UK New Sub Board for aircon

[Flanders]

Just to add a 100 amp switch or main RCD will not give you overload protection . An RCD is used to detected a fault current only and as others have said it's not a good idea to use an rcd to protect a distribution circuit.

 

[MattBaigent]

The "least trouble" option is to feed the new DB from a 63A MCB in the existing board, and then put in separate MCB for each A/C unit, or RCBO if the A/C units have any need for additional protection (cable routes not guaranteed deeper than 50mm behind wall surface, etc). For the minimal extra cost I would go RCBO.

You won't get much selectivity between the A/C final breakers and the 63A feed, but making it 63A C-curve or D-curve would help, and also checking what the A/C specifications say they really need in order to select the smallest acceptable MCB/RCBO as well.
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As already said, the A/C capacity is it heat-pumping ability and typically the input electrical power is less than half of that, but they do have motors starting and stopping so you might find the breaker specification is higher to allow for the surge.
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Final thought, have to checked the feed arrangement to the garage DB you are planning on attaching to? You need to check the sub-main cable has both the necessary current carrying capacity AND low enough impedance to meet the voltage drop requirements.
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A photo might help!

Reading the OP again it might be the mains supply enters at the garage, so no sub-main cable rating to worry about there, but the issue of looking at how the per-phase loads stack up still needs to be done.

the mains supply enters into the centre of the property and a sub main in the garage fed by a 25mm 3 core SWA. We were unable to enter the property due to the owners being out.

 

[davesparks]

the mains supply enters into the centre of the property and a sub main in the garage fed by a 25mm 3 core SWA. We were unable to enter the property due to the owners being out.

So you don't even know what size OCPD is protecting the submain?

I assume the galv adaptable box is being used as a spreader for the 25mm SWA? If so you could swap it for a larger box and put some of the bolted terminal type din rail terminals in there to split the feed to your new board.

 

[MattBaigent]

So you don't even know what size OCPD is protecting the submain?

I assume the galv adaptable box is being used as a spreader for the 25mm SWA? If so you could swap it for a larger box and put some of the bolted terminal type din rail terminals in there to split the feed to your new board.

unfortunately not. I am going there on Monday (hopefully) and we will be able to get inside and look.

Yes it is and yes, that can be done. Is that essentially the same principle as a Henley block? Would I need a fused switch isolator before each board or is this just personal preference?

also, thank you everyone for your input, it’s been really interesting reading it all.

 

[davesparks]

Yes it is and yes, that can be done. Is that essentially the same principle as a Henley block? Would I need a fused switch isolator before each board or is this just personal preference?

I guess you could say its the same principle, but I would say they are completely different things.
Any way you manage to hack Henley blocks into that installation is going to look like ,and likely be, a bodge

Why would you need a fused switch before each board? The question makes no sense to me, what is your logic behind suggesting this?

 

[pc1966]

There is a spare 20A breaker on the non-RCD side that could be changed to something bigger to feed a 2nd adjacent DB, but the bigger question is the total load here, and also how it is distributed between the incoming phases.

As @davesparks said earlier, you need to do a load study (maybe helped by any documentation on the original design?) to see what loads are present and decide if it is reasonable to take it from here.

That should also re-visit this DB feed cable to see if it is OK for the increased load here with the A/C added. If 25mm SWA as you say it is almost certainly fine to the 100A load limit of that existing DB for current carrying but the volt drop might be too much. Again if the original calculations are available that might be simple, otherwise you need to determine the cable length in addition to establishing the new load/diversity, etc.

Splitting the SWA out to feed two boards might be tricky to do in any neat way, so unless you really need to, then taking it from a breaker in that board would be simplest. But only if all calculations work out, of course!

 

[MattBaigent]

I guess you could say its the same principle, but I would say they are completely different things.
Any way you manage to hack Henley blocks into that installation is going to look like ,and likely be, a bodge

Why would you need a fused switch before each board? The question makes no sense to me, what is your logic behind suggesting this?

because I’m learning and just asking a question.

 

[David Prosser]

Looking at the potential load on the single phase board in the photo I think you need to check what else is fed of that phase from the three phase board. You may find that a new supply cable for the A/C of a different phase is a better idea.

 

[MattBaigent]

Looking at the potential load on the single phase board in the photo I think you need to check what else is fed of that phase from the three phase board. You may find that a new supply cable for the A/C of a different phase is a better idea.

ok, I will have a look at what is on the other boards. Thank you.

 

[davesparks]

because I’m learning and just asking a question.

It's a very specific question so you must have a reason for asking that question. So why do you think a fuse switch would be required?

It appears you aren't just learning but are attempting to complete work by getting the forum to do your designs for you.