Definition of own circuit

nicebutdim said:

Perhaps the OP should pose these questions to the IET.

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The key word is separate. It is common to see a sub CU off a say 40A beaker in the main CU. Similar arrangement is also done with garage CU units. So a 40A circuit has a number of sub circuits off it with one maybe being an immersion. So an immersion then does not have its own circuit back to the main CU. It is a circuit off a circuit, but regarded as separate and all above board. No different to taking an immersion off a final ring with its own mcb at the take off point.

Looks like a sense of humour is banned on this forum now then!

Steve-S said:

The key word is separate. It is common to see a sub CU off a say 40A beaker in the main CU. Similar arrangement is also done with garage CU units. So a 40A circuit has a number of sub circuits off it with one maybe being an immersion. So an immersion then does not have its own circuit back to the main CU. It is a circuit off a circuit, but regarded as separate and all above board. No different to taking an immersion off a final ring with its own mcb at the take off point.

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The key word is not separate it is final.

Sub circuit is not a currently recognised term, circuits are either final circuits or distribution circuits.

A distribution circuit is a circuit which feeds a distribution board or consumer unit which then in turn feeds other circuits.

A final circuit is a circuit which feeds final points of utilisation, either lighting points, sockets or fixed current using equipment.

A ring final circuit is, by definition, a final circuit and not a distribution circuit.

davesparks said:

Looks like a sense of humour is banned on this forum now then!


The key word is not separate it is final.

Sub circuit is not a currently recognised term, circuits are either final circuits or distribution circuits.

A distribution circuit is a circuit which feeds a distribution board or consumer unit which then in turn feeds other circuits.

A final circuit is a circuit which feeds final points of utilisation, either lighting points, sockets or fixed current using equipment.

A ring final circuit is, by definition, a final circuit and not a distribution circuit.

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Thanks. However it says:
"Should be supplied by their own separate circuit."

It does not say final.

Steve-S said:

Thanks. However it says:
"Should be supplied by their own separate circuit."

It does not say final.

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Yes a separate circuit, if it is not a seperate final circuit then it is not separate.

A distribution circuit is a type of circuit which, by definition, feeds multiple separate final circuits via a distribution board or consumer unit.

A final circuit does not feed any other circuit.

A ring final circuit cannot then feed another circuit.

Really needs the definitions in the regs to pin it down.

Steve-S said:

Really needs the definitions in the regs to pin it down.

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Yes I agree with you.

You should go and spent some time revising.

I have not found anything yet to counter my initial assessment. I may do, you never know.

Steve-S said:

Really needs the definitions in the regs to pin it down.

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The Wiring Regulations contain clear definitions for both distribution and final circuits in the section appropriately titled 'Definitions'.

Steve-S said:

I have not found anything yet to counter my initial assessment. I may do, you never know.

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It's not hard to find as 'Definitions' are afforded their own section in The Wiring Regulations - 'Section 2'.

Not so much a case of not having found anything yet, but one of deliberately ignoring regulatory instruction which does exactly what you 'haven't been able to find'.

You can read and understand The Wiring Regulations or you can continue to ignore them and form more of your own mad ideas - quite frankly it makes no odds to me. At this point I'm simply looking forward to the rapidly approaching moment when these threads stop appearing on this forum.

Mad is creating a new circuit to take an immersion off? my oh my! Such Entrenched views.

Steve-S said:

Mad is creating a new circuit to take an immersion off? my oh my! Such Entrenched views.

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The IET write The Wiring Regulations - as previously suggested it would be best to take this argument up with them. So far not a single member of this forum has expressed an opinion in favour of your idea, so it's unlikely that continuing to repeat the same action will yield a different result.

Steve-S said:

"I believe it’s been established that the regs don’t allow you to put an MCB on the ring."
It has not been established.

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Maybe I'm missing something but my reasoning is that there is only one reg about ring final circuits:


An MCB isn't an accessory to BS 1363.

Steve-S said:

Nephew has had a substantial solar array installed. There was no immersion cable run to the upstairs cylinder by the builder 20 years ago as it was heated an still heated by gas. So he naturally wants to heat it via electricity for free a lot of the time. On a cloudy day he can pull in around 3kW. To get a cable to it will be a nightmare when a little used final ring is adjacent.

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Thank you for giving the context. That would be a much better topic for a thread as about 10 questions immediately come to my mind.
If he ever adds battery storage and wants to use a solar diverter, he will want a dedicated circuit that the solar diverter can simply turn on and off, and he won't want the upstairs ring turning off when the batteries aren't fully changed.

timhoward said:

Maybe I'm missing something but my reasoning is that there is only one reg about ring final circuits:
View attachment 117652

An MCB isn't an accessory to BS 1363.


Thank you for giving the context. That would be a much better topic for a thread as about 10 questions immediately come to my mind.
If he ever adds battery storage and wants to use a solar diverter, he will want a dedicated circuit that the solar diverter can simply turn on and off, and he won't want the upstairs ring turning off when the batteries aren't fully changed.

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Also, I believe that with solar diverters, the load must be resistive.

timhoward said:

Maybe I'm missing something but my reasoning is that there is only one reg about ring final circuits:
View attachment 117652

An MCB isn't an accessory to BS 1363.

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H5 relating to an immersion heating over 15 litres of water says:

"Should be supplied by their own separate circuit."

Taking a radial off a final ring with its own ocpd (MCB or fuse):

1) Creates a circuit that can have more than one outlet.
2) Having more than one outlet the circuit is shared by the outlets hence not separate.
3) A separate circuit is serving only one outlet.

I have seen nothing yet to negate that above.

Steve-S said:

H5 relating to an immersion heating over 15 litres of water says:

"Should be supplied by their own separate circuit.

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I think the first thing to attempt to understand is the word “separate”.

If you plug an iron and a washing machine into a ring final circuit are those items on separate circuits or both on a ring final circuit ?

Steve-S said:

I have seen nothing yet to negate that above.

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I'll do my best to explain why I think your intended approach is flawed:

The regs document BS7671 has only one reg about ring final circuits and honestly it's pretty vague, amounting to use one of these protective devices, and this cable, and make sure you won't exceed the current carrying capacity of the cable for long periods.

To complement this there is an appendix 15 in BS7671 specifying ways to comply with the one regulation (433.1.204)
That says that the requirements can generally be achieved by (Fig 15a ii ) "Not supplying immersion heaters, comprehensive electric space heating or loads of a similar profile from the ring circuit." (direct quote, page 555)
Indent iii also advises to run dedicated radial circuits for various large loads over 2Kw.

To further complement the regs there is the on site guide. If you follow this to the letter you are likely to comply with the actual (more complicated) regulations.
As you have stated, the OSG also says to put immersion heaters on a separate circuit (paraphrased)
It makes no sense at all to interpret the OSG as you suggest as it would completely undermine appendix 15; the power is still coming from the same ring final circuit along the same cables, and the stated intention is not to exceed the ring final circuit's CCC for long periods.

The actual compliance comes down to the loading of the circuit, and it may be safe and comply today. It also may not be safe another day if the heating fails and the fan heaters come out.

This all amounts to a choice. You either do an installation that follows the advice in the regs and the on-site-guide that any competent electrician won't question, or you do something unusual that future electricians will do a double-take at, instantly think of the sentences in black and white in two places that advise not to do this, and get them wondering if it's compliant.

Even if you go ahead, I'm intrigued what your intention is for controlling it? Would he have to go upstairs when it's sunny and turn it on?