Cable or flex?

[Autolycus]

The (gas) boiler is around 8ft from the area where all the control wiring is to be brought together, in a different room.

The supply to its fcu will be surface-run behind some fixed furniture, then in trunking for the last 4ft to the boiler, and will be in 2.5 t&e. No problem so far.

Is there a presumption against running 4-core flex directly from the boiler's control side, through the trunking, and thence to the controls, as against using a shorter length of flex, then connecting to 3+e cable, as far as the controls? Pro: avoids the need for a flex outlet box / box with a choc block in; Cons: I suppose the flex is less mechanically protected than cable (extra protection where it goes through walls?); it somehow seems wrong to use flex for "fixed" wiring. The boiler fcu will be fused at 3A, the controls are all 240v, and there will be plenty of room for either in the trunking.

[Andrew Gabriel]

There's no prohibition against using flex for fixed wiring. It just usually isn't as convenient or cheap to do so. FWIW, I've got a 7 core flex running from my boiler to its control centre.

[Dave Liquorice]

The boiler?

If so the boiler does not need a FCU as the *whole* CH/HW control system will have a requirement to a single mains feed from DP switched (to provide proper isolation for maintenance, a SP fuse or an MCB is not suffcient) fused at 3A connection unit. Look at the ratings for thermostats, motorised valves, programmers etc as well for the 3A requirement.

All the boiler needs is a flex outlet, so run the T&E from the wiring centre to the flex outlet adjacent to the boiler and flex from there into the boiler.

[David Hansen]

On Wed, 23 Jan 2008 07:41:02 -0000 someone who may be "Autolycus" wrote this:-

Seems a trifle large and I doubt if it comes in three core and earth format.

There is a fair presumption against it, but I can't recall a prohibition.

As you are DIYing it you have the opportunity to do it to a higher standard than someone doing it for a living. If you have the cheaper three core and earth 1.0mm cable anyway then I would run this to a suitable point. At that point I would put either a triple pole "fan" isolator, or a standard cord outlet (with the additional wire connected via a terminal block inside). From there I would run suitable flex to the boiler. This gives a place to disconnect near the boiler which can be useful, for example if the flex is damaged one just has to replace a short length of flex.

[Autolycus]

David and Dave: sorry if it wasn't clear: the 2.5 t&e is the 16A RCD-protected radial circuit supplying the boiler fcu, and an adjacent socket. The fcu will be fused at 3A and will protect and provide isolation for the boiler and all its controls.

I'd prefer to avoid another switch, as it might one day lead to confusion as to which one isolates the boiler. Unfused, unswitched cord outlets aren't so easy to find, unless one goes to a cooker outlet, which I'd guess has rather large terminals to hold a bit of 0.75 flex.

I liked Andrew's re-assurance: I'll go with flex, but leave a small loop somewhere so that if the end is ever damaged someone could re-address this problem for themselves. B&Q want £8.48 for 5m of 4-core 0.75 flex, so I don't think I'll buy it there.

Thanks to all.

[David Hansen]

On Thu, 24 Jan 2008 08:11:04 -0000 someone who may be "Autolycus" wrote this:-

There have been two of these beside my boiler since December.

One takes the supplies (switched live and "permanent" live) to the boiler. The other takes the supply from the boiler to the boiler pump.

[Dave Plowman (News)]

Some grid switch systems include a cord outlet option.

[Dave Liquorice]

So the boiler is fed from one source of mains and the controls from another? *VERY* bad practice, the switched mains from the controls could or be live when the boiler is "off"...

Confused, yes I am.

A FCU does not provide isolation the fuse is only single pole. A DP switched, fused, connection unit does provided the contact gap when off is

[Autolycus]

No: all controls, including timeswitch, motorised valves, and the receiver for an rf thermostat, are (electrically-speaking) downstream of the boiler.

*VERY* bad practice, the switched mains from the controls could

Exactly, which is why I've done it as I have.

Hmmm.

I have checked that the connection unit I've used is DP, but I'll have to rely on the makers about the contact gap.