Cable size help

[hodgey1972]

Gents i've been to attend a melted 3ph 5pin socket and isolator at a commercial kitchen installed recently.
Cable is 15m 6mm 5c yy buried in a solid wall for a 16kw cooker.
If im honest im not getting the 3rd harmonic/neutral, and im getting confused with general adiabatic equations.
Can anyone help me work it out with the full equation?
Thanks in advance

 

[Darkwood]

Please show your equations then we will express where you have made an error (If you have), Just asking us to do the working out tends to lead to abuse of our good nature by some so appreciate that you show you are doing your sums even if they may be mistakes to correct

 

[ackbarthestar]

Well Hodgey,
1/ Calculate what you would expect the line current to be and then assess its maximum loading. What is it?
2/ Why would you expect to find harmonics from a basic resistive load in a kitchen?
3/ The cpc is the same size as both the neutral and and phase conductors. So what value did you get for the min. cpc csa for compliance?
4/ YY cable is essentially an industrial control cable. What are the thermal characteristics when buried in... whatever ?
5/ Are there any cpc/bonding connections to the outside world that might have allowed a larger than expected current to flow through the cpc/neutral under 'normal conditions?

I suggest you analyze this step by step so that you can find a solution

 

[hodgey1972]

ok, best i can find is re cable type is table 4f1a flexible cables non armoured inst method type a ,buried in wall
6mm =34amps
34 x 0.7298 derating factor =24a
volt drop neg 0.29?0.015x1.44x21x0.64
what else am i missing
and cable calc app indicating 4mm

 

[Richard Burns]

4F1A is probably your best table to work from and has a current carrying capacity of 34 Amps. The volt drop would be 6.7 mV/A/m
If the cable is buried in a solid wall then this would be reference method C and not require derating.
If it is in an insulating wall then that would be method A.
You should be applying diversity to the cooker circuit, depending on how it is operating, this would then give you your design current to size your (breaker and) cable.
The cause of overheating is most likely to be a loose connection rather than overheating the cable through normal current use.

 

[hodgey1972]

yes thats what i thought regarding loose connection but the connections are the push type, and l1 and n show signs of damage

 

[Darkwood]

4F1A is probably your best table to work from and has a current carrying capacity of 34 Amps. The volt drop would be 6.7 mV/A/m
If the cable is buried in a solid wall then this would be reference method C and not require derating.
If it is in an insulating wall then that would be method A.
You should be applying diversity to the cooker circuit, depending on how it is operating, this would then give you your design current to size your (breaker and) cable.
The cause of overheating is most likely to be a loose connection rather than overheating the cable through normal current use.

Diversity is for the installation demand and not to be used in final circuit cable calcs, any cable and circuit breaker should be sized in accordance to the stated full load current of the cooker with applicable cable calculations for install methods.

I must admit Richard.. its rare I found reason to pull you on advice :whatchutalkingabout, I expect it just reads wrong but it suggests applying diversity for a cooker circuit is done to find its cable size and mcb rating?

 

[Richard Burns]

This is for a cooker, I do not know what sort of cooker so I cannot specify but it is likely it will not all run at full power simultaneously.
You are stating that a 12kW single phase cooker could never be put on a 32A circuit breaker as the design current is 52A?
The design current is calculated to determine the expected maximum load drawn by the appliance, where there are several mutually incompatible elements within the appliance then the design current would be lower than the maximum current indicated by the overall power rating.

 

[Richard Burns]

yes thats what i thought regarding loose connection but the connections are the push type, and l1 and n show signs of damage

If the cables show no signs of damage then what melted, was this just because they had a heater under the socket?

 

[Darkwood]

This is for a cooker, I do not know what sort of cooker so I cannot specify but it is likely it will not all run at full power simultaneously.
You are stating that a 12kW single phase cooker could never be put on a 32A circuit breaker as the design current is 52A?
The design current is calculated to determine the expected maximum load drawn by the appliance, where there are several mutually incompatible elements within the appliance then the design current would be lower than the maximum current indicated by the overall power rating.

If you can put all the loads on together and they demand 12kw in your example then yes that is what I'm stating, regardless of the likely hood of it happening and even the Rheostat control once they all reach temp' you have to ensure your cable is rated for the full load current of the cooker. Diversity is used to gauge the average demand of multiple circuits to assess the KVA demand on the supply as you will not have several circuit running at their rated circuit value ever; unless deliberately trying to.

You've lost me on your thinking here, if an oven had mechanical isolation to limit load (its impossible to choose both options of say top and bottom oven because of switch options) this will have already reflected on the rating plate.

Where it is possible to demand 12kw than you design your cable and front end device to that rating - what do we do at xmas when you got an house full for dinner and every ring and oven is going full tilt - you'll probably trip said 32amp mcb and the cook ends up in tears... :uhoh2:

You suggestion is not unsafe but it leaves open possible nuisance tripping issues on the occasion when the cooker is used heavily.

Diversity on final circuits has to be given a lot of respect, say the oven is fully loaded you can argue that the rheostats will stop a long duration of a large overload will occur but you also have to take into account of accumulative heating of the cable where you have regular small overloads of a cable where the mcb doesn't meet its time curve to ever trip- this regular overloads as the rheostats click on/off can build up heat faster than dissapation occurs and damage the cable ... I do alot of industrial heating process with Elements which are subject to stat control and have come across many situes where cables have overheated even though the circuit protection would seemingly prevent this happening... maybe an extreme example but its the same but on a smaller scale.

 

[Richard Burns]

As you know I do not tend to push too hard however there are many domestic cookers that are installed in just the way I describe where the cable is designed for a current similar to, but not necessarily greater than, the load as designated by the power rating and the circuit breaker is rated significantly lower than the power rating. This is proven to work in the long term and without problems. I do not necessarily say it is the best design but it will work.

In designing a standard circuit for the vast majority of single appliances and equipment your method is certainly the correct one to choose.

 

[Darkwood]

Its the way I was taught back in the days, now I'll stand corrected if they are now teaching to apply diversity at the final circuit stage but find that too dependent on situe to give a blanket rule and again enlighten me if its the case but if they are not then its not good advice to give as it would contradict how they were taught.
I sympathise and I understand why it rarely becomes an issue because as you well know 95+% of the time the diversity will be correct but to design the circuit correctly you do not apply diversity to your final circuit and it should be the case you don't get a phone call on xmas day from a customer saying the mcb has tripped a few times and they have family round for dinner... I know which design method I would choose....

 

[Richard Burns]

Just my understanding of the real world, I too could be incorrect, I can certainly follow your approach and understand it and realise the level of safety and lack of inconvenience it provides.
I can also see that diversity will generally never be a problem, however I am reminded of my frustration and annoyance on seeing something that has been designed badly, even dangerously, and yet works perfectly (until there is a problem).
The main problem is that most circuits are designed to be very safe and problems do not occur, which leads people to think they are always safe, this is not the case; as this forum demonstrates admirably!