space for a second leisure battery..

Hi, on my 1998 Symphony i have two batteries under the drivers seat. Very tight but they fit.

-mojo- wrote:

SJR wrote:
I think it does make the larger cable pointless, as you might just as well have run the entire length in the smaller cable.

As I said earlier, the problem here is not going to be one of overheating - the key problem (if one exists) in this type of installation is voltage drop on a long(ish) run of cable. The long part in large diameter cable keeps the voltage drop to a minimum, and a short length of thinner cable will only have a minimal additional effect, precisely because it is short.

Despite what you suggest, the short length of thinner cable does not somehow negate the benefits of the bigger cable - when you think about it, the inline fuse itself is just a very thin bit of wire, but that does not make the thickness of the rest of the wire pointless!

Please believe me - this is not some random opinion of mine. I do actually understand the subject.

I agree, these are the 25amp fuses attached to the short length of cable recommended by the auto/electrical engineers.

I'm sure they know what they're talking about

-mojo- wrote:
Despite what you suggest, the short length of thinner cable does not somehow negate the benefits of the bigger cable - when you think about it, the inline fuse itself is just a very thin bit of wire, but that does not make the thickness of the rest of the wire pointless!

As fuses are to protect the weakest link in the chain, should any fuse fitted be rated for the short length of small diameter cable? 10 Amp?

If that is not the case, at what length of cable would the size of fuse used reflect the current carrying capacity of the cable?

Note; I ask this out of interest, not to challenge what has been said previously.

Hi

-mojo- wrote:
Despite what you suggest, the short length of thinner cable does not somehow negate the benefits of the bigger cable - when you think about it, the inline fuse itself is just a very thin bit of wire, but that does not make the thickness of the rest of the wire pointless!

Agreed.

I think we’re getting confused between voltage drops and current ratings. Voltage drop is dependent on the length of the cable and its gross sectional area. Thin cable will have a higher voltage drop per metre but a very small length will have a very small voltage drop.

Look a domestic shower. If the cable run is more than a few metres you probably find it’s wired with 10mm or even 16mm cable. Inside the unit the wires to the element will be much thinner. The thinner wire has almost no effect on the system because it’s so short.

matchlessman wrote:
As fuses are to protect the weakest link in the chain, should any fuse fitted be rated for the short length of small diameter cable? 10 Amp?

The rule is that if different sizes of cable are used the fuse must be capable of protecting the smallest cable in the run. As for the cable used in the fuse holder, the manufacturer should give a current rating for the complete fuse holder and that is the rule to go by, whatever size of cable is fitted. If a fuse holder has no manufactures information and no published specification I would avoid it! Also beware that the maximum rated fuse value may not be the same as the maximum continuous rated current value.

e.g. a typical datasheet:

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matchlessman wrote:
If that is not the case, at what length of cable would the size of fuse used reflect the current carrying capacity of the cable?

There is no rule. Unless you have a very good reason you should always use cable rated above the maximum current you expect to take (and at the temperature you expect it to be working at).

I think the only rule is “if in doubt get professional advice”.

Brod.

when making a cable sizing selection surely you work out the expected load, then select a cable that will handle the load, then take into account any other calculations, mV drop per amp per mtr, grouping factors, ambient temp, etc and upscale the cable CSA if needed?

I have two cars with the batteries in the boot ( I know these will have high load starting current) but these have large cables (35mm'ish). The connectors for these are large and difficult to remove/re-attach as the cable stays the same size. They would be much easier to work with if they changed to a smaller cable at the connection point. Why did the manufacturer not do this?

Regards,
SJR

SJR wrote:They would be much easier to work with if they changed to a smaller cable at the connection point. Why did the manufacturer not do this?

Probably primarily because it would mean using three components (the thick wire, the thin wire and the splice/joint) where one works Ok. It would probably also make that section physically too weak. I doubt they care how easy the vehicle is to work on at that level.

Thanks for that Mojo.

Sorry if I have mislead anyone with my incorrect "Knowledge". It looks like I have turned into an internet authority, one of the people that annoy me by spouting incorrect info as if were fact, I can't believe it.

SJR

SJR wrote:
Sorry if I have mislead anyone with my incorrect "Knowledge".

May I request that you read through the thread again?

Nobody has said that your original advice was incorrect, and I started my reply with the words "SJR is right in theory", and then went on to explain why, although you are correct that what appears in the photo is not ideal, the shortcomings of the installation are not - in my opinion - serious enough to be worth the effort of reworking.

If I were doing that installation myself I would have done exactly as you suggest - provided the right parts were available - and spliced the fuse carrier into the thick wire a short distance from the battery.

Hopefully you will accept this as an apology on my part for labouring a point perahps a little further than it needed to be!

Mojo,
There is no need for an appology, Thanks for the clarification though.

SJR