Cable Supplying an Induction Hob

[self builder 123]

Gents

Our electrican has installed a 6 sq.mm cable direct from the consumer unit to power our induction hob. A 40A MCB has been fitted.

The hob has 5 hot plates and is listed as having a total loading of 10.8kW.

I understand the logic behind diversity, but would like piece of mind that we could run all the hot plates ( if we wanted / needed to ) without worrying about tripping the MCB.

The cable is run in a timber frame through rockwool insulation.

I have been reading through the 2001 Amendment 1 - Table 4D5A - Installation Method 15.

Leaving aside the other factors that derate the current carrying capacity of the cable, the maximum current which a 6 sq.mm can support is 35A.

At 240Vac drawing 35A the maximum power is 8.4kW.

So based on this we will never be able to run all 5 hot plates at the same time ? Am I correct ?

I just wish that he had installed a 10 sq.mm cable. Was it a mistake not to as the 10 sq.mm can handle 47A ?

Further more I am concerned about the 40A MCB ?

Any help / advice would be much appreciated.

 

[jjrenolds]

Hi self builder, you have kinda answered your own question. Not only is your power consumption too high for the cable if everything is on, but you are protecting it with a mcb that is rated higher than the cables current carrying capacity.

Alway go with caution, although its been years since i did my 16th edition so i can't remember the current tables exactly, i always work on the basis that 6mm cable carries 32 amps. Remember this is your house you are talking about, in a worst case scenario you could burn your house down if a fault developed or the cable simply gets hot. I know personally i wouldn't risk it.

 

[Chri5]

The guy should have used 10mm.

It's a reinstall job using 10mm cable.

Although you might argue that the load to the hob is never likely to be 100%

Even if all x 5 plates are running simultaneously, whats the likely-hood of all of them being turned up to the highest setting continuously.

An interim might be to consider down sizing the MCB, then if demand is stretched beyond capacity you have the safety net of the MCB switching before any other events occur.

 

[self builder 123]

Hi

Thanks for your replies.

The maximum loading for the hob is listed as 10.8kW.

Looking at the individual hot plates specs they are as follows :

Front Left 1.8 to 2.5kW ( with boost function )
Back Left 1.8 to 2.5kW ( with boost function )
Centre 1.8 to 4.4kW ( with boost function )
Front Right 2.2 to 3.3kW ( with boost function )
Back Right 1.4 to 1.8kW ( with boost function )

So if you do not use the boost functions then I calculate the total load as being 9kW when all the 5 rings are on maximum.

So if you had all 5 rings on at 50% then the total loading would be 4.5kW ?

Is a 9kW draw a lot of power for a hob ?

Is it fair to say that you would never have all 5 rings on at the maximum setting ( you may have guessed that I am not a cook !!!! )


My plan would be to replace the 40A MCB with a 32A MCB.

At 240V the hob could safely draw ( 240 x 32 ) 7680W - based on Installation Method 15 in Table 4D5A the 6 sq.mm cable can provide 35A.

The 7680W ( 7.7kW ) still seems like a lot of power ?

I am concerned that the 6 sq.mm cable will restrict the full use of the 5 ring hob making it more of an ornament !!!!

Your feedback would be much appreciated.

Cheers

 

[Steve]

Even on the max setting, the hobs will cycle on and off on thermostats. So full load will rarely be drawn.

As long as the MCB/fuse protects the cable, then there wont be any danger.

 

[Adam_151]

I'd downsize the MCB, the rule for most cases is you have

three current figures:

Ib - design current of the circuit which may include diversity
In - nominal current of the fuse/ breaker
Iz - tabulated current for the cable for the method its installed (after any extra factors have been applied)

In must be greater than or equal to Ib, and Iz must be greater or equal to In.

(there are situations which allow In to be greater than Iz as long as Iz is still greater than Ib, but they are special cases and other things have to be calculated and are not relevant here)