Electrical cable for 9kW electric boiler

[Jack155Q4]

Hi all,

We have had fitted a 9kW electric boiler. The cable used from the double pole switch (fitted by an electrician with the RCOB) is 6mm - is 6mm suitable for this?

The cable is 25cm long and we run the boiler at under half demand.

It does not get hot just a slightly warm.

The electrician wants to come back and fit 10mm and a different termination box to accommodate it - costing £95 which is money we just don't have after the initial bill for the electrical work and and boiler install.

If we can safely avoid it we will.

Thanks,
Jack

 

[securespark]

Does the boiler use the full 9 k's all the time it's on?

Is the cable run through insulation or in a hot environment?

What size the RCBO?

 

[EFLImpudence]

The cable is 25cm long and we run the boiler at under half demand.

I shall assume you mean 25 metres.

The 6mm² cable will be suitable if installed in near ideal situation.
It must have been installed buried directly in plaster or clipped directly to walls or joists.
It must not be in conduit nor be in contact with any insulation.

Perhaps your electrician realises this and it does need uprating to 10mm².
However, if that is the case it should not have been energised until this was done.

EDIT What size is the cable from the Consumer Unit to the Double Pole Switch? Same applies to that.

 

[Jack155Q4]

Does the boiler use the full 9 k's all the time it's on?

Is the cable run through insulation or in a hot environment?

What size the RCBO?

I am not sure. It has a 10mm supply from it to the switch though.

Cable is not through insulation but is in the room with the boiler where the CU is too.

No idea is it is using the 9kW at all times - the dial is at about 45%.

To be honest we might as well just get it done, take the cost but then forget about it - it does seem never ending though!

 

[Jack155Q4]

The cable is 25cm long and we run the boiler at under half demand.

I shall assume you mean 25 metres.

The 6mm² cable will be suitable if installed in near ideal situation.
It must have been installed buried directly in plaster or clipped directly to walls or joists.
It must not be in conduit nor be in contact with any insulation.

Perhaps your electrician realises this and it does need uprating to 10mm².
However, if that is the case it should not have been energised until this was done.

Nope, I mean 25cm! The boiler is next to the switch in the same room on the same wall. Doesn't need clipping even.

 

[EFLImpudence]

Well in that case it will be alright if MCB or RCBO 40A.

Do you mean he wants £95 to replace 25cm of cable which, if needed, should have been done when fitted?

 

[Jack155Q4]

Well in that case it will be alright if MCB or RCBO 40A.

Do you mean he wants £95 to replace 25cm of cable which, if needed, should have been done when fitted?

He also wants to fit a different switch (to allow termination of the 10mm flex).

The RCBO was fitted with 10mm and the double pole - it was then left for the plumbers to commission the boiler (electrician seems uneasy about leaving the suitable cable ready for the boiler.

I am probably being scared in to doing this more than anything but when you don't know for sure who are you to believe!?!

 

[EFLImpudence]

9kW equates to 39.13A therefore it must have 40A MCB or RCBO. Does the breaker in the consumer unit which controls this circuit have B40 marked on it?

6mm² cable in the open is capable of conducting 47A.

The switch should be rated for 45A. So if all these conditions are met and installed satisfactorily then all is well.

 

[Jack155Q4]

9kW equates to 39.13A therefore it must have 40A MCB or RCBO. Does the breaker in the consumer unit which controls this circuit have B40 marked on it?

6mm² cable in the open is capable of conducting 47A.

The switch should be rated for 45A. So if all these conditions are met and installed satisfactorily then all is well.

B40 30mA is on the RCBO, switch is 45A.

The electrician tells me:

The 6mm T&E supply to the boiler from the 10mm supply will cause resistance.

This resistance will create excessive heat, this heat will create more cable resistance thus creating more heat, this in turn creates more cable resistance[vicious circle]. In a very short period of time this could create serious fault conditions.

 

[Jack155Q4]

Any thoughts on it currently being 6mm T&E and not heat resistant flex?

Thanks for all the help by the way!

 

[Stoday]

The electrician tells me:

The 6mm T&E supply to the boiler from the 10mm supply will cause resistance.

This resistance will create excessive heat, this heat will create more cable resistance thus creating more heat, this in turn creates more cable resistance[vicious circle]. In a very short period of time this could create serious fault conditions.

Wow! A thermal runaway! That's what happened at Fukushima. You house is at risk of meltdown.

The electrician is pulling your p*sser! As others in this thread have pointed out, the 6mm² cable is adequate. The cable rating takes the temperature rise of the cable and its consequent small resistance increase into account. The cable will get warm, but not hot enough to exceed its maximum permitted temperature.

 

[bernardgreen]

Tell your "electrician" that the equation for heat generation is

Watts = (voltage times voltage) divided by ohms

If the resistance increases and the voltage remains constant then the wattage reduces.

An electrician should know that.

 

[Jack155Q4]

He has great reviews on Trust a Trader but me being me I like to investigate everything online.

So, finally, T&E 6mm is also suitable and I don't need heat resistant flex?

 

[ban-all-sheds]

He has great reviews on Trust a Trader

Presumably written by people who also know little about electrical matters...

 

[ban-all-sheds]

He also wants to fit a different switch (to allow termination of the 10mm flex).

Err---

You have a 45A rated switch, which must surely therefore have terminals suitable for 10mm² conductors, and which already has a 10mm² cable on one side of it, so why does this clown think he needs to replace it?