UK New Sub Board for aircon

[MattBaigent]

Hi all,

Little about me. new to the forum and new to the industry. I have been a data engineer for 2 years specialising in fibre but I always wanted to go into power so I committed to it and paid for the course. I have completed my level 2 and unfortunately my level 3 has been delayed due to the virus.

A friend of mine got a job installing power for an additional 4 aircon units (we have already installed one for the client). According to the aircon installers, they want 3 on a 16a breaker, 1 on a 20a breaker and the one already installed is on a 20a breaker. Before knowing the sizes I suggested fitting a 63a breaker and have that feeding a sub board next to it with all the air con breakers in (There isn’t enough space In the existing board). After knowing the sizes I am now unsure if this will work, and most importantly, be safe. I then spoke to some friends who are electricians and one suggested fitting a Henley block to split the feed into 2 have have both boards on a 100A main switch. This seemed to make sense and looked like a way we could do it.

I am now wondering if I can fit the 100A main switch in the existing board and have it feed the sub board with just the aircon RCBOs inside?

I hope this makes sense? If not then please ask. Or if you know of a better way it could be done then please let me know. I am still very much learning and am open to any advice.

thank you,

matt
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Also, the property is a domestic house, 3 phase supplying the house which feeds a board in the garage (the one I’m working with) and multiple boards inside. I am still yet to find the wattage of the units to find the design current (the aircon installers said they usually run at around 6 Amps which I’m struggling to believe as all the ones I could find online were over 2.5kw which is just over 10A.

I have also looked at diversity but there is nothing specific to aircon. There is power & heating or space heaters and they’re the closest options.

 

[davesparks]

What does the electrician you are working with want to do?

 

[plugsandsparks]

Is the garage board single or three phase
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Oh, A/C units usually quote their output power, the input power is somewhat less. If you get the model Nos of the proposed a/c units the installation manuals often give both the recommended breaker size and maximum running current, this can help with your diversity calcs.

 

[MattBaigent]

What does the electrician you are working with want to do?

the guy I'm doing the work for isn't an electrician, I had a look at it and have been trying to decide on the best way of doing it so I can learn. I'll just be installing the trunking and running the cables, there is an electrician doing the board and will ultimately decide what they want/need
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Is the garage board single or three phase
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Oh, A/C units usually quote their output power, the input power is somewhat less. If you get the model Nos of the proposed a/c units the installation manuals often give both the recommended breaker size and maximum running current, this can help with your diversity calcs.

I have tried multiple times to get info on the units that are being installed but had no luck so far. if I were to have a 2 x 20A breakers and 3 x 16A breakers, I'm assuming this can not be fed by a 63A breaker because all the breakers add up to more.

 

[davesparks]

the guy I'm doing the work for isn't an electrician, I had a look at it and have been trying to decide on the best way of doing it so I can learn. I'll just be installing the trunking and running the cables, there is an electrician doing the board and will ultimately decide what they want/need

So you need to talk to them about what they want installed and how, I'm sure they will be able to tell you what they intend to do and why.
Us guessing from the very limited information you have provided is not going to be much help.

 

[plugsandsparks]

Is the garage DB, 3 phase or single phase ?
I install a/c and if the breaker sizes they are requesting are correct i would have no worries about running all off a 63A MCB, however if the garage board is 3 phase, then its easy to balance the loads, if not they you have balancing the loads to consider further upstream.
Hope this helps

 

[Wilko]

I am now wondering if I can fit the 100A main switch in the existing board and have it feed the sub board with just the aircon RCBOs inside?

Hi - if you did this, what would be the protection for the distribution circuit to the new board? For my money, it’s a no.

 

[MattBaigent]

Is the garage DB, 3 phase or single phase ?
I install a/c and if the breaker sizes they are requesting are correct i would have no worries about running all off a 63A MCB, however if the garage board is 3 phase, then its easy to balance the loads, if not they you have balancing the loads to consider further upstream.
Hope this helps

It’s a single phase board.

 

[davesparks]

It’s a single phase board.

So you may need to consider the effects on the balance of the upstream TP supply

 

[MattBaigent]

Hi - if you did this, what would be the protection for the distribution circuit to the new board? For my money, it’s a no.

I was initially intending on a 63A RCD but one of my questions was if that would be enough which is why I was asking if a 100A would be acceptable but I have not been able to find a 100A RCD
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So you may need to consider the effects on the balance of the upstream TP supply

Could you explain how this would effect the supply?

This is all a learning experience for me. If it’s too much for a Sunday evening then please don’t worry.

 

[westward10]

If the A/C installer has specified those protective devices it is highly unlikely four units will trouble a 63A protective device as previously stated. 2.5kw is not its power consumption but the heat capacity it can generate, it is doubtful they are consuming much more than 6-7A each probably less. Yes balancing of the three phase supply may be an issue so you need to access how the existing loads are connected across them, I suspect L1 is most heavily loaded so your new board needs to be connected from the lightest loaded phase.

