BCO and Cooker Circuit Update

[Andy Wade]

Seconded - unless overload protection is provided by a rewireable fuse (BS 3036) in which case we need Iz >= In/0.725.

[ARWadsworth]

It was a mixup between I2 and Iz that led to that statement in the Wiki., however there is nothing wrong with designing a circuit with Iz>1.45In where a user may alter the final load such as by changing an electric shower for a higher powered one. I just think it could be made clearer in the article.

[ARWadsworth]

I wonder if you can guess where I would insert the double oven? It need not matter that there is not yet available a double-oven-sized hole, I would make it fit.

I am still happy to speak to the guy (without making such suggestions) and point out that he is wrong.

I will also put pen to paper if needed.

[John Rumm]

Yup, be my guest ;-)

[John Rumm]

Ask him to explain how the cable could be overloaded when the current draw is limited by the 32A MCB and not the cooker. The MCB will peg it at 1.45 x 32 or lower.

[John Rumm]

Would a way forward here be for Phil to write to the BCO, and say something like "due to the conflict in advice I was receiving regarding the cooker feed, I have opted to engage an electrician to design the circuit. I have asked him to copy you with the design". That way if he disagrees with the design he can argue the toss with your tame sparks...

[ARWadsworth]

No problem, all Wiki pages are subject to review and revison.

[ARWadsworth]

The annoying thing is that Phil did take advice from an electrican before installing the circuit.

[TheScullster]

"ARWadsworth" wrote

John/Adam

I am so grateful for your continued interest and support on this issue - almost wishing I had just "got a man in" to do the kitchen though!

The situation now is...

Er indoors only ever wanted a single oven, so looking at the specs for Bosch appliances I can get oven and hob together at 10250w. BCO is happy to have a 45A breaker fitted which would give 10800w @ 240v (16th Ed OK he says).

So the next questions are....

Where can I get a 45A Hager MCB? Can I mix suppliers and put an MK one in (they seem more readily available) - will it fit my Clipsal box?

Yes I know this is the wrong approach and I should be fighting the corner etc., but I will need to keep the guy partly on side or risk having him find loads of issues/problems just out of spite.

Phil

[ARWadsworth]

Ask him if an NICEIC registered electrician can do the inspection for him?

Hager do not make 45A MCBs.

You could try one of the Part P firms to see if they will help you. I would avoid NAPIT if you want help from me as you are in Yorkshire and due to a small mix up with my right hand and the Yorkshire NAPIT inspector's throat (a B&Q kitchen install) things could be difficult.

[TheScullster]

"ARWadsworth" wrote

Thanks Adam, but what about mixing mcbs? Would an MK 45A jobbie fit in my clipsal box - it looks likely from the dimensions! And are there any issues mixing manufacturers of mcbs in the same box?

Phil

[TheScullster]

You mean to bring them in as a second opinion - I think John suggested tthat also. Think I could do to sleep on all this now. I did consider contacting other local councils to see if they have a resident electrical inspector. Whether they would help if they knew the reason for the questions though, I somewhat doubt.

Would you mind telling me your qualifications/experience please Adam? I remember you have done some inspection work for Doncaster council but have no idea as to your other quals. I trust from the tone of your posts you wouldn't object to being named as part of my technical support on this and other electrical issues? It would add credibility if the need arises to be able to state your experience and qualifications.

Thanks again

Phil

[ARWadsworth]

Stand your ground. Your BCO is wrong.

[Andy Wade]

You can always use a larger cable then necessary, provided the conductors will fit in the terminals. That wasn't really my point - the article contains some errors (the following quotes are from the article):

That should refer to the rated full load current (if given), otherwise divide the *VA* rating by 230. Diversity can be applied where permitted, e.g. for a cooker circuit :-).

BS 7671 doesn't deal in degrees of overload protection. You either have it or not, so delete the word "full".

No, Iz only needs to be >= I2/1.45.

Since the only two 'fusing factors' relevant to domestic work are 2.0 (for rewireable fuses) and 1.45 for other devices (i.e. cartridge fuses, MCBs and RCBOs) the article could be simplified by removing reference to I2. Then summarise requirements along these lines:

Case 1: overload protection required,

In >= Ib (OPD rating equal to or bigger than load)

and for OPD type = cartridge fuse, MCB or RCBO then

Iz >= In (cable rating (as-installed) equal to bigger than OPD)

and if OPD type = rewireable fuse then

Iz >= In / 0.725 (cable must be up-rated to allow for coarser protection from this type of fuse);

Case 2: overload protection not required, due to characteristics of load,

In >= Ib (as for case 1) Iz >= Ib (cable rating (as-installed) equal to bigger than load)

but if Iz < In an adiabatic calculation must be performed to verify fault (s/c) protection of the cable.

OPD = overcurrent protective device

[Tim Watts]

TLC for one - they can order it if it is Hager's catalogue.

[cynic]

Hager do make 45A devices and there is a 45A RCBO on ebay right now

[ARWadsworth]

NVQ 2381 & 2391 and 15 years experience full time (I was doing stuff before then but only part time for myself and friends)

[ARWadsworth]

won't fit in The Skullster CU:-(

[ARWadsworth]

I wonder what cable and MCB your local council use in rewires on their properties?

It is a while since I did any council work but Doncaster council used to use

6mm cable and a 32A MCB a few years ago.

[John Rumm]

ok, paraphrased that...

Yup, my fault - intoxicated by the exuberance of my own verbosity. Fixed.

[snip examples - now worked into the text]

Now we have a conflict with the intro para:

"Overload protection requires the cable be able to carry an overload that is 1.45 times the MCBs rated current. A typical example of this would be a lighting circuit with a 6 amp MCB. Although the circuit is designed to carry a maximum of 6 amps, the MCB will not trip instantly at 6 amps. In fact the MCB will take around 1 hour to trip when the current is (6 x 1.45)= 8.7 amps. So designing the circuit to carry 8.7A gives the overload protection needed should a householder change the light fittings for higher powered ones and start to exceed the 6A maximum design current."

I suggest a simplified wording such as:

"Overload protection is required when a cable may conceivably be required to carry a circuit current in excess of its design current for a period of time, either brought about by the circuits user, or by a predictable failure mode of equipment supplied by the circuit. e.g. a user connecting too many high power appliances to a socket circuit,

Any better suggestions?