help with wire thickness/amps

[dgym]

Hi all,
I've bitten the bullet (pun intended) and pulled out my loom and drawn up a map and measured etc etc with the view to making a new loom.
I'm a bit stuck when it comes to the thickness of the wire I need.
I'm looking on the holden page,
is anyone able to advise me on what kind of amps I will need for say the lighting, the fused green, the window motors?
I'm in a world of confusion after three days in a room with a loom.

[rgh0]

You need to match the wiring to the rating of the fuse you will be using to protect the circuits. How many items you put on one fuse dictates the fuse size required.

So you need layout your wiring diagram and fusing arrangements to figure out the load on each fuse then size the wiring to match that fuse as a starting point. Its all a bit iterative as you dont want to end up with lots of small loads all on one big fuse and then having to run heavier than needed wiring to each small load. Once that's done you need to check for voltage drop in each individual circuit to ensure the wire is big enough which it will normally be if you have done the first step.

Lotus of course got around this problem by using only a couple of fuses and small wiring that melted before the fuse

cheers
Rohan

[bob_rich]

Hi

There will be a lot of opinions on wiring current ratings.!! From the electrical point of view the wire should be rated safely to clear the fuse that protects the wire. The problem then is in many older cars a large fuse protected several loads so a odd fault that say trebled the load current may not necessarily clear the fuse but would overload the wire.

Vehicle Wiring Products and Polevolt catalogues give a good guide. The usual method in low voltage (12V) systems is to pick a wire so the volts drop is no more than 3% at working current. A link is attached to info on the 3% idea. These 3 approach should allow a suitable wire to be picked.

when I rewired my car I fixed modern blade fuses for each accessory.

As a guide I checked my log book from my rebuild and the following may help for the bigger loads
headlamps 13A : fog lamps 10A : cooling fan 10A for second or so falling back to 7.5A :
heater hi speed 8.5A : heater low speed 7.5A : window motors 16 to 20A :
windscreen wipers 8A hi speed : windscreen wipers low speed 6.5A : Horn 5A : (air horn pump could be 20A)
Most other loads were less than 6A

All above were with 12.5V from a DC supply that held the volts when the current was drawn. Allow around 15% to more when engine running. I found the biggest variable was the state of the window lift system this can vary quite a bit.

Doing your own loom means you can go conservative on the wire sizing --which I would recommend .

best of luck !

Bob

http://www.engineeringtoolbox.com/amps- ... d_730.html

[stugilmour]

Further to Rohan and Bob's excellent advice, take a look at the vehicle wiring products cable page and note the available colours. I used there thin wall cable as I was not fussed about originality.

http://www.vehicle-wiring-products.eu/s ... wall-cable

For most of the heavy loads Bob notes I used the 25 amp thin wall wire (headlamp power, air horn power, cooling fan, window lifts, etc.)

16.5 amp was good for most of the general wiring (instrument senders & warning lights, relay triggers, wipers, windshield washer, side lights, turn signals, etc.)

The black grounds can use the larger sizes as required, as most sizes come in black.

For the main loom power (Brown, Hot at all Times / Unfused) I think I used 50 amp. I added a fusable link like modern practice.

Another large one is the main ignition power (White, Hot in run & start / Unfused). Consider 33 or 39 amp.


HTH

Stu

[dgym]

Thanks for all the replies. I'm feeling like I've gotten in over my head a bit and about to give up and buy the standard loom!
BUT
Let me see if I understand the process of wire choice;
Based on my loom design I need to figure out the load on each fuse. Eg. 20amps
Then I need to choose a fuse that will blow when more current than the usual load occurs. Eg 25amps
Then I need to choose wire that will not cook before the fuse does. Eg 30amps.

Is my head in the right place?

Thanks again!

[bob_rich]

Hi again

Your reasoning is quite correct. The wire must be rated for greater than the fuse current. The problem occurs when
several loads are connected to a single fuse. In my +2S the 4 ( original) fuses under the centre dash board cover
had several loads on one fuse

As an example the 2nd fuse from the left, and using the current I measured, had the engine fan 7A to 10A, wiper 8.5A and heater 7.5A. All of these things use motors. The fuse needs to be sized so that all 3 units can run at the same time ( e.g. UK winter in traffic ) so 26A total suggests a 30A fuse. On the face of it 16A wire looks OK for each unit. BUT motors frequently fail by jamming/slowing and drawing excess current that may be say for example only 3 time normal . So the heater could have a fault and draw 22.5A overload the 16A wire and not blow the fuse.

So in this example to be safe you really need 30A wire on each of the accessories if you use a 30A fuse.

When I rewired my car I used a suitable fuse for each accessory and used modern blade fuses that have well defined clearing characteristics. ( good data on the Littlefuse web site) I was able to easily install a bank of 10 off blade fuses in the space that had the 4 off glass 1.25" fuses .

Also remember that in a car loom wires will be bunched together and this will further reduce their current ratings. Info is available on this from the catalogues in the above posts.

I calculated the overload capacity of wiring based on the IEE wiring regulations for domestic house wiring and this is quite helpful in selecting a fuse, or confirming that given a wide variety of overload situation from dead short to heavy load that the wire will cope OK.

Hope this helps and does not bore you to much! To be safe use a bigger wire if you have any concerns

best of luck

Bob

[RichardHawkins]

dgym,

I was horrified at the state of the wiring on my elan, so decided to replace and improve.

