Definition of own circuit

[timhoward]

"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.

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.

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.

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.

 

[mainline]

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.

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

 

[Steve-S]

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.

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.

 

[mainline]

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

"Should be supplied by their own separate circuit.

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 ?

 

[timhoward]

I have seen nothing yet to negate that above.

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?

 

[Steve-S]

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 Yes a thread in its aren't fully changed.

Yes, a thread in its own is worthy.

In this case far more than 3kW can be generated which the surplus power over 3Kw (immersion heater size) will be used by the whole house and not exported back to the grid. In short, all the solar electricity will be fed into the whole house.

An iBoost can be fitted adjacent to a cylinder. The iBoost monitors export to the grid, and when export is about 25 watts, the iBoost sends solar power it to the 1st immersion output (to immersion), when the water temp reaches max temperature, it then switches to the 2nd immersion (the bottom one). Then when the both are at max temp, the iBoost allows for export to the grid. If the iBoost has no water to heat it can only allow export. This is fine when having a cheaper fuel like gas available. When solar available a relay can be on the output of the iBoost to switch out the gas boiler.

 

[Steve-S]

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?

Thx.
Still it is full of recommendations, advice, etc. Nothing firm.

A point you bring up "future electricians will do a double-take".
I did a search on this forum finding one thread from a few years ago, that an electrician would not connect up an immersion as a control unit consuming hardly any current at all was to be taken off the same adjacent supply. The circuit was a dedicated radial from the CU with a 16A MCB. Of course the circuit was then shared, not separate. I realised that one way around it was to make the cable from CU a distribution circuit to a small CU at the cylinder with a 13A MCB for the immersion and an MCB for the control box. Hassle and expense to placate an awkward electrician which added no extra value in safety, but sorted.

Thinking on those lines A 4-way metal CU off the ring with a double pole 16A MCB. Off the MCB a 13A MCB for immersion and a 3A for the iBoost. But I think the iBoost is powered from the incoming immersion supply. That should stop a double-take.

 

[mainline]

Thx.
Still it is full of recommendations, advice, etc. Nothing firm.

A point you bring up "future electricians will do a double-take".
I did a search on this forum finding one thread from a few years ago, that an electrician would not connect up an immersion as a control unit consuming hardly any current at all was to be taken off the same adjacent supply. The circuit was a dedicated radial from the CU with a 16A MCB. Of course the circuit was then shared, not separate. I realised that one way around it was to make the cable from CU a distribution circuit to a small CU at the cylinder with a 13A MCB for the immersion and an MCB for the control box. Hassle and expense to placate an awkward electrician which added no extra value in safety, but sorted.

Thinking on those lines A 4-way metal CU off the ring with a double pole 16A MCB. Off the MCB a 13A MCB for immersion and a 3A for the iBoost. But I think the iBoost is powered from the incoming immersion supply. That should stop a double-take.

The more you think, the worse it gets.

 

[Steve-S]

An explanation of how you intend to wire this maybe helpful.

An option is off the upstairs 32A ring take a separate radial protected by a double pole MCB feeding the iBoost which switches from top to bottom immersions. Then a relay on each output of the iBoost to switch out the gas boiler. The iBoost communicates to its CT at the CU by wi-fi.

Gas boiler heats the cylinder to 56C with blending valve down to 53C for the DHW outlet temp using a blending valve. The iBoost will heat the cylinder to 75-80C, clawing as much free solar electricity as possible. If the sun is out first thing heating the cylinder, and the cylinder is depleted of DHW because of morning showers, the gas is cut out and the solar stays on heating cylinder.

 

[Steve-S]

The more you think, the worse it gets.

So says the resident jester. I do like fun.

 

[mainline]

Thinking on those lines A 4-way metal CU off the ring with a double pole 16A MCB. Off the MCB a 13A MCB for immersion and a 3A for the iBoost. But I think the iBoost is powered from the incoming immersion supply. That should stop a double-take.

I do hope you're not actually doing electrical work.

 

[mainline]

An option is off the upstairs 32A ring take a separate radial protected by a double pole MCB feeding the iBoost which switches from top to bottom immersions. Then a relay on each output of the iBoost to switch out the gas boiler. The iBoost communicates to its CT at the CU by wi-fi.

Gas boiler heats the cylinder to 56C with blending valve down to 53C for the DHW outlet temp using a blending valve. The iBoost will heat the cylinder to 75-80C, clawing as much free solar electricity as possible. If the sun is out first thing heating the cylinder, and the cylinder is depleted of DHW because of morning showers, the gas is cut out and the solar stays on heating cylinder.

This is the thorn in your side and not just for this design but seems to be for any design you think up.

The manufacture states that:
1. The electrical installation of this device must only be undertaken by a suitably trained and qualified electrician.

All local safety standards must be observed.

All work must satisfy Building/IEE Wiring regulations in force at the time.

 

[Steve-S]

Thank you. Correct on all points.