# Fuse calculation

Which is the best fuse to use (3A, 5A or 13A) with a 1.15kW electric fire at a potential difference of 240v
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1150/240=4.79 so theoretically a 5A fuse shoud do.
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Graham.
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Unless there is a surge on start up which can happen on heaters. I'm very surprised we do not get 7.5 amp fuses for this sort of thing. 13 amp ones go with a heck of a bang. You can get very slow blow fuses of course with a lot of that heat conducting stuff in them, but I'd guess a lot of fuses one sees in pound shops are dodgy in the extreme. Brian
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On Tuesday, March 20, 2018 at 2:48:07 AM UTC+11, Graham. wrote:

May be 5A will do, as "cold" heater element resistance would be higher than normal heating.
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On Thu, 22 Mar 2018 18:50:17 -0700, gopalansampath wrote:

Surely it's the other way round.
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On 23 Mar 2018 09:55:54 GMT, Bob Eager wrote:

AIUI the type of heater that is in an incandescent lamp - filament, of course - has low initial resistance so that it comes on 'instantaneously' whereas an electric fire/kettle/shower is pretty well constant. Cold inrush to a 3kW kettle+bad idea. The 2 elements in my electric shower, measured when cold of course, have the resistance corellating to the rated power. The electric kettle stays the same, although the meter won't show the power for the first cycle or two.
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On Friday, March 23, 2018 at 8:55:58 PM UTC+11, Bob Eager wrote:

OOps! I stand corrected. Surely fuse rating shall be higher than % Amps.
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On Friday, 23 March 2018 01:50:20 UTC, snipped-for-privacy@gmail.com wrote:

But according to regulations in the UK on changing fuses it should be either a 3A for below 720W or a 13amp for above 720W .
https://www.rospa.com/home-safety/uk/northern-ireland/electricity/plugs-fuses/choosing-fuses/
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whisky-dave wrote:

ROSPA doesn't represent "regulations", it's just advice, based on the ratings of fuses they expect people to have at hand, and even that page suggests 5A for some appliances ...
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On Friday, 23 March 2018 13:37:35 UTC, Andy Burns wrote:

As I said it's a bad question and in this university we have the regulations which tell us we must only use 3 & 13amp fuses, about 10 years ago I and others had the job of checking all fuses and replacving them with the 'correct ones' i.e 3 or 5, this is one reason why I have dozens of 5, 7 and 10 amp fuses, I even have 1 amp and 2 amp that I used in angle poised lamps with 40 or 60w bulbs. ANd don;t forget the FUSE is to protect the cable NOT the item of equipent so it really doesnlt matter what current rating the electric fire is, you put the fuse in that is correct for the cable.
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Nonsense!
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On Friday, 23 March 2018 17:46:17 UTC, Sam wrote:

Fact.
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On 23/03/2018 17:57, whisky-dave wrote:

Your view is too simplistic. Any fuse value below the rating of the cable will protect it. Therefore a "univeral" cable rated at say 5amps feeding an appliance rated at say 2 amps will still be protected by using a fuse suitable for the appliance.
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On 23/03/2018 18:07, Sam wrote:

If an appliance needs a fuse for protection it should be in the appliance. Plug fuses are not there to protect the appliance and can't be relied on to do so.
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On 23/03/2018 18:16, dennis@home wrote:

So?
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On 23/03/2018 18:17, Sam wrote:

So a 13A fuse is perfect protection if the cable is thick enough and a 3A if it isn't. Its illegal to supply anything unsafe for use with a 3A fuse. It may even be illegal to supply leads that are unsafe with a 13A fuse if the regulations have been passed.
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On Saturday, March 24, 2018 at 7:19:46 AM UTC+11, dennis@home wrote:

the item

e.

Makes sense absolutely.
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On Friday, 23 March 2018 18:17:55 UTC, Sam wrote:

the item

.
The idea that the plug fuse is solely to protect the flex is a pov much pro moted, but a proper look at it shows it doesn't really add up.
Of course you can take that approach, but fitting better chosen fuses does improve safety and costs zero extra, so is the more sensible thing to do. A s the owner of many historic electrical goods & the occasional 3rd world im port I'd hate to run any of them on a 13A fuse.
As ever such ideas lead to idiocy like people being ordered to replace appr opriate fuses with higher rated ones etc.
NT
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On Friday, 23 March 2018 18:07:13 UTC, Sam wrote:

It;s the one that is used.

So what, that wasn't thw question, not putting any fuse in would protect the cable even more.

What's a universal cable ?

That wasn't the question, and you don't feed the appliance, thatr's one thing our studetns get wrong too, the appliance takes current , current doesn't get pushed into thre appliance.
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On Monday, 26 March 2018 10:41:37 UTC+1, whisky-dave wrote:

I hope you jest. But knowing you...
NT
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