A single 16 amp ceeform socket outlet should be connected to an MCB not exceeding 16 amps. Although LESS than 16 amps is permitted, I would consider this to be poor practice in most situations, an ordinary user might expect to be able to use a full 16 amps.

Two or more 16 amp ceeform outlets may be protected by a 20 amp MCB.

Any appliance fitted with a  16 amp plug should be suitable for connection to a 20 amp circuit. The flexible cord fitted to the appliance may be sized according to the load, but not less than 0.75mm. Experience has shown that 0.75mm flex can be protected against short circuit by a 20 amp MCB. Overload protection should be by design of the appliance.

If you have a circuit feeding one or more 16A sockets you can use a 20A breaker if you like - the OSG annex H3 used to mention this explicitly.

H3
Radial final circuits using 16 A socket- outlets complying with 85 EN 60309-2 (BS 4343)

H3.1 General
Where a radial circuit feeds equipment the maximum demand of which, having allowed for diversity, is known or estimated not to exceed the rating of the overcurrent protective device and in any event does not exceed 20 A, the number of socket-outlets is unlimited.

H3.l Circuit protection

The overcurrent protective device should have a rating not exceeding 20 A.

And it was in the body of the older regs as well, though I cannot find a reference  quickly now. Actually you may like to know that the old BS4343 from which the commando plug derives its dimensions is pretty comfortably rated, and unless badly tarnished the 16A model is good for a quite a bit more than 20A without any distress. If only the authors of the 13A plug spec had included a similar slack, but they didn't, and it doesn't

Mike

But if I say wanted to connect to a heavily loaded consumer unit my initial reaction would be to put in something like a 20A breaker

Alternatively you could use a 16A MCB and a spacer each side to mitigate the warming from adjacent devices. In many cases, where the uprating of the protective device would have knock-on consequences, that may well be the preferred approach.

  - Andy.

Another option (granted not a first choice in many cases) might be a larger OCPD in the DB sized to take account of the group derating, with a closer (in this case 16A) device providing overload protection downstream. If the cable twixt the two were sized appropriately, you might then have created room for future expansion rather than using up multiple ways.

broadgage said:

A single 16 amp ceeform socket outlet should be connected to an MCB not exceeding 16 amps.

Why ... what is the difference between a single 16 A socket-outlet supplied by a 20 A circuit-breaker, and a circuit with two 16 A socket-outlets supplied by a 20 A circuit-breaker, in which one of the socket-outlets is not in use, or is pulling additional load (causing heating behind the terminals of an upstream socket-outlet that might be fully loaded) ?

The 20 A may be preferable because of inrush or starting currents in some cases?

Similarly, there is nothing to prevent a 30 A ring-final circuit with only one single or double 13 A BS 1363 socket-outlet, or a radial from a 16 A or 20 A circuit-breaker with a 13 A single or double socket-outlet ?

It also depends on the type of load - in general 16A sockets are not used in the same "plug and play" way as the 13A domestics, but there is some of the same uncertainty in some settings, like building sites and some kinds of workshops

Certainly in "known load"  installations (I'm thinking of theatrical lighting and some types of outdoor events ) where the lamp load for one socket is very much planned and fixed as cables get installed, the rules are pushed a bit. Splitters that take a single 32A plug to  a couple of 16A sockets , with no fusing down, are quite common, though they are  a source of some teeth whistling and clucking noises from 'real' sparks. There is an assumption that the folk doing the set-up  are not random public, and are capable of adding up amps and not doing something too silly so both tails get a similar load and not one side dangerously over 16A..

Mike.

mapj1 said:

It also depends on the type of load - in general 16A sockets are not used in the same "plug and play" way as the 13A domestics

Problem with 13 A domestics is that BS1362 fuses tend not to be able to cope with starting currents of chunky machines in a well-equipped home workshop. One solution, which my friend did, is to machine a piece of brass rod to the appropriate dimensions. :-0

I suspect that a 13A plug without the fuse can withstand much higher currents without overheating.  The fuse is the biggest source of heat.

I cannot disagree. I don't think that I have ever felt a hot BS 1363 plug even connected to say, a 3 kW heater.

Chris Pearson said:

I cannot disagree. I don't think that I have ever felt a hot BS 1363 plug even connected to say, a 3 kW heater.

1. What damages some socket-outlets? Occasionally, it's an overheating plug.
2. I've seen plugs on some appliances where multi-way adaptors have been overloaded, or there are broken conductors in the flexible cable supplied by the plug.