My approach was slightly different. I went looking for a vehicle that had the basis of what I wanted, but was larger. I chose a Skoda Felicia because the fuse/distribution board is behind and under the passenger glove box, which is where there is some space in the elan. All the design work with respect to wire sizes has been done, and as the vehicle is bigger all the wires should be long enough. In practice that was not correct and I had to replace more wire than I expected, as although there is more wire on the Skoda, some of the wires take different routes and are shorter than I needed.

I bought an MOT ( MOT is the roadworthy test in the UK) failed Skoda, a Haynes manual for the wiring diagram, and stripped the wiring out. Label everything cut nothing, I cut some wires that I thought I would not need, and regretted it. I sold the rest of the car for scrap at nearly the cost that I payed for it.

I used Vehicle Wireing Products www.vehicleproducts.co.uk and Auto Electric Supplies Ltd www.autoelectricsupplies.co.uk

I hope there are similar suppliers in Australia. The work took me a year. My electrical knowledge is rudimentary, I learned a lot, but whether it was worth the effort I am not sure.

Richard Hawkins

[dgym]

ok i think i'm getting closer to some progress, thanks again for all the replies.

I'd like to test for amps in my car's appendiges with the multimeter - hooking it up in line with each device to see what it's drawing.

the problem is that my wiring is all out, so there'll be no running the car.

Can I just hook up a little rig with my car battery and some alligator clips and go round the car with it and the multimeter testing all the equipment or will I get an innacurate reading from just the battery connected right to the item?

thanks again and again,
-jim

[billwill]

That should be accurate enough as you will apply say 25% extra safety factor on the figure that you measure anyway.

[Andy8421]

25% headroom may not be enough.

Resting voltage for a fully charged lead acid battery is in the order of 12.6V. A modern alternator will aim for a charging float voltage of 13.6V or so. The dynamo / regulator combination on an Elan will be no where near as precise, and could easily be above this voltage under no-load conditions.

You could easily get +10% extra voltage (and therefore +10% current) over the amount you measure just because the engine is running and the battery is under charge.

Second problem is that motors and bulbs draw substantially more current starting up than in steady state. In the case of the blower motor (for example), startup current could easily be two or three times the running current as the motor spins up. Bulbs are also problematic. The cold resistance of the filament is much less than the hot resistance and the bulb will draw high levels of 'inrush current' until the filament warms up. This could be many times the steady state.

These are transient draws, and given the slow-blow nature of car fuses it is quite possible that a fuse will be able to deliver much more current than its rated size for a brief period to allow the bulbs to warm up or motors to start. On the other hand the damn fuse may blow every time you switch the headlights on.

Selecting the right fuse is a combination of fuse blow characteristics, load startup dynamics and supply voltage variation. 25% over steady state may be enough, but don't be surprised if the fuse blows repeatedly.

[dgym]

wow this is epic.

[William2]

I was tempted to make up my own looms but by the time I had costed the wire and all the bullet connectors it seemed cheaper to buy the looms ready made.

[dgym]

I've already given up and called Sue Miller but there's a three month wait so here I am back at it.

[rgh0]

While you can make it very complicated in theory it gets much more simple in practice.

You will generally end up with two wire sizes - one size for big loads (e.g. headlamps)s and one for small loads (eg. instrument lights)
and two matching fuse sizes of maybe 30 amps and 15 amps.

The real advantages of making your own loom versus buying a replacement is:

1. You can put in relays to take the load off the againg and hard to replace switches
2. You can add fuses to minimize future problems
3. You can add extra earths to minimize earth fault problems
4. You can use modern multiple plug connectors ( suitably hidden if you wish) which are more relaible and easier to connect and disconnect. This can make things like dash removal much easier

The disadvantages are

1. Cost will probably be greater in the end probably than a standard replacement loom.
2. You will spend many hours making it
3. It will not look totally original unless you can find someone with a tape wrapper machine and you take efforts to hide the non standard plugs and fuses and relays ( this is a personal perception issue not a real issue but it may affect resale value perceptions for some people)
4. It is hard to source all the required individual wire colours if you want to replicate the original colour code system. You do need to produce a detailed wiring diagram for the colours used and other details you have changed in fuses and relays versus the original wiring diagram.


I made my own loom for my Elan back in 1980 as the old one was in a terrible state after 12 years with the main run of wires back to the rear of the car melted into a solid block of multicoloured plastic with a bunch of copper wires in it - however it surprisingly still worked when I pulled it out !!! My home made one has been total reliable for the last 37 years

cheers
Rohan

[dgym]

thanks for the pep talk Rohan,
seeing your car at Winton Historics was actually part of the inspiration for the re-wire. seeing your non original wiring I had a bit of a moment where I realised I could do whatever I wanted to my car in order to make it work for me. I don't even have to use Lucas connectors or fuse boxes!
I'd been a bit of an originality nut in the past but to be honest the general car admirier doesn't notice those things and it's only in our own heads that it's important.

So Im on a bit of a quest now to make my car safer and more reliable and with the hope of getting out on the track eventually. (not just hill climbs)

back to the loom, you are right about the theory being more complicated than the practice. I've spent days freaking out about it. I only really started to see the light of day when I started drawing up my own schematic.
I've done a first version - very messy but correct. Tomorrow I'm going to go and buy myself a really nice pencil and do a neater version. I find it quite relaxing when it's going right. I've found i'm starting to know the colours and circuits off buy heart. Which will be good for any trouble shooting.

I've added several relays and fuses. also a hazard switch (a must have in the freeway tunnels with no breakdown lanes!! ) And will do a battery cut off.

I'll post an image of the schematic when i'm done for any interested nerds

cheers!