Thread: Oven & Cooktop on same circuit
- 26th Jul 2012, 11:37 AM #1Member
- Join Date
- Jan 2010
- Sydney Australia
Oven & Cooktop on same circuit
Hi, We've just updated our 30year old appliances with a new oven and an induction cooktop. I've run through it briefly with the electrician but I have a couple more questions I wanted to throw out there. Reading through the manuals I've noted the requirement to have an isolation switch for the oven & the induction cooktop & am making preperations to order this.
At present, after destroying large amounts of plasterboard while removing the tiles I can confirm that the current circuit for the cooking appliances has one 6mm cable which runs from the mains into a junction box in my wall then splits into two 6mm cables. One for the oven and one for the cooktop.
There's been no discussion on changing this and wanted to get some confirmation on whether I need to re-assess this. To my knowledge the cooktop draws 30Amps & the cooktop is around 14.2Amps. Is that single cable run before the junction box sufficient to carry the load required. Also out of interest is there any reason why I couldn't use a single isolator switch for both appliances. If I need an elec to work on one of the cooking appliances or need to turn them off for my sons safety I see no reason why I need seperate switches unless there's guidelines that suggest otherwise.
- 26th Jul 2012, 12:30 PM #2
The cooktop only will need an isolation switch, or you can use it to control both. The 6mm2 should be enough but it depends on the size of the appliances. I'm not sure it a like for like replacement needs the fitting of an isolation switch but best to do so if possible.
- 26th Jul 2012, 03:02 PM #3Golden Member
- Join Date
- Sep 2010
Re your cable, you are right on the limit but of course the answer isn't straight forward.
Your cooking load seems to be 10.6kW based on a 240V supply. Table C4 in AS3000 allows you to assume a maximum demand of 25A for 8kW to 10kW, and 32A for 10kW to 13kW rather than the apparent load of 44.2A (The maximum demand allowance is based on the expectation that not everything is turned on at once).
You indicate that the cable from the board is 6sq mm. The rating of this varies with the environment, i.e. whether it is run in free air, in conduit, enclosed by thermal insulation etc. Assuming V75 (normal plastic) insulation twin sheathed cable, your existing cable is rated at 44A unenclosed, 37A in conduit, and 31A partly enclosed by thermal insulation (the most likely situation).
So, checking the rating of your new appliances and determining the load in Amps is quite important if your installation is to be "legal". Maybe a further conversation with your electrician is in order. Obviously you don't want to run a new circuit if you don't have to. This isn't an uncommon situation when people are replacing cooking appliance although I must say that the 30A+14.2A load seems to be quite a bit more than I would expect. Most domestic setups that I come across are in the order of 7.5kW or so, which is well below the rating of your cable.
- 26th Jul 2012, 04:02 PM #4Member
- Join Date
- Jan 2010
- Sydney Australia
Thanks for the feedback. Chalkyt, just out of interest these are the specs I have regarding oven and cooktop obtained from the installation guide.
I believe the cooktop is a little under 30Amps but regardless this is out of the book.
Pyro Oven .. 3400 W 220 - 240 V
Induction Cooktop .. 6950W @220-240/380-415V 2N~ (30A)
You are correct regarding the cabling. It's partially enclosed. Obviously my concern was total load exceeding the rating on the cable when sharing the cooktop & oven on a single cable. I'm going to assume I'll never hit that upper value as it'd be rare for me to be dialling the oven up full and using every element on the cooktop. I am a little more conscious of these values because of the new features offerred on these appliances such as pyroletic cleaning which hits 500deg and another feature being a powerboost function on the cooktop to heat the pot quicker. I can only assume all these features would introduce a bit of a whack in terms of power demands.
As per Pulse's comments the Aus standards only imply an isolation switch is required for the cooktop however the manufacturer states the need for one for their oven. Where do I stand on this? My electrician certainly didn't mention anything regarding these requirements then again it was a brief visit and hadn't looked at the appliances at the time.
Just curious to know if the manufacturers requirements supersede that of the AusStandards. Obviously I'd like to retain my 2year warranty.
---- Oven requirements.
a suitable isolating switch providing full disconnection from the mains power supply is
incorporated in the permanent wiring, mounted and positioned to comply with the local wiring
rules and regulations. The isolating switch must be of an approved type and provide a 3 mm air
gap contact separation in all poles (or in all active [phase] conductors if the local wiring rules
allow for this variation of the requirements)
I guess I'll have to revisit all this with my electrician.
- 27th Jul 2012, 04:29 PM #5Golden Member
- Join Date
- Sep 2010
I am guessing that your new appliances are european in origin or design. It is not uncommon for manufacturers to specify the highest requirements that might apply to whatever markets they sell into. Our "rules" (AS3000) only require a visible isolator for hotplates. This is to provide an instant "OFF" if everything goes pear shaped with the pan of boiling fat or whatever. Mind you, a nearby isolator for the oven would be useful, but not essential. I would be surprised if the absence of an isolator negated your warranty, but stranger things have happened!
As far as the cable is concerned, as I mentioned, the rules allow for a diversity factor (i.e. not everything is on at once) when determining the load on a final sub-circuit. The Table C4 that I mentioned yesterday is one way to deal with this. But the rules do allow you to use other methods. So, bear with me for the next paragraph or so.
Your equipment specification of 3400W + 6950W at 220 - 240V is not strictly correct. That load (in Watts) only applies at one voltage (probably 240V). What you need to do is determine the current drawn at the voltage supply at your place and then determine the actual load. Your electrician can do this for you or you can measure the resistance of the appliances when everything is turned on. The actual load (amps) is voltage divided by resistance (ohms). In theory, your oven draws 14.17A at 240V (3400/240=14.7) and so its resistance should be 16.9 ohms (240/14.17=16.9). If this is the actual resistance, and your supply voltage is only 235V, then the actual wattage (and hence current drawn) is really only 3270W. Applying this logic to both appliances comes up with a calculated load of 9936W, i.e less than 10000W and so your assessed maximum demand according to Table C4 is only 25A, and so below the rating of the cable.
You could mount and argument that if the Table C4 assessed maximum demand at 10000W is 25A and at 13000W is 32A, then interpolating the increase suggest that the assessed maximum demand goes up by 2.3A per 1000W Your 10350W load therefore suggests an assessed maximum demand of 25A + (350/1000x2.3A=0.81A) = 25.81A, still well below the rating of the cable.
As I said before, AS3000 isn't hard and fast on maximum demand and recognises that "the determination of diversity factors is not accurate for every installation and different installations of the same type may have significantly different load profiles that the designer needs to consider". So, the short answer seem to be that you need to discuss this with your electrician. Most likely he doesn't want to run a new circuit as much as you don't want him to, but at the end of the day, he has to sign off on it. Perhaps a chat between the electrician and your supply authority would help.
I hope the above alternatives help.
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