Which circuit is appropriate ?

[AttitudeAllStar]

Hi All,

I'm at the cabling stage of my renovation. I have an old boiler and pump which just plug into a twin external socket. That socket is wired back to a 3A FCU and on to a 20A MCB. That MCB is housed in a small CU in an old kitchen extension with three or four other MCB's, which is wired back to the main CU on it's own 40A MCB.

As I'm changing the location of the kitchen, the boiler/pump circuit will be removed from that small CU, and either joined as a fused spur on the hallway or new kitchen circuit, but I think regs might state it needs its own dedicated 3A fused circuit?

The options due to access and potential damage are:
(1) hallway 20A MCB (protected by a 63A RCD) which currently has just two twin sockets used by a washing machine and a dryer.
(2) kitchen 20A MCB (protected by 63A RCD) which will have a higher load - dishwasher, 4x available twin sockets, microwave, fridge freezer, kettle etc.
(3) Run a 3 core 2.5mm sq back to the CU through the ceiling joists (about 16 metre run), and have the 20A MCB moved to the main CU (or change to a 16A).

Option 3 is probably the best move?

AA

 

[Murdoch]

A gas boiler does not need a dedicated circuit but it does need to be fused down the 3A

 

[AttitudeAllStar]

A gas boiler does not need a dedicated circuit but it does need to be fused down the 3A

Thanks a lot for your reply - so if I were to link it in to say the hallway circuit - I've calculated this as
washing machine: 2kw
dryer: 3kw
boiler: 1.7kw (guessing)
pump: .1kw

Total 29.56A

Might be too much for the 20A breaker? As is, I'm not sure a 20A is appropriate for a dryer and w/machine because the total seems to run over the 20A.. which might lead to nuisance tripping if using the sockets as well as the machines at the same time.

If it's 29.56A, shouldn't it be a 32A breaker? The kitchen-to-be circuit is also on a 20A, I'm thinking that is too low for it to be able to handle the cluster of appliances running all at once.

 

[Murdoch]

Thanks a lot for your reply - so if I were to link it in to say the hallway circuit - I've calculated this as
washing machine: 2kw
dryer: 3kw
boiler: 1.7kw (guessing)
pump: .1kw

Total 29.56A

Might be too much for the 20A breaker? As is, I'm not sure a 20A is appropriate for a dryer and w/machine because the total seems to run over the 20A.. which might lead to nuisance tripping if using the sockets as well as the machines at the same time.

If it's 29.56A, shouldn't it be a 32A breaker? The kitchen-to-be circuit is also on a 20A, I'm thinking that is too low for it to be able to handle the cluster of appliances running all at once.

In one word - DIVERSITY

Your gas boiler will be about 50 w if that, including the central heating pump.

 

[SPECIAL LOCATION]

If it's 29.56A, shouldn't it be a 32A breaker? The kitchen-to-be circuit is also on a 20A, I'm thinking that is too low for it to be able to handle the cluster of appliances running all at once.

The MCB is to protect the cable.. MCB ratings must be lower than the max current capacity of the cable..

You do not increase an MCB rating because you think there may be a bigger load connected.

I hope the washing machine and drier are not plugged into the same double socket outlet.?

 

[binky]

I would run a separate cct for the boiler - failed boiler mid winter due to fault on socket circuit is a right PITA!

Washer drier will be Ok on a 20A, but again I would run a separate cct, preferably in 4mm so you can use it as a 32A cct if needed. Kettles can be 3kW alone these days, so that and drier running at 2.5kW tends to trip a 20A MCB

 

[Fleeting]

I assume you don't have an electrician on board?

 

[AttitudeAllStar]

The MCB is to protect the cable.. MCB ratings must be lower than the max current capacity of the cable..

You do not increase an MCB rating because you think there may be a bigger load connected.

I hope the washing machine and drier are not plugged into the same double socket outlet.?

