Load on a ring main

[Andy]

Hi,

I'm looking at purchasing a homebrew boiler. It's basically a stainless vessel with two integral 3kW heating elements. The elements are capable of being switched independently but there are times where I'd require both of them on. I intend to use the boiler in my garage which has sockets served via a RCD fusebox (don't know exact term for it) which is run as a spur off the downstairs ring main. The downstairs ringmain is connected to a 32A MCB, the house was constructed in

1990.

My question is, am I OK to operate the boiler with both 3kW elements on given the above configuration ? I'd obviously ensure that any additional load on the ringmain was minimised by switching off TV, PC, washing machines etc. 6kW would draw 25A - would my ringmain be OK with this ?

Cheers!

Andy.

[dom]

Presumably your garage is on a 16A spur from the ring main?

The answer is no. Your peak power demand is around 25A and requires a dedicated radial as well the appliance being wired in.

Can we ask what you're cooking up with a 6KW boiler?

[Andy]

Not sure, will check at home this evening.....

Darn.

Beer!

[Owain]

No, absolutely not.

No, because you'd be creating a high point load at one point on the ring. More to the point, you're running off a spur from a ring, which must not have more than one single or twin socket connected to it, and will have a max assumed intermittent load of 20A. In fact, you probably have an RCD fused connection unit, so the spur will be restricted to 13A max. (having the fused connection unit is the only way you'd be allowed to have more than one single or twin socket on the spur).

Plugging the appliance into 2 sockets simultaneously (even though presumably the heaters are entirely electrically separate) also means you do not have a single point of isolation for the appliance.

You require a dedicated >25A circuit from the consumer unit and a double pole isolator supplying both heaters.

Fittings and wiring in the garage may not be suitable for use in a steamy environment anyway.

Owain

[Christian McArdle]

Absolutely not. The load is completely unsuitable for your electrical installation.

It isn't clear if your spur is fused or unfused. A fused spur can supply 13A and an unfused spur is rated for 20A.

However, even if you ran only one of the elements, it still isn't allowed, as immersion heaters are not allowed on socket circuits, as they are inappropriate to the diversity calculations used to justify them.

Christian.

[David Hansen]

On Tue, 22 Aug 2006 14:18:48 +0100 someone who may be Owain wrote this:-

Fuse of up to 13A or a circuit breaker of up to 20A.

[David Hansen]

On 22 Aug 2006 05:54:53 -0700 someone who may be "Andy" wrote this:-

Quite a big boiler by the sound of it. Sounds like it is not portable equipment and thus should be connected permanently to the supply.

You will need a proper sub-main from your consumer unit to say a four way board in the garage. From there you can run circuits to the boiler, sockets, lights and so on.

The boiler should have a switch nearby. If it is vaguely portable you might consider fitting a 32A BS4343 plug to the lead, which can be plugged into a suitable socket.

Of you are in England or Wales beware of Mr Prescott.

[Andy]

No, it's very portable.

Sounds like waaay too much hassle so I won't bother with the purchase.

Thanks for all the advice chaps.

[The Medway Handyman]

There are carpet cleaning machines that are plugged into two sockets. One cable runs the 3kw water heater, the other runs the 2 x 1200 watt vac motors and the 100watt solution pump.

I have been told these could be dangerous, because in commercial buildings the single phase sockets could be taken from different phases of the 3 phase supply, so plugging in two cables from one machine could lead to a potential

415v shock.

Is that right?

[Andrew Mawson]

sockets. One

Potentially

AWEM

[robgraham]

What a bunch of spoil-sports -- there was a great chance for us all to get free beer, and you all blow it out of the water !!

Rob

[meow2222]

there are ways it can be run in the house off 2 sockets, each one on a separate ring, but... the house just isnt set up for this kind of load.

NT

[Andrew Gabriel]

One would presume such an appliance is designed with this in mind. Shocks are usually to earth anyway, so it's very difficult to imagine how such an appliance could give you a 415v shock in any kind of even remotely possible scenario.

[Guy King]

The message from "The Medway Handyman" contains these words:

As far as I know you shouldn't have nearby areas of the same building running off different phases.

[Owain]

Yes.

I suppose if the flexes were worn at the point of entry to the machine (which is not an uncommon fault) then the two phases could be exposed. Whilst statistically improbable, the consequences of a phase-phase shock should feature strongly in the H&SAWA Risk Assessment.

Owain

[Frank Erskine]

I remember an electric water still in one of the chemistry labs at school. It was made by a firm called Manesty and it had two mains plugs. I don't know whether or not the two sockets were on different phases; I assumed that the arrangement was merely to accomodate a current draw in excess of 13A. I'd have thought that since the sockets were clearly provided for the still that they'd be on separate radial circuits. No doubt it'll have been removed by now - distilled water is probably banned now under COSHH regulations ;-) I know that pupils used to be rebuked by "Jakey" for squirting distilled water at one another - "It costs a shilling a pint, boy!"

[Owain]

You'd be much better with a copper hot water cylinder hooked up to the CH boiler - that'd give you loads more heat than 6kW and be cheaper than the lekky to run.

What about a big gas ring (as used by tar-boilers) on a gas cylinder?

Owain

[dom]

Do you know any brand names?

Is it possible this was a "modification"?

I'd expect such a specialised machine to plug into an industrial 3 phase connector.

[Andrew Gabriel]

... which would make it useless nearly everywhere.

[The Medway Handyman]

Hydromist, now Truevox. And a few others.

No, off the shelf jobby

Virtually all cleaning machines are designed to plug into a 13 amp socket. At least 99%.