Ground/Earth Cable Size?

[tiger2]

I'm re-wiring a workroom that will have 3 receptacles and draw a total of 24 amps. No more than ~18 amps will be used at any time. The local electric shop has a PVC cable with 2.5 mm2 live and neutral wires and 1.5 mm2 earth wire. A 25 amp combo MCB/RCD will provide protection to this circuit.

My question is: will the 1.5 mm2 earth wire be OK or does it need to be larger (diameter)?

[Crossy]

Firstly you should have no more than a 20A breaker protecting 2.5mm² cable. If that's not going to be enough then consider splitting into two circuits. Remember a 20A breaker will happily run forever at 22A and will take nearly an hour to open at 25A.

The 1.5mm² ground conductor is generally accepted as being sufficient, however you should really do a PFC (Projected Fault Current) calculation and/or measure the loop impedance (special kit required). The issue being that on a long cable run a L to E fault may not result in sufficient current to open the breaker.

Provided the cable run is less than 20m or so I would not worry about doing any calculations or measurements particularly as you're using earth leakage protection.

[tiger2]

Thanks for the fast reply Crossy. This circuit will have a 9000 BTU

aircon, two pump units (1000W each) and power tools on it. The total

wattage is ~5280. The earth lead will be about 8-10 meters long.

I thought using 25A breaker(s) would be needed for the surge on starting

the pumps, aircon, etc

Do you still recommend a 20A breaker?

[Crossy]

Bangkok Cable rate their 2.5mm² VAF-GRD (T&E) at 22A in air, alternative fixing methods will only de-rate from this value. Hence 20A max breaker on 2.5.

20A at 220V is 4400W, your 2x1kW pumps plus 9000 BTA A/C (about 1-1.5kW draw) are going to put you quite close anyway and the switch-on surge if all three start together will likely drop the breaker (possibly even a 25A unit).

Conventional wisdom would put the A/C on it's own circuit, another for the two pumps and a final circuit for the outlets. All in 2.5 and all with 20A breakers.

If you must put them all on one circuit then try to obtain a C-curve MCB, these are intended for motor loads and have a much slower trip time on transient overloads.

It is unwise to simply put in a bigger breaker as you are reducing the overload protection of your circuit which in extreme cases could lead to cable failure and associated nasties.

UK regs do allow the use of oversize breakers if there is no possibility of overload (ie the load is fixed but has a large start surge) so if you split off the outlets from the A/C and pumps then those could go on a single feed with a breaker big enough that it won't open on start, but I really wouldn't recommend it.

The issue is that you never know what someone will add to a circuit later which could take the cable into overload but not open the oversize breaker.

Why not consider running a 6mm² sub-main from the main DB to a small DB in the workshop, you could protect the 6mm with a 40A breaker at the main DB. That would also put your RCD protection in the workshop where it can be easily reset.

[tiger2]

Hi Crossy

Why not consider running a 6mm² sub-main from the main DB to a small DB in the workshop, you could protect the 6mm with a 40A breaker at the main DB. That would also put your RCD protection in the workshop where it can be easily reset.

About an hour ago me and the electrician talked and decided on a plan. Now I've returned home and it appears you have telepathic powers because this is what we planned on doing

Main DB with a 40A MCB (BTicino thermal-magnetic) feeding a 6mm2 sub-main cable to a 4-slot DB (in the workshop) with a 25A (or 32A) RCD. We keep the 2.5mm wiring to the 3 receptacles. Yay or nay???

Thanks a bunch Crossy

Edited August 11, 2011 by tiger2

[Crossy]

Yay.

It's happened in the past, the telepathetic thing that is

Sadly I've not worked out how to profit from these powers

Actually it's more likely just common sense prevailing.

[electau]

Size of the earthing conductor is based on the size of the current carrying conductors for 2.5sqmm this is normally 2.5sqmm except where the earth conductor is laid up in the same sheath as the current carrying conductors. Ref AS/NZ3000. Table 5.1.

The earth loop L to E impedance can be carried out using a standard continuity tester. You have to check the earth continuity as one of the tests before connection to supply.

[Crossy]

Whilst the above is undoubtedly correct, I doubt if any of the non-sparks members of this board has a suitable instrument for measuring L-E loop impedance in their toolbox, hence my reference to special kit. Most of us who do any work at all on house wiring will have a cheap multimeter which is adequate for checking earth continuity where the cable runs are relatively short.

[electau]

Whilst the above is undoubtedly correct, I doubt if any of the non-sparks members of this board has a suitable instrument for measuring L-E loop impedance in their toolbox, hence my reference to special kit. Most of us who do any work at all on house wiring will have a cheap multimeter which is adequate for checking earth continuity where the cable runs are relatively short.

A multi meter is a continuity tester on the low ohms range for most practical applications. ( To get an accurate reading one requires a range of 0 to 3 ohms).