Induction Hob Cabling

[PeterW]

I’m installing an induction hob that has a max capacity/current of 7.4kw / 32A on the manufacturers information. Sparky wants to run a 40A RCBO to start with and from memory it’s run in 6mm T&E from the board (need to check..!) It’s a mix of run in the open web joists,  chased into walls, and run in a stud wall around the back of cabinets and around 12m in total. 

 

The cable terminates behind the stud where the hob is installed in a 40A isolator switch. From there it is less than a metre of flex to the hob. I’ve assumed that 4mm butyl flex would be fine here as it won’t get hot (hob base is max 40c) and it’s a very short run - but the 6-4 transition is niggling even though I know it should be fine ..!

 

6mm flex seems to be a bit like rocking horse poo so before I go searching for the holy grail, I’d just like to confirm the thinking ..!

 

 

 

 

 

[Jeremy Harris]

4mm² butyl should be fine, it's rated at 35A in free air and it's reasonable to assume some diversity on that 7.4 kW max that the hob could draw. IMHO. 

 

The one potential gotcha that slightly concerns me with our installation is that our supply voltage is always over 240 VAC, and with the PV generating on a nice sunny day we're often sat with the inverter limiting output because it's hit the 253 VAC limit.  It seems our inverter soft limits and just reduces output when it hits 253 VAC, rather than shut down, which is a nice feature, but it does mean that the house is often operating with a supply that's around 250 VAC in nice weather.  It probably doesn't effect a load like an induction hob, which almost certainly controls the true power to the coils, anyway, but it does impact on simple loads, like the resistive heater in the Sunamp, which runs at around over 8% above it's nominal power rating, and consequently draws over 8% more current than it's nominal current rating.

 

I'm still trying to get the DNO to drop the tap on the local transformer, as even in mid-winter we never see less than about 240 VAC on the supply.  I'm sure they could knock the supply voltage down by at least one 2.5% tap step and still stay well within tolerance.  I'm pretty sure they could drop two steps and still stay within tolerance, as we're almost at the end of the cable (there's only one house beyond us) and two steps would only drop the mid-winter, max load, voltage down to about 228 VAC, still well inside the 216.2 VAC to 253 VAC allowable tolerance band, yet would allow us to export more in summer and reduce the max voltage we see by a few volts.

[Onoff]

My 7.3KW induction is wired in 4mm butyl (from TLC).

 

https://www.tlc-direct.co.uk/Products/CA4TQ3slash50.html

 

It's on the original hob supply that's 6mm² up to the ccu. The hob instrictions said to wire in "3x2.5" which I wasn't happy with guess they were applying diversity. The 4mm was a pig to get in though, Actually fed from a 30A C50. A 32 would have been belt and braces but hey, it's a C50 - they never trip anyway!