Fuse Sizes

Submitted: Thursday, May 11, 2017 at 12:37
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Hi All, I'm doing some electrical work on my camper trailer, and I'm a bit confused about fuse sizes. I believe that the fuse size rating must be lower than the cable itself, but is there a way of working out the required fuse size for different size cables?

Thanks in advance,

Greg..
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Reply By: Member - Boobook - Thursday, May 11, 2017 at 12:47

Thursday, May 11, 2017 at 12:47
Look at this table

My advice is to use the next amperage up from 20% more than the load you are using. Then less than 75% of the ratings above.

Eg if the load is 10A, then use a 15A fuse. For a 15A fuse the minimum cable size would be 3 or preferably 4mm.

If the load is 5A then a 7.5A fuse would work and you could use 2.5mm cable.

This ignores voltage drop which is a different issue.



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Reply By: All Things Outdoors on Youtube - Thursday, May 11, 2017 at 14:48

Thursday, May 11, 2017 at 14:48
Check out the chart on this website http://bdfuses.com/fusesnwires.php
AnswerID: 611039

Reply By: cookie1 - Thursday, May 11, 2017 at 17:24

Thursday, May 11, 2017 at 17:24
Are you talking about Mains 240Vac or 12Vdc?
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Follow Up By: Greg A6 - Thursday, May 11, 2017 at 19:25

Thursday, May 11, 2017 at 19:25
Yes only 12v.
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Follow Up By: Nomadic Navara - Thursday, May 11, 2017 at 19:39

Thursday, May 11, 2017 at 19:39
It does not matter what the voltage is. It's the current that heats up the wire. You need to limit the current in the wire to a value that will not allow allow the wire to heat up and melt the insulation. If you look at the specification for any wire you will find the maximum current carrying capacity for that wire. You will often find it on the reel the wire comes on. Just use a breaker of fuse that is lower in value than that specified current.
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Follow Up By: cookie1 - Thursday, May 11, 2017 at 20:07

Thursday, May 11, 2017 at 20:07
It actually does matter with all due respect

If it was 240Vac then it has to be done by a qualified electrician and a COC issued.

Just be careful to get it right, if there is a fault which results in a fire, the insurer may deny the claim, always best to ensure the correct fuse for the gauge of wire and have the fuse as close as possible to the battery to protect the cable and ensure all cables are protected against heat and rubbing.
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Follow Up By: The Bantam - Tuesday, May 16, 2017 at 21:46

Tuesday, May 16, 2017 at 21:46
Cookie mate ....... voltage is utterly irrelivent to selection of fuse value.

yes different fuses and breakers are required for different purposes and required under different regulations ....... but voltage remains irrelivent for selection of fuse value.

cheers
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Reply By: Dean K3 - Thursday, May 11, 2017 at 19:44

Thursday, May 11, 2017 at 19:44
Think you need to work a tad backwards in this instance.

what loads do you want to run a a certain circuit whats it current draw per item led lighting next to bugger all whilst a 12v water heater will pull 20a or more.

once you worked this out then you can select correct wiring and fuse/breaker capacities.

As your dealing with DC lengths become double due to the flow between negative to positive wires this also determines cable size to minimise volt loss and resistance

One useful source is collyn rivers has a series of books out on designing systems etc.

I would suggest (and some will disagree here) try and use a circuit breaker narva and other make them. means they can be manually rest if a accidental overload does occur rather than blowing a fuse esp if its a higher current distribution cable.

IF ANY mains power is being installed ie GPO outlet to power battery charger for charging when at caravan park or maintenance charging at home then this needs 240 sparkie to install and certify
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Follow Up By: Dion - Friday, May 12, 2017 at 13:33

Friday, May 12, 2017 at 13:33
In relation to last paragraph, why can Mr Joe Average still purchase GPO's from just about any hardware store for D.I.Y?
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Follow Up By: cookie1 - Friday, May 12, 2017 at 15:17

Friday, May 12, 2017 at 15:17
Dion, it is something that sparkies are not a fan of but alas there is no law stopping it. I am aware of many issues of customers buying the bits and getting sparkies to install them, such as when it fails the hardware store will honour the warranty but you have to take it back to them, so you need to re-engage a sparky to remove it, then again to install the replacement, much easier to get a sparky to supply & install then it is covered by the sparky and is highly unlikely to supply cheap shyte as they are then legally responsible for anything that happens.

