Domestic remodel - Sanity check on a couple of things

[SparkyChick]

Hi all,

Currently quoting for a fairly extensive remodel of a kitchen diner, and I have a couple of things I'd appreciate a sanity check on.

1. Structural steel bonding - My brain is telling me based on what I've read and understood that I don't need to bond the new structural steels. They aren't in contact with the ground, but my heart is telling me that just seems wrong somehow. One of the beams is going to have it's cavity acting as trunking effectively since it's the path of least resistance from one side of the room to the other and so my thinking is, I should bond it to ensure a good earth path in fault conditions.

2. Sanity check on a cable size - New oven, 22.2A (max, no diversity factored in at all). 6mm can handle it, but obviously running a 20A breaker at that is not great long term, so 32A breaker required. Since 6mm can only handle 27A when in a ceiling with insulation (ref method I'm going to be encountering), my only choice is 10mm. Voltage drop is fine either way, it's just the potential overload situation. So 10mm is the correct choice as far as I'm concerned.

3. There is going to be some under cabinet lighting. To future proof, I am going to suggest that the supply for this (coming from a 6A lighting circuit) is exposed in the cupboards as 13A unswitched outlets since then the customer can change the lighting to their hearts content. My concern though is if they stick something other than lighting on it. 1.5mm is fine generally, and can handle 13A in the ceiling with insulation. So my brain is saying this is fine, but for some reason it just feels wrong. For that reason I was considering providing the supply by 5A round pin outlets, but that raises the issue of the 6A breaker. I have very limited experience of round pin outlets. 5A is obviously the continuous rating, try as I might, I can't find any information about overload characteristics, so I'm wondering what you guys think?

Whilst it's a fairly big job (by my standards so far), the rest of it is straight forward, it's just these three niggles I could do with a bit of feedback on.

Comments and thoughts welcome

 

[Pete999]

Can you feed the lights from the existing lighting circuit, and switch them from the existing switch position, obviously adding a new switch, just takng a N and SL to the under cabinet position, eliminating the need to have them fed via a 5a socket? just a thought

 

[westward10]

1. I would not bond the steels.
2. I would use 10.0 to anticipate future upgrades.
3. I have 2A lighting sockets on 6A devices, they are never likely to overload.

 

[SparkyChick]

Can you feed the lights from the existing lighting circuit, and switch them from the existing switch position, obviously adding a new switch, just takng a N and SL to the under cabinet position, eliminating the need to have them fed via a 5a socket? just a thought

The existing circuit feeds two separately switched pendants. It's going to be repurposed to switch downlighters as one circuit and then a central pendant and under cab lighting as another circuit.

Some parts of the lighting are a bit vague (like downlighter positions), so I was kind of trying to make my life easier and neater, how they do it then is up to them. But yes, I could just pop a junction box under the cabinet.

 

[Pete999]

The existing circuit feeds two separately switched pendants. It's going to be repurposed to switch downlighters as one circuit and then a central pendant and under cab lighting as another circuit.

Some parts of the lighting are a bit vague (like downlighter positions), so I was kind of trying to make my life easier and neater, how they do it then is up to them. But yes, I could just pop a junction box under the cabinet.

That's how mine are done, mind you I had to re do them as it was a Kev the Kitchen fitter job when I bought the house.

 

[telectrix]

1. IR test from steelwork to MET. if > 22K ohms, then it's not extraneous.

2.allowing for diversity, a 20A MCB will be fine, so 6mm.

3. 5A sockets but use a 10A MCB.

 

[Richard Burns]

1. If the structural steel does not introduce an earth potential into the property and the steels will not be accessible once the kitchen is finished then I can see no advantage in providing an electrical connection to a large piece of metal and potentially making live in a fault condition whereas it would not be live otherwise.
2. 10mm² sounds correct based on what you have described.
3. having a 13A socket on a lighting circuit is asking for trouble, you could label it 3A max or something or have a 3A fuse protecting it. If someone does overload it it is not dangerous as the circuit breaker will operate.
The 5A round pin sockets are routinely used in lighting circuits and should not cause a problem. I am not sure of the actual rating now, but after they were introduced there was some discussion about classifying them as 10A because they could handle the current, the chance of the under cupboard lighting exceeding 5A is miniscule. You could use click flow connectors under the cabinets for ease of connection.

 

[SparkyChick]

Thank you chaps.

@Richard Burns your comment about 13A socket on a lighting circuit is exactly why it just feels wrong. Thanks for the info about the rating, nice to know they have some headroom if things goes wrong.

 

[mattg4321]

I wouldn't be running 10mm2 for an oven with a max rating of 22A. Use the diversity calculation, that's what it's there for.

Don't worry about future proofing it too much. 99% of houses have 6mm2 for cooker circuit and nobody else will be quoting for 10mm2.

 

[Octopus]

I've put a 13A socket on the load side of a light switch in the past (and probably would in the future), the reason being those pesky transformers for under cupboard lights / plinth lights which are moulded into the 13A plug....

 

[SparkyChick]

The issue is more about the breaker than the cable.

22A on a 20A breaker is not a good plan in my opinion, and whilst the load will drop off once the unit is up to temperature and begins cycling, I know from my combination cooker, it can place full load on the circuit for quite some time.

The customer also had notes on the literature he showed me about the oven that stated it required a 32A breaker. Since a 32A breaker won't adequately protect a 6mm cable with the installation methods that are going to be encountered (in ceiling with 100mm of insulation - 6mm has a rating of 27A in that scenario), 10mm cable is the right choice because the breaker will adequately protect it for all encountered installation methods.

 

[Des 56]

Why place so much interest on factors governing the current carrying capacity then discarding accepted long standing diversity allowances
I agree with Tel 6mm

 

[SparkyChick]

Because it doesn't feel right to use a 20A breaker with a maximum load of 22A regardless of how long that load may be applied.

However, we're obliged to follow the manufacturers guidance. Just checked the website and it actually specifies a 30A fuse, so it's largely immaterial as a 30A fuse or 32A breaker won't protect a 6mm cable, so the choice is 10mm or 10mm because the decision is out of my hands.

 

[HandySparks]

1) I wouldn't bond a new steel unless you know that it's extraneous.

2) If the cooking appliance is an oven only (not a cooker with hob), and the max load is 22A, then 6mm² protected by a 32A breaker will be fine. The only time it's going to trip will be on a fault, not overload, so the insulation will be irrelevant. (Actually, with diversity, a 20A would probably be fine.)

3) I'd terminate the lighting cable to a small junction box on top of, or the underside of, the wall cupboards. It's what people expect to find in this sort of situation.

 

[mattg4321]

Because it doesn't feel right to use a 20A breaker with a maximum load of 22A regardless of how long that load may be applied.

However, we're obliged to follow the manufacturers guidance. Just checked the website and it actually specifies a 30A fuse, so it's largely immaterial as a 30A fuse or 32A breaker won't protect a 6mm cable, so the choice is 10mm or 10mm because the decision is out of my hands.

I'm fairly sure that manufacturers instructions are to be taken into account but not blindly followed. I'll be happy to be corrected if I'm wrong though.

Taking into account diversity, I make the oven 14A. There's no way in my opinion that it's necessary to run 10mm2 for this appliance. I'd be happy to fit 6mm2 on a 32A MCB and I'd have no trouble at all sleeping at night. As above, overload protection can be omitted when supplying a fixed load appliance.