Electrical Question

63a sub mains board off a 32a mcb?

I hired an electrician in to fit a sub mains board as I only had 1 spare way on my Consumer unit and was getting an extension which would be a kitchen and utility room, he has done a runner now that the job is nearly done and left the place a mess! I will at some point need to get a tradesman in to finish it but I noticed something today..

He's supplied the new 63A sub mains board with a 10mm armoured cable but tje MCB and my consumer unit that is feeding it is only a 32A?

The only things I have running off the sub mains right now is 1 10A radial lighting circuit, 1 32A kitchen ring circuit and 1 32A utility room circuit but I want to add in 2 fan assisted ovens rated at 2.8kw each and an electric hob (not sure of the rating yet as the delivery hasnt came) I did expain to the guy I was getting these things added in at a later date!

I think a 64A sub mains board could handle this? But surely not if its run off a 32A MCB, is there anyway to fix this that wont be super expensive??

5 Answers

Best Answer

Hello peter ,
The situation you have could be worse than you anticipate .
To determine what size cable you need to supply your sub-main is determined by the amount of current drawn when potentially all circuits are being used ( add to this given diversity factors ) at the same time , This would give you maximum current rating .
Your sub-main supply cable would need to be able capable at carrying this as a minimum requirement , ideally carry more .
Add to this calculation the enivonment that the cable is to be run ( clipped direct to surface , layed in conduit / trunking , buried in the wall , covered by loft insulation , etc ) again each different environment changes the cable current rating .
Once the cable rating has been determined then you can establish the breaking capacity of the fuse required to cover this sub-main ( This figure can not exceed to maximum carrying capacity of the supply cable ) .
The main fuse to the property would also need to be checked to ensure it can cover the additional loading .
The main tails and earthing conductor would also need to be checked .
There are various other things that should be looked at to ensure that everything is satisfactory within this altered installation .
Please get an approved electrician to give you a complete peace of mind explanation as to exactly what can be done and the options you have to achieve a safe and compliant installation .
wayne soppitt (WPS Electrical Services ) .

Answered 23rd Apr 2012


Member since 28 Feb 2011

Report him to his body ie Niceic ?
Anyway the 10mm cable could be supplied with a 50 a mcb ? Just need to check a few things such as length of run and total load supply
Hope this helps
If you need any further work,post on here and get an approved electricain like me to quote on the works
Thank you

Answered 23rd Apr 2012


Member since 8 Oct 2008

He should have used a double pole Henley block and installed a separate 63A switch fuse to create a proper sub main. Always check main fuse as is could only be a 60A. Why are people still getting away with this.

Answered 23rd Apr 2012


Member since 4 Nov 2011

"Best get an NICEIC electrician round to sort it out asap and ensure that the work is fully tested and notified to building control"

The electrician dose not have to be NICEIC registered any electrician who is partp registered will be fine!

Answered 30th Apr 2012

A Jeavons Electrical

Member since 7 Dec 2011

Do you mean that the main switch in the new consumer unit is rated at 63amp? only its unclear what the 63amp relates to?

If the SWA 10mm cable runs back to the meter position then the best way of doing this is to have a 100amp main switch and 50amp MCB installed and connect 16mm tails from this unit in with your existing meter tails using Henley blocks. This will be a much better arrangement and still leaves you a spare way on your old existing fuseboard.

Sounds like you were not using a registered electrician! - you need to be careful as if the local authority get wind of unregistered part P notifiable work, then they could fine you for breaching the regulations. ( up to £5000!)

Best get an NICEIC electrician round to sort it out asap and ensure that the work is fully tested and notified to building control.

Answered 23rd Apr 2012

Electrical Safety Services

Member since 17 Oct 2011

Need help with your project?

We have tradesmen ready to help you. Post a job, read reviews and hire today.

Post a job

Need some help?

Post a job on MyBuilder to find quality, local Electricians who can help you with your project.

Search all questions