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Old 10th Jan 2019, 7:53 pm   #1
gramophone1
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Default 13 amp fuse for 1.5mm mains flex?

I have wired up a trailing socket and plug with heavy duty 1.5mm rubber cable.
Can I use a 13 amp fuse with this cable size ?
The cable is specified upto 15 amps, but I suspect a 13 amp fuse will exceed its max load.

The length of cable is approx 2.5 metres

Any help appreciated
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Old 10th Jan 2019, 7:59 pm   #2
McMurdo
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Default Re: 13 amp fuse for 1.5mm mains cable

Some appliances designed for a 13A fuse only use 1.25mm cable. So 1.5 will be fine for a 3-bar fire etc. The main proviso for extension cables is that the length does not cause the earth loop impedance to go out of spec.
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Old 10th Jan 2019, 8:12 pm   #3
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Default Re: 13 amp fuse for 1.5mm mains cable

As McMurdo says - it'll probably be OK. Admittedly I wouldn't personally want to use it in a situation where you're continually pulling the full 13A load day-in-day-out, but that would be more of a concern over the historic issues with 13A plugs/sockets and the fuse-clips losing their temper through heating.

Fit a 10A fuse in the plug if you're paranoid. Lots of the cheap bargains/DIY-shop multi-way trailing sockets come with a 10A fuse in the plug.
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Old 10th Jan 2019, 8:25 pm   #4
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Default Re: 13 amp fuse for 1.5mm mains cable

You're correct. I just looked at the cable on an old brennenstuhl mains extension I have which is 1.25mm.

I am using it for my hi-fi, so no heavy loads. Meant to add it is a six socket extension.

Last edited by gramophone1; 10th Jan 2019 at 8:33 pm.
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Old 10th Jan 2019, 9:04 pm   #5
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Default Re: 13 amp fuse for 1.5mm mains cable

A 13 amp fuse should be fine.
1.5mm flex insulated with rubber or the commonly used plastics is rated at 15 amps or 16 amps.

A 13 amp fuse will pass more than 13 amps for some hours, but this fact has already been allowed for in determining cable ratings.

As others post, many ready made extension leads and heavy loading appliances use only 1.25mm flex. That IMHO is cutting things a bit fine, but seems OK in practice.
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Old 10th Jan 2019, 9:18 pm   #6
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Default Re: 13 amp fuse for 1.5mm mains cable

Personally, I am not in favour of 13A trailing sockets being fitted with 10A fuses. There is a greater likelihood of long, low level overload of the 10A fuse if a nominal 3kW of load is connected by mistake, which can lead to overheating if the fuse clips are less than perfect, before it actually blows. I would always assemble or purchase fully-rated multiway extensions and fuse them at 13A.

1.5mm is ideal for heavy duty 13A cables and is specified for most industrial applications. It is in any case the largest cable that 13A plugs are designed to accommodate. The constraint due to loop impedance can optionally be waived if a 30mA RCD is fitted at the supply end. That is how long extension cables such as 50M 1.25mm drums can be considered safe.
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Old 10th Jan 2019, 9:18 pm   #7
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Default Re: 13 amp fuse for 1.5mm mains cable

It will be fine, certainly under 3 metres. In fact, you could probably get away with 1mm cable over that run (unless you plan to use this extension lead in a daisy-chain).

The bulk resistivity of copper is 16.8e-9Ωm (that's ohms times metres; when we divide that by metres**2, we get ohms per metre). With an area of 1.5e-6m, that gives us (16.8 / 1.5)e(-9+6) = 1.12e-3Ω/m. So over the length of the cable, there will be 2.8mΩ in each conductor, for a total of 5.6mΩ. With 13A flowing through it, the voltage developed across the cable will be 0.0728V, giving a power dissipation of less than a watt (0.9464W, to be precise).

If we used 1.0mm cable, we would get 1.68e-3Ω/m, for a total of 8.4mΩ in the two conductors, dissipating 1.419W at 13A. That's still not much, considering the total surface area over which it is distributed (compare it to the 3W resistor any good engineer would choose to dissipate that much in a hypothetical circuit).
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Old 10th Jan 2019, 9:23 pm   #8
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Default Re: 13 amp fuse for 1.5mm mains cable

However, 1.0mm cable of normal types for this application, such as BS6500 flexible, has a widely recognised and accepted rating of 10A. Therefore, for the minimal cost difference, I would recommend at least 1.25mm so that the extension can be fully rated to 13A and made suitable for the purposes to which it could reasonably be put.
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Old 10th Jan 2019, 9:52 pm   #9
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Default Re: 13 amp fuse for 1.5mm mains cable

Thank you for all the replies, and advice.

I will stick with 13 amp fuse.
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Old 10th Jan 2019, 9:53 pm   #10
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Default Re: 13 amp fuse for 1.5mm mains cable

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Originally Posted by Lucien Nunes View Post
I would recommend at least 1.25mm so that the extension can be fully rated to 13A and made suitable for the purposes to which it could reasonably be put
Exactly, if I saw a 13A socket I would expect to be able to draw 13A from it.

The fuse in a 13A plug is to protect the cable, no more, no less.
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Old 10th Jan 2019, 10:24 pm   #11
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Default Re: 13 amp fuse for 1.5mm mains flex?

I am using Permaplugs. They are heavy duty and robust. You can pick them up cheaply. Maybe overkill for a plug. But I think they are excellent, and fit firmly in the socket. I think they are made by Masterplug.

