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Posted By Topic: Separate Oven and Hob wiring

PaulT
Dec 02 2018 20:27

I am wiring a kitchen alteration. There was a separate Oven and Hob each wired in 4mm TC+E with 20amp fuse for each. The home owner is now installing 2 x under-bench ovens (3.2 kW each) plus a Hob. Can I connect both ovens to the original double pole switch and upgrade protection to 32amp Circuit Breaker? The Hob could remain as is and already has a separate switch.
   

DougP
Dec 02 2018 20:56

If you mean connect two ovens to one of the 4mm2, and the hob to remain on the other 4mm2? That should be fine.

You said "20A fuse for each" - I would change those to a 25A circuit breaker each.

   

Satobsat
Dec 02 2018 23:39

Upgrading circuit protection will depend on circuit length and how the cable is installed.
   

AlecK
Dec 03 2018 08:36

The CCC of 4 mm2 partially surrounded is 31 A, so a 32 A mcb would be non-compliant.

If those kW ratings for the ovens are the FLC, then both ovens in use together will exceed 25A

So you really should be looking at running another cable
   

DougP
Dec 03 2018 09:58

Wouldn't diversity apply for the two ovens?

Being only a couple of amps over the 25A breaker, and at the partially surrounded rating for 4mm2..... barely compliant possibly..

   

PaulT
Dec 03 2018 11:08

DougP. Yes Hob on one fuse and 2 ovens on the other. Will replace fuses with 25amp breakers as suggested.

Just another thought. Can the feed from the switch to each oven be downsized to 2.5mm (seeing as the entire circuit is protected by a 25 amp breaker)? Same for the hob? The cable run (4mm) from the main switch board to the switches in the kitchen would be less than 5m?
   

AlecK
Dec 03 2018 12:27

Yes, we can apply diversity, so being borderline on ability to sustain the load is OK (though not ideal, as anyone wanting 2 ovens is likely to want to use them both at once, and the diversity isn't great as t'stat for each will be either entirely on or entirely off. Chances of both being "on. So I'd be looking to supply the highest load of each (bake, grill, fanbake; whatever setting has highest load).
3.4.1 para 3 requires partially surrounded at best, and this also applies to the section downstream of switch - which is NOT an isolator - (so 2.5 with CCC of 23 A would need the protection downsized to 20A, and thast's just not going to work with 2 x 27 A ovens!)
Reality is that there's unlikely to be BTI in an internal wall; so putting pedantic compliance to one side overheating the cable unlikely to ever become an issue. But if the actual installation condition is (or later becomes) fully surrounded you're down to 27 A, which is only enough for one of these ovens at a time.

Whenever we're tempted to cut a corner - often to save costs - it's always worth asking: "if the place burns down, and the insurer's lawyer suggests my work may have been the cause; can I defend my decision in (possibly coroner's) court?". You don't have to break any rules to end up on the wrong end of a charge. And civil damages is only "balance of probabilities" not "beyond reasonable doubt".


But when it comes to CCC & protecting conductors we can't apply diversity. We have NO discretion to say the load is "only a little bit" over the CCC. The calc in 2.5.3.1 is carved in stone; the circuit protection MUST be no greater than the CCC. So can't use a 32 A mcb if the CCC is 31A. Even if we don't think it will often be as high as that (diversity) protection of conductors is absolute.

   

pluto
Dec 03 2018 13:13

You should also to review if there is enough spares capacity on the mains of the installation.
Table C1 may be of assistance to having an educated guess on the m,aximum current likely under normal operating conditions.
   

PaulT
Dec 03 2018 13:48

AlecK. Thanks for the feedback. Just to clarify each oven is 3.2kW (total 6.4kW) so the max current at 240V would be 26.7A. Probably worth just running 4mm cable from the switch to each oven and protect with 32amp circuit breaker?
   

DougP
Dec 03 2018 15:02

I agree with Alec. But I think he missed the detail that you would run one 2.5 from the switch to each oven. Which I think he should agree is fine for the 13.xA load, and omission of overload protection.
   

AlecK
Dec 03 2018 15:34

SLV is 230 V, NOT 240 V.

Which increases current value derived by calculation from nominal power (3200 / 230 = 13.91 A, compared with 13.33 using 240 V); though not necessarily actual current which of course depends on resistance (oven being an almost purely resistive load.
So work from rated current @ 230 V, not from nominal power. And if the rated current is stated at 240 V (o; only given as power), first calculate resistance from that; then the FLC at 230 V.

Yes a reduced size of conductor can be used for each oven without having to downgrade circuit protection [using 2.5.3.4(b)]. But ONLY if direct-connected and NOT if using a socket for connection. I couldn't assume direct connection, so stated the requirement to protect the smaller cable.

The cost saving over a few metres will be minimal; so why not simplify things 9and avoid pushing any boundaries) by using 4 mm2.

Still no ratings for the new hob, so be careful as many modern hobs will not fit on the other 4 mm2 (even with diversity). I goy caught out a few years back with a "gas hob' on an island bench, and allowed only a nominal socket for the ignitor. When the unit turned up, it had 4 gas rings & ignitor (as expected. But it also had a 7 kW electric BBQ. Wasn't easy to get extra cables through after the floor was down.

Moral of that story is: Assume NOTHING; instead ASK QUESTIONS.
   

PaulT
Dec 03 2018 16:40

Thanks for all of the feedback guys. Very helpful.