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Posted By Topic: Oven and Cooktop on Under-rated circuit.

Feb 26 2019 18:46

Hello, While I am not an electrician I am an engineer by trade and have also been consulting with a family friend who shares my same thinking on this matter.

On a new build (2018) house, the oven and cooktop have been wired to a single 4mm, 32a circuit. From the specifications of the appliances, these have a max draw of 15a and 25a respectively.

I appreciate from a quick skim of the standards that there are diversity rules in place but we are both of the opinion that as these are both fixed appliances and likely to be used at the same time that this circuit is underrated?

Does anyone have any input as to whether this is an acceptable setup? This has come up due to wanting to change from a ceramic to induction cooktop with a similar 27a rating.

Feb 26 2019 21:00

I would assume that the original oven circuit was correctly sized @ 4mm, personally if I was doing it I would have used 6mm to allow for a future up-size of connected appliances.
More than likely 4mm was put in as it was compliant for the specified load and budget constraints at the time.

Without more details on the change in loading, the distance from the D/B and the method of installation it is not possible to tell if 4mm will still be big enough to handle the new loading.


Feb 26 2019 21:46

A 2018 home will be insulated such that all cables are partially surrounded. 12m would be a reasonable run from main switchboard to a cooking appliance.

A 12m run of 4mm TPS, partially surrounded and max 75 degrees conductor temperature is suitable for a load current of 27 Amps.

Your current and proposed cooking appliances will draw max loads of 9.2Kw and 9.6Kw respectively.

Table C4 of AS/NZS 3000 suggests (informative) that cooking appliances with max load 8-10Kw may be assessed as 25A for purposes of max demand.

If my assumptions are correct then your 4mm circuit, if protected by a 25A type C MCB, would be sufficient for both your existing and proposed cooking setup.

That said, I always run oven/range/hob circuits in 6mm TPS for reliability and future proofing.

Feb 27 2019 08:53

The circuit was never fit for purpose, even with no de-rating for BTI.
40-odd years ago as an apprentice we always used 6 mm2 for a range or a combination of separate hob & oven - which back then would typically be an 8 kW range or equivalent; so assessed at 20 A.

Yes the hob - which at 6 kW is not large by today's standards - can be assessed as 25A; so would maybe be OK with separate circuit in 4 mm2. But that makes 40 A when used together with the 3.5 kW oven.
The "starting" CCC (installed touching a surface) is 39 A; which may look close enough. But Under rules applying 2018, we must assume BTI, so best case is partially surrounded - only 31 A.

There is also bunching to consider - and there is only a tiny chance that this cable isn't run in a bunch with several other circuits for more than the 1m exemption. Say two circuits for other kitchen appliances, at least one for other sockets / appliances in adjacent areas and probably a lighting circuit. Looking at Table 22 that bunch of 5 circuits has a de-rating factor of 0.60; but to be fair maybe not all fully-loaded so use figure for 3 fully-loaded circuits (0.7).
So the actual CCC of the 4 mm2 is only 21.7 A.

So even a 25 A mcb is too high. Using a 32 A is bordering on criminally stupid.

Distance doesn't matter; it affects volt drop & EFLI which can lead to need for larger cable; but this cable simply isn't up to the job no matter how short the run.

Even 6 mm2, using 40 A for partially surrounded and same 3-circuit factor would be a 28 A CCC, so OK protected at 25 A and fit for purpose as long as larger appliances aren't used.

Most likely explanation is an installer who never gave the loadings a thought, nor applied any de-rating; but instead just used 4 mm2 to keep their tender price down.


Feb 27 2019 10:48

Could have been a poorly trained apprentice that was sent to wire the house up and thought that it was going to be a free standing range so ran 1 4mm 2C+E. I agree 4mm is too light. I just priced a house and with a 24m run including the switched portion of the run, partially surrounded and no grouping as I run these circuits separate to anything else, even each other. I ran a 4mm for the oven on a 20A breaker and 6mm for the hob on a 32A breaker. I also had elbow room on Volt Drop as I up-sized the mains for a future 32A EV charging point.

Feb 27 2019 11:23

Can't blame an apprentice. Their work is supposed to be supervised; and an apprentice didn't issue the CoC (incorrectly) stating that the work complied.

Feb 27 2019 18:07

I wasn't blaming the apprentice Aleck, I was having a dig at the Electricians and Employers that operate under the She'll Be Right philosophy sending apprentices out to do house wires with minimal training, supervision or guidance. Of course it could also have been done by an electrician without a clue.