4 consumer units on a single supply

[JonChas]

Hello, I would like some advice about the proposed approach to be taken by my electrician.
 

I have a single phase 100A supply to my property and need this to serve 4 separate consumer units. (A diversity calculations has been made).

My electrician has proposed using a Ryefield distribution board to split the supply to the 4 consumer units. He has informed me that a 5-way Henley block is not suitable for this application. 

However, I have seen many examples online where a Henley block has been used to distribute the supply to consumer units, but admittedly not to split the supply 4 ways.

Is he correct in stating that the Ryfield board is the only suitable appropriate approach for splitting the supply 4 ways?

Thanks,

Jonathan.

[ProDave]

Henley blocks don't provide isolation and over current protection of the submains, so your electrician is correct.

 

[JonChas]

Hi Dave,
 

Thanks so much for the response. 
 

The full layout is as shown (I didn't include full description previously for simplicity). 

There will be a fused switch at the meter and isolators for each consumer unit after the supply has been distributed. 
 

In this instance would Henley blocks work?
 

Thanks,

Jonathan.

[ProDave]

I would go straight from meter to henley blocks then feed each SWA with a switch fuse.

[Nickfromwales]

1 hour ago, ProDave said:

I would go straight from meter to henley blocks then feed each SWA with a switch fuse.

Yup +1.

 

Defo need to have those de-rated to say 60a per run vs just isolated x4. Can be the same device, just knock a way out for a 63a MCB in each one, and then you have a switch-fuse isolator.

 

Is this a very big property, eg to warrant 4 CU's?

[ProDave]

And unless the run to the sub boards is long, once fused down 16mm is plenty.

[JonChas]

Thanks for this.
 

Yes, I'll need the 4 CU's.
.

We chose for 25mm2 SWA to future proof it, I may at some point switch to a 3-phase if greater capacity is needed.

I want to avoid the Ryefield as it is a large expense which would be superfluous if I do switch to 3-phase.
 

Good to know you see no issue with the Henley blocks.

[Nickfromwales]

54 minutes ago, JonChas said:

Good to know you see no issue with the Henley blocks

No problem at all. 
 

There are benefits to be had with going with our advice here, some which are not immediately obvious but soon become apparent after opening a dialogue here for experts in their industry to bolster each decision you then go on to invest time and money into, safe in the knowledge that….  
 

 

 

 

 

 

 

 

 

 

 

 

 

……in the winter you can use them to keep your hands warm 🤣. 

[Gus Potter]

On 03/03/2025 at 20:21, JonChas said:

(A diversity calculations has been made).

 I assume you have gas hobs etc and showers running off a gas combie boiler and not on electric.

 

Potentially do you have 4 ovens running on xmas day.. make it make sense please or is BS 7176 not all DEI?

Edited Tuesday at 23:34 by Gus Potter

[JonChas]

Yes, the heating/hobs will be gas.
 

I do have a further question related to this if I can tag it on.

My DNO specifies that the meter tails between the meter position and the consumer unit should not be longer than 3 metres without the installation of an additional protective device at the nearest point to the supply inside the premises.
 

The consumer units will be over 3 metres from the meter and therefore would I need to keep the fused switch after the meter (before the Henley blocks split the supply) (as shown in my previous post)? If so, I assume I would have this fused switch as well as having the isolator switch with 63A fuse (or MCB) on each of the 4 feeds to the consumer units (after the Henley blocks)?

Edited Thursday at 21:09 by JonChas

[ProDave]

44 minutes ago, JonChas said:

My DNO specifies that the meter tails between the meter position and the consumer unit should not be longer than 3 metres without the installation of an additional protective device at the nearest point to the supply inside the premises.

Henley blocks and your bank of switch fuses need to be withing 3 metres of the suppliers meter.  Simple.

[Gus Potter]

3 hours ago, JonChas said:

My DNO specifies that the meter tails between the meter position and the consumer unit should not be longer than 3 metres without the installation of an additional protective device at the nearest point to the supply inside the premises.

I'm not a spark, rather an SE / Designer. So here is a laymans question! Mind you I've see odds an sods hence why I'm asking.