Thanks yes I should've mentioned the cable size is 2.5mm.. which I believe can handle between 25-35A..? all inherited old circuits.. and yes the washer and drier are on one twin socket! You are saying I need to separate them into two single sockets, one daisy chaining to the next? I can see how that is safer.

@Murdoch I didn't realise it would be that low, the boiler has an old oil burner. Diversity I get, but often times these will all be running together.

@Fleeting not yet, trying to get my head around things first so I can suggest alternatives if needed

@binky I'm veering towards a dedicated circuit for the boiler and pump on 2.5mm.. going back to a 20A MCB. and with a 3A FCU before the timer.

You all have me questioning now the existing radial circuit of the kitchen-to-be. It will have a F/F, microwave, dishwasher, extractor fan, kettle, and whatever else people decide to plug in. It is currently a 2.5mm circuit with a 20A breaker. For the hob and oven I've run in 6mm, presumably the sparks will place that on a 32A MCB. But I'm thinking now I probably need either another 20A for the kitchen, or replace the cabling with 4mm and put on a 32A.

Now is the time to do it while still at first fix.

 

[Murdoch]

Now is the time for the ELECTRICIAN to do it while still at first fix.

Edited that for you.

 

[binky]

2.5mm is good for 26amps max, unless connected as a ring cct. 20a should be fine for the kitchen appliances without the drier on the same cct.

 

[AttitudeAllStar]

Ok, so I took some time today to refresh my memory on diversity and loadings etc. I've come up with a plan that I think will be straightforward for the electrician and allows me to get on with laying my floor.

My current CU is a 2 way split load supplying a limit of 100A with 2x 63A RCDs.
RCD 1 Main on the bottom rail has 4x B10 (lighting circuits), 1xB40 (Mini CU), and 1xB40 RCBO (8.5kW shower).
RCD 2 has 5x B20 (one of which is an immersion only, the others are sockets).

The Mini CU has 1x C25 RCBO, 1x B10 (light), 2x B20 (2 twin sockets, boiler/ pump).

I have calculated the following loads (incl new kitchen plan and after moving cooker circuit to main CU):
RCD 1 =
B10, 5x lights (max .5kW);
B10, 6 lights (.6kW);
B10, 3 lights (.3kW);
B10, 4 lights (.4kW);
B32 Moved from Mini CU for new hob and oven circuit on 6mmsq cable, 10.3kW (oven 4.5kW, hob 5.8kW)
B40 Mini CU (keeping RCD, B20 (4.1kW) and B10 (4 lights, .4kW)).
RCBO B40 Shower (8.5kW)

Total 25100kW/230v = 109.1A

RCD 2 =
B20 Immersion 2.5kW;
B20 3 rooms, 4.1kW (using laptops, desk lamps, hairdryer, other tech, poss a small oil heater at 2kW)
B20 3 Rooms 4.1kW
B20 Hall sockets 4.2kW (1kW washer+3kW dryer+occasional dehumidifier .2kW)
B20 New Kitchen 4kW (kettle 2.5kW+Toaster 1.5kW+sockets for misc.)
New B20 circuit in 2.5mmsq, 3.6kW (Boiler/ pump .2kW+microwave .9kW+dishwasher 1.2kW+extractor fan .2kW+fridge-freezer .3kW+twin socket for poss hoover .7kW+small lamp .1kW)

Total 22500kW/230v = 97.8A

Total max potential demand = 206.9A

Now, with Diversity calculations:

RCD 1 =
B10, (max .5kW);
B10, (.6kW);
B10, (.3kW);
B10, (.4kW);
B32 Moved from Mini CU for new hob and oven circuit on 6mmsq cable, 10.3kW
B40 Mini CU (keeping RCD, B20 (4.1kW) and B10 (.4kW)).
RCBO B40 (8.5kW)

66% diversity on all lights = 2.2kWx.66==6.3A
Cooker diversity taking first 10A and 30% on rem = 20.4A
40% diversity on socket circuit = 4.1x.4=7.1A
No diversity applicable to shower = 37A

