iiyama
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| posted on 26/8/07 at 08:10 AM |
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current
Have continued with the bke loom theme, ie using a negative cable in the loom.
However Im not sure Ive used a big enough cable! Used a 2mm thinwall rated at 25A, all of the components have independant 1mm cable rated at 16.5A.
Suddenly had a thought this morning that I have run -ve cables from the fan, fuel pump, water pump, fan stat and headlights off of this 2mm and I dont
think its big enough. :O
Having said that Im not sure how DC current works.
Im I ok? Or do I need to rip this out and put 3mm in?
If its broke, fix it. If it aint broke, take it apart and find out how it works!
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caber
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| posted on 26/8/07 at 08:18 AM |
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I think your 25A might be undersized under some circumstances. I would suggest rather than ripping the 2mm out you add a 3mm and join them together,
the lowest resistance you can achieve in your earth path the better particularly where you have electronics or instruments that read voltage as the
effective voltage varies with the increased resistance of the earth path this could give some funny results when you are using high current, i.e.
headlights and fan at the same time. This would be a bugger of a problem to figure out if it happened when running.
There is a misunderstanding around that there is no current after a device in a circuit. That is wrong. The current or flow remains constant it is the
voltage or pressure that is reduced across a load.
Caber
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ReMan
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| posted on 26/8/07 at 08:59 AM |
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Are you saying each load, (light/fan) has it's own 2mm earth back to a central point/battery/chassis?
If that is so then it should be fine, as you say the cable is 25A and none of the INDIVIDUAL loads are that high.
If however you have commoned them so the final return point for all th loads is through ONE cable of 2mm then this may well be underated.
Col
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iiyama
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| posted on 26/8/07 at 09:16 AM |
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Thanks guys.
Yup, thats what I have done, 2mm running within the loom and then components run into it locally with 1mm.
Ill run another 2mm, (as I have some!), and use this for the headlights and fan. 25A should be plenty for them?
Trouble is I have no spec for the fan, (although thinking about it I should be able to find out as its a Pacet), or the water pump. Lights I can work
out, fuel pump is known.
One other thing while Im on the subject, does the chassis need to be run to -ve? At the moment it isnt but wouldnt take much to do it. Fuel tank is
back to -ve for the sender.
[Edited on 26/8/07 by iiyama]
If its broke, fix it. If it aint broke, take it apart and find out how it works!
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RazMan
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| posted on 26/8/07 at 09:25 AM |
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Headlights will draw 10A with std bulbs, fan will be about 10A, fuel & water pumps are probably around 5A each so 1mm is fine for these.
Definitely earth the chassis (and engine too) but use 6mm cable 
[Edited on 26-8-07 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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ReMan
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| posted on 26/8/07 at 09:31 AM |
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Does the chassis need to be run to -ve?
Technically, probably not, but you may run into trouble or make more work for youself, having to fit some seriously big link wires when it comes to
earthing the engine/starter/battery.
The main reason for earthing is to save you putting in all the return wires that you are doing!!
There may be a safety issue with static that makes for good practice earthing the fuel tank too, the whole theory of which escapes me right now
[Edited on 26/8/07 by ReMan]
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BenB
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| posted on 26/8/07 at 10:00 AM |
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Surely if you've got seperate feeds for the +ve wires then the simplest thing to do is to make the chassis -ve and just attach the wires to the
chassis....... You can always keep the "common earth" 2mm wire as a backup... I wouldn't rely on just the 2mm except as an emergency
backup though, the worse case scenario is going to pull way more than the max current of the wire....
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C10CoryM
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| posted on 26/8/07 at 04:19 PM |
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How is it you get wires rated at specific amps? As the wire gets longer, the restance gets higher, and the amperage it can handle goes down. Or is
it that automotive wire there has a max length of 20ft or so?
http://www.rbeelectronics.com/wtable.htm
Is a chart to help you pick out what thickness wire to use.
