Breaker and Wire Gauge required for new Oven/Microwave and Separate Induction Hob

Forums › Home Workshop DIY › Breaker and Wire Gauge required for new Oven/Microwave and Separate Induction Hob

allyshake

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Wannabe Geek

#319002 12-Mar-2025 13:23

I am purchasing the following two appliances in New Zealand:

Bosch Series 8 Built-In Compact Oven with Microwave Function CMG778NB1 Connected Load 3.6 kW Spec says 16A breaker required

https://assetscdn.loadbee.com/catalogue/stdzsexzpn4nf9e3/Documents/specsheet/en-NZ/CMG778NB1.pdf

Series 8 - induction cooktop - 60 cm - Black PXY675DC1E Connected Load 7.4 kW Spec docent say what breaker size needed

https://assetscdn.loadbee.com/catalogue/stdzsexzpn4nf9e3/Documents/specsheet/en-NZ/PXY675DC1E.pdf

I have one breaker and circuit available on my switchboard for connection. It is currently a 32A MCB Single Phase which was used for the old free standing oven. Not sure if its 4mm or 6mm sq TPS.

Can both these appliances be on the ONE circuit?

If they are on the one circuit what wire gauge will I need to run. The cable run will be less than 10m in total for the run from board to outlet.

What breaker will I need. Im planning on using an RCBO rather than just an MCB if that is OK.

Whilst there is no insulation in the walls Im planning to add it and there is insulation in the roof. So assume Enclosed in insulation for the sake of specifying switchgear.

Is it normal for these to be on a fixed connection or will they need to have an isolation switch at the wall? (There is one currently but it has a very old style plug connection which is probably original when the house was built circa 1962.

Thanks

Senecio

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#3353161 12-Mar-2025 13:33

From my experience, any decent sparky will insist on running a seperate circuit for each appliance. Both should then also have their own isolation switch.

elpenguino

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#3353165 12-Mar-2025 13:51

Cable selection is an electrician job.

What other factors have you left out of your calculations? For example, will the cable be surrounded by insulation?

One thing I can tell you is that 7.4 / .230 is 32 so I don't think you can have both appliances on one circuit.

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gregmcc

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#3353224 12-Mar-2025 14:38

allyshake:

I am purchasing the following two appliances in New Zealand:

Bosch Series 8 Built-In Compact Oven with Microwave Function CMG778NB1 Connected Load 3.6 kW Spec says 16A breaker required https://assetscdn.loadbee.com/catalogue/stdzsexzpn4nf9e3/Documents/specsheet/en-NZ/CMG778NB1.pdf Series 8 - induction cooktop - 60 cm - Black PXY675DC1E Connected Load 7.4 kW Spec docent say what breaker size needed https://assetscdn.loadbee.com/catalogue/stdzsexzpn4nf9e3/Documents/specsheet/en-NZ/PXY675DC1E.pdf I have one breaker and circuit available on my switchboard for connection. It is currently a 32A MCB Single Phase which was used for the old free standing oven. Not sure if its 4mm or 6mm sq TPS. Can both these appliances be on the ONE circuit? If they are on the one circuit what wire gauge will I need to run. The cable run will be less than 10m in total for the run from board to outlet. What breaker will I need. Im planning on using an RCBO rather than just an MCB if that is OK. Whilst there is no insulation in the walls Im planning to add it and there is insulation in the roof. So assume Enclosed in insulation for the sake of specifying switchgear. Is it normal for these to be on a fixed connection or will they need to have an isolation switch at the wall? (There is one currently but it has a very old style plug connection which is probably original when the house was built circa 1962. Thanks

Cable sizing, volt drop, derating factors are all things your electrician should take care of, for the average homeowner this is beyond what they are likely to know.

A fixed oven/cook top does not need a plug/socket arrangement.

No need for a RCBO as not required for a fixed appliance.

You can run both off the same circuit, ideally separate switches, again dependent on the actual wire size.

As far as the spec's on the cooktop, things don't add up, 4 zones at 3.7kw does not = the claimed connected load of 7.4kw, but keep in mind there is a diversity factor allowed for in AS/NZS3000.

The circuit breaker is there to protect the wiring, not the appliance on the end of it.

Get your electrician to do all this for you, that's why they spend 4 years learning the trade.

allyshake

3 posts

Wannabe Geek

#3353252 12-Mar-2025 16:51

Thanks for the speedy responses.

There will be insulation so Im assuming a derating of breakers. The power circuits are all on 20A MCB's currently and 2.5 cable so those will go down to 16A and 6A for the light circuits all 1mm cable.

Wanting to use RCBO's as Ill take advantage of the Resi9 MaxBar which is a nice tidy layout in the board.

Ive only got a small board now and will upgrade at a later date to a 2 x 15 Way but at present I only have 11 spots total on the Din rail.

As Im working my way through the Reno and progressively putting in insulation I just want it to be safer (thus smaller rated RCBO's). The wiring is in relatively good condition but its old, original and the type with no shielding on the earth core. Its all a little stiff too with age and the roof space gets very hot.

