4 little questions....

[StevieB]

Hi,

My first registered post

I need to change the consumer unit in the house before wiring in my workshop. After reading around and considering the replies to my last post I am left with 4 questions :?

1) I plan to have a 3 MCB box in the shop fed by armoured cable from the house on a 45A MCB. This will contain a 6A lighting circuit, a 30A ring circuit and a spare for upgrades/extra kit. Obviously an RCD is essential, but do I need one on the main house consumer unit and one on the workshop box or is one on the workshop box sufficient?

2) What rating RCD is recommended? Screwfix do boxes with 63A/30mA and 80A/30mA RCDs.

3) My existing house electrics system has a single switch to turn off all electric to the house before the consumer unit (which is an old push fit fuse job). I plan to leave this in place and just replace the consumer unit. Do I therefore need a double pole isolator on the consumer unit? (I am assuming the DP does the same as the single switch between the mains and the old consumer unit... if it doesnt what does a DP isolator do )

4) The existing old house consumer unit is big and chunky and I am worried when I fit a new one the existing wiring will be too short as the new unit will be smaller than the old one. Can I simply cut the wiring back and fit a row of junction boxes and thus extend the cable to the new consumer unit or is there a better way?

Many thanks for any help, and sorry these are not strictly woodworking related questions hopefully once the wiring is sorted I can get in and start making sawdust!

Cheers,

Steve.

 

[GrahamC]

Steve,

I'll have a go at each of your answers in turn...

1. An RCD in your shed alone would be fine, however if you are going to the effort of replacing your consumer unit in the house you should fit one there by good practice. The one in the house should cover ring mains and lights. Don't take your feed to the workshop from the RCD in the house as you dont want to trip the house from the workshop and vice versa.

2. The big number in the RCD rating is the maximum sustainable current, so 63A would be sufficient for your 45A feed to the workshop. 63A would also most likely be suitable for your house RCD if you are going to fit one.

3. You would not need a DP switch in your consumer unit as the existing one would do fine, however as with anything else age has an effect on switches. Ideally you should disconnect your main supply fuse and replace the existing switch too. However, your local electricity board own the fuse and it will be wired with a lead seal to encourage you not to remove it. The electricty board will probably want to charge you to remove and re-install this for you. You could always just do it anyway and plead ignorance later if they ever asked about it.

4. It would be poor practice to cut back existing cabling an add in junction boxes, however sometimes needs must. If you do this make sure that you use an appropriate rated junction box that meets at least the current rating of your main supply fuse. Don't use "chocolate block" connectors, but make sure you buy a proper connection unit. Other than using a junction box there is no magical way to lengthen short cables.

It might be possible though to lengthen the main live feed from your main switch to the consumer unit instead of all the tails from the consumer unit out to the rest of the house

Good luck

 

[StevieB]

Hi Graham,

Many thanks for the answers!

Unfortunately I dont think it will help to lengthen the main feed to the new unit instead of all the tails going out into the house since the feed from the old consumer unit goes up the wall and the returns come into the consumer unit from below Thus I am restricted by the height of the old unit, and I fear the new unit will be smaller :roll:

Thanks again,

Steve.