 

My though process is you have an incoming DNO connection rated at 100 amps. I guess as a layman that will be protected with a 100 amp sand type fuse just before the meter. This fuse is not really intended to protect you, rather it's to protect the electricity board cables. Sand fuses don't blow in the same way as say a fuse on a 13 amp plug. The sand adsorbs the heat and delays the blowing of the fuse. If you say have a 120 amp demand then sand will heat slowly and then the fuse will blow after a bit of time. Draw 400 amps and the fuse will not last so long. This is maybe why DIY shed welders wonder how they can get more bang for their buck! I think there is a table in BS 7671 ( the sparks regs) that explains how long it takes for sand fuses blow depending on how much you overload them.

 

The cable that comes into your house probably runs to a big local transformer. That cable might have a 600 amp fuse at the eletricity board transformer end.

 

Now I faced a similar dilemma. What the spark did was to fit a 100 amp fuse isolater switch just after the meter. Then run a thick wire armoured cable and split that at the consumer unit end, hence overcoming the 3.0 m restriction on the tails. I have more than one consumer unit in my house, one is mounted above the other.. long story but each consumer unit is capable of serving one dwelling. To be a bit different the spark and I agreed to wire the house using radial circiuts, it suits me but it's technically less efficient than using ring mains for example.

 

Now a couple of days ago the spark phoned me and said my industry (can't remember if it was SELECT or NAPIT) body wants to inspect some of my jobs, can the inspector come to look Gus? Inspector turns up and gives my spark work a clean bill of health.

 

 

[JonChas]

10 hours ago, ProDave said:

Henley blocks and your bank of switch fuses need to be withing 3 metres of the suppliers meter.  Simple.

Thanks for this Dave,
 

What I don't understand is how a 63A fuse on each of the 4 sub supplies would protect the incoming supply? 

This would not do the same as a 100A fuse after the meter?

[joth]

The incoming supply should be protected by the dno fuse on the cut out before the meter. 

 

[Nickfromwales]

4 hours ago, JonChas said:

Thanks for this Dave,
 

What I don't understand is how a 63A fuse on each of the 4 sub supplies would protect the incoming supply? 

This would not do the same as a 100A fuse after the meter?

The max you can draw is 100a, before the supply head fuse would ‘blow’ (it’ll take a lot more than 100a before it blows though).

 

Your 100a fuse protects the meter / DNO equipment, plus also limits your max possible draw on the network (grid). 
 

Each individual switch fuse after the Henley’s de-rates the 100a that would otherwise be available to each of the 4 legs.
 

Because you have 4 satellite fuse boards (CU) that are >3m away from the supply fuse you must limit the liability of each of the cables feeding each of the CU’s, eg in the event that a cable becomes mechanically damaged, or if any CU takes too much juice and would risk blowing the 100a fuse. You cannot have such a huge amount of power (current capacity) in a cable traversing a domestic dwelling. Just too risky. This is why the head / 100a fuse / meter are always on an outside / external wall or fed from a cable riding from the ground immediately under them. None of that has true earth protection, so can only ‘blow’ with over current or a catastrophic short circuit (mechanical damage directly linking live to neutral). Anything less than a direct short results in a spectacular and worryingly long lasting fireworks display as the fault arcs itself into infinity, literally. In that event you soon realise just how much raw power is available from that 100a fuse !!

 

De-rating allows each of the 4 legs to all be managed in terms of the max current carrying capacity vs the distance of the cable going from each switch fuse to each CU, and caps the max amount of current each can draw in the event of a failure.
 

If one CU was drawing 80a and the other three say 40a a piece then the main fuse would blow (rupture), and then you’d go blame the designer as it then should not have got past initial feasibility.

 

Each leg must have its own isolation and lower rated fuse, as you are not allowed to consider the DNO fuse as your primary means of protection.

[Alan Ambrose]

Well the OP doesn't say why  but I was planning something similar. Someone on BH mentioned a preference for putting the kitchen on a separate CU, which makes a lot of sense given the number of circuits. Similarly, the garage / car charger. Add in a shed with a bunch of stuff in it and ... I nearly forgot ... the house. And there you have 4. Or maybe you want a separate 'high integrity' / backup CU powered by batteries to backup the critical lowish power stuff.

 

I was planning similar with cat5 and switches to avoid running miles of cat5 all point-to-point from a single switch.