 

[Wilko]

I was initially intending on a 63A RCD but one of my questions was if that would be enough which is why I was asking if a 100A would be acceptable but I have not been able to find a 100A RCD
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Hi - If I understand correctly, you’re thinking of an option where the new board could be supplied from the existing board by a 100A switch. The switch will provide a point of isolation, but the new distribution circuit still needs overload and short circuit protection in accord with its conductors, loop impedance and load (fuse, mcb, rcbo etc). Good on you for thinking things through and asking .

 

[MattBaigent]

Hi - If I understand correctly, you’re thinking of an option where the new board could be supplied from the existing board by a 100A switch. The switch will provide a point of isolation, but the new distribution circuit still needs overload and short circuit protection in accord with its conductors, loop impedance and load (fuse, mcb, rcbo etc). Good on you for thinking things through and asking .

Yes, that is correct. So each one of the aircon circuits will be on an rcbo but are you saying that the circuit going from existing board to new one needs more protection than just a 100A main switch? Do you think a 63A RCD would be enough?

I also thought about installing a Henley block and then I would just have the main incoming going to both boards.

With regards to the Henley block option, would I need to install an isolation switch before each board or can I run the tails direct to the DB

 

[Wilko]

Yes, that is correct. So each one of the aircon circuits will be on an rcbo but are you saying that the circuit going from existing board to new one needs more protection than just a 100A main switch?
Yes

Do you think a 63A RCD would be enough?
Likely, but see @westward10 post #11

I also thought about installing a Henley block and then I would just have the main incoming going to both boards. With regards to the Henley block option, would I need to install an isolation switch before each board or can I run the tails direct to the DB.
A single point of isolation for the electrical installation is best. This could be an isolation switch immediately after the meter and before your new service connector block.

Hi - my 20p worth of thoughts are in the quote above .

 

[westward10]

Do they actually need additional rcd protection.

 

[davesparks]

I was initially intending on a 63A RCD but one of my questions was if that would be enough which is why I was asking if a 100A would be acceptable but I have not been able to find a 100A RCD

Putting the whole board on an RCD is a bad idea, the aircon units will very likely be tripping it quite often.

I'm not convinced by your plan of putting them on RCBO's either, why do they need rcd protection?
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Yes, that is correct. So each one of the aircon circuits will be on an rcbo but are you saying that the circuit going from existing board to new one needs more protection than just a 100A main switch? Do you think a 63A RCD would be enough?

I also thought about installing a Henley block and then I would just have the main incoming going to both boards.

With regards to the Henley block option, would I need to install an isolation switch before each board or can I run the tails direct to the DB

You need to explain this a lot better than you have, you've talked about splitting the tails in a Henley block, and about a three phase supply elsewhere.
Is this board being located at the incoming mains position, adjacent to an existing DB fed by a submain or is the new DB remote from all other DBs and itself fed from a submain?

 

[plugsandsparks]

4 X a/c units off one 30mA RCD no matter where you put it is bad design is likely to create nuisance tripping. individual RCBOs or MCBs are the way forward depending on your wiring from the board to the outdoor units.
As the garage is single phase then you need to clamp this phase at the 3 phase board and see what the load is one this phase first before banging 4 X a/c units in. You may need to jiggle a few single phase loads in the 3 phase board to get a best guess balance out of all three. They dont need to be perfect but you do need to do a max demand calc to ensure you are not hammering one phase.

 

[pc1966]

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.

 

[Ian1981]

Get a link kit that connects to the busbar or 100amp terminals that connect to the bus bar, tail off less than 3 meters into a switch fuse isolator, then from isolator to a DB containing your A/C circuits. Perhaps go polyphase and fit a 4 way DB as not to overload any one phase

 

[davesparks]

Get a link kit that connects to the busbar or 100amp terminals that connect to the bus bar, tail off less than 3 meters into a switch fuse isolator, then from isolator to a DB containing your A/C circuits. Perhaps go polyphase and fit a 4 way DB as not to overload any one phase

The tails length would need to be less than 3m total from consumers terminal if this is at the origin.
But otherwise yes, if a terminal block is available for the DB that is a viable option.
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4 X a/c units off one 30mA RCD no matter where you put it is bad design is likely to create nuisance tripping. individual RCBOs or MCBs are the way forward depending on your wiring from the board to the outdoor units.
As the garage is single phase then you need to clamp this phase at the 3 phase board and see what the load is one this phase first before banging 4 X a/c units in. You may need to jiggle a few single phase loads in the 3 phase board to get a best guess balance out of all three. They dont need to be perfect but you do need to do a max demand calc to ensure you are not hammering one phase.

Clamping the phase is no good, that only gives a momentary snapshot and not a picture of the overall loading.
A load study is the only sensible way to measure that.