Total Diversity of 70.9A on RCD 1

RCD 2 =
B20 2.5kW; diversity at 100% = 10.9A
B20 4.1kW = diversity 40% = 7.1A
B20 4.1kW = diversity at 40% = 7.1A
B20 4.2kW = diversity at 40% = 7.3A
B20 4kW = highest sockets load so diversity at 100% = 19.6A
New B20 circuit in 2.5mmsq, 3.6kW = diversity at 40% = 6.3A

Total Diversity of 58.3A on RCD 2

Total Max demand Diversity = 58.3+78.8 = 129.2A

So I figure the 63A RCDs are a problem, he will need to change at least one of them to a 80A RCD.

How it affects me is that I can see he might've suggested moving some of the new kitchen sockets on to the Immersion circuit (ok with regs as far as I can tell), but I would prefer him to put in a separate B20 with new radial. I was able to work out what cabling I'll need for the two separate kitchen radial circuits, 2.5mm is fine, and could see how the loadings might create problems if appliances are being used at the same time etc.
I now know I can go ahead and cover in my floor as there is just one 2.5mm circuit to be added through the ceiling; the other existing 2.5mm will suffice for one of the two kitchen radials.

I just hope my assumptions are correct!

 

[AttitudeAllStar]

I've just discovered that existing B20 on RCD2 for the new kitchen is a ring circuit. So another possible option, which would really save me a headache with the ceilings, is if it was changed to a B32 with a circuit max supply of about 30.4A (not considering diversity). That would allow me to daisy chain the kitchen sockets instead of running in a new one. I could then just add the boiler/pump and the extractor fan to the B20 Immersion circuit, although that would still have to be accommodated through the ceiling and before the CU.

Or, I could have another B20 added as planned, and split the ring into two radials, which come to think of it, would be easier from the point of view of ceiling damage. Either way I think it's a positive.

 

[SPECIAL LOCATION]

Dual RCD boards are a poor design in my opinion..
What type of RCD's are they?

 

[AttitudeAllStar]

Dual RCD boards are a poor design in my opinion..
What type of RCD's are they?

There's a single pole fuse carrier (Wohner brand) 63A, and a General Electric breaker which is either single or double I can't tell but not wide enough for more.

 

[Murdoch]

A photo of the cu might be handy

 

[binky]

Ok, so I took some time today to refresh my memory on diversity and loadings etc. I've come up with a plan that I think will be straightforward for the electrician and allows me to get on with laying my floor.

My current CU is a 2 way split load supplying a limit of 100A with 2x 63A RCDs.
RCD 1 Main on the bottom rail has 4x B10 (lighting circuits), 1xB40 (Mini CU), and 1xB40 RCBO (8.5kW shower).
RCD 2 has 5x B20 (one of which is an immersion only, the others are sockets).

The Mini CU has 1x C25 RCBO, 1x B10 (light), 2x B20 (2 twin sockets, boiler/ pump).

I have calculated the following loads (incl new kitchen plan and after moving cooker circuit to main CU):
RCD 1 =
B10, 5x lights (max .5kW);
B10, 6 lights (.6kW);
B10, 3 lights (.3kW);
B10, 4 lights (.4kW);
B32 Moved from Mini CU for new hob and oven circuit on 6mmsq cable, 10.3kW (oven 4.5kW, hob 5.8kW)
B40 Mini CU (keeping RCD, B20 (4.1kW) and B10 (4 lights, .4kW)).
RCBO B40 Shower (8.5kW)

Total 25100kW/230v = 109.1A

RCD 2 =
B20 Immersion 2.5kW;
B20 3 rooms, 4.1kW (using laptops, desk lamps, hairdryer, other tech, poss a small oil heater at 2kW)
B20 3 Rooms 4.1kW
B20 Hall sockets 4.2kW (1kW washer+3kW dryer+occasional dehumidifier .2kW)
B20 New Kitchen 4kW (kettle 2.5kW+Toaster 1.5kW+sockets for misc.)
New B20 circuit in 2.5mmsq, 3.6kW (Boiler/ pump .2kW+microwave .9kW+dishwasher 1.2kW+extractor fan .2kW+fridge-freezer .3kW+twin socket for poss hoover .7kW+small lamp .1kW)

