adding electric circuit for air con + general electric questions

[James]

Hi,

I am about to have a split type air conditioner installed which will require its own electric circuit. I have a 6 way fuse box at the moment which will soon be replaced by a 8 way. Co-incidentally I am going to have a split airconditioner installed which will need its own circuit (one of the 2 spare after the upgrade). The electrician installing the fuse box may not be able to actually route a cable from one of the free circuits to where the outside air con unit will be. This is for various reasons which I do not want to go into. I am sure he will be able to give advice, but I have not spoken to him regarding this yet.

As an alternative, one of the air con installers says he is an electrician as well. He sounds convincing, and on the air con front I have no reason to doubt what he says. So he said he could route the new circuit instead if the other electrician could not do it. I'm sure I can work something out from both of them, but I would like some advice if anyone can help.

My fuse box is in the garage is at the left side of the house, the air con will be on the right side. The air con installer proposes the following:

- Drill hole through garage wall to the outside

- Run the cable up to the loft outside wall, put the cable through the wall into the loft

- Run the cable throught the loft, drill thruogh the wall, run the cable down the outside wall to a 25 amp isolator.

The cable will be some kind of standard weatherproof outdoor cable according to him (he used the phrase 2.5 twin+earth). The air con electric specs are as follows (I don't know what this means really)

MFA 20AMPS LRA 7.3 AMPS

25 AMP isolator required

20 AMP circuit required

Is there anything that sticks out as 'illegal' or wrong about the above? Are there any other alternative ways to route a cable without tearing down internal walls? Bear in mind I do no know anything about routing electric cables in walls. My house is of brick construction, most walls are solid.

I have some general questions:

1. How do I know how much current my house can draw before it 'overloads'?
2. If the fuse box has 8 circuits with 200amp (completely random example) maximum load, 6 circuits equal 160amp. Does that mean the other two can be a combination to equal 40amp. For example 20+20 or 10+30?
3. Why do showers and cookers have their own circuits?

Thanks,

James

[Andy Wade]

First point: The work involves new circuits in a dwelling, so the new Part P of the Building Regs applies. The fist question for both your potential contractors is "are you a member of a Part P competent persons scheme." If not, a building notice should be submitted and the work inspected by your council's building control department, or someone they appoint.

1. It sounds like a long run. Have voltage drop and earth fault loop impedance calculations been done?
2. The route through the loft must avoid thermal insulation (unless shown by calculation to be OK).
3. It won't look very pretty.
4. I don't regard long runs of unprotected twin-and-earth cable as suitable for installation on an outside wall as part of a permanent installation. Either SWA or Hi-Tuf cable would be a better choice, or the use of conduit.

That's determined by the rating of your supply, and its main fuse. 100 amps is normal for anything built in the last 25 years or so. Older houses are likely to have 60 amp main fuses, or sometimes 80 amps.

No it's not as simple as that. Adding up all the circuit ratings would give you two or more times the supply rating in most cases. The principle of "diversity" is used to allow for the fact that the probability of all circuits being fully loaded at the same time is negligible. There are standard diversity factors (in the IEE On-Site Guide) which can be used to determine what is called the "assessed maximum demand."

Both are heavy current users and it would be impractical (or at least very inefficient use of copper) to combine them with other circuits. Also the division of an installation into circuits is there to avoid inconvenience and preserve safety in the event of one circuit failing or being taken out of use. This is why fixed lighting has its own circuits too.

HTH

[Pete C]

Hi,

It might be worth paying more for an 'inverter' type split, these can usually be run from a standard 13A socket and would save the wiring hassle.

Can be cheaper to run as they adjust to the cooling load better, one with a heat pump facility can take the chill off a room without needing to run the CH.

cheers, Pete.

[James]

It is actually an inverter split type that I am having. But is is a 5kw, and according the the installers it is a legal requirement for it to have its own circuit with a 20amp trip, if it was a 3.3 kw, this would not be the case. I do have a lot of equipment in my house running off the plug circuit anyway. The room where the air con will be installed is only 7ftx7ft, but a 5kw was suggested as it is full or computers and associated equipment, plus it has a south facing window. When I asked about overspeccing air conditioning units, the reply was that with inverters that is not so much of a problem these days.

Regarding andy's reply. I have not had any loop or drop calculation done, nor have they been suggested yet. I would say that the original plan for the cable route would be approximately 26 meters from the fuse box. When the electrician comes to do the fuse box I am going to ask if it is feasible to route it around the side of the house (outside), where there are cables already, like TV and sky ones, at least part of the way. I would say that run is about 15 meters. Is a loop and drop calculation a standard electrician thing, is there any expensive or specialist equipment required?

Also does anyone know if I need planning pemission for a split type air con unit?

Thanks,

James

[Stefek Zaba]

[ ... ]

It's a standard electrician thing, needing no equipment and only multiplies-and-divides-and-values-from-tables; but even more standard is for an electrician to avoid doing it by using one of the 'known good' Conventional Final Circuit arrangements listed in the On-Site Guide (the 'simple consequences of the Regs' book for domestic/light-commercial installs). For a 5kW load with a starting surge (I'm guessing here from general characteristics, not a deep knowledge of 'inverter' aircon kit!) I'm guessing a Type C MCB. The relevant maximum lengths from the OSG are:

2.5/1.5mmsq, type C MCB: 17m with TN-S earth, 27m with TN-C-S (PME), only suitable for surface-clipped cable mounting 4.0/1.5mmsq, type C MCB: 19m with TN-S earth, 42m with TN-C-S (PME), clipped-direct and in thermally-insulating wall OK 6.0/2.5mmsq, type C MCB: not listed for 20A MCB, but values for 30A MCB are 27m with TN-S earth, 42m for TN-C-S (PME)

The 2.5mmsq as well as being marginal for loop resistance, is right on the edge for voltage drop (26m, 20A, at 18mV/A/m drops 9.36V - too close to the 4%-of-nominal for comfort. So as expected, 2.5mmsq is too weedy on a number of grounds. The 4mmsq is closer to the mark, but as standard

4mmsq has the same size earth conductor as 2.5, doesn't give much longer as maximum length for a TN-S earth. 6mmsq will be comfortably within limits.

If you do decide to route outside, don't use 'normal' T&E - many people do, but I'd claim it's not adequately mechanically protected. (For a couple of feet running to a PIR 'security' light, massively overrated - no real issue; but carrying 20A - no.) You'd want to use either Hi-Tuf or metal-armoured - both of which have lower-impedance earth conductors too (for Hi-Tuf, all three cores are the same size; for SWA, the armour has a much greater cross-section than the live cores. Even with the shorter route, I'd be tempted to go for 4mmsq minimum, rather than the

2.5mmsq you could probably get away with: the (true) difference in material costs is small, and voltage-drop matters for motors.

All this should be taken into account by your electrician (whether that's the guy replacing your CU, or the aircon installer later connecting up to a spare way). And I hope you're planning to sleep in the room next door to all that kit and the aircon - I know I couldn't, with all that racket going on!

HTH - Stefek

[Pete C]

Hi,

You might want to look at solar film for the windows, and moving some non PC kit essential kit elsewhere in the house.

Toshiba might do a more powerful split that can run from a socket, and in a large house there could be enough spare capacity on the circuits feeding the room.

If you can measure what the PCs draw at the meter with not much else in the house running, your electrician should be able to advise on the above.

Split air cons don't require planning permission but locate the outdoor unit well away from yours and especially neighbours windows. If provision can be madefor retrofitting some suitable shielding all the better.

I expect that 5kW if the cooling output of the aircon and not the power input! :))

cheers, Pete.