Hope this helps
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Follow Up By: Dean K3 - Friday, May 12, 2017 at 20:55

Friday, May 12, 2017 at 20:55
Agree with what is being said about 240v mains or higher 440 3 phase.

chain of responsibility comes to mind as this applies to both agriculture and transport industries, but it doesn't apply to hardware stores,all they are required to have is a sign saying must be installed by licenced sparkie.

But then if i need to replace a globe inside my house technically a sparkie has to be called in but how many people apart from strata title and large multi storey apartment buildings with a onsite maintenance person/contractor does it ? .

If i go to a certain hardware store and without naming (defamation purposes) I can go purchase star pickets CCA treated logs and carry them home inside a station wagon.

If I purchase same items from a agricultural supply shop I am required to transport them on either trailer or ute only-same applies for chemicals used for weed/insect control.
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Follow Up By: 9900Eagle - Saturday, May 13, 2017 at 19:43

Saturday, May 13, 2017 at 19:43
It doesn't apply to electrical wholesalers either. I can purchase anything I want, so don't think it is just hardware stores.

If people live or work remote, I hope people realise that you can't afford to get a sparkie out for a small job at $2 a kilometre to change a gpo, fix a lead or replace a contractor, and then pay for the work done. Unless it is a major it isn't going to happen.
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Follow Up By: Nomadic Navara - Friday, Jun 09, 2017 at 13:20

Friday, Jun 09, 2017 at 13:20
In most cases it is no the ownership of particular goods and chattels that makes you a law breaker, its the use of those that makes you a transgressor.
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Reply By: Bob Y. - Qld - Thursday, May 11, 2017 at 20:27

Thursday, May 11, 2017 at 20:27
Wouldn't the load ie: fridge, water pump etc, determine the fuse size, and then the cable size?

Bob

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Follow Up By: Gronk - Thursday, May 11, 2017 at 21:39

Thursday, May 11, 2017 at 21:39
Yes, sort of......say it's a fridge that draws 5A, then a fuse of 10A would be plenty big enough, but the wire COULD be anything that can carry 10A.....BUT , for voltage drop or just because you want or have it laying around, a wire of a rating of 30, 40, or even a 100A can be used.

The fuse is primarily there to protect the wiring, but knowing the wiring is plenty big enough for the job at hand, I'll fuse to suit the appliance.
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Follow Up By: Bob Y. - Qld - Thursday, May 11, 2017 at 22:03

Thursday, May 11, 2017 at 22:03
Yeah, that's more or less the way I'd do it too, Gronk. Bigger the wire the better, without going overboard. Just doing a partial rewiring of my alloy canopy at the moment.

Bob

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Follow Up By: 9900Eagle - Saturday, May 13, 2017 at 19:44

Saturday, May 13, 2017 at 19:44
Hello Bob, met your neighbour at Tatts the other day. I hope she said hello.
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Follow Up By: Bob Y. - Qld - Saturday, May 13, 2017 at 20:59

Saturday, May 13, 2017 at 20:59
G'day Slow,

Did get a message passed on by her hubby, but he had it a bit mixed up. Was talking about "990" and that had me buggered. Didn't have a clue who it might have been! :-)

Where are you off too? Shouldn't be frequenting pubs, chatting up attractive young ladies...........though I couldn't think of anything much better than that, ha ha.

In Birdsville tonight, there was a campdraft today, and a rough Stock rodeo tonight and bronco branding tomorrow. Good to catch up with a few friends.

