16mm tails with 100A fuse ???

[Wilko]

Carrying that thought ... 16mm cable has 2.8mV voltage drop per A per m at 70C. So if 16mm tails were running the full 100A of the service fuse rating, they would have a voltage drop of 0.28V in each metre of the run. Since P=V.I that's 28W of heat to be radiated per metre of tails at 100A at 70C. Which isn't a number that seems a big ask (?). Of course this current level should not be allowed as it exceeds the BS7671 current rating for 16mm, just saying.

 

[soms]

When at college today doing the Periodic course I questioned the tutor on this. He said you were correct

As a Trainee I find this thread both very interesting and confusing. I think the more I read here and research things the more things become grey areas. I could no doubt delve into the BYB and be left not much the wiser as many things are open to some level of interpretation. This thread has also brought up more than one topic of technical debate.

The matter of not being able to consider a cable being clipped direct because it enters an enclosure I cannot find words for. As someone learning the trade, all I can think is how terribly confusing it would be in this was the 'correct' thing to do and where would you draw the line? As mentioned, would this apply to a cable entering a patress or junction box, where would it end? Then what alternative rating should you select?
Yes, CCC must be determined on the worst case factor but I had never seen or heard of this specific technicality before.
Next term I will be asking my tutors for their opinion on this point.

I must clarify for myself also the matter of the DNO fuse rating and the absence of providing protection on the customers side of the installation and how the tails should be sized. IIRC the former is mentioned in the BYB.
So does this mean the purpose of the service head fuse to provide some form of overload protection for the service cable or fault protection? What would there be to provide fault protection for?

 

[Wilko]

So does this mean the purpose of the service head fuse to provide some form of overload protection for the service cable or fault protection? What would there be to provide fault protection for?

Yes, the DNO's fuse is for protection of the DNO's equipment.

 

[Phil Thompson]

@soms you obviously need to read the byb a bit more if you are only seeing confusion coming here.

The 16th Edition was the same way, lots of grey areas and areas open to individual interpretation.

Now let's play devils advocate here,

If you actually consider that your tails are say 80cm long and you have 20cm inside the CCU, you have enclosed 25% of them. So if you enclosed 25% of a run of 10mm2 T&E and the rest was clipped direct, would you factor that into your cable rating? Or would you say it's only 25% so I won't factor it in?

 

[westward10]

Hello Soms it is good to think outside the box and get the old grey matter going as opposed to following the sheep blindly. See #54 for further confusion.

 

[soms]

@soms you obviously need to read the byb a bit more if you are only seeing confusion coming here.

The 16th Edition was the same way, lots of grey areas and areas open to individual interpretation.

Now let's play devils advocate here,

If you actually consider that your tails are say 80cm long and you have 20cm inside the CCU, you have enclosed 25% of them. So if you enclosed 25% of a run of 10mm2 T&E and the rest was clipped direct, would you factor that into your cable rating? Or would you say it's only 25% so I won't factor it in?

Yes, you are right, I should read it a bit more!

Hello Soms it is good to think outside the box and get the old grey matter going as opposed to following the sheep blindly. See #54 for further confusion.

Ah yes, reading #54 and looking at the tables got the grey matter working!

 

[marconi]

I realise this string of posts is about meter tails but it jogged my memory about the problem of 'thermal runaway' in buried cables. Here is a short interesting piece:

http://manuals.decagon.com/Application Notes/AN_Thermal Runaway.pdf

No doubt if thermal runaway is not considered there are/may be instances where this does/could happen above ground in other installations and installation methods.

To help you ponder remember:

For a steady state temperature Ohmic heating P = Thermal Heat flow Q

1. Ohmic heating P = Isquared R

2. Thermal heat flow Q = Tconductor-TAmbient/(Rt) were Rt is thermal resistance of insulation) in the units of degrees C/Watts and Q in Watts

The thermal resistance is series combination each of the layers of insulation.

For more detail look at the Neher-McGrath calculation of the ampacity of a condutor:

Understanding the Neher-McGrath Calculation and the Ampacity of Conductors - http://www.electrician2.com/ampacity.htm

Is a circular cross section the best shape for a conductor carrying a high current? If not, what might be better?

 

[Pete999]

I realise this string of posts is about meter tails but it jogged my memory about the problem of 'thermal runaway' in buried cables. Here is a short interesting piece:

http://manuals.decagon.com/Application Notes/AN_Thermal Runaway.pdf

No doubt if thermal runaway is not considered there are/may be instances where this does/could happen above ground in other installations and installation methods.

To help you ponder remember:

For a steady state temperature Ohmic heating P = Thermal Heat flow Q

1. Ohmic heating P = Isquared R

2. Thermal heat flow Q = Tconductor-TAmbient/(Rt) were Rt is thermal resistance of insulation) in the units of degrees C/Watts and Q in Watts

The thermal resistance is series combination each of the layers of insulation.

For more detail look at the Neher-McGrath calculation of the ampacity of a condutor:

Understanding the Neher-McGrath Calculation and the Ampacity of Conductors - http://www.electrician2.com/ampacity.htm

Is a circular cross section the best shape for a conductor carrying a high current? If not, what might be better?

An interesting and informative read Marconi, thanks.