When to use your own earth rods

[Countrypaul]

Based on the fact that the AC units draw much less current when operating that when starting, but the breaker has to be rated at 20A for each is there any reason why both could not be run off a single breaker less than 40A?

[Stinsy]

Based on the fact that the AC units draw much less current when operating that when starting, but the breaker has to be rated at 20A for each is there any reason why both could not be run off a single breaker less than 40A?

You can do it that way, however we are already disregarding the breaker rating and using the manufacturer-stated max power draw for max-demnd calcs so doing so doesn't help us here.

If you have 2x A2A HPs that require a 20A breaker and draw 9A at full chat, you can use the 9A rather than the 20A, but you can't apply diversity based on "I rarely use both at the same time".

[sharpener]

1. Don't modern AC units have inverter drives which provide an integral soft-start capability so don't need any over-rating of the breaker?

However do they then need expensive Type B RCDs bc of a possible DC fault from the inverter? Can you get one to fit the BG board (Wylex/Crabtree won't be available until spring 2025)?

2. IIRC if you have 32A possible PV going on to a single DB you have to consider that the max avail power = incoming supply + max PV = 132A which might exceed the rating of standard domestic DB busbar arrangement.

[Stinsy]

1. Don't modern AC units have inverter drives which provide an integral soft-start capability so don't need any over-rating of the breaker?

However do they then need expensive Type B RCDs bc of a possible DC fault from the inverter? Can you get one to fit the BG board (Wylex/Crabtree won't be available until spring 2025)?

2. IIRC if you have 32A possible PV going on to a single DB you have to consider that the max avail power = incoming supply + max PV = 132A which might exceed the rating of standard domestic DB busbar arrangement.

1. They still draw more power on startup than when running, just not as bad as I used to. The example I used above requires a 20A breaker and pulls 9A when running flat out.

You can also get type B RCDs for BG CUs: https://www.bgelectrical.uk/uk/circuit- ... evices/rcd

2. Agreed. It might be better to put the Solar on its own CU (or busbar).

[AGT]

I have from memory in my DB

Lighting 6amp
Lighting 6amp
Lighting 6 amp
Intruder 6amp
Data 6amp
Boiler 6 amp
Electric gate 6amp
Fridge Freezer 16 amp
Plus lots of power

Be lucky if I ever pull 6amp total on the 8 I listed so don’t get too hung up about adding MCB’s

[cojmh]

If max demand calcs come out at 106A then any spark would let that through with a roll of their eyes and a sigh…

However I have some problems with your methodology…

• Having DB3 and DB4 next to eachother in the outbuilding (similar to DB1 and DB2 in the house) is fine. Disregard my earlier concerns. good good - If I choose to do it

• The Eddi controls your immersion heater. You’re required to put this circuit as 100% if there is any way to manually activate the immersion heater as in some kind of “boost” function. If the IH can only be activated via diverted export then you can downrate the IH to 0A for max demand calcs. The Eddi and immersion heater is purely for solar diversion there will be no means to over-ride or run the immersion elsewhere. If we need hot water the boiler will be the source.

• What is the outbuilding? I have a garden office, it is on a 32A, but 20A would be fine. Having a workshop or something on a 20A might cause nuisance tripping. The outbuilding is actually two separate areas - one is for woodwork and one is for car work/metal work. There are things like air filtration/welding etc. and tools that use a lot of power - but this is my hobby and I work by myself so the maximum that will be on at a time is one "big" piece of equipment, air filtration/extraction and the lights (LED) maybe the radio too .... I will need to check the big saws etc. but I think 20A might be fine.

• AC units usually require a higher breaker rating than their running power because of the startup. EG a typical 3.5kW model might require a 16A breaker but only consume 5A when running (at max power). I heat a whole 5-bed house with a single “7kW” AC unit. You really don’t need one in every room! (Although two smaller units would be better!) Sounds like I am over specifying what I need then ... I will give this more thought. If you don't mind me asking how have you laid out your AC indoor units?

• You need to think really carefully before dropping the downstairs ring to 16A. What about the: washing machine, dishwasher, kettle, toaster, air frier, microwave, etc.? Nowadays it is common to put the kitchen/utility on one 32A ring and the rest of the downstairs on another. More thought required on this.

