How Do I Know Which Power Cables I Need?

[Dutchman6]

For example I need to choose a cable as cheap as possible for my pc 200w + fans 200w + microwave 1000w + light 120w = 1520w / 220v = 6.9 Ampere

Also I need to know if my existing cables are strong enough for what's on it now, for example I used to have a 3500w shower heater but now I have a 8000w model on the same cable. I already got new 40 Amp circuit breakers but have no idea what cables I have to pick!

In the hardware store they have grey and white cables, the inside of the white ones is a solid stray of copper and the grey ones are with the "little hairs" inside. But both white and grey often have the same text written on it: PVC 70*C VFF 2x2.5 SQ.MM. Table 10 another thinner cable sais PVC 70*C VFF 2x0.5 SQ.MM. Table 1, with the only small change being 2x0.5 instead of 2x2.5.

So how do I know what cable to pick for my load and if my existing cable is powerful enough? The personel in the hardware store had no clue and neither do I. I'd like to know how I can identify how many Watts a cable can handle by looking at what's written on it.

I don't know what can happen if the wiring is too weak, I suppose fire?

[Crossy]

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

1.5 mm2 cable - 15 amp maximum circuit breaker.

2.5 mm2 cable - 20 amp maximum circuit breaker.

4 mm2 cable - 25 amp maximum circuit breaker.

6 mm2 cable - 32 amp maximum circuit breaker.

10 mm2 cable - 40 amp maximum circuit breaker.

It's normal to wire outlets with 2.5mm2 cable protected by a 20A breaker. Your 8kW water heater should be on 10mm2 cable if you have a 40A breaker although it would be OK on 6mm with a 32A breaker as it's an intermittent load. The 32A will happily carry the slight overload without tripping, wiring showers like this is commonplace in the UK and allowed by the UK regulations.

It is extremely unwise to simply up the rating of the breaker without uprating the cable, yes the result can be conflagration

You should also consider your incoming supply and meter, if the meter is marked 5/15 you should upgrade to a 15/45 to avoid frying the poor thing (and the associated inaccuracy). Also verify the size of your incoming cables from the pole.

[Dutchman6]

Alright, so when the cable states 2x2.5 SQ MM does the whole thing count as 5 SQ MM or does it mean the cable is 2.5 SQ MM? I also saw markings like 4/2x2.5 SQ MM, not even the biggest cables state 10SQ MM... And like I said I have a big white solid cable for the heater that sais 2x2.5 SQ MM but so does my smaller grey cable with the copper hairs inside. I thought the white cables are for the bigger load.

Also I'd like some more explanation about the meter is marking 5/15. I've had the condo electrician place the new circuit breakers, you'd suppose he wouldn't do that if the cables outise of my room aren't capable of taking the load.

[Dutchman6]

I found this on the internet : Strand count and diameter: for instance, 16/0.2. This means 16 strands of 0.2 mm. diameter. To get the area of each strand, use π / 4 * D * D. Then multiply this by the number of strands. The area of a 16/0.2 cable would be 16 * π / 4 * 0.2 * 0.2 = 0.503 sq. mm.

If I calculate this for my cable that sais 2x2.5 I'd say it is 2 * π / 4 * 2.5 * 2.5 = 9.8 SQ MM which would mean I have a white cable strong enough for my load on the heater and a grey cable way too strong for the load on my sockets.

Can anyone tell me if this is the correct conclusion?

[Crossy]

Nope, not correct.

The colour of a cable has no bearing on its current carrying capability.

The cross sectional area of a cable marked 2.5SQ MM is, oddly enough, 2.5mm². The marking 2 x 2.5SQ MM means 2 cores of 2.5mm², this cable must have a breaker of no more than 20A.

Are you saying you have an 8kW heater on 2.5mm² cable protected by a 40A breaker? That cable must be getting pretty warm even after a 5 minute shower. As I noted earlier, the absolute minimum cable size for your 8kW heater would be 6mm² on a 32A breaker.

Sadly, your local sparks will likely not understand what he's doing and will simply follow your instructions. Luckily for you, modern cables are capable of handling massive overloads before anything nasty occurs, that doesn't mean however that routinely overloading your wiring is OK.

I strongly recommend that you return your electrical installation to its original as-designed state, then we can start discussing how you can safely upgrade your system.

[Dutchman6]

I can't return to the old installation of course. I have an absolute maximum load (something that will never happen) of 18 Amp on a wire marked 2x2.5 SQ MM on a 20 Amp breaker. Now I have the shower heater on the remaining 40 Amp breaker with a cable marked 2x2.5 SQ MM, I will replace this cable but what should the cable say I need in order to be 10sqmm because I've seen all the cables in the hardware store and none bear such an inscription, it's all 2x0.5 SQ MM to 2x2.5 SQ MM and some of the big white cables bear markings like 4/2x2.5 SQ MM.

I will also try to discuss the meter and the cable outside my room with the condo management and have it replaced. What would the desired 15/45 meter cost and where do I get it, Home Pro?

[Dutchman6]

Thanks a lot for your help Crossy!

I saw the 15/45 meters for 530thb and now understand they're for 15 - 45 Amp. Also I found a cable that indeed had 2x6 SQ MM on it for the heater.

I'm gonna have the condo electrician check for the cable in the hall but it's bloody 50m of cable that should be replaced if it's not strong enough, would a 4 SQ MM cable do for a 8500/220= 38 Amp load? It's just 8500w for 5 mins a day...

The room came with 16 Amp circuit breakers and 2x2.5 SQ MM cables for an 3500w aircon unit and a 3500w shower heater along with some light bulbs and a regrigerator. I don't understand how that explains itself on a 16Amp breaker.

[powderpuff]

Never use multi-strand wire for household wiring. Use solid core only.

Job 1. Upgrade the wire from the breaker going to the water heater to 2 X 10 sq.mm.

Edited December 20, 2010 by powderpuff

[Crossy]

As I noted previously 2 x 6mm2 (plus a ground) will be OK for the heater PROVIDED you replace the breaker with 32A.

With a 15/45 meter and an incoming breaker of 40A you must use 10mm2 minimum from meter to distribution board, however, it would be wise to use at least 16mm2 with which you could use a 63A incomer and be OK for any future load.

The meter is usually supplied either by the condo (if they provide the bill) or by MEA / PEA if you are billed direct, you should not attempt to replace your own meter.

EDIT continues here

Edited December 20, 2010 by Crossy