Boiler wiring

[pm]

A couple of q's on boiler wiring:

The boiler manual says to run the pump live, earth & neutral from three terminals on the boiler. I must also run live, neutral and earth to the wiring box for the controls. This needs to be fed from the same switch as the boiler so it is not possible to switch part of the system of and have the rest live.

I also need a switched live from the wiring box to the boiler.

I plan to put the wiring box and pump in the airing cupboard. The boiler will be in the kitchen.

First q - the boiler wiring must be 90deg heat resistant, I can only find flex that is high temp rated and I understand that flex mustn't be buried in walls. Why is this? For a fixed installation, can I run the flex in conduit that is buried? I would rather do this than surface mount.

Second q - ignoring the permanent L/N/E from the switch to the wiring box, according to the instructions I need four conductors going from the boiler to the airing cupboard - pump live, pump neutral, pump earth and switched live. All the heat resistant flex I've seen is three core. Can I get away with using the earth cable in the flex as the switched live and take the pump earth from the wiring box if I tag (or sleeve) the cable appropriately? To me this is similar to using the black connector for switched live in a lighting circuit.

Answers, comments and suggestions appreciated as always.

Pete

[Andrew Gabriel]

Flex isn't normally buried in walls, but there's nothing in the wiring regulations which forbids it. 90deg T&E is available; it has a white sheath with the new wiring colours (white sheath with old wiring colours doesn't mean 90deg). You would probably have to go to an electrical wholesaler or other specialist supplier though.

No, the cable needs an earth conductor in it even if it isn't used at the other end.

I suggest you go along to a wholesaler and see if you can get a small reel of 90deg triple and earth. You will probably have difficuly buying it by the metre.

[John White]

Electricians do not normally bury flex in walls - T&E cable is cheaper and we always have loads of it.

The "official" reason is that the thermal and mechanical properties of the flex can differ widely from those of a flat cable.

Most electricians will wire the boiler to a flex outlet plate

heat-resistant flex, and then to wire the rest of the installation in T&E cable.

No. Don't do this as anybody cutting into the cable in the middle would expect the bare conductor to be earth. Use 1.0mm or 1.5mm three-core and earth cable instead.

No that's using an insulated conductor not the earth. You should also add sleeving to indicate that the conductor is live and not neutral.

If you are in England or Wales then "Part P" will apply to this work.

John

[John Rumm]

The normal approach if you want it burried, would be to run it most of the way in T&E and terminate it at a flex connection unit. Then do the last bit in heat resistant flex.

I have seen people re-designate the earth in this way, and to be honest it is poor practice.

A better way would be to wire the bulk in three core and earth, and then use flex for the last bit. If you need more than the normal number of cores in a flex you can either use more than one of them, or get some "control flex" which is usually available in upto seven core. For example:

(note that is only a 70 degree one)

High temp:

[PM]

Thanks Andrew and the Johns, I have sourced some 4-way heat resistant flex, available by the metre, and bought two flex outlets.