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8ball

Junction box to divide power?

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Hi guys I am planning a new kitchen and have a question regarding power to a induction hob and a twin oven. The appliances will be placed at locations which require their own power supply as they are more than 3 metres apart on either side of the kitchen.

 

The existing cable from the CU is 10mm T&E with a 32A MCB and runs 4 metres and then gets reduced at a junction box to 6mm T&E and runs another 4 metres, this cable is currently running a twin oven (existing hob is gas).

 

So my question is can I just run another 6mm T&E from the junction box where the 10mm gets reduced to run the hob?

 

My calcs:

 

The Hob is 7.2kw and the double oven is 4.4kw = 11.6kw

 

11.6kw/240 = 48.3amps

 

Diversity

10% of 48.3amps = 4.83amps

 

30% of remaining = 13.041

 

5 amps added for control socket so I have a total of 18.041 amps

 

 

Any advice would be great ;-)

 

 

 

 

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I would be running a 10mm all the way to the hob, and 6mm all the way to the oven, on their own circuits from the CU.

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12 minutes ago, ProDave said:

I would be running a 10mm all the way to the hob, and 6mm all the way to the oven, on their own circuits from the CU.

 

 Thanks for the reply ProDave, can I ask why. I'm on a steep learning curve:| so understanding why I'm doing what I,m doing is important to me, I know detailed answers require more time but I would really appreciate it;)

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Simple answer is the total load would dictate a 50A circuit breaker which is way over the size you can use for a 6mm cable.

 

Also the Manufacturers Instructions for the oven probably say to use a 16A circuit breaker

 

If you really cannot re cable to the consumer unit, then it would be acceptable to feed the 10mm cable with a 50A mcb, then fit a mini consumer unit where the junction box is, with a 32A circuit to the hob and a 16A circuit to the oven.

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@8ball, in your calcs above its 30% of full load not "remaining". Appreciate it's still nom 13A!

 

Don't discount the Christmas dinner effect when considering diversity!

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My parents-in-law have two ovens which require a 16A supply each, with a double-pole switch to each oven. We took the existing 10mm2 cable from the old cooker which is protected with a 40A MCB in the consumer unit. This terminates in a small consumer unit with two 16A double-pole MCBs. This provides the necessary circuit protection, appliance protection and isolation requirements in a neat box, and it's much easier to wire than 10mm2 cable into backboxes. There are a couple of spare ways in the CU, so if they want to add anything in the future (over-unit lighting etc) there is plenty of scope to add an MCB without much work at all.

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I recently removed a 6-year-old double oven from the old kitchen. I was "entertained" to discover it was wired into a 13A plug :x

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11 minutes ago, richi said:

I recently removed a 6-year-old double oven from the old kitchen. I was "entertained" to discover it was wired into a 13A plug :x

 

The built in oven we were originally planning to order came with a 13 A lead and plug, but the design changed just before we placed the order and the replacement model that we ended up buying, needed a 16A wired supply.  It was nuisance, as I'd already made provision for a 13 A outlet to connect the built in oven to.  There was no problem with the cable rating on the radial, as it was 6mm² T&E.  What it did mean was that I had to fit an additional MCB in a small box inside on of the adjacent cupboards in order to provide protection and local switching for the oven and it's supply cable, something that would have been done by the 13 A outlet and plug if we'd had the original oven supplied.

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Yeah, but this oven had a plate on the back rating it at 40-something Amps. And it was obvious the plug wasn't standard-issue.

 

Mind you, the socket it was plugged into was at least a separate 50A circuit, but I bet the plug got toasty behind there when both ovens were heating up!

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Did it have an M6 bolt in the fuseholder in the 13A plug?

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Worth noting that the max continuous rating for a genuine (there are a LOT of fakes about....) BS1363 plug is about 10A, anything more for long periods of time and the plug will probably overheat.  The 13 A rating is an intermittent load one, not a continuous rating.

 

It's one reason that years ago "3 bar" electric fires became harder to find.  They were often rated at 1 kW per bar, on our old 240 VAC mains supply (before EU harmonisation).  This meant they were drawing around 12.5 A with all three bars on, which was fine when we had the old 15 A round pin sockets, but very marginal when we switched to the BS1363 13 A sockets.  The result was that there were a few issues with overheated plugs. 

 

When we harmonised our mains LV voltage standard with the EU  to 230 VAC (which was just a paper exercise in changing tolerances and ratings) power ratings for appliances were then changed to be at 230 VAC.  A 3 kW electric heater would now be designed to deliver 3 kW at the nominal 230 VAC, which gives a current of just over 13 A, above even the intermittent rating of a BS1363 plug.  Add in the factor that we didn't actually change our grid LV supply down to 230 VAC when we harmonised, we just adjusted the tolerance to be asymmetric (+6%, -10%), and so most supplies are still, in reality, around 240 VAC, then a 3 kW heater rated at the "new" 230 VAC standard would now deliver 3.13 kW. At the maximum allowable grid voltage of 253 VAC (230 VAC +10%) a heater designed to deliver 3 kW at 230 VAC would now draw about 14.35 A, way over the limit for a 13 A plug.

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