I am really impressed with the fuse holder. It holds the fuse really tightly. If your looking for a good quality mains plug. Try these . . . .
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Old 11th Jan 2019, 5:10 pm   #12
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Default Re: 13 amp fuse for 1.5mm mains flex?

Can I throw a spanner in here?

If it is only for the hifi I would suggest you fit a 3 Amp or may be 5 Amp fuse. A 3A fuse will give you 700/750 Watts and a 5A fuse will do 1100/1250 Watts depending on the voltage of your mains. (Then put a piece of red or black tape on the plug-top to remind you of the value fitted, so you do not try it with the electric fire too...)

I doubt if you add all the hifi together that it draws any thing like a kilo Watt?

Alan
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Old 11th Jan 2019, 5:26 pm   #13
Lucien Nunes
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Default Re: 13 amp fuse for 1.5mm mains flex?

What would be your motivation for fusing it down? The HiFi components will all be protected by their own fuses. The only thing that fuse is protecting is the extension cable itself, which is tough rubber and most unlikely to be damaged.
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Old 11th Jan 2019, 6:27 pm   #14
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Default Re: 13 amp fuse for 1.5mm mains flex?

It does mean that the fuse will blow faster in the event of a marginal overcurrent.
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Old 11th Jan 2019, 9:10 pm   #15
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Default Re: 13 amp fuse for 1.5mm mains flex?

I believe using a higher rated fuse lowers the impedance. Rubber is also a good dielectric. Please correct me if I am wrong. Rubber cable is very flexible. It is ideal for tight spaces behind the hifi.

This is the type of rubber cable used- Rubber Flexible Cable | H07RN-F Cable

As I mentioned, I have another mains extension with only 1.25mm cable at 13 amps, and never had a problem with it at all.

Last edited by gramophone1; 11th Jan 2019 at 9:31 pm.
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Old 11th Jan 2019, 10:45 pm   #16
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
Originally Posted by Lucien Nunes View Post
What would be your motivation for fusing it down? The HiFi components will all be protected by their own fuses. The only thing that fuse is protecting is the extension cable itself, which is tough rubber and most unlikely to be damaged.
Purely for one extra level of safety.
I'm generalising here and not aiming at the OP:
First I'm not overly keen on multiway blocks, though I use them within their limits.
Second we are all presuming that hifi and it's various plugs have the correctly rated fuses fitted. They may not be... I wonder how many have a 13 Amp fuse installed?

So 'down rating' the fuse to match the load seems like a simple precaution. Then if you do inadvertently overload the multiway or develop a fault in a lead you are better protected.

If you are sure you have the correct fuses installed in all the equipment plug tops, you check them for poor connections from time to time and so on, then a 13 Amp fuse will be fine, as you say, ''the only thing that fuse is protecting is the extension cable itself''.

Actually I cannot remember having a fuse blow in a plug for years, law of sod, but have seen a few overheated from loose or tinned connections...

gramophone, modern rubber cable is excellent. It rolls up nicely when not in use and is very tough. I use it often.
Not sure the fuse resistance is going to make much of a difference though.

Just my thoughts, I will go back under my (H&S) rock now

Alan
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Old 14th Jan 2019, 8:56 pm   #17
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Default Re: 13 amp fuse for 1.5mm mains flex?

The original question suggests a certain lack of understanding by suggesting that using a thicker cable can somehow overload the 13A fuse. The cable itself has no bearing on the load applied to the fuse, only the equipment connected via that cable. In general, cost and physical issues aside, thicker cable is better than thinner cable, end of.

Buried behind furniture, under carpets etc it will also run cooler, which is no bad thing either.

The safe current rating for any cable depends more on the insulation temperature rating than anything else- a silicone rubber insulated cable may well have a higher rating than a PVC insulated one for the same copper CSA. How and where it's routed also has a significant effect- look at the ratings for reeled vs unreeled extension leads.
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Old 15th Jan 2019, 12:37 am   #18
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Default Re: 13 amp fuse for 1.5mm mains flex?

About 10 years ago a bloke asked me to do a small job with a power tool and laid on an extension cable that reeled off just far enough to reach.
The remaining three quarters of the cable was melted onto the reel like cheese on toast.
It was toasted well and truly.
It had been switched off just before going short circuit and allowed to cool.
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Old 30th Jun 2019, 3:46 pm   #19
gramophone1
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Default Re: 13 amp fuse for 1.5mm mains flex?

Just an update. The mains extension is perfectly fine, and still running off a 13 amp fuse.

I .5mm is the maximum cable size, that both the plug, and the trailing socket will accommodate. I would have preferred using 2.5mm. I am not aware of any mains plugs that can accept a cable of that gauge. For hifi use, it has caused me no problems at all.
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Old 30th Jun 2019, 4:14 pm   #20
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Default Re: 13 amp fuse for 1.5mm mains flex?

Quote:
Originally Posted by snowman_al View Post
So 'down rating' the fuse to match the load seems like a simple precaution. Then if you do inadvertently overload the multiway or develop a fault in a lead you are better protected.

If you are sure you have the correct fuses installed in all the equipment plug tops,
Each item of hi fi will have a correct internal fuse. Each plug will have a fuse. How many more levels of safety do you want?

Fuses go in to plug bottoms, not their tops.
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