Fuse rating and cable length of 4 way socket? 2

[Cronus94]

Hi everyone, this is my first post on this site and hopefully you guys/girls can help me out

Ok so at work my boss (head engineer) and some electrical engineers are telling me different things.

So I have a 4 way desktop socket, each socket on the 4 way is fused individual all with a 5amp fuse. The cable is 1.5mm diameter and fused at 13amp in the plug (cable is standard 3 core).

My question is this, firstly should the individual fuses be lower ie 3.15amp (as 5amp x 4 sockets is great than 13amp fuse in plug)

My second question is if I was to change the cable to 2 meters long instead of 1.5meters would the cable diameter need to be 2mm (my boss says 2 meters long will need 2mm thick cable) but an electrical engineer says that 1.5mm thick cable will still be ok at 2m long cable

 

[zeusfaber]

Let's take these one at a time.

Q: What is the purpose of the fuse at this end of the system? A: Primarily to protect the circuit downstream of the fuse from overcurrent due to faults in the appliance and its flex; not to protect the upstream circuit and its fuses, nor to protect anything from marginal overloads.

On that basis, choose the fuses to be big enough for the expected individual loads, but no bigger than the rating of the cable downstream of the fuse. If you're worried that the loads are starting to exceed the rating of the circuit that's feeding the four-way, the solution is to plug fewer loads into it, rather than to reduce individual fuse ratings.

In practice, you will struggle to find a 3.15A (or even 3.25A) fuse with the appropriate certifications for a mains application. Since there's only a limited range of cable ratings available, there's a correspondingly limited range of fuses and 3.15A isn't one of them.

1.5 mm² is about as large a conductor as you can physically fit into the terminals of a BS 1363 plug and you'll regularly see it used in 13A extension leads very much longer than 2m. It's already a size up from the 1.25 mm² commonly used for 13A loads in the UK. As previously mentioned, cable comes in a limited range of sizes - and 2 mm² is a new one on me.

What actually happens if you use "too long" a stretch of cable at full rating? Provided the cable isn't tightly coiled, you don't have an overloading problem (the extra heat is dissipated over the increased length of the cable) - but you might start to lose voltage at the load end. Before you get too bothered about that, think a bit about how your actual supply voltage compares with the voltage range demanded by your loads. There's usually lots of margin (especially if you're in the UK, where power generally arrives at 10V over the nominal figure).

I used to have a boss like yours too. I learnt a lot from him - and have spent much of the last thirty years trying to put those lessons to good use by not behaving the same way.

A.

 

[ScottyUK]

Wow, some real strange advice from your workmates.

The flex needs to be capable of carrying the current which the 13A fuse will allow to pass. 1.5mm² 3-core PVC-insulated flex is rated for about 16A so provided that it isn't absurdly long then the 1.5mm² flex is OK. There's no need to individually fuse the outlets unless it's a real cheap and nasty p.o.s adaptor which can't handle 13A on any one outlet because the plugs for the individual loads (should) incorporate a suitably-sized fuselink selected to suit the load.

Oh, and ask your boss where to buy 2mm thick cable if that's what he thinks you need. Standard sizes are 0.5, 0.75, 1.0, 1.25, 1.5, 2.5mm².