Wiring Advise - Consumer Unit

[regedit]

I'm planning the electrics for a 2 storey townhouse which needs a new CU and rewiring. I'm a novice a DIY'er so would welcome constructive comments on my plan for the CU as below. Not entirely sure about RCDs - would I need 1 covering the aircon and water heaters, and 1 covering the sockets, light and kitchen circuits ? Have I used too many/too few MCB's ? Supply to the house is on 16mm cable.

AirCon Lounge – 20A MCB

AirCon Bed 1 – 20A MCB

AirCon Bed 2 – 20A MCB (future proof)

AirCon Bed 3 – 20A MCB (future proof)

Electric Shower Bath 1 – 35A MCB

Electric Water Heater Bath 2 – 35A MCB

Electric Water Heater Kitchen – 35A MCB (future proof)

Sockets Downstairs, excl. kitchen (6 double outlets) – 20A MCB

Sockets Upstairs (9 double outlets) – 20A MCB

Outside Circuit for Water Pump (1 double outlet) and Outside Lights (10) – 10A MCB

Kitchen Sockets (4 double outlets) and Extractor Fan (over hob) – 20A MCB

Kitchen Hob (ceramic hob) – 20A MCB

Lights Upstairs, all interior (10) – 10A MCB

Lights Downstairs, all interior (14), including kitchen – 10A MCB

[Crossy]

Looks reasonable.

What rating is your current meter and supply incoming breaker? These are what will limit your maximum demand.

You will also need RCD protection, an RCBO incomer perhaps.

20A seems small for the hob, check its requirements.

What cable sizes do you anticipate using?

I would put the pump on its own beaker.

[T_Dog]

Don't forget to think about the size of the wire that you will be using as well. Here is an excellent site that should help you integrate your needs with circuit breaker sizes and cable types/sizes. (Crossy is too modest usually to point you to it.)

http://www.crossy.co.uk/wiring/Circuit%20types.html

As an interesting note, I saw a new build being wired over the weekend. The outlets were connected with 1.5 square mm copper cable to a 32 amp circuit breaker. I pointed out that that the 1.5 mm wire could only handle about half of that breaker rating and the electrician pointed out two 1.5 square mm cables leaving the circuit breaker for two paths. Not good, as either path could have a short and fry the under-sized wire. Oh well.

[regedit]

Thanks for the advice. I've been through Crossy's informative guide several times in the past. It's a great help.

I've put my plan into a diagram - a split load CU - and noted the cable sizes I'm thinking of using (please see attached). I'll need to check on the meter rating.

Do these circuits seem to be on the right side (RCD and non-RCD protected) ?

Thanks.

CU Plan.pdf

[Crossy]

I doth reckon that will do the job just fine

[FarangBuddha]

I am currently working with a Thai electrician to do something similar to the op (I am retrofitting a current (crap) original electrical install...no RCD, everything tied into 6 circuit-breakers that are constantly tripping, several appliances that have blown out because of too much or too little power, etc.). Anyway, I have worked out the specs for my situation but something the op doesn't have in his install, I have a couple of bathroom fan/vent extractor units as well. (1) Should these be connected to the lighting or outlet circuits for their respective areas? Also, any suggestion as to the wire-guage to use?

Edited January 8, 2012 by FarangBuddha

[regedit]

I'm putting the bathroom extractor fans on the light circuit. The Mitsi fans I'm using come with 1mm cable tails so I'm assuming 1.5mm from the light circuit is ok.

Edited January 8, 2012 by regedit

[Crossy]

Yes, the lighting circuit is the place for the fan.

If it's the run-on timer type (turned on by the light switch then runs for several minutes after the light is turned off) it will need a permanent live as well as the switched live from the lights. In this case UK regs say it should have a 3-pole isolator like this one http://www.tlc-direct.co.uk/Products/CM2713.html but good luck in finding such a beast here. A note inside the fan housing next to the terminals reminding you that turning off the light does not kill all the lives will have to do.

[FarangBuddha]

Yes, the lighting circuit is the place for the fan.

If it's the run-on timer type (turned on by the light switch then runs for several minutes after the light is turned off) it will need a permanent live as well as the switched live from the lights. In this case UK regs say it should have a 3-pole isolator like this one http://www.tlc-direc...cts/CM2713.html but good luck in finding such a beast here. A note inside the fan housing next to the terminals reminding you that turning off the light does not kill all the lives will have to do.

Thanks for the info...have also re-reviewed Crossy's home electrics page for info re wire guages and CB amperages.

[FarangBuddha]

Yes, the lighting circuit is the place for the fan.

If it's the run-on timer type (turned on by the light switch then runs for several minutes after the light is turned off) it will need a permanent live as well as the switched live from the lights. In this case UK regs say it should have a 3-pole isolator like this one http://www.tlc-direc...cts/CM2713.html but good luck in finding such a beast here. A note inside the fan housing next to the terminals reminding you that turning off the light does not kill all the lives will have to do.