Total 22500kW/230v = 97.8A

Total max potential demand = 206.9A

Now, with Diversity calculations:

RCD 1 =
B10, (max .5kW);
B10, (.6kW);
B10, (.3kW);
B10, (.4kW);
B32 Moved from Mini CU for new hob and oven circuit on 6mmsq cable, 10.3kW
B40 Mini CU (keeping RCD, B20 (4.1kW) and B10 (.4kW)).
RCBO B40 (8.5kW)

66% diversity on all lights = 2.2kWx.66==6.3A
Cooker diversity taking first 10A and 30% on rem = 20.4A
40% diversity on socket circuit = 4.1x.4=7.1A
No diversity applicable to shower = 37A

Total Diversity of 70.9A on RCD 1

RCD 2 =
B20 2.5kW; diversity at 100% = 10.9A
B20 4.1kW = diversity 40% = 7.1A
B20 4.1kW = diversity at 40% = 7.1A
B20 4.2kW = diversity at 40% = 7.3A
B20 4kW = highest sockets load so diversity at 100% = 19.6A
New B20 circuit in 2.5mmsq, 3.6kW = diversity at 40% = 6.3A

Total Diversity of 58.3A on RCD 2

Total Max demand Diversity = 58.3+78.8 = 129.2A

So I figure the 63A RCDs are a problem, he will need to change at least one of them to a 80A RCD.

How it affects me is that I can see he might've suggested moving some of the new kitchen sockets on to the Immersion circuit (ok with regs as far as I can tell), but I would prefer him to put in a separate B20 with new radial. I was able to work out what cabling I'll need for the two separate kitchen radial circuits, 2.5mm is fine, and could see how the loadings might create problems if appliances are being used at the same time etc.
I now know I can go ahead and cover in my floor as there is just one 2.5mm circuit to be added through the ceiling; the other existing 2.5mm will suffice for one of the two kitchen radials.

I just hope my assumptions are correct!

diversity calcs are mostly rubbish! The more circuits the better, but diversity calcs would sugggest the way I rewire a house overloads the supply multiple times The more circuits the higher the figure, but more circuits is better for a whole host of reasons. What you actually need to look at is total load per cct, and total load of the main supply. Unless you want to run a commerical size arc welder its' highly unlikley you would ever overload the main supply in a standard house, but your understanding of how this works is also the downsisde of DIY - you don't, same as I don't understand medical stuff my doctor tells me. At the risk of sounding rude, get a sparky involved now, most of us are happy for you to do the donkey work, ie running cables in to save money, as long as you get us involved upfront, so we can supervise the work and design the ccts.

 

[SPECIAL LOCATION]

Dual RCD boards are a poor design in my opinion..
What type of RCD's are they?

There's a single pole fuse carrier (Wohner brand) 63A, and a General Electric breaker which is either single or double I can't tell but not wide enough for more.

So by the sounds of it you don't actually understand about types of RCD..???

It would probably be wise to consult a local electrician who can see first hand exactly what you do have...

and from that what is the most economical practical and BS7671 compliant solution to your needs..

It's quite difficult offering a practical solutions when limited information is provided by a non-electrically competent person. (Diversity calculations on post #11 are a classic example).

 

[Murdoch]

The current (pardon the pun) diversity guidelines were thought up when homes had 4-6 circuits max and are hopelessly unrealistic

Add up the breaker values and multiply by 0.25 and see what happens then

 

[AttitudeAllStar]

Thanks a lot for the replies guys, I've learned and re-learned so much in the past few days and have a guy coming out this eve to have a look. Will update!

 

[mort2376]

According to the IET at yesterday's techtalk diversity is something they are looking at for the next update.