• Having the other sockets circuits on 16A circuits is fine so long as you give it some thought. Will there really never be several high-power devices running simultaneously? The thing is that I am struggling to think of many high powered devices we use in the house. literally hair dryer, vacuum cleaner, maybe a gaming computer - everything else is pretty low power so all of these added together is 16A I suppose but
  it would be rare to be cleaning at the same time someone is drying their hair - but not inconceivable.

• I really don’t think it is sensible to put the outside socket on a 6A. People tend to plug high power devices into these (lawnmower, granny cable, etc.). Having any sockets circuit on a 6A is a bit unusual but permissible if it powered a single outlet intended and clearly labelled for a specific device only. Quite literally the only things that I have ever run from the outdoor socket at the front of the house is the hoover - 750W (I just checked) or the pressure washer - which is about 1KW I think or the Christmas lights (probably 250W) ..... anything bigger is petrol powered. So everything comes under the 6A and none are used at the same time.

Let’s do max demand calc for a typical installation using that BG board you’ve bought:

40A cooker - 10+9+5=24A
32A ring - 32A
32A ring - 32*0.4=12.8A
32A ring - 32*0.4=12.8A
16A outside socket - 16*0.4=6.4A
6A lights - 1A (for all lights)
6A lights - 0A
6A smokes - 0A

Total: 89A

So totally fine!

(BTW buy the nice BG din-rail blanks rather than the crappy ones that come with the board https://www.screwfix.com/p/british-gene ... lank/5390p) I will do

Answers and thoughts above

[cojmh]

As an additional thought - I think the only other area to approach is the AC units - as you pointed out.

I have looked at two multi split units which would manage every area in the house - but there are a couple of realities:

1. Not every room in the house will always be occupied - during the day I would expect down stairs to be cooled and not so much upstairs and vice versa at night. I accept that cooling might be left on by accident somewhere - but it is very very unlikely for both system to be running at max.
2. As the house has been significantly up graded in terms of insulation - this should also reduce the demand on the AC. From reading what others have achieved it seems this is likely?

Would this be adequate reasoning to apply a reduction factor on the power consumption by AC1 and AC2? Even a 20% reduction would bring the total under 100A?

Basically no. You can’t use this as justification to apply diversity. In very cold weather or when you return to a cold house you have to assume it’ll be cranking at the maximum power.

If the house is well-insulated then fit a smaller AC/HP system. As I said above you really don’t need one in every room. Here is a 4-head “7kW” multi-split that requires a 20A breaker but draws 9.2A running at full chat. You could install 4x 3.5kW internal units with it so you can direct all your heating/cooling to either upstairs or down as you wish: https://cooleasy.co.uk/cdn/shop/files/M ... 4618322948

2-units up and 2-units down really should be plenty!

I feel like the naughty school kid trying to test all possible avenues to fudge a palatable answer .... (3 Phase is not palatable at the indicated cost - unfortunately)

I will look into the AC more .... I worked on the logic of 1 AC per bedroom (so 5) and a couple of bigger ones for the living areas so using two multi-splits.

[Stinsy]

• I really don't think that you can put any kind of workshop on a 20A. All those powertools, many of which will be inductive (circular saws etc.) will cause frequent nuisance tripping, not to mention welders! Imagine how much of a ballache it'll be trecking to the house every so often to trip that darned 20A back on.

• My AC unit (or A2A HP) is at the top of the stairs. We'd had a few hot summers in a row (remember COVID?) and at the time I wasn't getting paid for export. So on sunny days any power over and above what I could store/use was "wasted". So I installed a mahoosive AC unit at the top of the stairs with the intention that the cold air would tumble down the stairs and cool the whole house. It was a bit of an extravagance, but I like a project and the fuel was free. The idea that it could be used for heating was secondary.
  
  However it ended up being better than I ever could have expected for heating. At "7kW" it is oversized even for my 5-bed house. Even sent to its lowest setting it cycles on all but the coldest days. The air seems to naturally circulate around the whole house, I guess the heat from appliances/people/cooking etc. downstairs no longer gets sucked straight up the stairs. The winter before last the A2A augmented my gas central heating, last winter the gas boiler sat completely unused (except for DHW).
  
  There is one bedroom that the heat didn't get to, the door is round a corner and behind the A2A. So I used a "silent" bathroom extractor and some insulated duct to shift air from the hallway to that bedroom. If I were designing from scratch I'd probably put in more ducting and some kind of manifold to shift warm air from the hallway to each bedroom. But to be hones this isn't needed.
  
  As I say, I wouldn't design a heating system this way. But certainly one up and one down would be plenty for most houses I'd guess.

