of between 2 and 4 % so it sound like there is some breathing room there which is always good.

Finally I think your 9 point set of rules for electrical installation is really useful with some really good points (point 5 especially). Not sure if there is some way of making that more accessible to the forum so it doesn't get lost when the thread dies???

Nick

 

[StarCruiser]

Thanks Nick, always happy to help where I can. 2-4% is a good aim on the volt drop and in reality you probably won’t run absolutely everything at once. Even if the lights do dip a bit when the pump starts you’re unlikely to worry too much. There are factors for de rating in insulation or in bunches that you may need to be aware of. For each circuit apart from 1 circuit on its own, as a rule of thumb derate them all by 10% per circuit. In other words for 3 bunched circuits they can all only carry 70% loading so if your load is above this then go up a size. You can ignore any cables that are going to carry 30% or less of their rated capacity.

 

[nick_the_fish]

Right, All the equipment for the new re-wire has arrived and work has commenced.

However, what is the best way to wire a relay for the airhorn/ Spotlights/ Underbonnet lighting etc?

Option 1. Is it best to connect the switch to the earth 85 and splice from a fused battery connection into 86 and 30, then running the accessory from 87?

Option 2. Or to switch from a seperate fused battery connection into 86?

I had a look on good ol' google and both seem acceptable, but if i'm gonna do it i'd like to do it right.

Any thoughts?

 

[StarCruiser]

I’d prefer option 2 as you can fuse a control circuit. Option 1 can be prone to faults to earth triggering the relay.