Electrical cable for 9kW electric boiler

[JohnW2]

Looks like a referee may be needed ...

[Err... What he [bernardgreen] said was "If the resistance increases and the voltage remains constant then the wattage reduces."
Ahem... anybody who can read should know that.

Bernard did indeed say that, but he really shouldn't have done - because in the situation being discussed, the relevant voltage (that across the length of cable in question) would not remain constant.

If the resistance increases and the current remains constant then the wattage increases.

But the current doesn't remain constant does it. Anybody with GCSE physics should know that.

Well, what Stoday said was far closer to the situation under discussion than what Bernard had written. As Stoday said, the current through the entire circuit (including the bit of cable of interest) would be almost entirely dictated by factors other than the impedance of the bit of cable in question. Hence, in practical terms, the current through (not voltage across) the length of cable in question would be essentially constant. That being the case, increasing the impedance of that bit of cable would result in an increase of voltage (not unchanged voltage) across the bit of cable and hence an increase, not decrease, in the the power disipated in that bit of cable.

So I reckon that, on this occasion, Stoday wins, by a relatively large head, and even you are only second

Kind Regards, John.

 

[bernardgreen]

Bernard did indeed say that, but he really shouldn't have done - because in the situation being discussed, the relevant voltage (that across the length of cable in question) would not remain constant.

I was correcting the "electricians" false statement.

In the situation being discussed the equation is complex as its is a constant voltage ( incoming mains ) connected across a number of resistors in series. These are the 10mm cable (R10) , the 6 mm cable (R6) and the element in the shower (Rs ) and altering the resistance of any one of these resistors will affect the current flowing and therefore the voltages across all the other resistors in the circuit. As Rs tends to be the highest resistance it has most but not total control of the current.

 

[ban-all-sheds]

That'll teach me to pay proper attention to context.

Sorry, Stoday.

 

[JohnW2]

I was correcting the "electricians" false statement.... In the situation being discussed the equation is complex as its is a constant voltage ( incoming mains ) connected across a number of resistors in series. These are the 10mm cable (R10) , the 6 mm cable (R6) and the element in the shower (Rs ) and altering the resistance of any one of these resistors will affect the current flowing and therefore the voltages across all the other resistors in the circuit. As Rs tends to be the highest resistance it has most but not total control of the current.

Needless to say, I agree completely with all that. The reason I said that you 'really shouldn't have talked about constant voltage' was that, in the context of the thread, you were giving the impression (although, I admit, not actually saying it) that if one increased your R6 or R10, the effect would be to decrease the power dissipated in the corresponding cable - whereas you know as well as I do that, in practice, the opposite would be the case (i.e. the power dissipated in the cable would increase) - since Rs>>R6 (or R10), and hence the voltage drop across the cable in question would increase whilst the current would change very little.

Kind Regards, John.

 

[mtommy]

I'm sorry to dig up an old thread but I could really do with some advice regarding these electric boilers

Our amptec C900 9kw boiler blew up in spectacular fashion just before xmas - In fact so badly we have had to order a new unit

The main incoming wiring caught fire and burnt out the first 12" of cable at the boiler end and also destroyed the PCB

Here is my dilemma and where I need some help if anyone can advise

The supply cable is a 6mm cable all the way from the CU
The run is only about 6 - 7 meters in total from CU to boiler and goes through a 3mm double pole switch
The run goes through a plaster board suspended ceiling and then enters the room where the boiler is through a wooden stud wall which is covered in plaster board too
As far as I am aware there is no insulation around the wire or trunking - however the room where the boiler is situated does become quite warm

This installation has been in now for 7 years without any known faults that we know of

The manufacturer has advised me that a 9kw boiler should have a 10mm cable from the CU to the boiler and also have heat resistant cable at the boiler end

I have the original electrician coming next week to examine the issue but he has already told me on the phone that 6mm cable is sufficient for our installation

We are obviously very concerned about this so any advice would be very much appreciated - The pics of the damage are below with the main cable already removed - The bare burnt cable is the live wire

Thanks Mark

 

[EFLImpudence]

6mm² cable is adequate as long as it runs through NO thermal insulation.

The manual -
http://www.heatraesadia.com/docs/Amptec_-_Issue_9.pdf
quotes 9kW @ 240V which is 37.5A.
although the manual states that this should have a 50A MCB which is confusing as you cannot have a 50A MCB with 6mm².

What sized MCB do you have on the circuit ay present?

40A MCB would be sufficient although the manufacturer's instructions should be followed.
The only way you could do that is by installing 10mm² cable.

 

[bernardgreen]

The condition of the Live terminal screw on the PCB connector is the clue to the cause of the over heating.

The Live connection has become loose and/or corroded and the resultant poor joint has become hot.

 

[mtommy]

Thanks for the reply

The MCB is a crabtree B40 - 61/B40 but I am unsure what this means?

The 6mm cable as far as I know does not go through any thermal insulation - However the room that its does travel through easily gets to 26 degrees which must heat the void in the wall and ceiling

The cable also goes around the output pipe of the boiler which runs at 65 - 70 degrees

Would any of this make any difference?

 

[EFLImpudence]

Apart from being near the hot pipe, ...

I would think every thing is quite acceptable as the damage is limited to the boiler area.

 

[mtommy]

The condition of the Live terminal screw on the PCB connector is the clue to the cause of the over heating.

The Live connection has become loose and/or corroded and the resultant poor joint has become hot.

Yes when I saw the damage I also believed it to be a poor connection but I cannot understand how this happened

When we decorated about 18 months ago I checked the connections (visibly) and there were no signs of any deterioration at all

In fact the heating system has worked with no issues for many years

 

[mtommy]

Apart from being near the hot pipe, ...

I would think every thing is quite acceptable as the damage is limited to the boiler area.

Thanks for the advice - I will see what the electrician says about the damage when he visits next week. Its always nice to have a second opinion when things go wrong

It makes me wonder why the manufacturer made such an issue regarding the cable size when I spoke to them - Its not like the unit is still under warranty