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

[D Skelton]

Right listen here whoever as abducted the cut throat no nonsense straight talking suffer no fools, Mr D Skelton and replaced him with this imposter, who is now being nice and cuddly and thoughtful to everyone. stop right now and give him back, we miss him!!
Good God next he will be singing, come by yah my lord!

I was in a good mood lol. To be fair, the OP says in a previous post that he's been to college and done his time. He's misguided yes, but at least he's put the effort in to be a spark and not taken the easy route. If this had been the case, the tone of my response may well have differed somewhat. He just needs a shove in the right direction and I'm sure he'll make a decent spark. We were all there once, in fact, I'm still there. The more you learn about this trade, the more you realise how much you don't know. Ok you progress up the ranks so to speak, but you will always be learning from those who know more than you.

Im sure normal service will resume in good time Glenn

 

[rattlehead85]

there's thousands of showers out there asking for trouble then. 9.5kW = 40A. 6mm ccc is 47A. what's the problem?

Agreed if it is clipped direct to joists or a wall , some installations shower cables are run in surface trunking method B which according to BS7671 states a c.c.c of 38A for 6mm t&e and thats before using a further derating factor if enclosed with other cables.
Therefore using 6mm cable under certain circumstances is incorrect and also you have an issue with MCB selection. Applying a 40a mcb on a cable only capable of carrying 38A is a C2 non compliance.

 

[telectrix]

Applying a 40a mcb on a cable only capable of carrying 38A is a C2 non compliance.

have to correct you there. shower is a fixed load, so OCPD only needed to protect against short circuit. not necessary for overload protection.

 

[davesparks]

Applying a 40a mcb on a cable only capable of carrying 38A is a C2 non compliance.

We've just done this one in this thread already!

If the MCB has been installed to provide fault protection only then it could be perfectly compliant providing the size of the CPC and Zs are OK.

 

[rattlehead85]

Applying a 40a mcb on a cable only capable of carrying 38A is a C2 non compliance.

have to correct you there. shower is a fixed load, so OCPD only needed to protect against short circuit. not necessary for overload protection.

The point i'm trying to make tel is that if the 6mm cable has been run in surface trunking with other cables which have further derated the c.c.c of that cable say to 32-33a and the shower in question pulls 36 amps the cable is therefore overloaded and cannot be protected against overload by the 40a device installed. I understand what you are saying Tel and i agree but sometimes there are factors where overload can become an issue even with a fixed shower load.

 

[D Skelton]

The point i'm trying to make tel is that if the 6mm cable has been run in surface trunking with other cables which have further derated the c.c.c of that cable

In a domestic situation, the chances of having to apply a grouping factor, regardless of the number of cables bunched together in a bit of trunking, are slim to none.

 

[rattlehead85]

In a domestic situation, the chances of having to apply a grouping factor, regardless of the number of cables bunched together in a bit of trunking, are slim to none.

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???

 

[davesparks]

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???

What overload current? we are talking about a shower supply!

And you don't just add up the number of cables in the trunking to get your grouping factor.

 

[rattlehead85]

What overload current? we are talking about a shower supply!

Read back through the post i wrote beforehand.

 

[rattlehead85]

What overload current? we are talking about a shower supply!

And you don't just add up the number of cables in the trunking to get your grouping factor.

Obviously not.

 

[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.