Overrated MCBs?

[AndyDavis]

I've got a MK Quickline Square D (metal) consumer unit set-up like (reading left to right)

1. DP Isolator 100A
2. Immersion (Type 2 M6 MCB, 20A)
3. Smoke alarm (Type 2 M6 MCB, 16A)
4. Lights down (Type 2 M6 MCB, 6A)
5. Lights up (Type 2 M6 MCB, 6A)
6. Blank (DVMMY 100A)
7. RCCB 100A (Protects 8-13 80A Max)
8. Cooker (Type 2 M6 MCB, 45A)
9. Ring Down (Type 2 M6 MCB, 32A)
10. Up (Type 2 M6 MCB, 32A)
11. Blank
12. Blank
13. Blank

I'm told that the unit has over rated MCBs. What does that mean? What can I do to fix it?

 

[SwindonSpark]

AndyDavis wrote:
I'm told that the unit has over rated MCBs.

By whom?

 

[AndyDavis]

The unit's at my mum's and she had a NICEIC man round to do a routine check.

 

[Adam_151]

The smoke alarm circuit dosn't need to be rated at 16A, the seriousness of this depends on the cable used and how it is installed, whether you trust that smoke alarms are a fixed load and incapable of overloading (which I'd guess they are) and finally and stipulations by the makers of the smoke detectors as to what the circuit should be fused at

 

[AndyDavis]

I'm not sure what "over rated MCBs" is on about? Does it mean the total of the MCBs is too high, then what should it be. I thought it was because the 3 MCBs to the right of the RCCB (45 + 32 + 32 = 109) was too high. If thats the case then can you put the Cooker on a 32A MCB because my mum cooks with gas anyway.

A 16A MCB does sound a bit high for two smoke alarms, you could put a 3KW electric heater on that. But if you put the smoke alarms on a 6A MCB will that solve all the problems?

 

[Lectrician]

I would say he has noticed the smokes on a 16, and the immersion on a 20. They are usually on a 6 and 16 respectively.

Adding the totals of the MCB's isn't a factor these days.

The cooker only needs RCD protection if the panel has a socket outlet, and that socket outlet could reasonably be expected to be used outdoors.

MK Square D?? One or the other - sounds like Square D to me, possibly with MK MCB's in it? They do have the same foot print.

 

[asc]

The cooker mcb may be considered a little oversized at 45a for adequate cable protection if the circuit is run in 6mm2 t+e

 

[plugwash]

note that just because someone is in the NICEIC doesn't make them a non-cowboy.

be especially wary of free electrical inspections. they often exist to try and sell overpriced work thats not really required to householders who don't known about electrics.

 

[AndyDavis]

He says theres two problem MCBs

1. Smoke alarm should on on 6A because the wire is 1.5mm
2. The cooker should be on 32A because the wire is 6mm

Does that sound right? Whats the best place to get a table of figues, for cables in walls, conduits, etc., and not just free air.

 

[ban-all-sheds]

He says theres two problem MCBs

1. Smoke alarm should on on 6A because the wire is 1.5mm
2. The cooker should be on 32A because the wire is 6mm

Does that sound right?

Yes - 16A and 45A are optimistic for those cable sizes.

Is there a table somewhere that tells you the maximum current through the different size wires?

See Page 6 here.