Bob
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Follow Up By: 9900Eagle - Sunday, May 14, 2017 at 04:28

Sunday, May 14, 2017 at 04:28
WA mate. In the Curry at the moment and caught up with an old mate I haven't seen for 18yrs. He just retired from the road and is loving it.

I hope they didn't mistake you for a cleanskin micky and rope you to the panel.

All the best and take it easy on the way back from Birdsville.

Cheers.
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Reply By: swampy - Thursday, May 11, 2017 at 21:43

Thursday, May 11, 2017 at 21:43
hi
Greg A6
dc fuse sizing
fuse is sized to the end product consumption
eg interior lights if 8 amps go 10 amp fuse
12v dc wiring
sized according to the current /amps capacity over the distance to prevent v/drop.
eg max volt drop for most applications is 3%
cable size is calculated from there
240v A/C fuses protect the cable
12v dc fuses protect the end product . The cable is oversize [easily carries the amp load]

Remember the dc cable advertised current rating.
Commonly refered to as a fire rating /melt down /unusable at this level of current draw ,volt drop to high at this point anyway .
AnswerID: 611052

Follow Up By: Dion - Friday, May 12, 2017 at 13:36

Friday, May 12, 2017 at 13:36
Pigs posterior about fuses in a 12VDC circuit protecting the end product!!
The fuse or circuit breaker in any powered circuit is to protect the power source from overheating the conductors. Applies to both AC and DC, in any voltage from mV to kV. End products are protected by thermal protection devices - completely different kettle of fish.
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Follow Up By: swampy - Friday, May 12, 2017 at 18:39

Friday, May 12, 2017 at 18:39
hi
How do u think automotive 12v works !!!
The component /consumer has a fuse just above its rated consumption
The cable is rated above this .
A blown fuses prevents component from drawing any further current than it needs therefore by blowing it protects the feed in circuit.
OR
12dc fuses protect/isolate both the end product and wiring in case of failure in the circuit or product .
Its been that way ever since cars were invented !!
Next time read the explanation fully and understand it fully .
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Follow Up By: cookie1 - Friday, May 12, 2017 at 21:22

Friday, May 12, 2017 at 21:22
Swampy, interested to know if you are actually a qualified Auto Electrician, reason I ask is that I was always taught ( A Class Electrician) that the fuse on the installed cable is to protect the cable not the actual devices powered from the circuit as I took from your statement above
"240v A/C fuses protect the cable
12v dc fuses protect the end product . The cable is oversize [easily carries the amp load]"

On the thermal rating of the cable I was also taught that we use the rating of the cable to then de-rate for the ambient temperatures, not the temperature of the cable once we draw too much current, hence I always assumed that the cable current ratings are ambit figures for the common users such as what is in AS3000 but the onus is then on us (who are legally responsible) to calculate the correct size cable for the load given the distance, anticipated current, derating factors - installation type, ambient temperatures etc.which we have AS3008 to work with

I am curious to see if Auto Sparkies are taught differently so no offence intended
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Reply By: Steve in Kakadu - Friday, May 12, 2017 at 21:49

Friday, May 12, 2017 at 21:49
So after all the expert answers above, the easiest way to answer this question is to take it to an auto electrician.

Just sayin.
AnswerID: 611085

Reply By: silvercruiser03 - Saturday, May 13, 2017 at 17:58

Saturday, May 13, 2017 at 17:58
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Follow Up By: Steve in Kakadu - Saturday, May 13, 2017 at 20:41

Saturday, May 13, 2017 at 20:41
That's GOLD
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Reply By: The Bantam - Tuesday, May 16, 2017 at 21:51

Tuesday, May 16, 2017 at 21:51
the philisophy of fuse selection varies greatly depending on the area involved.

The automotive manufacturers push the limits far harder than the building industry electrical industry.

In the proper electrical business the current carrying capacity of cables is worked starting from 10 amps per square mm ....... some in the automotive business will push it twice that hard ...... I don't think that is clever.

cheers
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