16mm T+E for Loft Extension, (en- suit shower. )

[sparkyjohn1]

Just wondering if any sparks out there would have any obligations or concerns about a proposed installation for loft conversion,, ( see drawing for details..)..
Hear loads of conflicting theories about cable sizing for showers , some of them are borderline at best... and different installation methods etc... this is my proposed method, might seem a bit overkill.. but seems to tick every box regarding cable sizing , length of run,,, installation method ( in trunking..).. and diversity.. I'd rather be well clear of borderline to be honest...
Quite a big House.. concerns with the run to the opposite corner for feeding the shower... Didn't really fancy terminating two lots of 16mm in a shower pull cord switch to be honest,,, 10mm can be a nuisance at times,, I hope this method would satisfy every criteria..??

 

[Risteard]

sometimes there are factors where overload can become an issue even with a fixed shower load.

But that's NOT overload. That's incorrect selection and erection of the cable.

 

[davesparks]

Read back through the post i wrote beforehand.

I did and can't see what you are talking about?
If the cable rating after factors are applied is 33A and the load is 36A then it is the wrong size cable, yes the cable is overloaded but this is due to poor installation design not an overload fault.

You made the statement that a cable rated at 38A connected to a 40A MCB is wrong and would attract a C2 on an EICR, this has been challenged and now you are back-pedalling and moving the goal posts!

 

[rattlehead85]

But that's NOT overload. That's incorrect selection and erection of the cable.

And incorrect selection because...the cable cannot adequately run the load connected under the conditions it is installed which is an overload condition of the cable.

 

[rattlehead85]

I did and can't see what you are talking about?
If the cable rating after factors are applied is 33A and the load is 36A then it is the wrong size cable, yes the cable is overloaded but this is due to poor installation design not an overload fault.

You made the statement that a cable rated at 38A connected to a 40A MCB is wrong and would attract a C2 on an EICR, this has been challenged and now you are back-pedalling and moving the goal posts!

How am i back peddling. If i go to a job and find a shower cable rated at 38a which after investigating the way it is run i find that its c.c.c is further reduced and the shower pulls more than its calculated value the 40a mcb isn't capable of protecting that cable against the overload condition in existance. My calculations have proved this and i attribute a C2 as required.
If a cable is sized to carry a fixed load under the conditions it is installed and is proved as such by calcs then i attribute no code. I gave an example and said why i gave the code thats all nothing to do with back peddling mate!

 

[davesparks]

You made a statement at post #82 that "Applying a 40a mcb on a cable only capable of carrying 38A is a C2 non compliance."

 

[rattlehead85]

I then went on to explain the point i was making. Tel observed my statement and i expanded the point...i didnt back peddle at any point...if you decide i did then thats your view. Im sure in a court of law i would be given the chance to explain the point. We'd all be condemned in your eyes lol.

 

[D Skelton]

Even when the cable is packed in a bit of trunking like they normally are so much so the cable cannot dissipate the heat generated by the overload current it is carrying in the first place.....you'd ignore this???

When have you ever felt a piece of trunking hot to the touch because the cables within aren't able to dissipate the heat generated by current flow? I never have.

The fact is, a grouping factor as shown in BS7671 is only applied to cables under prolonged and steady state operation at 100% of load factor. In the rare case that you find a cable under prolonged and steady state operation in a domestic situation, the chances are it will be operating at less than 30% of it's grouped rating anyway, so can be ignored for the purposes of calculating a correction factor for grouped cables.

My point being throughout is that during the selection process, where overload protection is afforded by the nature of the equipment under operation, then you will be applying your correction factors, if any, to the design current of the circuit. Once a cable has been selected so that it's tabulated current carrying capacity is greater than that of the corrected design current, you have due diligently performed your duty as a designer. Now, if a correction factor need not apply to the design current, then it should not be factored into the equation.

A cable that has been badly selected and erected in the first place will be overloaded sure, but a cable that has been correctly selected and erected taking into account the type and nature of the load and the method of it's installation, will not suddenly become overloaded just because it is then placed in a piece of trunking with another cable or two.

 

[Gavin A]

When have you ever felt a piece of trunking hot to the touch because the cables within aren't able to dissipate the heat generated by current flow? I never have.

The fact is, a grouping factor as shown in BS7671 is only applied to cables under prolonged and steady state operation at 100% of load factor. In the rare case that you find a cable under prolonged and steady state operation in a domestic situation, the chances are it will be operating at less than 30% of it's grouped rating anyway, so can be ignored for the purposes of calculating a correction factor for grouped cables.

storage heater circuits would be the main domestic circuits to watch for I'd have thought.

 

[Engineer54]

extra C / U because of cable run,, thought might as well utilise sockets and lighting to this one,,, as it's local..... and compliant,,

And totally unnecessary!!

Why anyone would want to pull a 10mm T&E through joists etc (6mm is bad enough) when there just isn't any need, ...well i just don't know!!

 

[D Skelton]

storage heater circuits would be the main domestic circuits to watch for I'd have thought.

Definitely one to watch for, but the chances are, your storage heaters will only be on when your other circuits are unloaded, and vice versa, and even then, most of the time your storage heaters will be run individually in 2.5mm, which when on, are only operating at roughly 50% of load factor.

 

[Maddison]

There's no such thing as "pushing the boundaries" in a compliant circuit. Either a cable can carry that amount of current or it can't. I think you will find that stated values err on the conservative side too. I have seen showers fed in 2.5mm cable that have not caught fire.
I always sleep easy as I do my calcs and install to them.
Not sure where your lack of confidence in calcs stems from, but hey, if you like overspeccing that's your preference, nothing against you either.
I have never installed more than a 6mm to any cooker or hob, and only last week saw a new induction hob with a 13A plug top as supplied.

I worked with with another sparky a few years back and he had to install this range cooker. Him and the boss came to the conclusion that a 16mm was what was needed. Couple days later I show up like have you lot never heard to diversity.... F**kinghell. They ended up having to put in a 3 phase rotary isolator just to be able to terminate the cables.. I think there's going over the top and just not thinking he was a good sparky in all fairness