• I think we are at the stage with the max-demand where we need to be more creative! There is no way you can power everything you want to with a single-phase supply. You can't put that outside socket on a 6A, you just can't. And the workshop needs way more than 20A. And your downstairs ring really does need to be 32A.
  
  However there is a solution! Why don't you put DB2 completely off grid, powered by batteries charged from the grid?
  
  SoMain Consumer Unit: DB1
  Cooker - 40A -10+9+5=24A (assuming a socket)
  Outbuilding - 16A (feeding hybrid inverter functioning as battery charger in outbuilding)
  PV - 32A - 0A (negligible demand)
  Zappi - 32A - 0A (installed with wired CT and set to cap max demand)
  Eddi - 16A - 0A (installed with wired CT and set up to cap max demand)
  AC 1 - 16A - 7A - Based on this one : https://cooleasy.co.uk/products/lg-stan ... EJ4zkD_wqs
  AC 2 - 16A - 7A - ased on this one : https://cooleasy.co.uk/products/lg-stan ... EJ4zkD_wqs
  Hybrid inverter - 16A (functioning as battery charger for DB2).
  
  TOTAL: 70A
  
  The feed to the outbuilding would go straight to a hybrid inverter that would be programmed to start charging the batteries when they get down to 20% (or whatever) and stop charging them at 80% or whatever. You could then have a 2nd hybrid inverter that takes power out of the batteries to power the DB in the outbuilding. It can be as powerful as you like.
  
  You can have exactly the same setup in the main house for DB2.

[cojmh]

Thank you for the thoughts, but can I just clarify to see I am understanding correctly.

Consumer Unit 1 (DB1) as you described connected directly to grid supply.

From DB1 there is a 16A connection to a hybrid inverter (HI1) that charges Battery Stack 1 (BS1) in the house at programmed intervals and then another Hybrid Inverter (HI2) that supplied Consumer unit 2 (DB2) in the house from batteries only.

Additionally there is another 16A connection to a hybrid inverter (HI3) that charges Battery Stack 2 (BS2) in the out building at programmed intervals and then further hybrid inverter (HI4) will supply Consumer unit 3 (DB3) in the outbuilding from batteries only.

There is no need for a DB4 as DB3 is completely off grid.

Effectively we are just putting DB2 and DB3 on big UPS's made from inverters and battery stacks?

My questions are:

• Do HI1 and HI3 need to be hybrid inverters or would AC inverters do as there does not seem to be any PV requirement?

• If I add solar panels to the outbuilding - something I would like to do - would these be added on to HI4?

• How would you calculate a sensible size for the battery stacks BS1 and BS2? I already have 4 x Pylontech US3000C on my existing PV array

I need to think this through as it seems it would work - but is a big capital investment.

[Stinsy]

Thank you for the thoughts, but can I just clarify to see I am understanding correctly.

Consumer Unit 1 (DB1) as you described connected directly to grid supply.

From DB1 there is a 16A connection to a hybrid inverter (HI1) that charges Battery Stack 1 (BS1) in the house at programmed intervals and then another Hybrid Inverter (HI2) that supplied Consumer unit 2 (DB2) in the house from batteries only.

Additionally there is another 16A connection to a hybrid inverter (HI3) that charges Battery Stack 2 (BS2) in the out building at programmed intervals and then further hybrid inverter (HI4) will supply Consumer unit 3 (DB3) in the outbuilding from batteries only.

There is no need for a DB4 as DB3 is completely off grid.

Effectively we are just putting DB2 and DB3 on big UPS's made from inverters and battery stacks?

My questions are:

• Do HI1 and HI3 need to be hybrid inverters or would AC inverters do as there does not seem to be any PV requirement?

• If I add solar panels to the outbuilding - something I would like to do - would these be added on to HI4?

• How would you calculate a sensible size for the battery stacks BS1 and BS2? I already have 4 x Pylontech US3000C on my existing PV array

I need to think this through as it seems it would work - but is a big capital investment.

You are correct in your understanding of the plan!

I suggested hybrid inverters just because they are cheap commodity items and you can add solar later. You absolutely could use AC inverters instead if you are not planning to add solar. If you are planning to add solar then a hybrid is the correct choice.

Battery size depends. If you’re not worried about ToU then the size of the pack is more about discharge power than size. Those US3000C packs are rated at c. 1.8kW each so 3x would be 5.4kW. Alternatively you could build a pack of LF280k cells, these can be very economical indeed.