The fans just go on and off when the light is turned on and off...so no cut-off delay so I guess it is just another loop on the light on-off switch circuit?

[Crossy]

Yes, the lighting circuit is the place for the fan.

If it's the run-on timer type (turned on by the light switch then runs for several minutes after the light is turned off) it will need a permanent live as well as the switched live from the lights. In this case UK regs say it should have a 3-pole isolator like this one http://www.tlc-direc...cts/CM2713.html but good luck in finding such a beast here. A note inside the fan housing next to the terminals reminding you that turning off the light does not kill all the lives will have to do.

The fans just go on and off when the light is turned on and off...so no cut-off delay so I guess it is just another loop on the light on-off switch circuit?

Yeah, just put it in parallel with the light as if it was another bulb. Don't forget the ground to the fan if it needs one.

[FarangBuddha]

What cable size is appropriate for split-unit air conditioners of 12k BTU and less...would a 25 amp CB and 4mm cabling be acceptable? The reason I ask is that if I use a 32 or larger CB, 6mm cabling is specified and such large cabling is only available in the in the ground cabling at my local supplier. The salesman insisted that 4mm was sufficient and this also seems to be stated in the instructional postings for cabling installs posted around the electrical goods department.

Edited January 9, 2012 by FarangBuddha

[Crossy]

What cable size is appropriate for split-unit air conditioners of 12k BTU and less...would a 25 amp CB and 4mm cabling be acceptable? The reason I ask is that if I use a 32 or larger CB, 6mm cabling is specified and such large cabling is only available in the in the ground cabling at my local supplier. The salesman insisted that 4mm was sufficient and this also seems to be stated in the instructional postings for cabling installs posted around the electrical goods department.

A 12,000 BTU A/C will draw about 1.3kW (5.5A) from the supply so it will be more than happy on 2.5mm² cable and a 20A breaker, if you want to future proof for a bigger unit then 4mm² will be good for up to 50k + BTU.

Note that if you approach the rating of the cable you will have to use an oversized breaker (or a C curve breaker rated for motor use). This is not an issue so long as the breaker provides short-circuit protection (you'll need to do the sums) as overload protection is not required.

Important note. If your run is more than 15-20m then voltage drop on start is going to be more of an issue than the actual rating of the cable. Assume 3-4 times run current on start (unless it's an inverter unit) and allow 10% drop on start.

[FarangBuddha]

What cable size is appropriate for split-unit air conditioners of 12k BTU and less...would a 25 amp CB and 4mm cabling be acceptable? The reason I ask is that if I use a 32 or larger CB, 6mm cabling is specified and such large cabling is only available in the in the ground cabling at my local supplier. The salesman insisted that 4mm was sufficient and this also seems to be stated in the instructional postings for cabling installs posted around the electrical goods department.

A 12,000 BTU A/C will draw about 1.3kW (5.5A) from the supply so it will be more than happy on 2.5mm² cable and a 20A breaker, if you want to future proof for a bigger unit then 4mm² will be good for up to 50k + BTU.

Note that if you approach the rating of the cable you will have to use an oversized breaker (or a C curve breaker rated for motor use). This is not an issue so long as the breaker provides short-circuit protection (you'll need to do the sums) as overload protection is not required.

Important note. If your run is more than 15-20m then voltage drop on start is going to be more of an issue than the actual rating of the cable. Assume 3-4 times run current on start (unless it's an inverter unit) and allow 10% drop on start.

Is it correct to assume that for standard sized point of service water heating units, individual 20a CBs/2.5mm cabling is also appropriate...as seems to be specified in the instructions for the 4000w Fagor unit I am installing? (And if so, why the recommended 32a CBs/4mm cables for the OP's water heater installs?)

Edited January 10, 2012 by FarangBuddha

[Crossy]

Is it correct to assume that for standard sized point of service water heating units, individual 20a CBs/2.5mm cabling is also appropriate...as seems to be specified in the instructions for the 4000w Fagor unit I am installing? (And if so, why the recommended 32a CBs/4mm cables for the OP's water heater installs?)

4,400 W is the maximum that will run on a 20A breaker and 2.5mm² cable, so whilst your 4kW heater will be just fine there's no upgrade path. Our OP wants 32A breakers either because he wants 6kw heaters or to be future proof, 4mm² cable would be appropriate for those circuits.

The standard 3.5 or 4kW heaters are that size so they will run on a basic 5/15 supply (just), and are only barely adequate for a farang style hot shower.

Those in the cooler regions and who like high flow hot showers may even wish to go for 6mm² cable on 40A breakers which will allow an 8kW heater.

Both 6 and 8kW heaters are readily available.