Also, dont forget that as the amperage goes up from the length of wire, it will get closer to the fuses max capacity. I have books on all this stuff
with simple ways to calculate it all out. I should have a look for them.
Cheers.
"Our watchword evermore shall be: The Maple Leaf Forever!"
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ReMan
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| posted on 27/8/07 at 09:18 AM |
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quote:
Originally posted by C10CoryM
How is it you get wires rated at specific amps? As the wire gets longer, the restance gets higher, and the amperage it can handle goes down. Or is
it that automotive wire there has a max length of 20ft or so?
http://www.rbeelectronics.com/wtable.htm
Is a chart to help you pick out what thickness wire to use.
Also, dont forget that as the amperage goes up from the length of wire, it will get closer to the fuses max capacity. I have books on all this stuff
with simple ways to calculate it all out. I should have a look for them.
Cheers.
Sorry mate , I think youve got a few facts mixed up!
Wires are rated for their maximum safe current carrying capacity. At this point this has nothing to do with the length you use.
However, as the wire gets longer, the resistance gets higher, this REDUCES the amount of current that can flow,for a fixed voltage, but the thicker
the wire the less resistance. Therefore to maintain a good current flow at a distance you may use a bigger (read heavier more expensive) wire.
Either that, OR raise the voltage. Part of the reason that trucks for instance use 24V .
There is nothing special about automotive wire, other than its price
The amperage flowing goes down with the length of the wire and the wire does not change the fuse rating, only the load determines this.
HTH
[Edited on 27/8/07 by ReMan]
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iiyama
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| posted on 27/8/07 at 10:02 AM |
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This is a highly informative thread!
Reason Ive placed a -ve cable(s!) within the loom is that the original bike loom had one to start with. The Solenoid is right on top of the battery
which in turn is only 350mm or so from the starter motor. The length of the cable from the +ve on the battery is around 150mm and is the original bike
cable. Ill have to get cables to go from the solenoid to the starter and from the -ve on the battery to ground the engine. These will be around
350mm.
The two reasons I placed a -ve cable within the loom are :-
1/ as already mentioned the bike loom had one.
2/ I know its an entierely different car, but the cabling within the Gen RX-7 was exteremely complex, lots of relays and other stuff for all sorts of
electrics, twin turbos added to the complexity. So most of us ran a large cable around the engine bay directly from the -ve on the battery to help
with bad chassis grounding. Now part of this I guess was to do with the age of the cars, mine was a '94.
I will probably ground the chassis as a matter of course, fuel tank and engine are connected directly to the -ve terminal on the battery.
Anyway, keep the discussion going! Im learning a little more with every post!
Cheers guys!
If its broke, fix it. If it aint broke, take it apart and find out how it works!
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C10CoryM
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| posted on 28/8/07 at 11:25 PM |
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Yes, you're right. Sorry for any confusion. Wish I would remember not to post when Im tired .
None the less, wire length does affect how much current can go through it w/o melting. Resistance from the longer wire makes heat and will melt the
insulation and/or the conductor. I guess if you have specific amperage ratings it can handle that current until the resistance gets so high it
becomes open? Hard to imagine. I wonder just how long a wire has to get before it can't conduct current at 14.5V
Cheers.
quote:
Originally posted by ReMan
Sorry mate , I think youve got a few facts mixed up!
Wires are rated for their maximum safe current carrying capacity. At this point this has nothing to do with the length you use.
However, as the wire gets longer, the resistance gets higher, this REDUCES the amount of current that can flow,for a fixed voltage, but the thicker
the wire the less resistance. Therefore to maintain a good current flow at a distance you may use a bigger (read heavier more expensive) wire.
Either that, OR raise the voltage. Part of the reason that trucks for instance use 24V .
There is nothing special about automotive wire, other than its price
The amperage flowing goes down with the length of the wire and the wire does not change the fuse rating, only the load determines this.
"Our watchword evermore shall be: The Maple Leaf Forever!"
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