Maximum load on RCD?

[richgearguy]

Hi - I'd like to add another radial circuit for sockets (32A or 40A) to the RCD side of my CU, and I'm wondering if it's ok to do this, given that there are already three MCBs installed on the RCD side, as below. The RCD is 80A / 30mA.

50A shower 32A ring main (sockets) 16A radial (sockets)

It seems that I'm nearing the 80A capacity of the RCD already. Is this too much load on the RCD?

Cheers.

[Christian McArdle]

I wouldn't myself. Then I don't like sharing RCDs between circuits anyway. I would use RCBOs on any socket circuits and only use the shared RCD for circuits that don't really matter much, like the shower.

You could certainly add your new circuit using an RCBO on the non-RCD side, assuming your consumer unit manufacturer makes one that you have room for.

Christian.

[tinnews]

You can apply 'diversity' to the above which says that although the total load if everything was fully loaded is more than the rating of the switch you can add more on the basis that it won't be turned on all the time.

There are quite specific rules for diversity and what 'average' load you should apply for various types of circuit. (I think you'll find them in the On Site Guide among other places).

The reality is that you should add up the likely maximum load and decide whether 80A is enough.

[Andy Wade]

Yes, but - assuming TN earthing - there is no need for the shower to be on the RCD side. In fact it's specifically recommended that the RCD side of a split-load CU should only feed socket circuits. If possible transfer the shower to the non-RCD side[*] and then you will have plenty of capacity for another socket circuit on the RCD side.

[*] When doing this (a) make sure that the supplementary bonding in the bath/shower room is up to scratch and (b) ensure that the earth fault loop impedance (Zs) for the shower circuit is properly tested in order to meet a maximum 5 second disconnection time in the event of an earth fault.

[David Hansen]

On 27 Oct 2006 07:41:54 -0700 someone who may be [e-mail] wrote this:-

What sort of sockets are you going to connect to a 40A protective device?

[richgearguy]

Thanks for all of your replies. I'm running a 6mm cable to my utility room, to feed three double sockets, then terminating at a fourth double socket (MK masterseal) on the outside of the house. The 3 double sockets in the utility room will feed a washing machine (2kW), tumble drier (3kW), iron (2kW) and the outside socket a garden shredder (2kW). So it's a high load, though unlikely all appliances will be on at the same time.

Cheers

[David Hansen]

On 27 Oct 2006 09:01:49 -0700 someone who may be [e-mail] wrote this:-

Good luck getting that size of cable into the terminals.

Perhaps a study of the black art of maximum demand and diversity is in order. A 30/32A ring or radial circuit will cope happily with those appliances, even if they are all turned on at the same time.