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Replacing 3 Phase Cooker With Induction hob

Postby Toomanyjobs » Sat May 04, 2019 4:17 pm

Hi everybody - some advice needed

I've removed a faulty 3 phase cooker.

The supply is via a 3 pole 20A breaker, the neutral is unswitched.

I want to replace it with an induction hob that can be wired for 2 or 3 phases, and a single phase oven. The combined draw is about the same as the old cooker.

If at all possible I want to avoid replacing the supply cable.

What's the best way to tackle this?
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Postby Mr White » Sat May 04, 2019 10:17 pm

Is this for a commercial property? (It's unusual to find 3 phase in a domestic property) If it is commercial I would suggest you call an electrician.
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Postby Toomanyjobs » Sat May 04, 2019 11:15 pm

Property had 3 phase installed to run workshop machinery. It appears that the previous owner simply took advantage of the 3 phase supply on the other side of the kitchen wall and installed a professional cooker for his wife's cake baking hobby.

In any case I now want to install domestic equipment. I could install a standard range cooker as they can be wired for 3 phase, but we want an integrated hob and oven.

I know how to make it work - I just can't find any regs that specifically cover this situation. I don't want to do anything that's not compliant. I'm happy to pay a professional to connect it up I just need to know if I'm going to need to simply replace breakers, or if I need a supplimentary consumer unit with RCD's or whatever.

Living where I do it's a big ask to get someone out to quote for a small job so I want to get everything I need in place so whoever I get in to connect it has an easy job.
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Postby collectors » Sun May 05, 2019 8:21 am

I am 99% sure you will need a separate supply for the oven. You would need to post more info from the spec tags on each unit. IE: wattage. Would also need to know the size of cable there at the moment. 20amp is only the switch size/capacity. If its only fed by 2.5mm 3 phase cable then no to running an oven off this size cable.
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Postby Toomanyjobs » Tue May 07, 2019 7:37 am

[quote="collectors"]I am 99% sure you will need a separate supply for the oven. You would need to post more info from the spec tags on each unit. IE: wattage. Would also need to know the size of cable there at the moment. 20amp is only the switch size/capacity. If its only fed by 2.5mm 3 phase cable then no to running an oven off this size cable.[/quote]

I had a better look - the 3 pole mcb is 30 a. The cable is run in singles that I measured as imperial size 7/044.

Hob is labelled 6800w but no info on how that is split betewwn phases.

Oven is 5600w single phase only.

Looks like the line cables will be OK, but the neutral is undersized?

I think I've answered my own question.

Given that it's singles, I might as well pull in another neutral and change the breakers to single pole RCBO?
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Postby collectors » Tue May 07, 2019 9:05 am

Its hard to say without seeing this type of thing, but you won't get away with a single RCBO as it may have to take 12.4kw = 51 amps. Better off having 2 circuit's at 26.6 amp & 23.3 amp.
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Postby Toomanyjobs » Tue May 07, 2019 7:06 pm

collectors wrote:Its hard to say without seeing this type of thing, but you won't get away with a single RCBO as it may have to take 12.4kw = 51 amps. Better off having 2 circuit's at 26.6 amp & 23.3 amp.


I was suggesting using 3 x RCBO's in place of the existing 3 pole mcb, creating 3 single phase circuits, (if I run extra neutrals) then I'll wire the equipment as single phase.

When I started this thread I thought it was cabled with one 4 core cable. I've had a better look now the lights are working and it's all singles which makes it easier to re-do

Thanks everyone who replied.
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