Conductor size required for long run to 9.1kw kitchen hob.

[Pdaz]

I'm extending my kitchen and plan to install a large five zone ceramic hob. It is rated at 9.1kw.

My consumer unit is located at the other side of the house so it's a long run.

I've used the diversity rule of thumb to calculate the current draw and I plan to use 2 core and earth from the breaker to a UK style (MK) cooker hob connection switch rated for 50amp.

I'd like to confirm the size of conductor required for a run of approx 15m between the switch and the breaker.

Is 10mm2 adequate for this installation ? Or do I need to use 16mm2 ?

This would only be for the hob. I also have an oven rated at 3.5kw which I plan to run off a seperate switch/breaker.m

My calc for the hob

9.1 kw = 39.56 amps

- 10amps = 29.56

X 30% = 8.86 + 10 =18.86amps.

Any help from sparky types would be appreciated. I'd rather over spec as there are certain to be times when I'm running all five zones.

[Crossy]

10A + 30% of remaining load is standard hob diversity for the UK. Comes out about 19A

At the full 9.1kW 10mm² cable will only give you a 2.7V (1.2%) drop, I suggest that's rather over specified.

I reckon you'd be OK on 6mm², which at 9.1kW would drop 4.4V (2%).

6mm² VAF-GRD from Bangkok Cable is rated at 38A in air, so unless it's buried in insulation you'll be fine. Use a 40A breaker.

Does the hob instruction leaflet give you any clues?

[Pdaz]

Thanks for the reply Crossy. It's going to run in PVC conduit with a bunch of other power conductors through roof insulation. Maybe I will check for an 8mm2 cable to be on the safe side. Cost difference will be next to nothing and plenty of space so size not an issue.

[Forkinhades]

Unfortuately you cannot get 8mm2 cable

If you was to run the cable on its own in its own pvc pipe, then 6mm will be fine with ambient temperature at 40 C

However you will be grouping the cable with other cables which in effect derates the cable, and you would need 10mm

Ambient temperature of 40 C will derate by 0.87

Grouping of cables will derate by 0.85

Calc done with 4% voltage drop

2 calcs attached

regards

Hob 6mm.pdf

Hob 10mm.pdf

Also when calculating a load for a single appliance, diversity should not be taken into account.

Edited October 4, 2013 by Forkinhades

[Crossy]

Agree with Forky, if you're going to group cables in a conduit you'll need the next size up and 8mm² is not available

[Pdaz]

Thanks again for the input. I downloaded the manual but as I expected info is limited " consult a certified tradesman" However it does say to use 6mm2 cable from the switch to the hob. This is meant to be less than 1.5m. So for my run of 15m to the breaker I guess 10mm is the answer.

Interesting to hear diversity is only considered for multiple appliancies. I believe electric hobs could be an exception as they are seldom used at full load. ie all burners at max simultaneously ?

I was originally going to run my oven and hob off the same connection. 9.1 + 3.5 kw for a total of 12.6 kw. using diversity as a rule this would appear to be fine on 10mm but I prefer to have a larger safety factor and run two seperate lines.

Even using high grade materials and imported kit everything still seems to have a much shorter life cycle here. Hate to think about the houses wired with tape wrapped joints and no earth.

[Crossy]

Interesting to hear diversity is only considered for multiple appliancies. I believe electric hobs could be an exception as they are seldom used at full load. ie all burners at max simultaneously ?

I think that is the general feeling in the UK trade, 10A + 30% of remaining load is the oft quoted 'standard'.

However, there is no one 'right' way to do it, if you post on the IET 'regs' forum I'll bet they say 6mm is just fine, but we have to factor in the higher ambient in sunny Thailand.

[Forkinhades]

For a run of only 15 meters, the extra cost will not be that much more for 10mm, attach a 4mm with it for your oven. If you have RCD protection then you do not need to have extra protection of conduit.

[Forkinhades]

^ that said. If installed clipped direct and not bunched with other cables 6mm for hob and 4mm for oven.

Or you can just lash a 10mm across the ceiling with a 4mm attached ..........hehe, let us know what you want to do, and how, then precise calcs can be obtained.

[Pdaz]

Cheers guys, appreciate your knowledge. Think I'll go with 10mm plus a 4mm for the oven. Haven't got any ceilings in at the moment so easy to do in conduit. All my existing wiring is UK style 2 core and earth run in the yellow Thai conduit. All the boxes are entered thru the correct size gland and all terminations done with American style 'wire nuts'

It's a little over spec'd compared to the norm here but I like to know it's been done correctly.

[Forkinhades]

If you run 2 pipes to carry the 2 circuits, you can use 6mm for the hob