An inspector put one RCBO on the board for the bathroom which I put underfloor (300W) heating in which required the RCBO.

But as the board is currently a mashup of different breakers and different brands they don't fit nicely and the comb connecting the phase doesn't sit nicely along one plane. Its just untidy and not ideal.

The main reason Im asking about the oven/microwave and the separate Hob is I don't want to buy the appliances and then find I can't connect them without upgrading the entire switchboard.

I realize this is the likely outcome as I'll upgrade the heat pump too which isn't on its own circuit but just daisy chained off a socket. Im adding a second heat pump (a Hypercore) so that will go on a separate circuit too but then the same thing goes as Ill have two heat pumps but only room for them on the one circuit.

Yes I hear you all already just saying bite the bullet and upgrade the board but Im on a tight budget as I live on my army disability pension and am just Reno a room at a time as I can afford it.

My experience of local electricians (I live in a small town not near a main centre) is that they try and upsell and won't do work unless you agree to their approach and whilst I understand the safety element considering they do the CoC it irks me when you can't get a job done to suit current needs as apposed to upgrading to something well beyond the actual requirements.

Thanks for everyones input.

Goosey

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#3353267 12-Mar-2025 18:01

Sounds, like you need to consider a rewire and new switch board.

how old is the house?

and even though you haven’t said or implied yet, we do hope you will engage a electrician to sort out your hob & oven. (Not hope, but you have to)!
They would then be able to advise best course of action for the remainder of what you want to do…

allyshake

3 posts

Wannabe Geek

#3353287 12-Mar-2025 19:36

circa 1962 it was built. Im doing the outside wall insulation and adding an extra 45mm service cavity for added insulation and to reduce thermal bridging. Perfect opportunity to install new TPS.

Yes ultimately it needs a new board but I need to work my way around the Reno first room by room with the insulation and extra service cav.

Just wanted to get an idea of what I need and source the switchgear etc before getting the electrician in.

I can buy the switchgear at a third of the price quoted by electricians to supply and install.

However even though its verified NZ sourced Schneider breakers etc electricians I have spoken to say they will only do a job if they supply everything.

I accept they are in business to make a profit but that is not the issue. 300% mark up on parts is extortion.

But yes I know the laws regarding what a home owner can do which obviously excludes entering a switchboard.

I just noted the absolute birds nest when an electrical inspector put in an RCBO for me for the bathroom underfloor heating.

The board isn't the old style with wire fuses so it has been updated probably ten years ago and does have MCB's and a couple of RCBO's and when I bought the house before the Lines company would enliven the house I had to have it inspected which was OK.

However I got a heat pump installed and they just daisy chained from a socket in the lounge for the connection which is OK BUT from reading the regs etc given that there is insulation covering the TPS Im not entirely happy with that solution, plus ultimately I want a stove and a cooktop as I don't have either and have been here 7 years now.

Its a bitch living on a veterans pension and trying to make ends meet.

cheers

I took this photo when the inspector was putting in the RCBO on the far left.

John19612

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Master Geek

#3353873 14-Mar-2025 07:12

allyshake:

I accept they are in business to make a profit but that is not the issue. 300% mark up on parts is extortion.

You wouldn't like the hospitality industry then.

The price is the price.

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Handle9

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#3353878 14-Mar-2025 08:02

John19612:

allyshake:

I accept they are in business to make a profit but that is not the issue. 300% mark up on parts is extortion.

You wouldn't like the hospitality industry then.

The price is the price.

The electrical industry has a real problem with transparency. The “trade” price in most wholesalers is a deliberate fiction made up to convince contractors they are getting a good deal.

When the Bunnings retail price is significantly cheaper than “trade price” with an 80% discount you know the whole thing is a bit of a crock.

John19612

68 posts

Master Geek

#3353881 14-Mar-2025 08:13

Handle9:

The “trade” price in most wholesalers is a deliberate fiction made up to convince contractors they are getting a good deal.

This is quite true, take this example of 2.5mm sqr, 2C+E TPS as pretty typical:

However, expecting an Electrician, or any tradesman, to give anything close to cost price on parts is a fantasy.

gregmcc

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#3353882 14-Mar-2025 08:22

John19612:

Handle9:

The “trade” price in most wholesalers is a deliberate fiction made up to convince contractors they are getting a good deal.

This is quite true, take this example of 2.5mm sqr, 2C+E TPS as pretty typical:

However, expecting an Electrician, or any tradesman, to give anything close to cost price on parts is a fantasy.

I know this is heading off topic, do you except anyone who sells retail to provide this kind of a breakdown for the materials for the product they sell. As far a cable goes, the electrical supply companies have this stupidly high price and discount it for those in the trade, if Joe public turns up and wants to buy from them and pay retail that's their problem.

John19612

68 posts

Master Geek

#3353883 14-Mar-2025 08:26

gregmcc:

As far a cable goes, the electrical supply companies have this stupidly high price and discount it for those in the trade, if Joe public turns up and wants to buy from them and pay retail that's their problem.

This is true of virtually any supply chain items in virtually all industries.